High-intensity Femtosecond Laser Research Articles

Observation of ultrafast impact ionization in diamond driven by mid-infrared femtosecond pulses

We report on the observation of ultrafast impact ionization in monocrystalline diamond driven by high-intensity mid-infrared femtosecond laser pulses. The measurements are based on monitoring the excited carrier population during and after the interaction of the pre-excited sample with a strong infrared pulse by transient transmission spectroscopy and photoluminescence measurements. A twofold increase in the initial carrier population due to impact ionization is observed with the peak infrared intensity of 2.5 TW/cm2. The experimental results are supported by numerical simulations of electron dynamics using time-dependent density functional theory, which show that the electrons in the conduction band reach the energy threshold for impact ionization during the interaction with the infrared pulse.

Polarization-dependent micro-structure fabrication with direct femtosecond laser writing on plastic polarizer films.

Iodine-doped polyvinyl alcohol (IDPVA) film has been widely used as a plastic polarizer due to its great linear dichroism. We found that the anisotropic character of the plastic polarizer can be permanently damaged upon exposure of high intensity femtosecond laser pulses. This process is a two-photon-induced chemical reaction and denominated as two-photon-induced isotropy (TPII). The TPII effect can form a high polarization contrast on the base of the original IDPVA films. With this property, polarization-sensitive diffractive optical elements are fabricated in IDPVA films. The low cost of the IDPVA film and the high polarization contrast of TPII make it a promising new candidate for femtosecond laser fabrication of polarization-selective optical elements.

Interference rings induced by ultrafast laser pulse in GaAs crystal

Gallium arsenide (GaAs) is an important semiconductor material with direct bandgap and clear-cut absorption edge. High-quality crystal is available, which has excellent performances in the field of high power transmission and luminescence because of high laser damage threshold and high thermal conductivity. Nonlinear optics is of great significance for developing the laser technology, spectroscopy, and optoelectronic devices. The Z-scan technology is a nonlinear measurement method based on the analysis of single beam transmittance. Previous studies mainly focused on the measurement of the nonlinear absorption coefficient of the sample by open-aperture configuration of Z-scan and the nonlinear refractive index of the sample by shut-aperture configuration of Z-scan. We build a dual-channel Z-scan system with variable optical path difference, equipped together with a high speed camera, to observe the spatial distribution of transmitted light. Besides, we find the interference rings induced by nonlinear absorption in semiconductor GaAs crystals. Using three kinds of lasers (continuous-wave laser, 80 MHz femtosecond pulsed laser, 1 kHz femtosecond pulsed laser) to illuminate the GaAs crystal separately, multi-level interference rings come out only when a GaAs crystal wafer is illuminated by a femtosecond pulsed laser. In the single-channel Z-scan experiment, we observe that the interference rings contract or expand regularly when changing pulsed laser intensity incident on the surface of GaAs crystal. The higher the intensity of pulse, the more interference rings appear, and the maximum exiting angle becomes larger. Nonlinear effect of high intensity femtosecond pulsed laser locally changes the refractive index of GaAs crystals, resulting in optical path difference (Kerr lens effect). However, the Kerr lens generated by ultrafast light pulse in GaAs crystal cannot focus a beam as done by an ideal thin lens, leading the transmitted light to form interference rings instead. By analyzing the variation of the interference rings, the nonlinear absorption coefficient and refractive index of GaAs crystal can be obtained. In the dual-channel Z-scan experiment, different interference rings are induced in the GaAs crystal as the path difference between the two pulses changes, as done by the nonlinear transmission power. Thus we obtain the formation time of the interference rings and ascribe it to the ultrafast relaxation process of GaAs carriers.

All-optical, self-focused laser beam array for parallel laser surface processing.

A single femtosecond laser beam can be focused to process material surfaces. However, the low processing efficiency by a single laser beam prevents broad applications of femtosecond laser surface processing. Here we have demonstrated an array of high-intensity femtosecond laser beams that self-focus in a piece of glass medium, enabling high-efficiency, millimeter scale parallel laser surface processing. Experimental and simulation results suggest that the laser fluence of each focused laser beamlet in the array is more than 1.2 J/cm2, which is sufficient for micro- or nano-structure laser processing. To prove the concept, nanoscale structures on silicon surface are generated by the self-focused laser beam array, which may improve the efficiency of black silicon fabrication for photovoltaic devices.

Surface-Plasmon-Assisted Cooper Pair Formation in Ordered Gold Films at Room Temperature

Electron emission induced by surface plasmon polaritons (SPPs) or simply surface plasmons excited by high-intensity femtosecond laser irradiation of an ordered Au surface has been studied by time-of-flight (TOF) analysis of the emitted electrons. Significant differences have been found compared to the same analysis of smooth surfaces. The SPP field enhancement has been significantly increased in hot spots which have the same periodicity as the surface. In addition to the increased field enhancement, periodic narrow peaks have been detected in the TOF spectrum. In the laser intensity range, where the high-energy peak is present, some of (also periodic in time) less frequent narrow peaks are about twice as large as the remaining ones, indicating the detection of two electrons (Cooper pairs) at about the same time, namely within the resolution of the TOF spectrometer.

Generation of a strong reverse shock wave in the interaction of a high-contrast high-intensity femtosecond laser pulse with a silicon target

We present ultrafast pump-probe reflectivity and Doppler spectrometry of a silicon target at relativistic laser intensity. We observe an unexpected rise in reflectivity to a peak approximately ∼9 ps after the main pulse interaction with the target. This occurs after the reflectivity has fallen off from the initially high “plasma-mirror” phase. Simultaneously measured time-dependent Doppler shift data show an increase in the blue shift at the same time. Numerical simulations show that the aforementioned trends in the experimental measurements correspond to a strong shock wave propagating back toward the laser. The relativistic laser-plasma interaction indirectly heats the cool-dense (ne≥1023 cm−3 and Te∼10 eV) target material adjacent to the corona, by hot electron induced return current heating, raising its temperature to around 150 eV and causing it to explode violently. The increase in reflectivity is caused by the transient steepening of the plasma density gradient at the probe critical surface due to this explosive behavior.

Stochastic mechanics for ponderomotive effect of a spin-less electron interacting with a high-intensity plane wave laser field

We discuss the relativistically covariant ponderomotive effect of a spin-less electron by a high-intensity laser field of a plane wave in the quantum regime. At first we recall the relativistic ponderomotive effect in classical mechanics. Then against that, we will investigate quantum mechanical expression of the ponderomotive effect by relativistic stochastic mechanics, where, we also propose relativistic stochastic mechanics to draw a trajectory of a spin-less electron. After giving a strict solution of stochastic mechanics, its slowly varying motion is examined as its ponderomotive effect in the quantum regime. Finally, we will see the universality of the ponderomotive effect acting on a spin-less electron interacting with a present high-intensity femtosecond laser, via the dressed (effective) mass of a spin-less electron bridging between classical and quantum mechanics.

Relativistic Interaction of Long-Wavelength Ultrashort Laser Pulses with Nanowires

We report on experimental results in a new regime of a relativistic light-matter interaction employing mid-infrared (3.9-micrometer wavelength) high-intensity femtosecond laser pulses. In the laser generated plasma, the electrons reach relativistic energies already at rather low intensities due to the fortunate lambda^2-scaling of the kinetic energy with the laser wavelength. The lower intensity suppresses optical field ionization and creation of the pre-plasma at the rising edge of the laser pulse efficiently, enabling an enhanced efficient vacuum heating of the plasma. The lower critical plasma density for long-wavelength radiation can be surmounted by using nanowires instead of flat targets. In our experiments about 80% of the incident laser energy has been absorbed resulting in a long living, keV-temperature, high-charge state plasma with a density of more than three orders of magnitude above the critical value. Our results pave the way to laser-driven experiments on laboratory astrophysics and nuclear physics at a high repetition rate.

Formation of channels with changed refractive index at the filamentation of femtosecond laser radiation in quartz glass

The paper considers the phenomenon of filamentation during the propagation of femtosecond laser radiation in transparent solid media as a tool for structuring quartz glasses, contributing to the formation of channels with a modified refractive index. This approach allows changing the intensity distribution in the transverse profile of the laser beam as it passes through the sample. The spatial intensity distribution is determined by the geometry of the formed channels. The use of high-intensity femtosecond laser radiation contributes to the formation of intensity distribution channels.

Ultrafast spin dynamics and inverse spin Hall effect in nanostructures with giant spin-orbit coupling

The features of ultrafast spin dynamics excitation using high-intensity femtosecond laser pulses in magnetic materials are reviewed. Key mechanisms of a pump pulse action on the spin system of a magnetic material are discussed, including the inverse Faraday effect in yttrium-iron garnet; induced magnetic anisotropy in the thulium orthoferrite, and the thermal driving of a magnetic system out of equilibrium in the case of the metallic ferrimagnet GdFeCo, which is a promising material for magnetic memory and terahertz spintronics. It was shown that, apart from using conventional magneto-optical methods for probing the magnetization dynamics in magnetic heterostructures, the inverse spin Hall effect can also be used, an approach that potentially enables the development of memory elements in which the ultrafast optical control of magnetization is combined with electrical detection.

Controlled volume production of simultaneously B/N co-doped reduced graphene oxide nanoflakes using femtosecond laser ablation

B and N co-doped reduced graphene oxide (B/N-rGO) nanoflakes were produced from a mixed aqueous solution of graphene oxide nanoflakes and boron nitride quantum dots, using high intensity femtosecond laser ablation. We observed a controlled production of B/N-rGO nanoflakes with an average flake size of 2.3 μm to 2.7 μm, with up to 5.9 at% boron and 4.1 at% nitrogen co-doping. The B/N-rGO nanoflakes were obtained in as low as 30 min, at ambient room temperature, and without the use of harsh chemicals.

Femtosecond laser-assisted surface modification of tungsten with 1015 W/cm2 intensity in air and vacuum ambience

The present study deals with modification of tungsten surface with high-intensity (∼1015 W/cm2) femtosecond laser in air and vacuum surrounding. Specific findings include: (a) the applied intensities produce crater-like damages with depth ∼31.0 μm in vacuum and ∼7.2 μm in air; (b) peripheral region of the damages is diffuse in air atmosphere unlike vacuum where it is more sharply defined; (c) hydrodynamic features, while present in vacuum, are dramatically reduced in air; (d) characteristic chemical surface changes occur at the surface, and (e) plasma forms in front of the target in both surroundings. The obtained results imply that the employed intensities, in a certain sense, can be used for simulation of some processes in the fusion reactor and for highly precise material removal and tungsten surface processing (by fine tuning of given fs laser parameters).

Nonlinear Polarization and Low-Dissipation Ultrafast Optical Switching in Phosphorene

Fundamental limits to the computation using silicon-based devices stimulate other emerging and viable alternatives. Manipulation of the electrical and optical properties with the ultrafast and intense electric fields offers one of such alternatives. Here, we study the interaction of high-intensity pulsed femtosecond laser of various intensities and carrier frequencies with a monolayer phosphorene using the real-time real-space time-dependent density functional theory. The nonlinear induced currents are entirely reversible but are phase-shifted compared to low-intensity lights within the period of incident laser. We observe optical Kerr effect associating the changes in the number of free charge carriers with the change in the refractive index of the material, which subsequently characterizes the material’s ability of dielectric switching. The transient changes in refractive index are reversible up to a certain threshold, suggesting that the properties can be switched on the timescale of an optical period,...

A setup for micro-structured silicon targets by femtosecond laser irradiation

We present a process to fabricate micro-structured thin silicon foils with highly light absorbing properties over a broad wavelength region based on short-pulse laser treatment. We employ this fascinating technique to the fabrication of targets for high-intensity laser-plasma experiments. A polished silicon wafer is processed with high-intensity femtosecond laser pulses resulting in conical spikes within the irradiated region. Height, distance, and shape of these spikes depend primarily on the number, energy, central wavelength and duration of the incident laser pulses, as well as the ambient medium. This method is of great value for the future development of target fabrication. The broad parameter base offers a huge selection regarding shape and dimensions of the resulting structure. It can be included in existing laser operations within the manufacturing chain while offering a high degree of customisability. Within the reach of higher repetition rates of high-power laser systems and an increased number of requested targets, this manufacturing method can be scaled while being cost efficient and easily adaptable.

Femtosecond laser pulse modification of amorphous silicon films: control of surface anisotropy

A one-dimensional surface relief with a 1.20 ± 0.02 µm period was formed in amorphous hydrogenated silicon films as a result of irradiation by femtosecond laser pulses (1.25 µm) with a fluence of 0.15 J cm−2. Orientation of the formed structures was determined by the polarization vector of the radiation and the number of acting pulses. Nanocrystalline silicon phases with volume fractions from 40 to 67% were detected in the irradiated films according to the analysis of Raman spectra. Observed micro- and nanostructuring processes were caused by surface plasmon–polariton excitation and near-surface region nanocrystallization, respectively, in the high-intensity femtosecond laser field. Furthermore, the formation of Si-III and Si-XII silicon polymorphous modifications was observed after laser treatment with a large exposure dose. The conductivity of the film increased by three orders of magnitude at proper conditions after femtosecond laser nanocrystallization compared to the conductivity of the untreated amorphous surface. The conductivity anisotropy of the irradiated regions was also observed due to the depolarizing contribution of the surface structure, and the non-uniform intensity distribution in the cross-section of the laser beam used for modification.

800 nm高能量飞秒激光脉冲刻写长周期光纤光栅机理

800 nm高能量飞秒激光脉冲刻写长周期光纤光栅机理

Spontaneous emission of atoms in a strong laser field

The spontaneous emission of an atomic system in the field of a high-intensity femtosecond laser pulse is considered within the framework of a consistent quantum-mechanical approach based on an examination of the interaction of a quantum system with a set of quantized field modes in a vacuum state. Both even and odd harmonics of the driving field are shown to appear in the atomic emission spectrum, and the mechanisms of their generation are elucidated. A comparison with the semiclassical theory of laser harmonic generation is made. The semiclassical approach in describing the spontaneous emission in strong laser fields, especially under conditions of significant depletion of the ground (initial) state in a laser field, is shown to be limited.

MV/cm terahertz pulses from relativistic laser-plasma interaction characterized by nonlinear terahertz absorption bleaching in n-doped InGaAs.

We have developed a tabletop intense broadband terahertz (THz) source in the medium frequency range (≤ 20 THz) based on the interaction of a high-intensity femtosecond laser with solid targets at relativistic laser intensities. When an unpolished copper target is irradiated with a high-intensity femtosecond laser, a maximum of ~2.2 μJ of THz pulse energy is collected and detected with a calibrated pyroelectric detector. The THz spectrum was measured by using a series of bandpass filters, showing a bandwidth of ~7.8 THz full-width at half-maximum (FWHM) with a peak at ~6 THz. With tight focusing to reach high field strengths, we have demonstrated THz nonlinearity exemplified by THz absorption bleaching in a heavily n-doped InGaAs thin film, which enabled us to estimate the peak electric field of the THz pulses. We simulated the experimentally observed bleaching by employing a THz pulse having a bandwidth similar to that measured in our experiments and a temporal profile recoded in single-shot electro-optic detection. Through the simulations, we estimate a peak electric field associated with the THz pulses to be 2.5 MV/cm.

Characteristics of femtosecond laser pulses propagating in multiply ionized rare gases

A three-dimensional non-adiabatic model in combination with a rate equation model is used to characterize high-intensity (>1016W/cm2) femtosecond laser pulse propagation in atomic gas medium. In these conditions the atoms in the target gas will become multiply charged ions, while the laser pulse will propagate in a medium with high electron concentration created by itself. We obtain pulse characteristics in time, frequency and space domain for two representative cases: 800nm and 267nm (third harmonic) multi-cycle pulses which are both of practical importance. We show that the spatial-temporal variation of the refractive index in the macroscopic medium is the primary reason for pulse temporal/spectral and spatial shaping during propagation.

Range extension in laser-induced breakdown spectroscopy using femtosecond–nanosecond dual-beam laser system

We extend the detection range of laser-induced breakdown spectroscopy by combining high-intensity femtosecond laser pulses with high-energy nanosecond CO2 laser pulses. The femtosecond laser pulses ionize the molecules and generate filament in air. The free electrons generated in the self-confined plasma channel by the femtosecond laser serve as the seed electrons which cause efficient avalanche ionization in the nanosecond CO2 laser field. We show that the detection distance has been extended by three times with the assistance of femtosecond laser filamentation.