Single Femtosecond Research Articles

Simultaneous 3-D Surface Profiling of Multiple Targets by Repetition Rate Scanning of a Single Femtosecond Laser

We present an integrated scheme of 3-D surface profile measurements made at multiple sites concurrently by employing only a single fiber femtosecond laser as the common light source of low coherence scanning interferometry. This versatile use of an ultrashort mode-locked laser is enabled by linear scanning control of the pulse repetition rate on the source site, while diverse forms of unequal-path, non-symmetric measurements are taken with nanometer precision for different targets simply by delivering fr-scanned pulses through a fiber network. This proposed scheme has no restriction on the number of interferometer sites being integrated concurrently, allowing more diverse industrial applications of ultrashort lasers despite increased system cost and complexity.

Ultrafast Magnetization Reversal in DyFeCo Thin Film by Single Femtosecond Laser Pulse

Abstract—The possibility of the local all-optical magnetization reversal in a thin ferrimagnetic rare-earth—transition metal film (DeFeCo) by linearly and circularly polarized 35 fs laser pulses is demonstrated experimentally. Conditions of the exposure to unit pulse and pulse sequence without an applied external magnetic field are used. The found peculiarities of switching in DyFeCo differ substantially from the results obtained previously for rare-earth—transition metal alloys. The minimum nominal fluence for the magnetization reversal is 3 mJ/cm2.

An unstabilized femtosecond semiconductor laser for dual-comb spectroscopy of acetylene.

Dual-comb systems based on two optical frequency combs of slightly different line spacing emerge as powerful tools in spectroscopy and interferometry. Semiconductor lasers have a high impact in continuous-wave tunable laser spectroscopy. Here we demonstrate the first dual-comb interferometer based on a single femtosecond semiconductor laser: a dual-comb modelocked optically pumped external-cavity surface-emitting laser (MIXSEL) ideally suited for a 1 to 10 GHz comb spacing. At a center wavelength of 1.03 µm (9709 cm-1) we measured acetylene gas transmittance with a resolution of 2.7 GHz in 100 ms with residual errors of less than 3% using thousand comb lines without aliasing effects.

Single shot femtosecond laser ablation of molybdenum

Molybdenum is a high-strength refractory material widely used in industry; its behavior at high temperatures causes considerable theoretical and practical interest. We present the results of experimental studies of femtosecond laser ablation of molybdenum produced by pulses with duration of 160 fs at a moderate intensity of 1012 W/cm2. The values of single-shot ablation and cavitation thresholds, morphology of craters and surface expansion dynamics were measured using ultrafast chirped interferometry. The experimental data are compared to the results time-dependent heat flow calculations based on two-temperature model with the temperature dependent thermodynamic and transport coefficients in the range up to 20 kK.

Single shot femtosecond laser nano-ablation of CVD monolayer graphene

We investigate ablation of CVD monolayer graphene by femtosecond pulses in the single shot regime. We show that the ablation probability of flat graphene drastically reduces for small illumination diameters even if the ablation threshold is exceeded. However, the presence of graphene wrinkles enhances the ablation probability. This is interpreted in terms of electron and energy diffusion within the graphene layer. This differentiated behavior is a drawback for single shot laser nanopatterning. The morphology of the holes with minimal diameter depends on the fluence distribution at ablation threshold. Strong fluence gradients due to strong focussing produce an explosive folding of graphene during ablation.

Femtosecond laser induced concentric semi-circular periodic surface structures on silicon based on the quasi-plasmonic annular nanostructure

In this study, we report polarization-dependent concentric circular periodic surface structures on Si induced by a single shot femtosecond (fs) laser pulse based on pre-processed quasi-plasmonic annular-shaped nanostructure. An abnormal annular-shaped energy deposition of the fundamental fs laser pulse can be found by using dual-wavelength superposition of the fundamental frequency (ω) and the second-harmonic frequency (2ω) of an fs Ti:sapphire laser, which is confirmed by real beam shape detection. Based on the annular-shaped energy distribution of dual-wavelength fs laser, a concentric quasi-plasmonic corral nanostructure can be imprinted on the Au thin film. Surface plasmon polaritons (SPPs) excitations on the planar metallic nanostructures enable the manipulation of light on subwavelength scales. Thus, the pre-processed concentric quasi-plasmonic corral nanostructure can act as a precursor for the subsequent SPPs excitation and propagation by the fs laser irradiation. Using this technique, polarization-dependent semi-circular periodic surface structures on silicon can be found by the irradiation of fs laser pulse with only one shot. This research provides an additional freedom for the laser induced periodic surface structure (LIPSS) modulation based on the modulation of SPPs excitation and propagation, which plays an important role in the formation of LIPSS.

Single pulse femtosecond laser ablation of silicon – a comparison between experimental and simulated two-dimensional ablation profiles

Abstract Ultrashort laser pulses are widely used for the precise structuring of semiconductors like silicon (Si). We present here, for the first time, a comparative study of experimentally obtained and numerically simulated two-dimensional ablation profiles based on parameters of commercially relevant and widely used near-infrared and diode pumped femtosecond lasers. Single pulse laser ablation was studied at a center wavelength of 1040 nm and pulse duration of 380 fs (FWHM) in an irradiating fluence regime from 1 J/cm2 to 10 J/cm2. Process thresholds for material transport and removal were determined. Three regimes, scaling with the fluence, could be identified: low and middle fluence regimes and a hydrodynamic motion regime. By comparing the simulated and experimental ablation profiles, two conclusions can be drawn: At 2 J/cm2, the isothermal profile of 3800 K is in excellent agreement with the observed two-dimensional ablation. Thus exceeding a temperature of 3800 K can be accepted as a simplified ablation condition at that fluence. Furthermore, we observed a distinct deviation of the experimental from the simulated ablation profiles for irradiated fluences above 4 J/cm2. This points to hydrodynamic motion as an important contributing mechanism for laser ablation at higher fluences.

Energy deposition of single femtosecond filaments in the atmosphere: erratum.

In this erratum the funding section of Opt. Lett.41, 3908 (2016)OPLEDP0146-959210.1364/OL.41.003908 has been updated.

Ultrafast strong laser filamentation and applications in high-temperature combustion field

The propagation of ultrafast strong laser in optical media such as ambient air can induce a dynamical balance between Kerr self-focusing and defocusing of plasma, resulting in a unique nonlinear phenomenon—laser filamentation, which have shown promising applications in areas such as atmospheric sensing, THz generation, and weather control. In this article, we present an overview of ultrafast strong laser filamentation in combustion fields and its potential application in combustion diagnostics, which is of particular significance in rationalizing the physical and chemical nature of combustion systems such as auto engines for efficient combustion of fuels with low-pollution products. We first introduce the dynamical processes and propagation properties of ultrafast strong laser filamentation in combustion fields, in which the determination of the two fundamental physical parameters, critical power and clamping intensity, for femtosecond laser filamentation in combustion flames is presented. Although these values in combustion flames are found to be smaller than in air, the intensity clamped in flame filaments are enough to induce multiple photon excitation and ionization of combustion intermediates such as CN, C2 and CH free radicals, and atomic C and H to fluoresce. The fluorescence signals are sensitive to the position of interaction of the filament with the flame that indeed reflects the concentration distributions of the species to be sensed. We then present the underlying mechanisms of ultrafast strong laser filamentation induced nonlinear spectroscopy, in which the femtosecond filament-induced flame fluorescence emissions are found to mainly originate from the interaction of femtosecond laser pulses with the combustion intermediates existing in the combustion environment, but not from the fragmentation of parent fuel molecules. In particular, due to the high nonlinear properties of femtosecond laser filamentation in combustion flames, filament- induced nonlinear flame fluorescence technique could be applied for simultaneous identification of multiple combustion intermediates in different fuel flames by comparing the signal ratios between the intermediates, providing the possibility of this technique in application to various combustion conditions that strongly depend on the fuel species. Interestingly, with the femtosecond laser filament excitation, it is demonstrated that in flames, some combustion species can be populated inverted, generating lasing actions by observing the backward fluorescence signals as a function of filament length, which provides a way to overcome the quenching of specific species in combustion diagnostics, and thus to improve signal to noise ratio in the combustion monitoring. It is also demonstrated that filament-induced fluorescence of flames can provide real-time monitoring of combustion intermediates by using a single shot femtosecond laser pulse, which is very important in combustion diagnostics because of the turbulent nature of combustion flames. We finally give an overview of the current research status and progress of ultrafast laser filamentation in application to the diagnostics of high-temperature combustion fields, and discussed both current challenges and a future perspective. Since high-power femtosecond laser system with high repetition rate of up to 10 kHz is available, the abovementioned results using femtosecond laser filamentation in flames reveal the possibility for high-speed monitoring of multiple combustion intermediates by means of femtosecond laser filament excitation.

Optical gating and streaking of free electrons with sub-optical cycle precision

The temporal resolution of ultrafast electron diffraction and microscopy experiments is currently limited by the available experimental techniques for the generation and characterization of electron bunches with single femtosecond or attosecond durations. Here, we present proof of principle experiments of an optical gating concept for free electrons via direct time-domain visualization of the sub-optical cycle energy and transverse momentum structure imprinted on the electron beam. We demonstrate a temporal resolution of 1.2±0.3 fs. The scheme is based on the synchronous interaction between electrons and the near-field mode of a dielectric nano-grating excited by a femtosecond laser pulse with an optical period duration of 6.5 fs. The sub-optical cycle resolution demonstrated here is promising for use in laser-driven streak cameras for attosecond temporal characterization of bunched particle beams as well as time-resolved experiments with free-electron beams.

单光束飞秒激光诱导玻璃内部纳米光栅机理及应用研究进展

单光束飞秒激光诱导玻璃内部纳米光栅机理及应用研究进展

Energy deposition of single femtosecond filaments in the atmosphere.

We present spatially resolved measurements of energy deposition into atmospheric air by femtosecond laser filaments. Single filaments formed with varying laser pulse energy and pulsewidth were examined using longitudinal interferometry, sonographic probing, and direct energy loss measurements. We measure peak and average energy absorption of ∼4 μJ/cm and ∼1 μJ/cm for input pulse powers up to ∼6 times the critical power for self-focusing.

Invited Article: Multiple-octave spanning high-energy mid-IR supercontinuum generation in bulk quadratic nonlinear crystals

Bright and broadband coherent mid-IR radiation is important for exciting and probing molecular vibrations. Using cascaded nonlinearities in conventional quadratic nonlinear crystals like lithium niobate, self-defocusing near-IR solitons have been demonstrated that led to very broadband supercontinuum generation in the visible, near-IR, and short-wavelength mid-IR. Here we conduct an experiment where a mid-IR crystal is pumped in the mid-IR. The crystal is cut for noncritical interaction, so the three-wave mixing of a single mid-IR femtosecond pump source leads to highly phase-mismatched second-harmonic generation. This self-acting cascaded process leads to the formation of a self-defocusing soliton at the mid-IR pump wavelength and after the self-compression point multiple octave-spanning supercontinua are observed. The results were recorded in a commercially available crystal LiInS2 pumped in the 3-4 μm range with 85 fs 50 μJ pulse energy, with the broadest supercontinuum covering 1.6-7.0 μm. We measured up 30 μJ energy in the supercontinuum, and the energy promises to scale favorably with an increased pump energy. Other mid-IR crystals can readily be used as well to cover other pump wavelengths and target other supercontinuum wavelength ranges.

Nanocrystalline and Polyicosahedral Structure of a Nanospike Generated on Metal Surface Irradiated by a Single Femtosecond Laser Pulse

Short pulse laser irradiation of metal targets can trigger a cascade of highly nonequilibrium processes leading to the formation of unique surface structures of interest to various practical applications. In this paper, we report the results of a large-scale atomistic simulation predicting the generation of a ∼200 nm long frozen nanospike on the surface of a Ag target irradiated by a femtosecond laser pulse. The simulation provides detailed information on the mechanisms responsible for the formation of the nanospike and the processes that define its complex nanostructure. The competing contributions of the epitaxial regrowth of the solid part of the target and the homogeneous nucleation of new crystallites triggered by the strong undercooling of the liquid regions are found to produce a remarkable variability of the structural motifs coexisting in different regions of the frozen nanospike. The homogeneous solidification, in particular, proceeds along two distinct paths selected at the nucleation stage and...

Supercontinuum accumulation along a single femtosecond filament in fused silica

The spectral evolution of a filament formed by femtosecond laser pulses in fused silica is experimentally investigated. It is found that the evolution of the supercontinuum (SC) spectrum along the single filament is closely related with its intensity and length. The energy conversion efficiency of fundamental laser pulse to SC generation increases monotonically with different rates as the filament length increases, showing an accumulating effect along the filament. The results also show that the short-wavelength cutoff of the SC spectra is more sensitive to the intensity of the filament than the conversion efficiency of the SC, which is contributed mainly by the efficiently ionized zone of the filament.

单光束飞秒激光诱导的电子态密度分布对双周期纳米光栅的影响

单光束飞秒激光诱导的电子态密度分布对双周期纳米光栅的影响

Field-free molecular orientation by two-color femtosecond laser pulse and time-delayed THz laser pulse

We propose a scheme for achieving molecular orientation steered by the combination of a nonresonant two-color femtosecond laser pulse and a time-delayed THz laser pulse. It is shown that the molecular negative and positive orientation can be significantly enhanced by applying this scheme compared to the single femtosecond or THz laser pulse. The effects of the intensity of the two-color femtosecond laser pulse and the delay time between the femtosecond laser pulse and the THz laser pulse on molecular orientation are discussed. The molecular orientation also depends on the carrier-envelope phase of the THz laser pulse and the initial rotational temperature of the molecule. The maximal negative and positive orientation degrees calculated are and , respectively.

High-Performance Laser Beam Homogenizer Based on Double-Sided Concave Microlens

Microlens array (MLA) has attracted increasing interest as its application in micro-optical device and system. In this letter, we proposed a new approach that applies rotational displacement MLA to the laser homogenization system. These are nonregular arrays consisting of close-packed concave MLAs on two sides with specific rotation angle. And, the double-sided MLAs diffusers with new design parameter were successfully fabricated using single pulse femtosecond laser assisted chemical wet etching. Simulation and experimental results reveal the good homogenization performance of the double-sided MLAs. In addition, we fabricated diffusers with different rotation angles (θ), and when θ = 60°, the diffuser obtains the best optical performance.

Hybrid single-source online Fourier transform coherent anti-Stokes Raman scattering/optical coherence tomography.

We demonstrate a multimodal optical coherence tomography (OCT) and online Fourier transform coherent anti-Stokes Raman scattering (FTCARS) platform using a single sub-12 femtosecond (fs) Ti:sapphire laser enabling simultaneous extraction of structural and chemical ("morphomolecular") information of biological samples. Spectral domain OCT prescreens the specimen providing a fast ultrahigh (4×12 μm axial and transverse) resolution wide field morphologic overview. Additional complementary intrinsic molecular information is obtained by zooming into regions of interest for fast label-free chemical mapping with online FTCARS spectroscopy. Background-free CARS is based on a Michelson interferometer in combination with a highly linear piezo stage, which allows for quick point-to-point extraction of CARS spectra in the fingerprint region in less than 125 ms with a resolution better than 4 cm(-1) without the need for averaging. OCT morphology and CARS spectral maps indicating phosphate and carbonate bond vibrations from human bone samples are extracted to demonstrate the performance of this hybrid imaging platform.

Nanostructuring of the surface of silicate glass by femtosecond laser pulses in the UV range

The electron dynamics and surface ablation of silicate glass excited by single UV femtosecond pulses have been studied by photoluminescence and contact ultrasound diagnostics. At low energy densities of the laser radiation, the photoluminescence signal reveals the linear character of the photogeneration of the electron–hole plasma, accompanied by its radiative recombination, carrier autolocalization, and point-defect generation. At high energy densities, rapid Auger recombination of the dense electron–hole plasma saturates the luminescence output, allowing only ultrasound diagnostics of the photoexcitation and subsequent thermal processes, including spallation and fragmentation ablation. The results of the experimental studies of the electron dynamics are confirmed by the results of its numerical modelling in terms of the quantum kinetic model, based on a solution of the Boltzmann equation. Surface nanolattices are generated on the surface of the glass in the multipulse laser-excitation regime.