Showing papers on "Laser published in 2009"
TL;DR: In this paper, the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated.
Abstract: The optical conductance of monolayer graphene is defined solely by the fine structure constant, α = (where e is the electron charge, is Dirac's constant and c is the speed of light). The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band is demonstrated. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the graphene thickness. These results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth, and wideband tunability.
2,217 citations
TL;DR: In this paper, the basic physics of laser pulse evolution in underdense plasmas is also reviewed, including the propagation, self-focusing, and guiding of laser pulses in uniform density channels and with preformed density channels.
Abstract: Laser-driven plasma-based accelerators, which are capable of supporting fields in excess of 100 GV/m, are reviewed. This includes the laser wakefield accelerator, the plasma beat wave accelerator, the self-modulated laser wakefield accelerator, plasma waves driven by multiple laser pulses, and highly nonlinear regimes. The properties of linear and nonlinear plasma waves are discussed, as well as electron acceleration in plasma waves. Methods for injecting and trapping plasma electrons in plasma waves are also discussed. Limits to the electron energy gain are summarized, including laser pulse diffraction, electron dephasing, laser pulse energy depletion, and beam loading limitations. The basic physics of laser pulse evolution in underdense plasmas is also reviewed. This includes the propagation, self-focusing, and guiding of laser pulses in uniform plasmas and with preformed density channels. Instabilities relevant to intense short-pulse laser-plasma interactions, such as Raman, self-modulation, and hose instabilities, are discussed. Experiments demonstrating key physics, such as the production of high-quality electron bunches at energies of 0.1-1 GeV, are summarized.
2,108 citations
Posted Content•
TL;DR: In this paper, the authors demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band.
Abstract: The optical conductance of monolayer graphene is defined solely by the fine structure constant. The absorbance has been predicted to be independent of frequency. In principle, the interband optical absorption in zero-gap graphene could be saturated readily under strong excitation due to Pauli blocking. Here, we demonstrate the use of atomic layer graphene as saturable absorber in a mode-locked fiber laser for the generation of ultrashort soliton pulses (756 fs) at the telecommunication band. The modulation depth can be tuned in a wide range from 66.5% to 6.2% by varying the thickness of graphene. Our results suggest that ultrathin graphene films are potentially useful as optical elements in fiber lasers. Graphene as a laser mode locker can have many merits such as lower saturation intensity, ultrafast recovery time, tunable modulation depth and wideband tuneability.
2,039 citations
TL;DR: The experimental observation of curved plasma channels generated in air using femtosecond Airy beams, where the tightly confined main intensity feature of the axially nonsymmetric laser beam propagates along a bent trajectory, leaving a curved plasma channel behind.
Abstract: Plasma channel generation (or filamentation) using ultraintense laser pulses in dielectric media has a wide spectrum of applications, ranging from remote sensing to terahertz generation to lightning control. So far, laser filamentation has been triggered with the use of ultrafast pulses with axially symmetric spatial beam profiles, thereby generating straight filaments. We report the experimental observation of curved plasma channels generated in air using femtosecond Airy beams. In this unusual propagation regime, the tightly confined main intensity feature of the axially nonsymmetric laser beam propagates along a bent trajectory, leaving a curved plasma channel behind. Secondary channels bifurcate from the primary bent channel at several locations along the beam path. The broadband radiation emanating from different longitudinal sections of the curved filament propagates along angularly resolved trajectories.
746 citations
TL;DR: It is shown that short-pulse laser-induced classical ripples on dielectrics, semiconductors, and conductors exhibit a prominent "non-classical" characteristic-in normal incidence the periods are definitely smaller than laser wavelengths, which indicates that the simplified scattering model should be revised.
Abstract: We show that short-pulse laser-induced classical ripples on dielectrics, semiconductors, and conductors exhibit a prominent "non-classical" characteristic-in normal incidence the periods are definitely smaller than laser wavelengths, which indicates that the simplified scattering model should be revised. Taking into account the surface plasmons (SPs), we consider that the ripples result from the initial direct SP-laser interference and the subsequent grating-assisted SP-laser coupling. With the model, the period-decreasing phenomenon originates in the admixture of the field-distribution effect and the grating-coupling effect. Further, we propose an approach for obtaining the dielectric constant, electron density, and electron collision time of the high-excited surface. With the derived parameters, the numerical simulations are in good agreement with the experimental results. On the other hand, our results confirm that the surface irradiated by short-pulse laser with damage-threshold fluence should behave metallic, no matter for metal, semiconductor, or dielectric, and the short-pulse laser-induced subwavelength structures should be ascribed to a phenomenon of nano-optics.
735 citations
TL;DR: In this paper, the formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSss) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130
Abstract: The formation of nearly wavelength-sized laser-induced periodic surface structures (LIPSSs) on single-crystalline silicon upon irradiation with single or multiple femtosecond-laser pulses (pulse duration τ=130 fs and central wavelength λ=800 nm) in air is studied experimentally and theoretically. In our theoretical approach, we model the LIPSS formation by combining the generally accepted first-principles theory of Sipe and co-workers with a Drude model in order to account for transient intrapulse changes in the optical properties of the material due to the excitation of a dense electron-hole plasma. Our results are capable to explain quantitatively the spatial periods of the LIPSSs being somewhat smaller than the laser wavelength, their orientation perpendicular to the laser beam polarization, and their characteristic fluence dependence. Moreover, evidence is presented that surface plasmon polaritons play a dominant role during the initial stage of near-wavelength-sized periodic surface structures in fem...
555 citations
TL;DR: In this article, the history of the discovery of the nonlinear optical crystal KBe2BO3F2 (KBBF) is described, with a full description of its growth, space structure, optical properties, and capability to generate deep-ultraviolet (UV) harmonic generation.
Abstract: This review introduces in detail the history of the discovery of the nonlinear optical crystal KBe2BO3F2 (KBBF), with a full description of its growth, space structure, optical properties, and capability to generate deep-ultraviolet (UV) harmonic generation. Several important applications developed recently using all-solid-state deep-UV light sources, such as super-high-resolution laser photoemission spectrograph and 193 nm photolithography, will be reviewed.
542 citations
TL;DR: An update on the latest developments in laser-based breath analysis is presented, finding that laser spectroscopic detection techniques not only have high-sensitivity and high-selectivity, as equivalently offered by the MS-based techniques, but also have the advantageous features of near real-time response, low instrument costs, and POC function.
Abstract: Breath analysis, a promising new field of medicine and medical instrumentation, potentially offers noninvasive, real-time, and point-of-care (POC) disease diagnostics and metabolic status monitoring. Numerous breath biomarkers have been detected and quantified so far by using the GC-MS technique. Recent advances in laser spectroscopic techniques and laser sources have driven breath analysis to new heights, moving from laboratory research to commercial reality. Laser spectroscopic detection techniques not only have high-sensitivity and high-selectivity, as equivalently offered by the MS-based techniques, but also have the advantageous features of near real-time response, low instrument costs, and POC function. Of the approximately 35 established breath biomarkers, such as acetone, ammonia, carbon dioxide, ethane, methane, and nitric oxide, 14 species in exhaled human breath have been analyzed by high-sensitivity laser spectroscopic techniques, namely, tunable diode laser absorption spectroscopy (TDLAS), cavity ringdown spectroscopy (CRDS), integrated cavity output spectroscopy (ICOS), cavity enhanced absorption spectroscopy (CEAS), cavity leak-out spectroscopy (CALOS), photoacoustic spectroscopy (PAS), quartz-enhanced photoacoustic spectroscopy (QEPAS), and optical frequency comb cavity-enhanced absorption spectroscopy (OFC-CEAS). Spectral fingerprints of the measured biomarkers span from the UV to the mid-IR spectral regions and the detection limits achieved by the laser techniques range from parts per million to parts per billion levels. Sensors using the laser spectroscopic techniques for a few breath biomarkers, e.g., carbon dioxide, nitric oxide, etc. are commercially available. This review presents an update on the latest developments in laser-based breath analysis.
501 citations
TL;DR: Results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking and stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser.
Abstract: We report on large energy pulse generation in an erbium-doped fiber laser passively mode-locked with atomic layer graphene. Stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser. Our results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking.
491 citations
TL;DR: In this paper, it was shown that a single 50-fs laser pulse couples efficiently to a ferromagnetic film during its own propagation, indicating that the material polarization induced by the photon field interacts coherently with the spins.
Abstract: The quest for ultrafast magnetic processes has triggered a new field of research—femtomagnetism: using femtosecond laser pulses to demagnetize ferromagnetic metallic thin films. Despite being the subject of intense research for over a decade, the underlying mechanisms that govern the demagnetization remain unclear. Here, we investigate how an ultrashort laser pulse couples to the spin of electrons in ferromagnetic metals. It is shown that a single 50-fs laser pulse couples efficiently to a ferromagnetic film during its own propagation. This result indicates that the material polarization induced by the photon field interacts coherently with the spins. The corresponding mechanism has its origin in relativistic quantum electrodynamics, beyond the spin–orbit interaction involving the ionic potential. In addition, this coherent interaction is clearly distinguished from the incoherent ultrafast demagnetization associated with the thermalization of the spins. We forecast that the corresponding coherent self-induced processes are the dawn of a new era for future research in magnetism. Femtosecond laser pulses can demagnetize ferromagnetic metallic thin films on an ultrafast timescale. Studying how a single optical pulse interacts with a magnetic film now provides a better understanding of this so-called femtomagnetism.
486 citations
TL;DR: In this article, a graphene-polymer nanocomposite membrane was fabricated and first used to mode lock a fiber laser, and stable mode-locked solitons with 3 nJ pulse energy, 700 fs pulse width at the 1590 nm wavelength have been directly generated from the laser.
Abstract: Due to its unique electronic property and the Pauli blocking principle, atomic layer graphene possesses wavelength-independent ultrafast saturable absorption, which can be exploited for the ultrafast photonics application. Through chemical functionalization, a graphene-polymer nanocomposite membrane was fabricated and first used to mode lock a fiber laser. Stable mode locked solitons with 3 nJ pulse energy, 700 fs pulse width at the 1590 nm wavelength have been directly generated from the laser. We show that graphene-polymer nanocomposites could be an attractive saturable absorber for high power fiber laser mode locking.
TL;DR: In this article, an erbium-doped fiber laser passively mode-locked with atomic layer graphene was used to generate large energy pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs.
Abstract: We report on large energy pulse generation in an erbium-doped fiber laser passively mode-locked with atomic layer graphene. Stable mode locked pulses with single pulse energy up to 7.3 nJ and pulse width of 415 fs have been directly generated from the laser. Our results show that atomic layer graphene could be a promising saturable absorber for large energy mode locking.
TL;DR: In this paper, a calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments is presented.
Abstract: We present a practical implementation of calibration-free wavelength-modulation spectroscopy with second harmonic detection (WMS-2f) for measurements of gas temperature and concentration in harsh environments. The method is applicable to measurements using lasers with synchronous wavelength and intensity modulation (such as injection current-tuned diode lasers). The key factors that enable measurements without the on-site calibration normally associated with WMS are (1) normalization of the WMS-2f signal by the first harmonic (1f) signal to account for laser intensity, and (2) the inclusion of laser-specific tuning characteristics in the spectral-absorption model that is used to compare with measured 1f-normalized, WMS-2f signals to infer gas properties. The uncertainties associated with the calibration-free WMS method are discussed, with particular emphasis on the influence of pressure and optical depth on the WMS signals. Many of these uncertainties are also applicable to calibrated WMS measurements. An example experimental setup that combines six tunable diode laser sources between 1.3 and 2.0 mum into one probe beam for measurements of temperature, H(2)O, and CO(2) is shown. A hybrid combination of wavelength and frequency demultiplexing is used to distinguish among the laser signals, and the optimal set of laser-modulation waveforms is presented. The system is demonstrated in the harsh environment of a ground-test scramjet combustor. A comparison of direct absorption and 1f-normalized, WMS-2f shows a factor of 4 increase in signal-to-noise ratio with the WMS technique for measurements of CO(2) in the supersonic flow. Multidimensional computational fluid-dynamics (CFD) calculations are compared with measurements of temperature and H(2)O using a simple method that accounts for the influence of line-of-sight (LOS) nonuniformity on the absorption measurements. The comparisons show the ability of the LOS calibration-free technique to gain useful information about multidimensional CFD models.
TL;DR: In this paper, the mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of laser excitation, electron−phonon equilibration, and electron heat conduction.
Abstract: The mechanisms of short pulse laser interactions with a metal target are investigated in simulations performed with a model combining the molecular dynamics method with a continuum description of laser excitation, electron−phonon equilibration, and electron heat conduction. Three regimes of material response to laser irradiation are identified in simulations performed with a 1 ps laser pulse, which corresponds to the condition of stress confinement: melting and resolidification of a surface region of the target, photomechanical spallation of a single or multiple layers or droplets, and an explosive disintegration of an overheated surface layer (phase explosion). The processes of laser melting, spallation, and phase explosion are taking place on the same time scale and are closely intertwined with each other. The transition to the spallation regime results in a reduction of the melting zone and a sharp drop in the duration of the melting and resolidification cycle. The transition from spallation to phase e...
Journal Article•
TL;DR: A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented, and a mixture of helium and trace amounts of nitrogen gas was used.
Abstract: A method, which utilizes the large difference in ionization potentials between successive ionization states of trace atoms, for injecting electrons into a laser-driven wakefield is presented. Here a mixture of helium and trace amounts of nitrogen gas was used. Electrons from the K shell of nitrogen were tunnel ionized near the peak of the laser pulse and were injected into and trapped by the wake created by electrons from majority helium atoms and the L shell of nitrogen. The spectrum of the accelerated electrons, the threshold intensity at which trapping occurs, the forward transmitted laser spectrum, and the beam divergence are all consistent with this injection process. The experimental measurements are supported by theory and 3D OSIRIS simulations.
02 Apr 2009
TL;DR: In this article, the authors present a survey of laser processing and its application in industrial laser processing, focusing on the following: 1.1 Basic Atomic Structure. 2.2 Planar Resonators. 3.3 Discontinuities. 4.4 Generalized Spherical Resonators (GRS). 5.5 Frequency Stabilization.
Abstract: SECTION I: PRINCIPLES OF INDUSTRIAL LASERS. Chapter 1: Laser Generation. 1.1 Basic Atomic Structure. 1.2 Atomic Transitions. 1.3 Lifetime. 1.4 Optical Absorption. 1.5 Population Inversion. 1.6 Threshold Gain. 1.7 Two-Photon Absorption. 1.8 Summary. Problems. Chapter 2: Optical Resonators. 2.1 Standing Waves in a Rectangular Cavity. 2.2 Planar Resonators. 2.3 Confocal Resonators. 2.4 Generalized Spherical Resonators. 2.5 Concentric Resonators. 2.6 Stability of Optical Resonators. 2.7 Summary. Problems. Chapter 3: Laser Pumping. 3.1 Optical Pumping. 3.2 Electrical Pumping. 3.3 Summary. Problems. Chapter 4: Rate Equations. 4.1 Two-Level System. 4.2 Three-Level System. 4.3 Four-Level System. 4.4 Summary. Problems. Chapter 5: Broadening Mechanisms. 5.1 Line-Shape Function. 5.2 Line-Broadening Mechanisms. 5.3 Comparison of Individual Mechanisms. 5.4 Summary. Problems. Chapter 6: Beam Modification. 6.1 Quality Factor. 6.2 Q-Switching. 6.3 Q-Switching Theory. 6.4 Mode-Locking. 6.5 Laser Spiking. 6.6 Lamb Dip. 6.7 Summary. Problems. Chapter 7: Beam Characteristics. 7.1 Beam Divergence. 7.2 Monochromaticity. 7.3 Beam Coherence. 7.4 Intensity and Brightness. 7.5 Frequency Stabilization. 7.6 Beam Size. 7.7 Focusing. 7.8 Radiation Pressure. 7.9 Summary. Problems. Chapter 8: Types of Lasers. 8.1 Solid State Lasers. 8.2 Gas Lasers. 8.3 Dye Lasers. 8.4 Semiconductor (Diode) Lasers. 8.5 Free Electron Laser. 8.6 New Developments in Industrial Laser Technology. 8.7 Summary. Problems. Chapter 9: Beam Delivery. 9.1 The Electromagnetic Spectrum. 9.2 Reflection and Refraction. 9.3 Birefringence. 9.4 Brewster Angle. 9.5 Polarization. 9.6 Mirrors and Lenses. 9.7 Beam Expanders. 9.8 Beam Splitters. 9.9 Beam Delivery Systems. 9.10 Summary. Problems. SECTION II: ENGINEERING BACKGROUND. Chapter 10: Heat and Fluid Flow During Laser Processing. 10.1 Energy Balance During Processing. 10.2 Heat Flow in the Workpiece. 10.3 Fluid Flow in Molten Pool. 10.4 Summary. Problems. Chapter 11: The Microstructure. 11.1 Process Microstructure. 11.2 Discontinuities. 11.3 Summary. Problems. Chapter 12: Solidification. 12.1 Solidification Without Flow. 12.2 Solidification With Flow. 12.3 Rapid Solidification. 12.4 Summary. Problems. Chapter 13: Residual Stresses and Distortion. 13.1 Causes of Residual Stresses. 13.2 Basic Stress Analysis. 13.3 Effects of Residual Stresses. 13.4 Measurement of Residual Stresses. 13.5 Relief of Residual Stresses and Distortion. 13.6 Summary. Problems. SECTION III: LASER MATERIALS PROCESSING. Chapter 14: Background on Laser Processing. 14.1 System-Related Parameters. 14.2 Process Efficiency. 14.3 Disturbances that Affect Process Quality. 14.4 General Advantages and Disadvantages of Laser Processing. 14.5 Summary. Problems. Chapter 15: Laser Cutting and Drilling. 15.1 Laser Cutting. 15.2 Laser Drilling. 15.3 New Developments. 15.4 Summary. Problems. Chapter 16: Laser Welding. 16.1 Laser Welding Parameters. 16.2 Welding Efficiency. 16.3 Mechanism of Laser Welding. 16.4 Material Considerations. 16.5 Weldment Discontinuities. 16.6 Advantages and Disadvantages of Laser Welding. 16.7 Special Techniques. 16.8 Specific Applications. 16.9 Summary. Problems. Chapter 17: Laser Surface Modification. 17.1 Laser Surface Heat Treatment. 17.2 Laser Surface Melting. 17.3 Laser Direct Metal Deposition. 17.4 Laser Physical Vapor Deposition. 17.5 Laser Shock Peening. 17.6 Summary. Problems. Chapter 18: Laser Forming. 18.1 Principle of Laser Forming. 18.2 Process Parameters. 18.3 Laser Forming Mechanisms. 18.4 Process Analysis. 18.5 Advantages and Disadvantages. 18.6 Applications. 18.7 Summary. Problems. Chapter 19: Rapid Prototyping. 19.1 Computer-Aided Design. 19.2 Part Building. 19.3 Post-Processing. 19.4 Applications. 19.5 Summary. Problems. Chapter 20: Lasers in Medical and Nano Manufacturing. 20.1 Medical Applications. 20.2 Nanotechnology Applications. 20.3 Summary. Chapter 21: Sensors for Process Monitoring. 21.1 Laser Beam Monitoring. 21.2 Process Monitoring. 21.3 Summary. Problems. Chapter 22: Processing of Sensor Outputs. 22.1 Signal Transformation. 22.2 Data Reduction. 22.3 Pattern Classification. 22.4 Summary. Problems. Chapter 23: Laser Safety. 23.1 Laser Hazards. 23.2 Laser Classification. 23.3 Preventing Laser Accidents. 23.4 Summary.
TL;DR: In this paper, the authors survey the development of high-order harmonic generation of femtosecond laser pulses by means of laser-produced plasmas and discuss the prospects for applying HHG as a short-wavelength coherent optical tool.
Abstract: The investigation of high-order harmonic generation (HHG) of femtosecond laser pulses by means of laser-produced plasmas is surveyed. This kind of harmonic generation is an alternative to the HHG in gases and shows significantly higher conversion efficiency. Furthermore, with plasma targets there is no limitation on applicable laser intensity and thus the generated harmonics can be much more intense. In principle, harmonic light may also be generated at relativistic laser intensity, in which case their harmonic intensities may even exceed that of the focused laser pulse by many orders of magnitude. This phenomenon presents new opportunities for applications such as nonlinear optics in the extreme ultraviolet region, photoelectron spectroscopy, and opacity measurements of high-density matter with high temporal and spatial resolution. On the other hand, HHG is strongly influenced by the laser-plasma interaction itself. In particular, recent results show a strong correlation with high-energy electrons generated during the interaction process. The harmonics are a promising tool for obtaining information not only on plasma parameters such as the local electron density, but also on the presence of large electric and magnetic fields, plasma waves, and the (electron) transport inside the target. This paper reviews the theoretical and experimental progress on HHGmore » via laser-plasma interactions and discusses the prospects for applying HHG as a short-wavelength, coherent optical tool.« less
TL;DR: In this article, a simple and easily-controlled method for fabricating stainless steel-based superhydrophobic surfaces is presented, which consists of microstructuring stainless steel surfaces by irradiating samples with femtosecond laser pulses and silanizing the surfaces.
TL;DR: The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays, making tabletop, fully coherent, multi-keV X-ray sources feasible.
Abstract: We show how bright, tabletop, fully coherent hard X-ray beams can be generated through nonlinear upconversion of femtosecond laser light. By driving the high-order harmonic generation process using longer-wavelength midinfrared light, we show that, in theory, fully phase-matched frequency upconversion can extend into the hard X-ray region of the spectrum. We verify our scaling predictions experimentally by demonstrating phase matching in the soft X-ray region of the spectrum around 330 eV, using ultrafast driving laser pulses at 1.3-μm wavelength, in an extended, high-pressure, weakly ionized gas medium. We also show through calculations that scaling of the overall conversion efficiency is surprisingly favorable as the wavelength of the driving laser is increased, making tabletop, fully coherent, multi-keV X-ray sources feasible. The rapidly decreasing microscopic single-atom yield, predicted for harmonics driven by longer-wavelength lasers, is compensated macroscopically by an increased optimal pressure for phase matching and a rapidly decreasing reabsorption of the generated X-rays.
TL;DR: In this paper, the physical mechanisms underlying the refractive index modification and the different laser systems used to induce such modification are discussed, and a thorough review of the photonic devices demonstrated with the femtosecond laser microfabrication technique is presented.
Abstract: In this paper we review the micromachining of photonic devices in several materials by means of ultrashort laser pulses. The physical mechanisms underlying the refractive index modification and the different laser systems used to induce such modification are discussed. A thorough review of the photonic devices demonstrated with the femtosecond laser microfabrication technique is presented. In particular, this paper is focused on photonic devices based on optical waveguides. The devices are organized into two categories: passive and active devices. In the former category power splitters, directional couplers, interferometers and Bragg gratings are reviewed, while in the latter amplifiers and lasers are discussed. Finally, conclusions and future perspectives of femtosecond laser micromachining in photonics are provided.
TL;DR: In this article, the role of the laser parameters such as pulse energy and duration and peak power have been investigated to join 3-mm thick Ti6Al4V using the Lumonics JK760TR Nd:YAG pulsed laser.
TL;DR: A new light source based on having alkaline-earth atoms in an optical lattice collectively emit photons on an ultranarrow clock transition into the mode of a high Q resonator has the potential to improve the stability of the best clocks by 2 orders of magnitude.
Abstract: We propose a new light source based on having alkaline-earth atoms in an optical lattice collectively emit photons on an ultranarrow clock transition into the mode of a high Q resonator. The resultant optical radiation has an extremely narrow linewidth in the mHz range, even smaller than that of the clock transition itself due to collective effects. A power level of order 10;{-12} W is possible, sufficient for phase locking a slave optical local oscillator. Realizing this light source has the potential to improve the stability of the best clocks by 2 orders of magnitude.
JILA1
TL;DR: A high-power optical-parametric-oscillator (OPO) based frequency comb in the mid-IR wavelength region that employs periodically poled lithium niobate and is singly resonant for the signal is presented.
Abstract: We present a high-power optical-parametric-oscillator (OPO) based frequency comb in the mid-IR wavelength region. The system employs periodically poled lithium niobate and is singly resonant for the signal. It is synchronously pumped by a 10 W femtosecond Yb:fiber laser centered at 1.07 microm. The idler (signal) wavelength can be continuously tuned from 2.8 to 4.8 microm (1.76 to 1.37 microm) with a simultaneous bandwidth as high as 0.3 microm and a maximum average idler output power of 1.50 W. We also demonstrate the performance of the stabilized comb by recording the heterodyne beat with a narrow-linewidth diode laser. This OPO is an ideal source for frequency comb spectroscopy in the mid-IR.
TL;DR: An experimental study of the high harmonic yield over a wavelength range of 800-1850 nm found that the scaling at constant laser intensity is lambda(-6.3+/-1.1) in Xe and lambda(-(5-6)) in Kr over the wavelength range, somewhat worse than the theoretical predictions.
Abstract: Using longer wavelength laser drivers for high harmonic generation is desirable because the highest extreme ultraviolet frequency scales as the square of the wavelength. Recent numerical studies predict that high harmonic efficiency falls dramatically with increasing wavelength, with a very unfavorable lambda(-(5-6)) scaling. We performed an experimental study of the high harmonic yield over a wavelength range of 800-1850 nm. A thin gas jet was employed to minimize phase matching effects, and the laser intensity and focal spot size were kept constant as the wavelength was changed. Ion yield was simultaneously measured so that the total number of emitting atoms was known. We found that the scaling at constant laser intensity is lambda(-6.3+/-1.1) in Xe and lambda(-6.5+/-1.1) in Kr over the wavelength range of 800-1850 nm, somewhat worse than the theoretical predictions.
TL;DR: It is shown that apart from the conventional dissipative Soliton operation, an all-normal-dispersion fiber laser can also emit square-profile dissipative solitons whose energy could increase to a very large value without pulse breaking.
Abstract: We report on the generation of 281.2 nJ mode locked pulses directly from an erbium-doped fiber laser mode-locked with the nonlinear polarization rotation technique. We show that apart from the conventional dissipative soliton operation, an all-normal-dispersion fiber laser can also emit square-profile dissipative solitons whose energy could increase to a very large value without pulse breaking.
TL;DR: Electrons ionized from an atom or molecule by circularly or elliptically polarized femtosecond omega and 2omega pulses exhibit different trajectory orientations as the relative phase between the two pulses changes, and the polarization of the terahertz wave emitted during the ionization process was found to be coherently controllable through the optical phase.
Abstract: Electrons ionized from an atom or molecule by circularly or elliptically polarized femtosecond omega and 2omega pulses exhibit different trajectory orientations as the relative phase between the two pulses changes. Macroscopically, the polarization of the terahertz wave emitted during the ionization process was found to be coherently controllable through the optical phase. This new finding can be completely reproduced by numerical simulation and may enable fast terahertz wave modulation and coherent control of nonlinear responses excited by intense terahertz waves with controllable polarization.
TL;DR: In this article, a model for coupled radiation transfer and thermal diffusion is proposed, which provides a local temperature field for single-line scanning of a laser beam over a thin layer of metallic powder placed on a dense substrate of the same material.
Abstract: A model for coupled radiation transfer and thermal diffusion is proposed, which provides a local temperature field. Single-line scanning of a laser beam over a thin layer of metallic powder placed on a dense substrate of the same material is studied. Both the laser beam diameter and the layer thickness are about 50 μm. The typical scanning velocity is in the range of 10―20 cm/s. An effective volumetric heat source is estimated from laser radiation scattering and absorption in a powder layer. A strong difference in thermal conductivity between the powder bed and dense material is taken into account. The above conditions correspond to the technology of selective laser melting that is applied to build objects of complicated shape from metallic powder. Complete remelting of the powder in the scanned zone and its good adhesion to the substrate ensure fabrication of functional parts with mechanical properties close to the ones of the wrought material. Experiments with single-line melting indicate that an interval of scanning velocities exists, where the remelted tracks are uniform. The tracks become "broken" if the scanning velocity is outside this interval. This is extremely undesirable and referred to as the "balling" effect. The size and the shape of the melt pool and the surface of the metallurgical contact of the remelted material to the substrate are analyzed in relation to the scanning velocity. The modeling results are compared with experimental observation of laser tracks. The experimentally found balling effect at scanning velocities above ∼20 cm/s can be explained by the Plateau―Rayleigh capillary instability of the melt pool. Two factors destabilize the process with increasing the scanning velocity: increasing the length-to-width ratio of the melt pool and decreasing the width of its contact with the substrate.
TL;DR: By demonstrating a general technique to implement reflecting or absorbing boundaries, this work produces evidence that whispering-gallery-like modes or true photonic-crystal states can be alternatively excited and provides a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal–metal waveguides.
Abstract: Semiconductor lasers based on two-dimensional photonic crystals generally rely on an optically pumped central area, surrounded by un-pumped, and therefore absorbing, regions. This ideal configuration is lost when photonic-crystal lasers are electrically pumped, which is practically more attractive as an external laser source is not required. In this case, in order to avoid lateral spreading of the electrical current, the device active area must be physically defined by appropriate semiconductor processing. This creates an abrupt change in the complex dielectric constant at the device boundaries, especially in the case of lasers operating in the far-infrared, where the large emission wavelengths impose device thicknesses of several micrometres. Here we show that such abrupt boundary conditions can dramatically influence the operation of electrically pumped photonic-crystal lasers. By demonstrating a general technique to implement reflecting or absorbing boundaries, we produce evidence that whispering-gallery-like modes or true photonic-crystal states can be alternatively excited. We illustrate the power of this technique by fabricating photonic-crystal terahertz (THz) semiconductor lasers, where the photonic crystal is implemented via the sole patterning of the device top metallization. Single-mode laser action is obtained in the 2.55-2.88 THz range, and the emission far field exhibits a small angular divergence, thus providing a solution for the quasi-total lack of directionality typical of THz semiconductor lasers based on metal-metal waveguides.
TL;DR: In this article, a femtosecond X-ray streak camera with a pump-probe scheme was realized using a pump and probe scheme that samples the transient response of matter to ionizing soft X-rays radiation in the presence of an intense synchronized terahertz field.
Abstract: A few-femtosecond X-ray streak camera has been realized using a pump–probe scheme that samples the transient response of matter to ionizing soft X-ray radiation in the presence of an intense synchronized terahertz field. Borrowing its concept from attosecond metrology, the femtosecond X-ray streak camera fills the gap between conventional streak cameras with typical resolutions of hundreds of femtoseconds and streaking techniques operating in the sub-femtosecond regime. Its single-shot capability permits the duration and time structure of individual X-ray pulses to be determined. For several classes of experiments in time-resolved spectroscopy, diffraction or imaging envisaged with novel accelerator- and laser-based short-pulse X-ray sources this knowledge is essential, but represents a major challenge to X-ray metrology. Here we report on the single-shot characterization of soft X-ray pulses from the free-electron laser facility FLASH. A streak camera for characterizing the ultrashort X-ray pulses produced by a free-electron laser is reported. The scheme has a single-shot capability, a resolution of a few femtoseconds and is expected to become a useful tool for X-ray metrology, including experiments involving time-resolved spectroscopy and imaging.
TL;DR: The dynamics of the acceleration of ultrathin foil targets by the radiation pressure of superintense, circularly polarized laser pulses is investigated by analytical modeling and particle-in-cell simulations and it is shown that for "optimal" values of the foil thickness only a thin layer at the rear side is accelerated by radiation pressure.
Abstract: The dynamics of the acceleration of ultrathin foil targets by the radiation pressure of superintense, circularly polarized laser pulses is investigated by analytical modeling and particle-in-cell simulations. By addressing self-induced transparency and charge separation effects, it is shown that for "optimal" values of the foil thickness only a thin layer at the rear side is accelerated by radiation pressure. The simple "light sail" model gives a good estimate of the energy per nucleon, but overestimates the conversion efficiency of laser energy into monoenergetic ions.