Showing papers on "Laser published in 2013"
TL;DR: An overview of the state of the art of ion acceleration by laser pulses as well as an outlook on its future development and perspectives are given in this article. But the main features observed in the experiments, the observed scaling with laser and plasma parameters, and the main models used both to interpret experimental data and to suggest new research directions are described.
Abstract: Ion acceleration driven by superintense laser pulses is attracting an impressive and steadily increasing effort. Motivations can be found in the applicative potential and in the perspective to investigate novel regimes as available laser intensities will be increasing. Experiments have demonstrated, over a wide range of laser and target parameters, the generation of multi-MeV proton and ion beams with unique properties such as ultrashort duration, high brilliance, and low emittance. An overview is given of the state of the art of ion acceleration by laser pulses as well as an outlook on its future development and perspectives. The main features observed in the experiments, the observed scaling with laser and plasma parameters, and the main models used both to interpret experimental data and to suggest new research directions are described.
1,221 citations
TL;DR: In this paper, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers and the excitation wavelength in the visible range (488, 514 and 647 nm).
Abstract: The Raman scattering of single- and few-layered WS2 is studied as a function of the number of S-W-S layers and the excitation wavelength in the visible range (488, 514 and 647 nm). For the three excitation wavelengths used in this study, the frequency of the A1g(Γ) phonon mode monotonically decreases with the number of layers. For single-layer WS2, the 514.5 nm laser excitation generates a second-order Raman resonance involving the longitudinal acoustic mode (LA(M)). This resonance results from a coupling between the electronic band structure and lattice vibrations. First-principles calculations were used to determine the electronic and phonon band structures of single-layer and bulk WS2. The reduced intensity of the 2LA mode was then computed, as a function of the laser wavelength, from the fourth-order Fermi golden rule. Our observations establish an unambiguous and nondestructive Raman fingerprint for identifying single- and few-layered WS2 films.
1,213 citations
TL;DR: Compared to the conventional flame emission spectroscopy, LIBS atomizes only the small portion of the sample by the focused laser pulse, which makes a tiny spark on the sample, and capturing the instant light is a major skill to collect sufficient intensity of the emitting species.
Abstract: ■ CONTENTS General Information: Books, Reviews, and Conferences 640 Fundamentals 641 Interaction of Laser Beam with Matter 641 Factors Affecting Laser Ablation and LaserInduced Plasma Formation 642 Influence of Target on the Laser-Induced Plasmas 642 Influence of Laser Parameters on the LaserInduced Plasmas 643 Laser Wavelength (λ) 643 Laser Pulse Duration (τ) 643 Laser Pulse Energy (E) 645 Influence of Ambient Gas on the Laser-Induced Plasmas 645 LIBS Methods 647 Double Pulse LIBS 647 Femtosecond LIBS 651 Resonant LIBS 652 Ranging Approaches 652 Applications 654 Surface Inspection, Depth Profiling, and LIBS Imaging 654 Cultural Heritage 654 Industrial Analysis 655 Environmental Monitoring 656 Biomedical and Pharmaceutical Analysis 658 Security and Forensics 659 Analysis of Liquids and Submerged Solids 660 Space Exploration and Isotopic Analysis 662 Space Exploration 662 Isotopic Analysis 662 Conclusions and Future Outlook 663 Author Information 664 Corresponding Author 664 Notes 664 Biographies 664 Acknowledgments 664 References 664
847 citations
TL;DR: In this article, a new field of direct femtosecond laser surface nano/microstructuring and its applications is reviewed, where the authors present a review of the current state-of-the-art in this field.
Abstract: This paper reviews a new field of direct femtosecond laser surface nano/microstructuring and its applications. Over the past few years, direct femtosecond laser surface processing has distinguished itself from other conventional laser ablation methods and become one of the best ways to create surface structures at nano- and micro-scales on metals and semiconductors due to its flexibility, simplicity, and controllability in creating various types of nano/microstructures that are suitable for a wide range of applications. Significant advancements were made recently in applying this technique to altering optical properties of metals and semiconductors. As a result, highly absorptive metals and semiconductors were created, dubbed as the “black metals” and “black silicon”. Furthermore, various colors other than black have been created through structural coloring on metals. Direct femtosecond laser processing is also capable of producing novel materials with wetting properties ranging from superhydrophilic to superhydrophobic. In the extreme case, superwicking materials were created that can make liquids run vertically uphill against the gravity over an extended surface area. Though impressive scientific achievements have been made so far, direct femtosecond laser processing is still a young research field and many exciting findings are expected to emerge on its horizon.
822 citations
TL;DR: In this paper, a unified formalism is presented for the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser accelerated electrons.
Abstract: Relativistic interaction of short-pulse lasers with underdense plasmas has recently led to the emergence of a novel generation of femtosecond x-ray sources. Based on radiation from electrons accelerated in plasma, these sources have the common properties to be compact and to deliver collimated, incoherent, and femtosecond radiation. In this article, within a unified formalism, the betatron radiation of trapped and accelerated electrons in the so-called bubble regime, the synchrotron radiation of laser-accelerated electrons in usual meter-scale undulators, the nonlinear Thomson scattering from relativistic electrons oscillating in an intense laser field, and the Thomson backscattered radiation of a laser beam by laser-accelerated electrons are reviewed. The underlying physics is presented using ideal models, the relevant parameters are defined, and analytical expressions providing the features of the sources are given. Numerical simulations and a summary of recent experimental results on the different mechanisms are also presented. Each section ends with the foreseen development of each scheme. Finally, one of the most promising applications of laser-plasma accelerators is discussed: the realization of a compact free-electron laser in the x-ray range of the spectrum. In the conclusion, the relevant parameters characterizing each sources are summarized. Considering typical laser-plasma interaction parameters obtained with currently available lasers, examples of the source features are given. The sources are then compared to each other in order to define their field of applications.
634 citations
TL;DR: In this article, a review of approaches aiming at translating this success in optical microscopy to optical lithography is presented, and basic principles and limitations, possible depletion mechanisms and recent lithography experiments by various groups are summarized.
Abstract: Direct laser writing has become a versatile and routine tool for the mask-free fabrication of polymer structures with lateral linewidths down to less than 100 nm. In contrast to its planar counterpart, electron-beam lithography, direct laser writing also allows for the making of three-dimensional structures. However, its spatial resolution has been restricted by diffraction. Clearly, linewidths and resolutions on the scale of few tens of nanometers and below are highly desirable for various applications in nanotechnology. In visible-light far-field fluorescence microscopy, the concept of stimulated emission depletion (STED) introduced in 1994 has led to spectacular record resolutions down to 5.6 nm in 2009. This review addresses approaches aiming at translating this success in optical microscopy to optical lithography. After explaining basic principles and limitations, possible depletion mechanisms and recent lithography experiments by various groups are summarized. Today, Abbe's diffraction barrier as well as the generalized two-photon Sparrow criterion have been broken in far-field optical lithography. For further future progress in resolution, the development of novel tailored photoresists in combination with attractive laser sources is of utmost importance.
545 citations
TL;DR: In this article, the Fabry-Perot cavity was designed to optimize the material quality and minimize surface recombination of (Al)GaAs nanowires, which is an important material system for infrared lasers that is extensively used for conventional lasers.
Abstract: Near-infrared lasers are important for optical data communication, spectroscopy and medical diagnosis. Semiconductor nanowires offer the possibility of reducing the footprint of devices for three-dimensional device integration and hence are being extensively studied in the context of optoelectronic devices1, 2. Although visible and ultraviolet nanowire lasers have been demonstrated widely3, 4, 5, 6, 7, 8, 9, 10, 11, progress towards room-temperature infrared nanowire lasers has been limited because of material quality issues and Auger recombination12, 13. (Al)GaAs is an important material system for infrared lasers that is extensively used for conventional lasers. GaAs has a very large surface recombination velocity, which is a serious issue for nanowire devices because of their large surface-to-volume ratio14, 15. Here, we demonstrate room-temperature lasing in core–shell–cap GaAs/AlGaAs/GaAs nanowires by properly designing the Fabry–Perot cavity, optimizing the material quality and minimizing surface recombination. Our demonstration is a major step towards incorporating (Al)GaAs nanowire lasers into the design of nanoscale optoelectronic devices operating at near-infrared wavelengths.
502 citations
TL;DR: In this article, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed.
Abstract: Whispering gallery mode (WGM) optical microresonators have attracted intense interests in the past decades. The combination of high quality factors (Q) and small mode volumes of modes in WGM resonators significantly enhances the light-matter interactions, making them excellent cavities for achieving low threshold and narrow linewidth lasers. In this Review, the progress in WGM microcavity lasers is summarized, and the laser performance considering resonator geometries and materials as well as lasing mechanisms is discussed. Label-free detection using WGM resonators has emerged as highly sensitive detection schemes. However, the resolution is mainly limited by the cavity Q factor which determines the mode linewidth. Microcavity lasers, due to their narrow laser spectral width, could greatly improve the detection resolution. Some recent developments in sensing using microcavity lasers are discussed.
482 citations
TL;DR: In this article, the use of colloidal quantum dots as a gain medium in a vertical-cavity surface-emitting laser has been investigated, achieving single-exciton gain in the red, green, and blue wavelengths.
Abstract: The methods and materials described herein contemplate the use films of colloidal quantum dots as a gain medium in a vertical-cavity surface-emitting laser. The present disclosure demonstrates a laser with single-exciton gain in the red, green, and blue wavelengths. Leveraging this nanocomposite gain, the results realize a significant step toward full-color single-material lasers.
419 citations
TL;DR: In this paper, high-strained germanium on silicon samples with up to 3.1% uniaxial strain is fabricated and then investigated by Raman spectroscopy.
Abstract: Highly strained germanium on silicon samples with up to 3.1% uniaxial strain are fabricated and then investigated by Raman spectroscopy. During optical pumping, changes in both the emission wavelength and output power are observed, indicating that bandgap modification and optical gain are occurring.
401 citations
TL;DR: The results demonstrate that the microfiber-based TI photonic device can operate as both the high nonlinear optical component and the SA in fiber lasers, and could also find other applications in the related fields of photonics.
Abstract: We report on the generation of passive harmonic mode locking of a fiber laser using a microfiber-based topological insulator (TI) Bi2Te3 saturable absorber (SA) The optical deposition method was employed to fabricate the microfiber-based TISA By virtue of the excellent nonlinear optical property of the proposed TISA, the fiber laser could operate at the pulse repetition rate of 204 GHz under a pump power of 126 mW, corresponding to the 418th harmonic of fundamental repetition frequency The results demonstrate that the microfiber-based TI photonic device can operate as both the high nonlinear optical component and the SA in fiber lasers, and could also find other applications in the related fields of photonics
TL;DR: Lasers with ultrashort pulses are shown to be particularly useful tools for the production of nanocluster films and the important question of the film stoichiometry relative to that of the target will be thoroughly discussed in relation to the films reported in the literature.
Abstract: Laser ablation of dielectrics by ultrashort laser pulses is reviewed. The basic interaction between ultrashort light pulses and the dielectric material is described, and different approaches to the modeling of the femtosecond ablation dynamics are reviewed. Material excitation by ultrashort laser pulses is induced by a combination of strong-field excitation (multi-photon and tunnel excitation), collisional excitation (potentially leading to an avalanche process), and absorption in the plasma consisting of the electrons excited to the conduction band. It is discussed how these excitation processes can be described by various rate-equation models in combination with different descriptions of the excited electrons. The optical properties of the highly excited dielectric undergo a rapid change during the laser pulse, which must be included in a detailed modeling of the excitations. The material ejected from the dielectric following the femtosecond-laser excitation can potentially be used for thin-film deposition. The deposition rate is typically much smaller than that for nanosecond lasers, but film production by femtosecond lasers does possess several attractive features. First, the strong-field excitation makes it possible to produce films of materials that are transparent to the laser light. Second, the highly localized excitation reduces the emission of larger material particulates. Third, lasers with ultrashort pulses are shown to be particularly useful tools for the production of nanocluster films. The important question of the film stoichiometry relative to that of the target will be thoroughly discussed in relation to the films reported in the literature.
TL;DR: Laser frequency combs can be harnessed for coherent anti-Stokes Raman spectroscopy and spectro-imaging and are expected to enable new applications for nonlinear microscopy but also benefit other nonlinear spectroscopic techniques.
Abstract: A new multiplex technique of coherent anti-Stokes Raman spectro-imaging with two laser frequency combs is shown to record molecular spectra of broad bandwidth on a microsecond scale.
TL;DR: In this paper, a seeded free-electron laser with a two-stage harmonic upshift configuration provided tunable and coherent soft-X-ray pulses with energies of tens of microjoules and a low pulse-to-pulse wavelength jitter at wavelengths of 10.8 nm and below.
Abstract: A seeded free-electron laser with a two-stage harmonic upshift configuration provided tunable and coherent soft-X-ray pulses. The configuration produced single-transverse-mode, narrow-spectral-bandwidth femtosecond pulses with energies of several tens of microjoules and a low pulse-to-pulse wavelength jitter at wavelengths of 10.8 nm and below.
TL;DR: A significant increase in laser-driven electron energy to the multi-GeV level by utilizing a 30-fs, 1-PW laser system and a dual-stage laser-wakefield acceleration scheme was applied to boost electron energies to over 3 GeV with a single PW laser pulse.
Abstract: Laser-wakefield acceleration offers the promise of a compact electron accelerator for generating a multi-GeV electron beam using the huge field gradient induced by an intense laser pulse, compared to conventional rf accelerators. However, the energy and quality of the electron beam from the laser-wakefield accelerator have been limited by the power of the driving laser pulses and interaction properties in the target medium. Recent progress in laser technology has resulted in the realization of a petawatt (PW) femtosecond laser, which offers new capabilities for research on laser-wakefield acceleration. Here, we present a significant increase in laser-driven electron energy to the multi-GeV level by utilizing a 30-fs, 1-PW laser system. In particular, a dual-stage laser-wakefield acceleration scheme (injector and accelerator scheme) was applied to boost electron energies to over 3 GeV with a single PW laser pulse. Three-dimensional particle-in-cell simulations corroborate the multi-GeV electron generation from the dual-stage laser-wakefield accelerator driven by PW laser pulses.
Patent•
20 May 2013
TL;DR: In this paper, a hand-supportable laser scanning bar code symbol reading system is presented. But the system is not suitable for reading bar code symbols in the presence of static sensors.
Abstract: Method of and system for reading bar code symbols using a hand-supportable laser scanning bar code symbol reading system supporting an improved level control over the length and intensity characteristics of laser scan lines projected onto scanned objects, at any instant in time, in a manner dependent the detected location, distance or range of the scanned object in the scanning field of the system during system operation. The system includes a laser scanning module that projects the laser scanning beam through a light transmission window, and across a laser scanning field in which an object is located, while the laser scanning beam is blanked out during the laser scanning cycle according to a laser source blanking function that is determined by the estimated distance, or data representative thereof.
TL;DR: In this article, a measure-and-sort approach was used to achieve sub-10 fs root-mean-squared (rms) error measurement at hard X-ray FELs, far beyond the 100-200 fs rms jitter limitations.
Abstract: Recently, few-femtosecond pulses have become available at hard X-ray free-electron lasers Coupled with the available sub-10 fs optical pulses, investigations into few-femtosecond dynamics are not far off However, achieving sufficient synchronization between optical lasers and X-ray pulses continues to be challenging We report a 'measure-and-sort' approach, which achieves sub-10 fs root-mean-squared (rms) error measurement at hard X-ray FELs, far beyond the 100-200 fs rms jitter limitations This timing diagnostic, now routinely available at the Linac Coherent Light Source (LCLS), is based on ultrafast free-carrier generation in optically transparent materials Correlation between two independent measurements enables unambiguous demonstration of ~6 fs rms error in reporting the optical/X-ray delay, with single shot error suggesting the possibility of reaching few-femtosecond resolution
TL;DR: In this paper, a few-layer TI: Bi2Se3 (2-4 layer thickness) is firstly fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm2 measured by the Z-scan technique.
Abstract: Passive Q-switching of an ytterbium-doped fiber (YDF) laser with few-layer topological insulator (TI) is, to the best of our knowledge, experimentally demonstrated for the first time. The few-layer TI: Bi2Se3 (2–4 layer thickness) is firstly fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm2 measured by the Z-scan technique. The optical deposition technique is used to induce the few-layer TI in the solution onto a fiber ferrule for successfully constructing the fiber-integrated TI-based saturable absorber (SA). By inserting this SA into the YDF laser cavity, stable Q-switching operation at 1.06 μm is achieved. The Q-switched pulses have the shortest pulse duration of 1.95 μs, the maximum pulse energy of 17.9 nJ and a tunable pulse-repetition-rate from 8.3 to 29.1 kHz. Our results indicate that the TI as a SA is also available at 1 μm waveband, revealing its potential as another broadband SA (like graphene).
TL;DR: A proof-of-principle experiment demonstrating dielectric laser acceleration of nonrelativistic electrons in the vicinity of a fused-silica grating and the demonstration of the inverse Smith-Purcell effect in the optical regime.
Abstract: A proof-of-principle experiment demonstrating dielectric laser acceleration of nonrelativistic electrons in the vicinity of a fused-silica grating is reported. The grating structure is utilized to generate an electromagnetic surface wave that travels synchronously with and efficiently imparts momentum on 28 keV electrons. We observe a maximum acceleration gradient of $25\text{ }\text{ }\mathrm{MeV}/\mathrm{m}$. We investigate in detail the parameter dependencies and find excellent agreement with numerical simulations. With the availability of compact and efficient fiber laser technology, these findings may pave the way towards an all-optical compact particle accelerator. This work also represents the demonstration of the inverse Smith-Purcell effect in the optical regime.
TL;DR: In this article, the authors proposed a new class of coherent optical frequency electromagnetic wave amplifiers that deliver intense coherent and directional surface plasmons well below the diffraction barrier, which can enhance significantly light-matter interactions.
Abstract: : Plasmon lasers are a new class of coherent optical frequency electromagnetic wave amplifiers that deliver intense coherent and directional surface plasmons well below the diffraction barrier. The strongly confined electric fields in plasmon lasers can enhance significantly light-matter interactions and bring fundamentally new capabilities to bio-sensing, data storage photolithography and optical communications.
TL;DR: The results indicate that the TI as a SA is also available at 1 μm waveband, revealing its potential as another broadband SA (like graphene).
Abstract: Passive Q-switching of an ytterbium-doped fiber (YDF) laser with few-layer topological insulator (TI) is, to the best of our knowledge, experimentally demonstrated for the first time. The few-layer TI: Bi2Se3 (2-4 layer thickness) is fabricated by the liquid-phase exfoliation method, and has a low saturable optical intensity of 53 MW/cm2 measured by the Z-scan technique. The optical deposition technique is used to induce the few-layer TI in the solution onto a fiber ferrule for successfully constructing the fiber-integrated TI-based saturable absorber (SA). By inserting this SA into the YDF laser cavity, stable Q-switching operation at 1.06 {\mu}m is achieved. The Q-switched pulses have the shortest pulse duration of 1.95 {\mu}s, the maximum pulse energy of 17.9 nJ and a tunable pulse-repetition-rate from 8.3 to 29.1 kHz. Our results indicate that the TI as a SA is also available at 1 {\mu}m waveband, revealing its potential as another wavelength-independent SA (like graphene).
TL;DR: In this paper, a review of diode laser-based spectroscopy is presented, which covers the developments made so far and illustrates the properties of free-running diode lasers.
Abstract: Spectroscopy using tunable diode lasers is an area of research that has gone through a dramatic evolution over the last few years, principally because of new exciting approaches in the field of atomic and molecular spectroscopy. This article attempts to review major recent advancements in the field of diode laser based spectroscopy. The discussion covers the developments made so far in the field of diode lasers and illustrates comprehensively the properties of free-running diode lasers. Since the commercially available free-running diode lasers are not suitable for high-precision spectroscopic studies, various techniques developed so far for converting these free-running diode lasers into true narrow linewidth tunable laser sources are discussed comprehensively herein. The potential uses of diode lasers in different spectroscopic fields and their extensive list of applications have also been included, which may be interesting for the novice and the advanced user as well.
01 Nov 2013-Materials Science and Engineering A-structural Materials Properties Microstructure and Processing
TL;DR: In this article, a systematic analysis of the main parameters for the selective laser melting (SLM) of a commercial stainless steel 316L powder was conducted to improve the mechanical properties and dimensional accuracy of the fabricated parts.
Abstract: In this work, a systematic analysis of the main parameters for the selective laser melting (SLM) of a commercial stainless steel 316L powder was conducted to improve the mechanical properties and dimensional accuracy of the fabricated parts. First, the effects of the processing parameters, such as the laser beam scanning velocity, laser power, substrate condition and thickness of the powder layer, on the formation of single tracks for achieving a continuous melting and densification of the material were analysed. Then, the influence of the environmental conditions (gas nature) and of the preheating temperature on the density and dimensional accuracy of the parts was considered. The microstructural features of the SLM SS 316L parts were carefully observed to elucidate the melting-solidification mechanism and the thermal history, which are the basis of the manufacturing process. Finally, the mechanical properties of the corresponding material were also determined.
Patent•
07 Aug 2013
TL;DR: In this paper, a method for manufacturing laser scanners is presented, in which the light-deflecting assemblies are configured to drive their light-consuming assemblies at a fixed drive frequency, such that operation of the laser scanners remains within engineering tolerances even through extreme environmental changes.
Abstract: A method for manufacturing laser scanners is provided. Laser scanners are configured to drive their light-deflecting assemblies at a fixed drive frequency. By selecting for inclusion in the laser scanners only those light-deflecting assemblies that have resonant oscillation frequencies falling within a specified range, operation of the laser scanners remains within engineering tolerances even through extreme environmental changes. Moreover, the method results in laser scanner having greater unit-to-unit consistency during operation even in extreme environments.
TL;DR: This work reports a nanotube-mode-locked all-fiber ultrafast oscillator emitting three wavelengths at the central wavelengths of about 1540, 1550, and 1560 nm, which are tunable by stretching fiber Bragg gratings, agreeing well with the numerical simulations.
Abstract: Multi-wavelength lasers have widespread applications (e.g. fiber telecommunications, pump-probe measurements, terahertz generation). Here, we report a nanotube-mode-locked all-fiber ultrafast oscillator emitting three wavelengths at the central wavelengths of about 1540, 1550 and 1560 nm, which are tunable by stretching fiber Bragg gratings. The output pulse duration is around 6 ps with a spectral width of ~0.5 nm, agreeing well with the numerical simulations. The triple-laser system is controlled precisely and insensitive to environmental perturbations with <0.04% amplitude fluctuation. Our method provides a simple, stable, low-cost, multi-wavelength ultrafast-pulsed source for spectroscopy, biomedical research and telecommunications.
TL;DR: This work presents the first digital laser comprising an electrically addressed reflective phase-only spatial light modulator as an intra-cavity digitally addressed holographic mirror, and shows that it can digitally control the laser modes with ease, and demonstrates real-time switching between spatial modes in an otherwise standard solid-state laser resonator.
Abstract: Customizing the output beam shape from a laser invariably involves specialized optical elements in the form of apertures, diffractive optics and free-form mirrors. Such optics require considerable design and fabrication effort and suffer from the further disadvantage of being immutably connected to the selection of a particular spatial mode. Here we overcome these limitations with the first digital laser comprising an electrically addressed reflective phase-only spatial light modulator as an intra-cavity digitally addressed holographic mirror. The phase and amplitude of the holographic mirror may be controlled simply by writing a computer-generated hologram in the form of a grey-scale image to the device, for on-demand laser modes. We show that we can digitally control the laser modes with ease, and demonstrate real-time switching between spatial modes in an otherwise standard solid-state laser resonator. Our work opens new possibilities for the customizing of laser modes at source.
TL;DR: In this article, the authors describe an approach to selecting parameters for high-density (>99 %) parts using 316L stainless steel, which is a powder-based additive manufacturing process where a 3D part is produced, layer by layer, by using a high-energy laser beam to fuse the metallic powder particles.
Abstract: Selective laser melting is a powder-based, additive-manufacturing process where a three-dimensional part is produced, layer by layer, by using a high-energy laser beam to fuse the metallic powder particles. A particular challenge in this process is the selection of appropriate process parameters that result in parts with desired properties. In this study, we describe an approach to selecting parameters for high-density (>99 %) parts using 316L stainless steel. Though there has been significant success in achieving near-full density for 316L parts, this work has been limited to laser powers 99 %, with the density reducing rapidly at high speeds due to insufficient melting, and less rapidly at low speeds due to the effect of voids created as the process enters keyhole mode.
TL;DR: The results suggest that a magnetic material's microstructure can be engineered to control transient laser-excited spins, potentially allowing faster (~ 1 ps) spin reversal than in present technologies.
Abstract: Ultrafast laser techniques have revealed extraordinary spin dynamics in magnetic materials that equilibrium descriptions of magnetism cannot explain. Particularly important for future applications is understanding non-equilibrium spin dynamics following laser excitation on the nanoscale, yet the limited spatial resolution of optical laser techniques has impeded such nanoscale studies. Here we present ultrafast diffraction experiments with an X-ray laser that probes the nanoscale spin dynamics following optical laser excitation in the ferrimagnetic alloy GdFeCo, which exhibits macroscopic all-optical switching. Our study reveals that GdFeCo displays nanoscale chemical and magnetic inhomogeneities that affect the spin dynamics. In particular, we observe Gd spin reversal in Gd-rich nanoregions within the first picosecond driven by the non-local transfer of angular momentum from larger adjacent Fe-rich nanoregions. These results suggest that a magnetic material's microstructure can be engineered to control transient laser-excited spins, potentially allowing faster (~ 1 ps) spin reversal than in present technologies.
TL;DR: The findings suggest that, alternatively, group-II–VI semiconductors with strong exciton–LOP coupling could be harnessed to achieve laser cooling and open the way to optical refrigeration based on semiconductor.
Abstract: Net laser cooling from 290 kelvin to about 250 kelvin is achieved in semiconductor cadmium sulphide ‘nanobelts’ and attributed to strong coupling between excitons and longitudinal optical phonons. Laser cooling of solids, or optical refrigeration, is attractive as a route to compact, cryogen-free and vibration-free refrigeration devices. Laser cooling, based on removing heat due to blue-shifted emission, has been reported previously in rare-earth-metal-doped glasses and crystals. Now Jun Zhang et al. demonstrate a substantial net laser cooling of a semiconductor CdS nanobelt — by about 40 K from 290 K pumped by a 514-nm laser. This achievement opens up a route to optical refrigeration based on semiconductors, where the mechanisms involve excitonic rather than atomic resonances. Laser cooling media based on II–VI semiconductors are potentially highly efficient, capable of achieving extremely low temperatures and readily integrated into optoelectronic devices. Optical irradiation accompanied by spontaneous anti-Stokes emission can lead to cooling of matter, in a phenomenon known as laser cooling, or optical refrigeration, which was proposed by Pringsheim in 19291. In gaseous matter, an extremely low temperature can be obtained in diluted atomic gases by Doppler cooling2, and laser cooling of ultradense gas has been demonstrated by collisional redistribution of radiation3. In solid-state materials, laser cooling is achieved by the annihilation of phonons, which are quanta of lattice vibrations, during anti-Stokes luminescence. Since the first experimental demonstration in glasses doped with rare-earth metals4, considerable progress has been made, particularly in ytterbium-doped glasses or crystals: recently a record was set of cooling to about 110 kelvin from the ambient temperature, surpassing the thermoelectric Peltier cooler5,6. It would be interesting to realize laser cooling in semiconductors, in which excitonic resonances dominate7,8,9, rather than in systems doped with rare-earth metals, where atomic resonances dominate. However, so far no net cooling in semiconductors has been achieved despite much experimental10,11,12 and theoretical7,8,9,13,14 work, mainly on group-III–V gallium arsenide quantum wells. Here we report a net cooling by about 40 kelvin in a semiconductor using group-II–VI cadmium sulphide nanoribbons, or nanobelts, starting from 290 kelvin. We use a pump laser with a wavelength of 514 nanometres, and obtain an estimated cooling efficiency of about 1.3 per cent and an estimated cooling power of 180 microwatts. At 100 kelvin, 532-nm pumping leads to a net cooling of about 15 kelvin with a cooling efficiency of about 2.0 per cent. We attribute the net laser cooling in cadmium sulphide nanobelts to strong coupling between excitons and longitudinal optical phonons (LOPs), which allows the resonant annihilation of multiple LOPs in luminescence up-conversion processes, high external quantum efficiency and negligible background absorption. Our findings suggest that, alternatively, group-II–VI semiconductors with strong exciton–LOP coupling could be harnessed to achieve laser cooling and open the way to optical refrigeration based on semiconductors.