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# Laser

15.10.2021
09:55 Laser treatment shows potential to transform glaucoma management in Africa

Laser treatment has the potential to transform the management of glaucoma in Africa, and to prevent more people from going irreversibly blind, particularly in regions with high disease prevalence and incidence, suggests new research published in Lancet Global Health.

09:45 Identification of electrostatic two-stream instabilities associated with a laser-driven collisionless shock in a multicomponent plasma. (arXiv:2110.07175v1 [physics.plasm-ph])

Electrostatic two-stream instabilities play essential roles in an electrostatic collisionless shock formation. They are a key dissipation mechanism and result in ion heating and acceleration. Since the number and energy of the shock-accelerated ions depend on the instabilities, precise identification of the active instabilities is important. Two-dimensional particle-in-cell simulations in a multicomponent plasma reveal ion reflection and acceleration at the shock front, excitation of a longitudinally propagating electrostatic instability due to a non-oscillating component of the electrostatic field in the upstream region of the shock, and generation of up- and down-shifted velocity components within the expanding-ion components. A linear analysis of the instabilities for a C2H3Cl plasma using the one-dimensional electrostatic plasma dispersion function, which includes electron and ion temperature effects, shows that the most unstable mode is the electrostatic ion-beam two-stream

09:45 Tutorial on laser linewidths. (arXiv:2110.06965v1 [physics.optics])

In this tutorial, the physical origins and mathematical analyses of laser linewidths are reviewed. The semi-classical model is based on an equation for the light-mode amplitude that includes random source terms, one term for each process that affects the amplitude (stimulated and spontaneous emission, stimulated absorption, and facet and material loss). Although the source terms are classical, their assigned strengths are consistent with the laws of quantum optics. Analysis of this equation shows that the laser linewidth is proportional to the sum of the (positive) source strengths for all gain and loss processes. Three-level and semiconductor lasers have broader linewidths than comparable four-level lasers, because stimulated absorption and the stimulated emission that compensates it both contribute to the linewidth.

08:06 New trial shows vaginal laser therapy treatments may not work at all

This laser is supposed to rejuvenate vaginal tissue. But scientists say it’s no better than a placebo.

00:40 Laser treatment could significantly improve glaucoma care in Africa, potentially at no extra cost

Laser treatment has the potential to transform the management of glaucoma in Africa, and to prevent more people from going irreversibly blind, particularly in regions with high disease prevalence and incidence, suggests new research. Conducted in Tanzania, the research is the first randomised controlled trial exploring the use of the laser treatment, Selective Laser Trabeculoplasty (SLT), for patients with glaucoma in Sub-Saharan Africa.

14.10.2021
05:18 Diode pumped silicate fiber for visible laser emission. (arXiv:2110.06515v1 [physics.optics])

We demonstrate yellow laser emission using silicate glass fiber as the gain medium. By employing core pumping using widely available GaN diode lasers with emission at 445 nm, we show that Dy3+-doped aluminosilicate glass fiber can be readily excited creating sufficient gain at 581 nm. In this proof of concept demonstration, the maximum output power generated was 3 mW with a slope efficiency approximately 1.8% with respect to injected pump.

05:18 Valley polarization control in WSe2 monolayer by a single-cycle laser pulse. (arXiv:2110.06433v1 [physics.optics])

Abstract The valley degree of freedom in two-dimensional materials provides an opportunity to extend the functionalities of valleytronics devices. Very short valley lifetimes demand the ultrafast control of valley pseudospin. Here, we theoretically demonstrate the control of valley pseudospin in WSe2 monolayer by single-cycle linearly polarized laser pulse. We use the asymmetric electric field controlled by the carrier-envelope phase (CEP) to make the valley polarization between K and K'-point in the Brillouin zone (BZ). Time-dependent density functional theory with spin-orbit interaction reveals that no valley asymmetry and its CEP dependence is observed within the linear-optical limit. In the nonlinear-optical regime, linearly polarized pulse induces a high degree of valley polarization and this polarization is robust against the field strength. Valley polarization strongly depends and oscillates as a function of CEP. The carrier density distribution forms nodes as the laser

13.10.2021
07:54 Thermal runaway of silicon-based laser sails. (arXiv:2110.06185v1 [physics.optics])

Laser sail-based spacecraft -- where a powerful earth-based laser propels a lightweight outer-space vehicle -- have been recently proposed by the Breakthrough Starshot Initiative as a means of reaching relativistic speeds for interstellar spacetravel. The laser intensity at the sail required for this task is at least 1 GW$\cdot$m$^{-2}$ and, at such high intensities, thermal management of the sail becomes a significant challenge even when using materials with low absorption coefficients. Silicon has been proposed as one leading candidate material for the sail due to its low sub-bandgap absorption and high index of refraction, which allows for low-mass-density designs. However, here we show that the temperature-dependent bandgap of silicon combined with two-photon absorption processes can lead to thermal runaway for even the most optimistic viable assumptions of the material quality. From our calculations, we set bounds on the maximum laser intensities that can be used for a thermally

07:54 Femtosecond laser micromachining for integrated quantum photonics. (arXiv:2110.06162v1 [quant-ph])

Integrated quantum photonics, i.e. the generation, manipulation and detection of quantum states of light in integrated photonic chips, is revolutionizing the field of quantum information in all applications, from communications to computing. Although many different platforms are being currently developed, from silicon photonics to lithium niobate photonic circuits, none of them has shown the versatility of femtosecond laser micromachining (FLM) in producing all the components of a complete quantum system, encompassing quantum sources, reconfigurable state manipulation, quantum memories and detection. It is in fact evident that FLM has been a key enabling tool in the first-time demonstration of many quantum devices and functionalities. Although FLM cannot achieve the same level of miniaturization of other platforms, it still has many unique advantages for integrated quantum photonics. In particular, in the last five years, FLM has greatly expanded its range of quantum applications with

07:54 Machine learning for laser-induced electron diffraction imaging of molecular structures. (arXiv:2110.06097v1 [physics.chem-ph])

Ultrafast diffraction imaging is a powerful tool to retrieve the geometric structure of gas-phase molecules with combined picometre spatial and attosecond temporal resolution. However, structural retrieval becomes progressively difficult with increasing structural complexity, given that a global extremum must be found in a multi-dimensional solution space. Worse, pre-calculating many thousands of molecular configurations for all orientations becomes simply intractable. As a remedy, here, we propose a machine learning algorithm with a convolutional neural network which can be trained with a limited set of molecular configurations. We demonstrate structural retrieval of a complex and large molecule, Fenchone (C$_{10}$H$_{16}$O), from laser-induced electron diffraction (LIED) data without fitting algorithms or ab initio calculations. Retrieval of such a large molecular structure is not possible with other variants of LIED or ultrafast electron diffraction. Combining electron diffraction

07:29 Laser vagina menopause therapy shows no benefit in trial

The treatment, meant to rejuvenate and treat dryness, worked no better than a placebo procedure.

07:19 Laser vagina menopause therapy shows no benefit in trial

The treatment, meant to rejuvenate and treat dryness, worked no better than a placebo procedure.

12.10.2021
10:43 Stable positron acceleration via laser-augmented blowouts in two-column plasma structures. (arXiv:2110.05226v1 [physics.plasm-ph])

We propose a setup for efficient positron acceleration consisting of an electron driver and a laser pulse creating a two-fold plasma column structure. The resulting wakefield is capable of stably accelerating positron bunches over long distances even when evolution of the driver is considered. The scheme is studied by means of particle-in-cell simulations. Further, the analytical expression for the accelerating and focusing fields are obtained, showing the equilibrium lines along which the witness bunch is accelerated.

10:43 Intermittent excitation of parametric instabilities enhanced by synergistic effect in broadband laser plasma interaction. (arXiv:2110.04750v1 [physics.plasm-ph])

A new evolution pattern for parametric instabilities in the non-linear stage driven by a broadband laser is studied with kinetic particle-in-cell simulations. It is found that an intermittent excitation of parametric instabilities caused by the high-intensity pulses, which are generated by the laser intensity variation of the broadband laser. The synergism between these high-intensity pulses reduces the Landau damping through the co-propagation of the electron-plasma waves and hot electrons, promoting the transition from convective to absolute stimulated Raman scattering. These intermittently excited parametric instabilities, especially absolute instability, will produce over-expected hot electrons compared with linear theory. However, comparisons between the energy of scattered lights for various bandwidth lasers indicate suppression of parametric instabilities with large laser bandwidths. Our works provided an opportunity to advance the understanding of the broadband laser-plasma

10:43 Bogoliubov theory of the laser linewidth and application to polariton condensates. (arXiv:2110.04611v1 [physics.optics])

For a generic semi-classical laser dynamics in the complex Ginzburg-Landau form, we develop the Bogoliubov approach for the computation of the linewidth. Our method provides a unifying perspective of the treatments by Henry and Petermann: both broadening mechanisms are ascribed to the non-orthogonality of the Bogoliubov modes, which live in a space with doubled degrees of freedom. As an application, the method allows to study the interplay of interactions and spatial inhomogeneity typical of polariton condensates: the concept of gain guided versus conservative modes is illustrated by varying the width of the pump spot and the frequency of the trap; it is shown that the traditional theory of the Henry and Petermann factors fails dramatically in the presence of a nonlinear refractive index (i.e. polariton-polariton interactions). In particular, we point out that the Henry's single mode approach performs poorly for an inhomogeneous condensate because the exact density-density correlator

10:43 Time shifting deviation method enhanced laser interferometry: ultrahigh precision localizing of traffic vibration using urban fiber link. (arXiv:2110.04449v1 [physics.ins-det])

Using fiber network as a huge sensing system will enrich monitoring methods of public infrastructures and geological disasters. With traditional cross-correlation method, laser interferometer has been used to detect and localize the vibration event. However, the random error induced by cross-correlation method limits the localization accuracy, and makes it not suitable for ultrahigh precision localizing applications. We propose a novel time shifting deviation (TSDEV) method, which has advantages over cross-correlation method in practicability and localization accuracy. Three experiments are carried out to demonstrate the novelty of the TSDEV method. In lab test, vibration localization accuracy of ~2.5 m is realized. In field tests, TSDEV method enhanced interferometry is applied to monitor the urban fiber link. Traffic vibration events on the campus road and Beijing ring road have been precisely localized and analyzed, respectively. The proposed technique will extend the function of

11.10.2021

The gunship, nicknamed "Hell in the Sky," packs a serious punch with three side-firing weapons, including a 25mm Gatling gun, a 40mm Bofors cannon, and a 105mm howitzer. The fourth will be the AHEL, a chemical energy weapon, unleashing concentrated pulses of light to transfer energy to the target, quickly heating it and damaging it.

05:59 Imaging with Diffractive Axicons Rapidly Milled on Sapphire by Femtosecond Laser Ablation. (arXiv:2110.04113v1 [physics.optics])

Fabrication of large area (sub 1 cm cross-section) micro-optical components in a short period of time (approximately 10 min) and with lesser number of processing steps is highly desirable and cost-effective. In the recent years, femtosecond laser fabrication technology has revolutionized the field of manufacturing by offering the above capabilities. In this study, a fundamental diffractive optical element, binary axicon i.e axicon with two phase or amplitude levels, has been designed in three configurations namely conventional axicon, photon sieve axicon (PSA) and sparse PSA and directly milled onto the Sapphire substrate. The fabrication results revealed that a single pulse burst fabrication can produce a flat and smooth profile than pulse overlapped fabrication which gives rise to surface damage and increased roughness. The fabricated elements were processed in IsoPropyl Alcohol and Potassium Hydroxide to remove debris and redeposited amorphous Sapphire. An incoherent illumination

10.10.2021
00:13 A better black hole laser may prove a circuitous 'Theory of Everything'

Researchers propose quantum circuit black hole lasers to explore Hawking radiation.

08.10.2021
10:55 LLNL researchers observe laser-driven tin ejecta microjet interactions

The experimental observations of high-velocity particle-laden flow interactions has been sparse, given the difficulty of generating high-velocity flows

07:06 $\Lambda$-enhanced gray molasses in a tetrahedral laser beam geometry. (arXiv:2110.03064v1 [physics.atom-ph])

We report observation of sub-Doppler cooling of lithium using an irregular-tetrahedral laser beam arrangement, which is produced by a nanofabricated diffraction grating. We are able to capture 11(2) % of the lithium atoms from a grating magneto-optical trap into $\Lambda$-enhanced $D_1$ gray molasses. The molasses cools the captured atoms to a radial temperature of 60(9) $\mu$K and an axial temperature of 23(3) $\mu$K. In contrast to results from conventional counterpropagating beam configurations, we do not observe cooling when our optical fields are detuned from Raman resonance. An optical Bloch equation simulation of the cooling dynamics agrees with our data. Our results show that grating magneto-optical traps can serve as a robust source of cold atoms for tweezer-array and atom-chip experiments, even when the atomic species is not amenable to sub-Doppler cooling in bright optical molasses.

07:06 Divergent effects of laser irradiation on ensembles of nitrogen-vacancy centers in bulk and nano-diamonds: implications for biosensing. (arXiv:2110.02955v1 [physics.optics])

Ensembles of negatively charged nitrogen vacancy centers (NV-) in diamond have been proposed for sensing of magnetic fields and paramagnetic agents, and as a source of spin-order for the hyperpolarization of nuclei in magnetic resonance applications. To this end, strongly fluorescent nanodiamonds represent promising materials, with large surface areas and dense ensembles of NV-. However, surface effects tend to favor the less useful neutral form, the NV0 centers. Here, we study the fluorescence properties and optically detected magnetic resonance (ODMR) of NV- centers as a function of laser power in strongly fluorescent bulk diamond and in nanodiamonds obtained by nanomilling the native material. In bulk diamond, we find that increasing laser power increases ODMR contrast, consistent with a power-dependent increase in spin-polarization. Surprisingly, in nanodiamonds we observe a non-monotonic behavior, with a decrease in ODMR contrast at higher laser power that can be ascribed to more

07.10.2021
20:07 Researchers observe laser-driven tin ejecta microjet interactions

The experimental observations of high-velocity particle-laden flow interactions has been sparse, given the difficulty of generating high-velocity flows of many particles. These observations play an important role in understanding a wide range of natural phenomena, ranging from planetary formation to cloud interactions.

09:52 Femtosecond laser inscriptions in Kerr nonlinear transparent media: dynamics in the presence of K-photon absorptions, radiative recombinations and electron diffusions. (arXiv:2110.02562v1 [physics.optics])

Femtosecond lasers interacting with Kerr nonlinear optical materials, propagate in form of filaments due to the balance of beam diffraction by self-focusing induced by the Kerr nonlinearity. Femtosecond laser filamentation is a universal phenomenon that belongs to a general class of processes proper to ultrashort lasers processing systems, associated with the competition between nonlinearity and dispersion also known to promote optical solitons. The present work considers a model describing femtosecond laser inscriptions in a transparent medium with Kerr nonlinearity. Upon inscription, the laser stores energy in the optical material which induces an electron plasma. The model consists of a cubic complex Ginzburg-Landau equation, in which an additional K-order nonlinear term takes into account K-photon absorption processes. The complex Ginzburg-Landau equation is coupled to a time first-order nonlinear ordinary differential equation, accounting for time evolution of the plasma density.

05.10.2021
08:10 Optimized laser ion acceleration at the relativistic critical density surface. (arXiv:2110.01257v1 [physics.plasm-ph])

In the effort of achieving high-energetic ion beams from the interaction of ultrashort laser pulses with a plasma, volumetric acceleration mechanisms beyond Target Normal Sheath Acceleration have gained attention. A relativisticly intense laser can turn a near critical density plasma slowly transparent, facilitating a synchronized acceleration of ions at the moving relativistic critical density front. While simulations promise extremely high ion energies in in this regime, the challenge resides in the realization of a synchronized movement of the ultra-relativistic laser pulse ($a_0\gtrsim 30$) driven reflective relativistic electron front and the fastest ions, which imposes a narrow parameter range on the laser and plasma parameters. We present an analytic model for the relevant processes, confirmed by a broad parameter simulation study in 1D- and 3D-geometry. By tayloring the pulse length plasma density profile at the front side, we can optimize the proton acceleration performance

08:10 Phonon-laser ultrasensitive force sensor. (arXiv:2110.01146v1 [quant-ph])

Developing nano-mechanical oscillators for ultrasensitive force detection is of great importance in exploring science. We report our achievement of ultrasensitive detection of the external force regarding the radio-frequency electric field by a nano-sensor made of a single trapped $^{40}$Ca$^{+}$ ion under injection-locking, where squeezing is additionally applied to detection of the smallest force in the ion trap. The employed ion is confined stably in a surface electrode trap and works as a phonon laser that is very sensitive to the external disturbance. The injection-locking drove the ion's oscillation with phase synchronization, yielding the force detection with sensitivity of 347 $\pm$ 50 yN/$\sqrt{Hz}$. Further with 3 dB squeezing applied on the oscillation phase variance, we achieved a successful detection of the smallest force to be 86.5 $\pm$ 70.1 yN.

08:10 Quantum optics of strongly laser--driven atoms and generation of high photon number optical cat states. (arXiv:2110.01032v1 [quant-ph])

Recently we have demonstrated the quantum nature of light in strongly laser driven atoms, and we have shown how the process of high harmonic generation can be used for the creation of highly non-classical light states, in particular superpositions of two coherent states, i.e., optical Schr\"{o}dinger "cat" states [M. Lewenstein et al., Generation of optical Schr\"odinger cat states in intense laser-matter interactions, Nat. Phys. (2021)]. Here, we investigate the quantum optical description of the interaction, incorporating many atoms and the back-action of the high harmonic generation or above threshold ionization processes on the coherent state of the driving laser field. We show how the conditioning on high harmonic generation and above threshold ionization can lead to non-classical light states, and we discuss the key parameters that can be used for controlling and characterizing the non-classical states obtained after high harmonic generation. The theoretical results have been

04.10.2021
15:11 What’s the Biggest Laser in the World?

I tried to find the single biggest laser in the world, but it turns out I'm spoiled for choice. The post What’s the Biggest Laser in the World? appeared first on ExtremeTech.

03:54 Terahertz annular antenna driven with a short intense laser pulse. (arXiv:2110.00325v1 [physics.plasm-ph])

Generation of terahertz radiation by an oscillating discharge, excited with short laser pulses, may be controlled by geometry of the irradiated target. In this work, an annular target with a thin slit is considered as an efficient emitter of secondary radiation when driven by a short intense laser pulse. Under the irradiation, a slit works as a diode, which is quickly filled by dense plasma, closing the circuit for a travelling discharge pulse. Such a diode defines the discharge pulse propagation direction in a closed contour, enabling its multiple passes along the coil.The obtained oscillating charge efficiently generates terahertz waves with a maximum along the coil axis and controllable characteristics.

01.10.2021
15:20 New laser meets demanding requirements for driving cutting-edge attosecond light sources

Researchers have combined a fiber-laser system with recent advancements in multi-pass cells to create a laser with a unique combination of few-cycle pulses at high average power, pulse energy and repetition rate and with stable carrier envelope phase (CEP) operation. These characteristics make the new laser ideal for driving next generation attosecond sources, such as those at the Extreme Light Infrastructure (ELI) in Europe.

04:38 All-optical probe of three-dimensional topological insulators based on high-harmonic generation by circularly-polarized laser fields. (arXiv:2109.15291v1 [cond-mat.mes-hall])

We report the observation of a novel nonlinear optical response from the prototypical three-dimensional topological insulator Bi$_2$Se$_3$ through the process of high-order harmonic generation. We find that the generation efficiency increases as the laser polarization is changed from linear to elliptical, and it becomes maximum for circular polarization. With the aid of a microscopic theory and a detailed analysis of the measured spectra, we reveal that such anomalous enhancement encodes the characteristic topology of the band structure that originates from the interplay of strong spin-orbit coupling and time-reversal symmetry protection. Our study reveals a new platform for chiral strong-field physics and presents a novel, contact-free, all-optical approach for the spectroscopy of topological insulators. The implications are in ultrafast probing of topological phase transitions, light-field driven dissipationless electronics, and quantum computation.

04:38 Ionization states for the multi-petawatt laser-QED regime. (arXiv:2109.14871v1 [physics.atom-ph])

A paradigm shift in the physics of laser-plasma interactions is approaching with the commissioning of multi-petawatt laser facilities world-wide. Radiation reaction processes will result in the onset of electron-positron pair cascades and, with that, the absorption and partitioning of the incident laser energy, as well as the energy transport throughout the irradiated targets. To accurately quantify these effects, one must know the focused intensity on target in-situ. In this work, a new way of measuring the focused intensity on target is proposed based upon the ionization of Xe gas at low ambient pressure. The field ionization rates from Phys. Rev. A 59, 569 (1999) and from Phys. Rev. A 98, 043407 (2018), where the latter rate has been derived using quantum mechanics, have been implemented for the first time in the particle-in-cell code SMILEI [Comput. Phys. Commun. 222, 351-373 (2018)]. A series of one- and two-dimensional simulations are compared and shown to reproduce the charge

04:38 Enantioselective chiral orientation induced by a combination of a long and a short laser pulse. (arXiv:2109.14693v1 [physics.optics])

Enantioselective orientation of chiral molecules excited by a shaped picosecond laser pulse and a delayed femtosecond pulse is considered. Using quantum mechanical simulations, we demonstrate a strong field-free enantioselective orientation along the laser propagation direction. In addition, we use a classical model to reproduce the enantioselective orientation. Moreover, the analysis of the corresponding classical system allows understanding the qualitative features of the induced enantioselective orientation. The strong enantioselective orientation may be used for the separation of chiral enantiomers using inhomogeneous electrostatic fields.

30.09.2021
07:21 Fermi pencil beams and off-axis laser detection. (arXiv:2109.14023v1 [math.AP])

This paper concerns the reconstruction of properties of narrow laser beams propagating in turbulent atmospheres. We consider the setting of off-axis measurements, based on light detection away from the main path of the beam. We first model light propagation in the beam itself by macroscopic approximations of radiative transfer equations that take the form of Fermi pencil beam or fractional Fermi pencil beam equations. Such models are effective in the small mean-free-path, large transport-mean-free path regime. The reconstruction of their constitutive parameters is also greatly simplified compared to the more accurate radiative transfer equations or (fractional) Fokker-Planck models. From off-axis measurements based on wide-angle single scattering off the beam, we propose a framework allowing us to reconstruct the main features of the beam, and in particular its direction of propagation and the location of the emitting source.

07:21 Laser-Induced Electronic and Vibronic Dynamics in the Pyrene Molecule and its Cation. (arXiv:2109.14397v1 [physics.chem-ph])

Among polycyclic aromatic hydrocarbons, pyrene is widely used as an optical probe thanks to peculiar ultraviolet absorption and infrared emission features. Interestingly, this molecule is also an abundant component of the interstellar medium, where it is detected via its unique spectral fingerprints. In this work, we present a comprehensive first-principles study on the electronic and vibrational response of pyrene and its cation to ultrafast, coherent pulses in resonance with their optically active excitations in the ultraviolet region. The analysis of molecular symmetries, electronic structure, and linear optical spectra is used to interpret transient absorption spectra and kinetic energy spectral densities computed for the systems excited by ultrashort laser fields. By disentangling the effects of the electronic and vibrational dynamics via \textit{ad hoc} simulations with stationary and moving ions, and, in specific cases, with the aid of auxiliary model systems, we rationalize

07:21 Branched flow of intense laser light in porous media. (arXiv:2109.14327v1 [physics.plasm-ph])

Branched flow is an interesting phenomenon that can occur in diverse systems. It is usually considered a linear phenomenon in that the flow does not alter the medium properties. Branched flow of laser light has recently been discovered. This provides an opportunity for investigating whether nonlinear branched flow can also occur. Here we found by two-dimensional particle-in-cell simulations that intense laser light propagating in a porous medium can indeed form light branches. In particular, dynamic ionization induced by the laser can enhance the density variations along the laser path and thus the light branching. However, too-intense lasers can suppress branching by smoothing the electron density. The branching properties and the location of the first branching point agrees well with an analysis based on a Schr\"odinger-like equation for the laser electric field. Branched flow of intense light in plasma opens up a new realm of intense laser-matter interaction, which would arouse

07:21 Beam-Beam Interaction in a Dielectric Laser Accelerator Electron-Positron Collider. (arXiv:2109.14221v1 [physics.acc-ph])

We examine through numerical calculation the collision of counter-propagating trains of optically spaced electron/positron microbunches in a 1 TeV collider scenario for a dielectric laser accelerator (DLA). A time-dependent envelope equation is derived for arbitrary number of bunches in the classical limit, with inclusion of the radiation reaction force (RRF). Example parameters are examined based on a constrained luminosity relation that takes into account the bunch charge for optimal efficiency, material damage limits, and power constraints. We find that for initially identical counter-propagating Gaussian bunch trains the periodic temporal structure leads to a peak in luminosity with number of bunches. For longer bunch trains, the enhancement then decreases inversely with number of bunches. The corresponding fractional energy loss of the beam is found to be of order 1.75\%, which is reduced to 0.35\% when the nonlinear radial dependence of the transverse force is included, with an

07:21 A Robust and Novel Linear Fiber Laser Mode-locked by Nonlinear Polarization Evolution in All-polarization-maintaining Fibers. (arXiv:2109.14202v1 [physics.optics])

We demonstrate a novel, robust and compact fiber laser mode-locked by nonlinear polarization evolution (NPE) in polarization-maintaining (PM) fibers. The reflectivity of the artificial saturable absorber (SA) is analyzed to explain the mode-locking mechanism in the laser cavity. Experimentally, three linear laser schemes that feature repetition rates 94 MHz, 124 MHz and 133 MHz are systematically investigated. When the pump power is 1100 mW, the 124-MHz laser cavity delivers highly stable pulses with a single-pulse energy of 0.92 nJ. After the compression, the pulse duration obtained from the 124-MHz fiber laser is 250 fs, while the corresponding transform-limited pulse duration is 124 fs. The highest fundamental repetition rate that could be achieved in our experiment is 133 MHz, as mentioned above. The noise characterization has been performed with different cavity lengths and therefore different net-cavity dispersion. The 68-fs timing jitter and the 0.01% relative intensity noise

07:21 Fermi pencil beams and off-axis laser detection. (arXiv:2109.14023v1 [math.AP])

This paper concerns the reconstruction of properties of narrow laser beams propagating in turbulent atmospheres. We consider the setting of off-axis measurements, based on light detection away from the main path of the beam. We first model light propagation in the beam itself by macroscopic approximations of radiative transfer equations that take the form of Fermi pencil beam or fractional Fermi pencil beam equations. Such models are effective in the small mean-free-path, large transport-mean-free path regime. The reconstruction of their constitutive parameters is also greatly simplified compared to the more accurate radiative transfer equations or (fractional) Fokker-Planck models. From off-axis measurements based on wide-angle single scattering off the beam, we propose a framework allowing us to reconstruct the main features of the beam, and in particular its direction of propagation and the location of the emitting source.

29.09.2021
10:32 Targetless Extrinsic Calibration of Stereo Cameras, Thermal Cameras, and Laser Sensors in the Wild. (arXiv:2109.13414v1 [cs.RO])

The fusion of multi-modal sensors has become increasingly popular in autonomous driving and intelligent robots since it can provide richer information than any single sensor, enhance reliability in complex environments. Multi-sensor extrinsic calibration is one of the key factors of sensor fusion. However, such calibration is difficult due to the variety of sensor modalities and the requirement of calibration targets and human labor. In this paper, we demonstrate a new targetless cross-modal calibration framework by focusing on the extrinsic transformations among stereo cameras, thermal cameras, and laser sensors. Specifically, the calibration between stereo and laser is conducted in 3D space by minimizing the registration error, while the thermal extrinsic to the other two sensors is estimated by optimizing the alignment of the edge features. Our method requires no dedicated targets and performs the multi-sensor calibration in a single shot without human interaction. Experimental

10:10 Laser Cooling of a Yb Doped Silica Fiber by 18 Kelvin From Room Temperature. (arXiv:2109.13872v1 [physics.optics])

A ytterbium doped silica optical fiber has been cooled by 18.4K below ambient temperature by pumping with 20W of 1035nm light in vacuum. In air, cooling by 3.6K below ambient was observed with the same 20W pump. The temperatures were measured with a thermal imaging camera and differential luminescence thermometry. The cooling efficiency is calculated to be 1.2+-0.1%. The core of the fiber was codoped with Al3+ for an Al to Yb ratio of 6:1, to allow for a larger Yb concentration and enhanced laser cooling.

10:10 Satellite-assisted laser magnetometry with mesospheric sodium. (arXiv:2109.13555v1 [physics.atom-ph])

Magnetic field sensing provides crucial insights into various geophysical phenomena such as atmospheric currents, crustal magnetism, and detection of oceanic circulation. In this paper, a method for remote detection of magnetic fields using mesospheric sodium with an assisting satellite is proposed. Sodium atoms in the mesosphere are optically pumped with a ground-based laser beam. A satellite-borne detector is used to measure magneto-optical rotation of the polarization of a probe laser beam by the sodium atoms. This sensitive magnetometry method benefits from direct detection of laser photons and complements the existing space- and aircraft-borne techniques by probing magnetic fields at intermediate altitudes inaccessible to those.

10:10 Electron Acceleration Using Twisted Laser Wavefronts. (arXiv:2109.13553v1 [physics.plasm-ph])

Using plasma mirror injection we demonstrate, both analytically and numerically, that a circularly polarized helical laser pulse can accelerate highly collimated dense bunches of electrons to several hundred MeV using currently available laser systems. The circular-polarized helical (Laguerre-Gaussian) beam has a unique field structure where the transverse fields have helix-like wave-fronts which tend to zero on-axis where, at focus, there are large on-axis longitudinal magnetic and electric fields. The acceleration of electrons by this type of laser pulse is analysed as a function of radial mode number and it is shown that the radial mode number has a profound effect on electron acceleration close to the laser axis.Using three-dimensional particle-in-cell simulations a circular-polarized helical laser beam with power of 0.6 PW is shown to produce several dense attosecond bunches. The bunch nearest the peak of the laser envelope has an energy of 0.47 GeV with spread as narrow as 10\%,

10:10 Characteristics of room temperature bipolar photoconductance in 150 GHz probe transients obtained from normal and irradiated silicon illuminated by 532 nm laser. (arXiv:2109.13326v1 [cond-mat.mtrl-sci])

A negative kink in excess conductivity is observed in p-type non-degenerate (moderate dopant concentration) silicon wafers when excited by a very narrow pulse of 532 nm laser appearing just after the complete positive decay of dark conductivity voltage. Most of the Si samples are pristine, and 3 of them are irradiated with gamma, proton, and chlorine ion beams respectively. These transients were examined using a time-resolved millimeter-wave conductivity apparatus (TRmmWC ) and the radiofrequency (RF) voltage response (after laser cut-off) consistently reveals a positive peak with nominal decay to zero followed by a negative kink. This negative photoconductivity (NPC) kink develops just after the complete decay of the positive photoconductivity (PPC) and lasts typically ~ 36 us. We present some data on general characteristics obtained from a set of normal (pristine doped Si) wafers and the gamma- and ion beam irradiated silicon (comparing with the parent pristine sample responses) for

10:10 Resonant phase matching by oblique illumination of a dielectric laser accelerator. (arXiv:2109.13300v1 [physics.acc-ph])

In dielectric laser-driven accelerators (DLA), careful tuning of drive-laser wavelength and structure periodicity is typically required in order to hit the resonant condition and match the phase velocity of the accelerating wave to the electron beam velocity. By aggressively detuning (up to 30 mrad) the angle of incidence of the drive laser on a double grating DLA structure, we show that it is possible to recover resonant phase matching and maximize the energy modulation of an externally injected 6 MeV beam in a 800 nm period structure driven using a 780 nm laser. These results show that it is possible to power DLA structures away from their design working point, and excite accelerating fields in the gap with phase profiles that change by a relatively large amount period-to-period. This flexibility is a key feature of DLAs and a critical element in the realization of phase-modulation based ponderomotive focusing to demonstrate MeV energy gain and large capture in a single DLA stage.

28.09.2021
05:46 Laser-induced thermal source for cold atoms. (arXiv:2109.13071v1 [physics.atom-ph])

We demonstrate a simple and compact approach to laser cool and trap atoms based on laser ablation of a pure solid granule. A rapid thermalisation of the granule leads to a fast recovery of the ultra-high vacuum condition required for a long trapping lifetime of the cold gas. We give a proof-of-concept of the technique, performing a magneto-optical trap on the 461 nm $^1S_0\rightarrow^1P_1$ transition of strontium. We get up to 3.5 million of cold strontium-88 atoms with a trapping lifetime of more than 4 s. The lifetime is limited by the pressure of the strontium-free residual background vapour. We also implement an original configuration of permanent magnets to create the quadruple magnetic field of the magneto-optical trap. This laser ablation technique can be generalized to other atomic elements such as transition metals and lanthanide atoms, and shows a strong potential for applications in quantum technologies ranging from quantum computing to precision measurements such as outdoor

05:46 Mono-elemental saturable absorber in mode-locked fiber laser: A review. (arXiv:2109.13024v1 [physics.optics])

Two-dimensional mono-elemental material is an excellent saturable absorber candidate with low saturation intensity, large modulation depth, high nonlinearities, and fast recovery time of excited carriers. Typically, these mono-elemental material with two-dimensional structure possesses tunable bandgap from metallic to semiconducting according to different number of layers. The successful application of these materials as the saturable absorber has exploited the development of mode-locked fiber lasers. Therefore, this review is intended to provide an up-to-date information to the development of mono-elemental saturable absorber for the advances in mode-locked fiber laser, with emphasis on their material properties, synthesis process and material characterization. Meanwhile, issues and challenges of the review research topic will be highlighted and addressed with several concrete recommendations.

05:46 Recent advances of MXene saturable absorber for near-infrared mode-locked fiber laser. (arXiv:2109.13011v1 [physics.optics])

To date, MXene has been discovered for its viability as alternatives to conventional saturable absorber such as carbon nanotube and graphene. The characteristics of high nonlinear saturable absorption, astounding modulation depth, flexible bandgap tunability, and high electron density near Fermi level are the fundamentals of the MXene as an excellent saturable absorber candidate. In particular, the research effort contributed to MXene in nonlinear ul-trafast optics are extensively growing because MXene comprises one of the largest families in 2D nanomaterials that provides huge combination possibilities by forming a class of metal carbide or metal nitride with 2D layered structure. Herein, this review summarizes the recent development on synthesis and material characterization of the MXene, the studies on its nonlinear saturable absorption and the application of the MXene saturable absorber in near-infrared mode-locked fiber laser. Finally, some issues and challenges as well as future

05:46 Scissor-cross ionization injection in laser wakefield accelerators. (arXiv:2109.12527v1 [physics.acc-ph])

We propose to use a frequency doubled pulse colliding with the driving pulse at an acute angle to trigger ionization injection in a laser wakefield accelerator. This scheme effectively reduces the duration that injection occurs, thus high injection quality is obtained. Three-dimensional particle-in-cell simulations show that electron beams with energy of ~500 MeV, charge of ~40 pC, energy spread of ~1% and normalized emittance of a few millimeter milliradian can be produced by ~100 TW laser pulses. By adjusting the angle between the two pulses, the intensity of the trigger pulse and the gas dope ratio, the charge and energy spread of the electron beam can be controlled.

05:46 Observation of high efficiency Betatron radiation from femtosecond petawatt laser irradiated near critical plasmas. (arXiv:2109.12467v1 [physics.plasm-ph])

We present an experimental demonstration of high conversion efficiency Betatron x-ray radiation from petawatt laser irradiated near critical plasmas. Direct laser acceleration serves as the dominant regime when laser pulse of ~5e20 W/cm2 intensity is focused into plasmas with electron density of 3e20 /cm3. Electron beam with a charge of ~35 nC is accelerated up to a maximum energy of 70 MeV and emit x-rays when oscillating in the laser field. The deduced energy conversion efficiency from laser to x-rays is up to 1e-4, orders of magnitude higher than other betatron regimes. Enhancement of acceleration and radiation with sharp plasma density boundary is also obtained and further interpretated with 2D particle-in-cell simulations.

24.09.2021
18:46 Tiny lasers acting together as one: Topological vertical cavity laser arrays

An international research team uses topological platform to demonstrate coherent array of vertical lasers.

18:38 World premiere for virtual laser lab "femtoPro"

In everyday life, we know lasers from numerous applications such as the laser printer or the supermarket scanner. Industrially, lasers are used in material processing for cutting, drilling and labeling, and in medicine for diagnostic and therapeutic procedures. Laser spectroscopy methods are also indispensable in scientific research.

15:59 Tiny lasers acting together as one: Topological vertical cavity laser arrays

Israeli and German researchers have developed a way to force an array of vertical cavity lasers to act together as a single laser—a highly effective laser network the size of a grain of sand. The findings are presented in a new joint research paper published online by the prestigious journal Science on Friday, September 24.

14:31 Tiny lasers acting together as one: Topological vertical cavity laser arrays

Researchers have developed a way to force an array of vertical cavity lasers to act together as a single laser - a highly effective laser network the size of a grain of sand.

11:44 Laser treatment shows potential for reducing industrial chemical processing for vehicles

Long-lasting protection from corrosion is essential for materials used for vehicles and aircraft to ensure structural integrity amid

09:41 Gamma-Ray Flash in the Interaction of a Tightly Focused Single-Cycle Ultraintense Laser Pulse with a Solid Target. (arXiv:2109.11401v1 [physics.plasm-ph])

We employ the $\lambda^3$ regime where a near-single-cycle laser pulse is tightly focused, thus providing the highest possible intensity for the minimal energy at a certain laser power. The quantum electrodynamics processes in the course of the interaction of the ultraintense laser with a solid target are studied via three-dimensional particle-in-cell simulations, revealing the generation of copious $\gamma$-photons and electron-positron pairs. The parametric study on the laser polarisation, target thickness and electron number density shows that the radially polarised laser provides the optimal regime for $\gamma$-photon generation. By varying the laser power in the range of 1 to 300 petawatt we find the scaling of the laser to $\gamma$-photon energy conversion efficiency. The laser-generated $\gamma$-photon interaction with a high-Z target is further studied by using Monte Carlo simulations revealing further electron-positron pair generation and radioactive nuclides creation.

23.09.2021
22:02 Laser treatment shows potential for reducing industrial chemical processing for vehicles

Long-lasting protection from corrosion is essential for materials used for vehicles and aircraft to ensure structural integrity amid extreme operating conditions. Two chemical pre-treatment processes are widely used in industrial settings to prepare for coating adhesion and protect aluminum alloy surfaces against corrosion. While highly regulated, both processes use large quantities of hazardous compounds with known environmental and health risks.

08:28 LLNL explores laser beam shaping to improve metal 3D printing

05:14 Optically Pumped AlGaN Double Heterostructure Deep-UV Laser by Molecular Beam Homoepitaxy: Mirror Imperfections and Cavity Loss. (arXiv:2109.10515v1 [physics.optics])

We demonstrate the first optically pumped sub-300 nm UV laser structures grown by plasma-assisted molecular beam epitaxy on single-crystal bulk AlN. The edge-emitting laser structures fabricated with the AlN/AlGaN heterostructures exhibit multi-mode emission with peak gain at ~284 nm. Having the goal of electrically injected, continuous wave deep-UV AlGaN laser diodes in mind, with its intrinsic material challenges of achieving sufficient optical gain, the optical cavity loss of a laser diode should be minimized. We derive an expression to quantify the effect of mirror imperfections, including slant and surface roughness on the optical mirror loss of a Fabry-P\'erot cavity. It is found that the optical imperfection loss is a superlinear function of the RMS roughness and slant angle of the facets, and also scales as the inverse wavelength squared of the principal lasing mode. This highlights the importance of device processing optimization as Fabry-P\'erot cavities couple to lower

05:01 Researchers use new laser-assisted technology for bioprinting adult neuron cells

A group of researchers including a Concordia PhD student have developed a new method of bioprinting adult neuron cells. They're using a new laser-assisted technology that maintains high levels of cell viability and functionality.

05:01 An Optimal Control Problem for Single-Spot Pulsed Laser Welding. (arXiv:2109.10788v1 [math.OC])

We consider an optimal control problem for a single-spot pulsed laser welding problem. The distribution of thermal energy is described by a quasilinear heat equation. Our emphasis is on materials which tend to suffer from hot cracking, such as aluminum alloys. A simple indicator for the occurrence of hot cracks is the velocity of the solidification front. We therefore formulate an optimal control problem whose objective contains a term which penalizes excessive solidification velocities. The control function to be optimized is the laser power over time, subject to pointwise lower and upper bounds. We describe the finite element discretization of the problem and a projected gradient scheme for its solution. Numerical experiments for material data representing the EN AW 6082-T6 aluminum alloy exhibit interesting laser pulse patterns which perform significantly better than standard ramp-down patterns.

22.09.2021
18:39 Laser beam shaping improves metal 3D printing

Researchers are exploring alternative shapes to the Gaussian beams commonly employed in high-power laser printing processes such as laser powder bed fusion (LBPF).

18:09 Using laser beam shaping to improve metal 3D printing

While laser-based 3D printing techniques have revolutionized the production of metal parts by greatly expanding design complexity, the laser beams traditionally used in metal printing have drawbacks that can lead to defects and poor mechanical performance.

07:59 Real-time imaging of laser-driven nanoplasma expansion. (arXiv:2109.09815v1 [physics.plasm-ph])

The free expansion of a planar plasma surface is a fundamental non-equilibrium process relevant for various fields but as-yet experimentally still difficult to capture. The significance of the associated spatiotemporal plasma motion ranges from astrophysics and controlled fusion to laser machining, surface high-harmonic generation, plasma mirrors, and laser-particle acceleration. Here, we show that x-ray coherent diffractive imaging can surpass existing approaches and enables the quantitative real-time analysis of the sudden free expansion of nanoplasmas. For laser-ionized SiO$_2$ nanospheres, we resolve the formation of the emerging nearly self-similar plasma profile evolution and expose the so far inaccessible shell-wise expansion dynamics including the associated startup delay and rarefaction front velocity. Our results establish time-resolved diffractive imaging as an accurate quantitative diagnostic platform for tracing and characterizing plasma expansion and indicate the

21.09.2021
10:31 Impact of Surface and Pore Characteristics on Fatigue Life of Laser Powder Bed Fusion Ti-6Al-4V Alloy Described by Neural Network Models. (arXiv:2109.09655v1 [cond-mat.mtrl-sci])

In this study, the effects of surface roughness and pore characteristics on fatigue lives of laser powder bed fusion (LPBF) Ti-6Al-4V parts were investigated. The 197 fatigue bars were printed using the same laser power but with varied scanning speeds. These actions led to variations in the geometries of microscale pores, and such variations were characterized using micro-computed tomography. To generate differences in surface roughness in fatigue bars, half of the samples were grit-blasted and the other half machined. Fatigue behaviors were analyzed with respect to surface roughness and statistics of the pores. For the grit-blasted samples, the contour laser scan in the LPBF strategy led to a pore-depletion zone isolating surface and internal pores with different features. For the machined samples, where surface pores resemble internal pores, the fatigue life was highly correlated with the average pore size and projected pore area in the plane perpendicular to the stress direction.

10:31 Analog FM free-space optical communication based on a mid-infrared quantum cascade laser frequency comb. (arXiv:2109.09465v1 [physics.app-ph])

Quantum cascade laser frequency combs are nowadays well-appreciated sources for infrared spectroscopy. Here their applicability for free-space optical communication is demonstrated. The spontaneously-generated intermodal beat note of the frequency comb is used as carrier for transferring the analog signal via frequency modulation. Exploiting the atmospheric transparency window at 4 $\mu$m, an optical communication with a signal-to-noise ratio up to 65 dB is realized, with a modulation bandwidth of 300 kHz. The system tolerates a maximum optical attenuation exceeding 35 dB. The possibility of parallel transmission of an independent digital signal via amplitude modulation at 5 MS/s is also demonstrated.

10:31 Mitigation of substrate coupling effects in RF switch by localized substrate removal using laser processing. (arXiv:2109.09452v1 [physics.app-ph])

With the evolution of radio frequency (RF)/microwave technology, there is a demand for circuits which are able to meet highly challenging RF frontend specifications. Silicon-on-insulator (SOI) technology is one of the leading platforms for upcoming wireless generation. The degradation of performance due to substrate coupling is a key problem to address for telecommunication circuits, especially for the high throw count switches in RF frontends. In this context, a novel technique for local substrate removal is developed to fabricate membranes of mm-sized RF switch which allows for total etching of silicon handler. RF characterization of membranes reveal a superior linearity performance with lowering of 2nd harmonic by 17.7 dB and improvement in insertion losses by 0.38 dB in comparison with High-Resistivity SOI substrates. This improvement leads to a significant increase in frontend efficiency. These results demonstrate a new route for optimization of circuit performance using

10:31 Experimental Observation of Partial Parity-Time Symmetry and Its Phase Transition with a Laser-Driven Cesium Atomic Gas. (arXiv:2109.09275v1 [physics.optics])

Realization and manipulation of parity-time (PT) symmetry in multidimensional systems are highly desirable for exploring nontrivial physics and uncovering exotic phenomena in non-Hermitian systems. Here, we report the first experimental observation of partial PT (pPT) symmetry in a cesium atomic gas coupled with laser fields, where a two-dimensional pPT-symmetric optical potential for probe laser beam is created. A transition of the pPT symmetry from an unbroken phase to a broken one is observed through changing the beam-waist ratio of the control and probe laser beams, and the domains of unbroken, broken, and non-pPT phases are also discriminated unambiguously. Moreover, we develop a technique to precisely determine the location of the exceptional point of the pPT symmetry breaking by measuring the asymmetry degree of the probe-beam intensity distribution. The findings reported here pave the way for controlling multidimensional laser beams in non-Hermitian systems via laser-induced

03:48 Low-level laser treatment can stimulate hair follicles and hair growth, expert finds

A world-renowned hair loss expert from Chula has discovered that low-level laser treatment can stimulate hair follicles and hair growth.

20.09.2021
06:09 A quantum-chemical perspective on the laser-induced alignment and orientation dynamics of the CH$_3$X (X = F, Cl, Br, I) molecules. (arXiv:2109.08413v1 [physics.chem-ph])

The laser-induced alignment and orientation (A&O) dynamics of the CH$_3$X (X = F, Cl, Br, I) molecules is investigated. Best practices for A&O simulations are determined, involving the quantum-chemical determination of molecular parameters necessary for accurate dynamics simulations. Special emphasis is given to the role the molecular properties and pulse parameters play in laser-induced dynamics, providing insight into possible control schemes for preferred A&O dynamics. It is shown that for accurate A&O dynamics simulations it is necessary to employ large basis sets and high levels of electron correlation when computing rotational constants, dipole moments, and polarizabilities. Benchmark-quality values of these molecular parameters are presented for the CH$_3$X (X = F, Cl, Br, I) molecules, obtained with the help of the focal-point analysis (FPA) technique and explicit electronic-structure computations utilizing the gold-standard CCSD(T) approach and basis sets up to quintuple-zeta

17.09.2021
09:52 Hisense анонсировала первый в мире сворачивающийся лазерный телевизор Rollable Screen Laser TV

Китайская компания Hisense, известная производством телевизоров различных типов, объявила о скором начале продаж, по её словам, первого в мире лазерного телевизора со сворачивающимся дисплеем. Объявление было сделано в ходе проходившего недавно мероприятия Global Laser Display Technology and Industry Development Forum. gizmochina.com

07:42 Single-camera Two-Wavelength Imaging Pyrometry for Melt Pool Temperature Measurement and Monitoring in Laser Powder Bed Fusion based Additive Manufacturing. (arXiv:2109.07472v1 [eess.SY])

Melt pool (MP) temperature is one of the determining factors and key signatures for the properties of printed components during metal additive manufacturing (AM). The state-of-the art measurement systems are hindered by both the equipment cost and the large-scale data acquisition and processing demands. In this work, we introduce a novel coaxial high-speed single-camera two-wavelength imaging pyrometer (STWIP) system as opposed to the typical utilization of multiple cameras for measuring MP temperature profiles through a laser powder bed fusion (LPBF) process. Developed on a commercial LPBF machine (EOS M290), the STWIP system is demonstrated to be able to quantitatively monitor MP temperature and variation for 50 layers at high framerates (> 30,000 fps) during a print of five standard fatigue specimens. High performance computing is employed to analyze the acquired big data of MP images for determining each MPs average temperature and 2D temperature profile. The MP temperature

06:37 Method and portable bench for tests of the laser optical calibration system components for the Baikal-GVD underwater neutrino Cherenkov telescope. (arXiv:2108.00097v2 [astro-ph.IM] UPDATED)

The large-scale deep underwater Cherenkov neutrino telescopes like Baikal-GVD, ANTARES or KM3NeT, require calibration and testing methods of their optical modules. These methods usually include laser-based systems which allow to check the telescope responses to the light and for real-time monitoring of the optical parameters of water such as absorption and scattering lengths, which show seasonal changes in natural reservoirs of water. We will present a testing method of a laser calibration system and a set of dedicated tools developed for Baikal- GVD, which includes a specially designed and built, compact, portable, and reconfigurable scanning station. This station is adapted to perform fast quality tests of the underwater laser sets just before their deployment in the telescope structure, even on ice, without darkroom. The testing procedure includes the energy stability test of the laser device, 3D scan of the light emission from the diffuser and attenuation test of the optical

06:37 Single-camera Two-Wavelength Imaging Pyrometry for Melt Pool Temperature Measurement and Monitoring in Laser Powder Bed Fusion based Additive Manufacturing. (arXiv:2109.07472v1 [eess.SY])

Melt pool (MP) temperature is one of the determining factors and key signatures for the properties of printed components during metal additive manufacturing (AM). The state-of-the art measurement systems are hindered by both the equipment cost and the large-scale data acquisition and processing demands. In this work, we introduce a novel coaxial high-speed single-camera two-wavelength imaging pyrometer (STWIP) system as opposed to the typical utilization of multiple cameras for measuring MP temperature profiles through a laser powder bed fusion (LPBF) process. Developed on a commercial LPBF machine (EOS M290), the STWIP system is demonstrated to be able to quantitatively monitor MP temperature and variation for 50 layers at high framerates (> 30,000 fps) during a print of five standard fatigue specimens. High performance computing is employed to analyze the acquired big data of MP images for determining each MPs average temperature and 2D temperature profile. The MP temperature

16.09.2021
09:05 Observations of Ultrafast Superfluorescent Beatings in a Cesium Atomic Vapor Excited by Femtosecond Laser Pulses. (arXiv:2109.07294v1 [physics.atom-ph])

Spontaneous emission from individual atoms in vapor lasts nanoseconds, if not microseconds, and beatings in this emission involve only directly excited energy sublevels. In contrast, the superfluorescent emissions burst on a much-reduced timescale and their beatings involve both directly and indirectly excited energy sublevels. In this work, picosecond and femtosecond superfluorescent beatings are observed from a dense cesium atomic vapor. Cesium atoms are excited by 60-femtosecond long, 800 nm laser pulses via two-photon processes into their coherent superpositions of the ground 6S and excited 8S states. As a part of the transient four wave mixing process, the yoked superfluorescent blue light at lower transitions of 6S - 7P are recorded and studied. Delayed buildup time of this blue light is measured as a function of the input laser beam power using a high-resolution 2 ps streak camera. The power dependent buildup delay time is consistently doubled as the vapor temperature is lowered

15.09.2021
17:26 NASA provides laser for LISA mission

Finding the biggest collisions in the universe takes time, patience, and super steady lasers.

11:58 Laser loops create ultrafast electric currents in solid materials

Theoreticians predict that a unique laser source could produce highly controllable electric currents in any bulk material. This work yields new insights for the development of ultrafast opto-electronic devices, for more efficient photovoltaics, and for the study of electron behavior in solids.

04:41 Laser-assisted binding of ultracold polar molecules with Rydberg atoms in the van der Waals regime. (arXiv:2109.06411v1 [physics.atom-ph])

We study ultracold long-range collisions of heteronuclear alkali-metal dimers with a reservoir gas of alkali-metal Rydberg atoms in a two-photon laser excitation scheme. In a low density regime where molecules remain outside the Rydberg orbits of the reservoir atoms, we show that the two-photon photoassociation (PA) of the atom-molecule pair into a long-range bound trimer state is efficient over a broad range of atomic Rydberg channels. As a case study, we obtain the PA lineshapes for the formation of trimers composed of KRb molecules in the rovibrational ground state and excited Rb atoms in the asymptotic Rydberg levels $n^{2}S_j$ and $n^{2}D_j$, for $n=20-80$. We predict atom-molecule binding energies in the range $10-10^3$ kHz for the first vibrational state below threshold. The average trimer formation rate is order $10^8\, {\rm s}^{-1}$ at 1.0 $\mu$K, and depends weakly on the principal quantum number $n$. Our results set the foundations for a broader understanding of exotic long

13.09.2021
14:34 New laser captures energy like noise-cancelling headphones

Physicists at The Australian National University (ANU) have developed extremely powerful microscopic lasers that are even smaller than the wavelength of the light they produce.

10:02 Deterministic loading of a single strontium ion into a surface electrode trap using pulsed laser ablation. (arXiv:2109.04965v1 [quant-ph])

Trapped-ion quantum technologies have been developed for decades toward applications such as precision measurement, quantum communication and quantum computation. Coherent manipulation of ions' oscillatory motions in an ion trap is important for quantum information processing by ions, however, unwanted decoherence caused by fluctuating electric-field environment often hinders stable and high-fidelity operations.. One way to avoid this is to adopt pulsed laser ablation for ion loading, a loading method with significantly reduced pollution and heat production. Despite the usefulness of the ablation loading such as the compatibility with cryogenic environment, randomness of the number of loaded ions is still problematic in realistic applications where definite number of ions are preferably loaded with high probability. In this paper, we demonstrate an efficient loading of a single strontium ion into a surface electrode trap generated by laser ablation and successive photoionization. The

10:02 Controlling nonlinear interaction in a many-mode laser by tuning disorder. (arXiv:2109.04902v1 [physics.optics])

A many-mode laser with nonlinear modal interaction could serve as a model system to study many-body physics. However, precise and continuous tuning of the interaction strength over a wide range is challenging. Here, we present a unique method for controlling lasing mode structures by introducing random phase fluctuation to a nearly degenerate cavity. We show numerically and experimentally that as the characteristic scale of phase fluctuation decreases by two orders of magnitude, the transverse modes become fragmented and the reduction of their spatial overlap suppresses modal competition for gain, allowing more modes to lase. The tunability, flexibility and robustness of our system provides a powerful platform for investigating many-body phenomena.

10:02 Molecular Laser-Cooling in a Dynamically Tunable Repulsive Optical Trap. (arXiv:2109.04589v1 [physics.atom-ph])

Recent work with laser-cooled molecules in attractive optical traps has shown that the differential AC Stark shifts arising from the trap light itself can become problematic, limiting collisional shielding efficiencies, rotational coherence times, and laser-cooling temperatures. In this work, we explore trapping and laser-cooling of CaF molecules in a ring-shaped repulsive optical trap. The observed dependences of loss rates on temperature and barrier height show characteristic behavior of repulsive traps and indicate strongly suppressed average AC Stark shifts. Within the trap, we find that $\Lambda$-enhanced gray molasses cooling is effective, producing similar minimum temperatures as those obtained in free space. By combining in-trap laser cooling with dynamical reshaping of the trap, we also present a method that allows highly efficient and rapid transfer from molecular magneto-optical traps into conventional attractive optical traps, which has been an outstanding challenge for

10.09.2021
16:32 High average output power achieved in PAPS photocathode drive laser system

The photocathode drive laser is one of the key parts of the beam test system of the Platform of Advanced Photon Source (PAPS). Recently, the researchers from Institute of High Energy Physics (IHEP) of the Chinese Academy of Sciences obtained the output power exceeding 116 W after the main amplifier of the laser. The study was published in Optics Express.

14:20 Leica's new flying robot laser scanner

The company is mastering reality capture by pairing sensors with advanced robots.

04:22 Comprehensive analysis of spherical bubble oscillations and shock wave emission in laser-induced cavitation. (arXiv:2109.04372v1 [physics.flu-dyn])

The dynamics of spherical laser-induced cavitation bubbles in water is investigated by plasma photography, time-resolved shadowgraphs, and single-shot probe beam scattering enabling to portray the transition from initial nonlinear to late linear oscillations. The frequency of late oscillations yields the bubble's gas content. Simulations with the Gilmore model using plasma size as input and oscillation times as fit parameter provide insights into experimentally not accessible bubble parameters and shock wave emission. The model is extended by a term covering the initial shock-driven acceleration of the bubble wall, an automated method determining shock front position and pressure decay, and an energy balance for the partitioning of absorbed laser energy into vaporization, bubble and shock wave energy, and dissipation through viscosity and condensation. These tools are used for analysing a scattering signal covering 102 oscillation cycles. The bubble was produced by a plasma with 1550 K

04:22 Efficient broadband THz generation in BNA organic crystal at Ytterbium laser wavelength. (arXiv:2109.04274v1 [physics.optics])

In this work, we demonstrate BNAs high potential for efficient generation of high power THz using ytterbium laser wavelengths. We study the generation theoretically and experimentally using laser wavelength of 960-1150 nm. Broadband pulses of 0-7 THz and high efficiency of 0.6% are demonstrated.

04:22 Cold CH radicals for laser cooling and trapping. (arXiv:2109.03953v1 [physics.atom-ph])

Ultracold CH radicals promise a fruitful testbed for probing quantum-state controllable organic chemistry. In this work, we calculate CH vibrational branching ratios (VBRs) and rotational branching ratios (RBRs) with ground state mixing. We subsequently use these values to inform optical cycling proposals and consider two possible radiative cooling schemes using the $X^{2}\Pi \leftarrow A^{2}\Delta$ and $X^{2}\Pi \leftarrow B^{2}\Sigma^{-}$ transitions. As a first step towards laser cooled CH, we characterize the effective buffer gas cooling of this species and produce $\sim5\times10^{10}$ CH molecules per pulse with a rotational temperature of 2(1) K and a translational temperature of 7(2) K. We also determine the CH-helium collisional cross section to be $2.4(8)\times10^{-14}$ cm$^{2}$. This value is crucial to correctly account for collisional broadening and accurately extract the in-cell CH density. These cold CH molecules mark an ideal starting point for future laser cooling and

04:22 Multi-frequency telecom fibered laser system for potassium laser cooling. (arXiv:2109.03864v1 [physics.atom-ph])

We describe a compact and versatile multi-frequency laser system for laser-cooling potassium atoms, by frequency doubling a fiber-optic telecom beam ($\simeq 1534$ or $1540$~nm). Low-power fiber-based telecom lasers and components generate a single beam containing the cooling and repumper half frequencies, subsequently amplified by high-power amplifier. A final free-space SHG stage generates a single beam with typically 2.5 W at quasi-resonant frequencies ($\simeq 767$ or $770$~nm) with high-quality mode and ready for laser cooling. This allowed to trap up to $4\times10^9$ $^{41}$K atoms with fast loading times (2.5 s) at sub-Doppler temperatures of 16~$\mu$K. This opens promising perspectives towards versatile and transportable ultracold atom setups.

09.09.2021
07:05 Laser ablation of Fe$_2$B target enriched in 10B content for boron neutron capture therapy. (arXiv:2109.03608v1 [physics.gen-ph])

The technique of laser ablation in liquids is applied to produce of Boron-containing nanoparticles from ablation of a Fe$_2$B bulk target enriched in 10B isotope. Laser ablation of the target in liquid isopropanol results in partial disproportionation to free Fe and Boron while nanoparticles of Fe$_2$B are also presented. Nanoparticles are magnetic and can be collected using a permanent magnet. Average size of nanoparticles is of 15 nm. The content of 10B in generated nanoparticles amounts to 76,9 %. Nanoparticles are biocompatible and can be used in Boron Neutron Capture Therapy.

08.09.2021
18:58 High-fidelity laser-free universal control of trapped ion qubits

15:52 New laser-based microphone calibration system

Researchers at the National Institute of Standards and Technology (NIST) have conducted the first demonstration of a faster and more accurate way to calibrate certain kinds of microphones.

09:55 Optimized laser-assisted electron injection into a quasi-linear plasma wakefield. (arXiv:2109.03053v1 [physics.plasm-ph])

We present a novel electron injection scheme for plasma wakefield acceleration. The method is based on recently proposed technique of fast electron generation via laser-solid interaction: a femtosecond laser pulse with the energy of tens of mJ hitting a dense plasma target at $45^o$ angle expels a well collimated bunch of electrons and accelerates these close to the specular direction up to several MeVs. We study trapping of these fast electrons by a quasi-linear wakefield excited by an external beam driver in a surrounding low density plasma. This configuration can be relevant to the AWAKE experiment at CERN. We vary different injection parameters: the phase and angle of injection, the laser pulse energy. An approximate trapping condition is derived for a linear axisymmetric wake. It is used to optimise the trapped charge and is verified by three-dimensional particle-in-cell simulations. It is shown that a quasi-linear plasma wave with the accelerating field $\sim$ 2.5 GV/m can trap

09:55 Influence of CO2-laser pulse parameters on 13.5 nm extreme ultraviolet emission features from irradiated liquid tin target. (arXiv:2109.02999v1 [physics.plasm-ph])

Laser-produced plasma (LPP) induced during irradiation of a liquid tin droplet with diameter of 150 um and 180 um by CO2 laser pulse with various pulse durations and energies is considered. The two-dimensional radiative magnetohydrodynamic (RMHD) plasma code is used to simulate the emission and plasma dynamics of multicharged ion tin LPP. Results of simulations for various laser pulse durations and 75-600 mJ pulse energies with Gaussian and experimentally taken temporal profiles are discussed. It is found that if the mass of the target is big enough to provide the plasma flux required (the considered case) a kind of dynamic quasi-stationary plasma flux is formed. In this dynamic quasi-stationary plasma flux, an interlayer of relatively cold tin vapor with mass density of 1-2 g/cm3 is formed between the liquid tin droplet and low density plasma of the critical layer. Expanding of the tin vapor from the droplet provides the plasma flux to the critical layer. In critical layer the plasma

09:55 Initial energy deposition and initiation mechanism of nanosecond laser damage caused by KDP surface micro-defects. (arXiv:2109.02784v1 [physics.app-ph])

To enable an exploration of the initiation mechanism of nanosecond laser damage on a potassium dihydrogen phosphate (KDP) surface, a defect-assisted energy deposition model is developed that involves light intensity enhancement and a sub-band gap energy level structure. The simulations provide an explanation on why the laser-induced damage threshold (LIDT) of the KDP crystal is two orders of magnitude lower than the theoretical value. The model is verified by use of the transient images that appear during the laser damage. In addition, the dimensions of the "dangerous" surface defects that are the most sensitive to the laser damage are proposed. This work enables clarification on the initial energy deposition (IED) and initiation mechanism of the nanosecond laser damage caused by the KDP surface defects on micro-nano scale. It is helpful in understanding the laser-matter interactions and to improve the processing technique for high quality optical components.

09:55 Adaptive four-level modeling of laser cooling of solids. (arXiv:2109.02702v1 [physics.optics])

Laser cooling of rare-earth doped solids has been demonstrated across a wide range of material platforms, inspiring the development of simple phenomenological models such as the four-level model to elucidate the universal properties of laser cooling under various operating conditions. However, these models usually require the input of full absorption spectra that must be provided experimentally or by additional complicated atomic modeling. In this letter, we propose that a four-level model, when extended to admit effective energy levels adaptive to the pumping photon energy, can accurately predict the cooling efficiency as a function of temperature and pumping frequency using only few inputs such as the absorption coefficient measured at a single frequency and temperature. Our model exploits the quasi-equilibrium properties of the excitation of rare-earth ions for the determination of the effective four energy levels. The model is validated against published experimental results for a

09:55 Arm locking using laser frequency comb. (arXiv:2109.02642v1 [physics.optics])

In this work, we describe an updated version of single arm locking, and the noise amplification due to the nulls can be flexibly restricted with the help of optical frequency comb. We show that, the laser phase noise can be divided by a specific factor with optical frequency comb as the bridge. The analytical results indicate that, the peaks in the science band have been greatly reduced. The performance of the noise suppression shows that the total noise after arm locking can well satisfy the requirement of time delay interferometry, even with the free-running laser source. We also estimate the frequency pulling characteristics of the updated single arm locking, and the results suggest that the pulling rate can be tolerated, without the risk of mode hopping. Arm locking will be a valuable solution for the noise reduction in the space-borne GW detectors. We demonstrate that, with the precise control of the returned laser phase noise, the noise amplification in the science band can be

07.09.2021
22:51 New powerful laser passes field test

A powerful experimental laser developed by the European Southern Observatory (ESO), TOPTICA Projects and other industry partners passed a key test last month at the Allgaeuer Volkssternwarte Ottobeuren observatory in Germany. The adaptive-optics laser has important additional capabilities compared to existing systems. It is to be installed at the European Space Agency's (ESA) Optical Ground Station in Tenerife, Spain, in the frame of the ESO–ESA Research & Development collaboration. The higher laser power and its chirping system will lead to significant improvements in the sharpness of astronomical images taken with ground-based telescopes. The technology also opens the door for developments in laser satellite communication.

14:48 Laser-powered robot weeder zaps without poison

Company building a better weeder nets big bucks for next gen ag solution.

06:15 Socially-Aware Multi-Agent Following with 2D Laser Scans via Deep Reinforcement Learning and Potential Field. (arXiv:2109.01874v1 [cs.RO])

Target following in dynamic pedestrian environments is an important task for mobile robots. However, it is challenging to keep tracking the target while avoiding collisions in crowded environments, especially with only one robot. In this paper, we propose a multi-agent method for an arbitrary number of robots to follow the target in a socially-aware manner using only 2D laser scans. The multi-agent following problem is tackled by utilizing the complementary strengths of both reinforcement learning and potential field, in which the reinforcement learning part handles local interactions while navigating to the goals assigned by the potential field. Specifically, with the help of laser scans in obstacle map representation, the learning-based policy can help the robots avoid collisions with both static obstacles and dynamic obstacles like pedestrians in advance, namely socially aware. While the formation control and goal assignment for each robot is obtained from a target-centered

03.09.2021
19:09 Testing 1-2: New laser-based microphone calibration measures up

Researchers have conducted the first demonstration of a faster and more accurate way to calibrate certain kinds of microphones. The technique, which uses lasers to measure the velocity at which a microphone's diaphragm vibrates, performs well enough to overtake one of the main calibration methods used.

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