Electronic excitation in femtosecond laser interactions with wide-band-gap materials

Effects of the ultrashort laser excitations of wide-band-gap materials are investigated. Single-, double-, and multiple-shot cases are considered with a particular focus on the control over the transient reflectivity changes and the energy deposition rate. We show that the history of laser excitations affects not only the ionization process and the final number of the conduction-band electrons, but also determines the reflectivity time evolution and the rate of laser energy deposition into the target. Based on the obtained calculation results, both thermal effects and structural modifications can be better controlled.     

Ultrafast blue light emission from SiC nanowires

Cubic silicon carbide (SiC) nanowires are synthesized in a catalyst-assisted process. The nanowires with diameter of - 40 nm exhibit strong blue light emission at room temperature under ultraviolet (UV) femtosecond laser excitation. The photon energy of peak emission is higher than the energy bandgap of cubic SiC which shows involvement of quantum confinement effect. The ultrafast fluorescence is deconvoluted by Monte-Carlo method. The results show two ultrafast decay processes whose lifetimes are about 26 and 567 ps respectively. The mechanisms of such ultrafast processes are discussed.     

Simulations of light antinucleus–nucleus interactions

Creations of light anti-nuclei (anti-deuterium, anti-tritium, anti-³He and anti-⁴He) are observed by collaborations at the LHC and RHIC accelerators. Some cosmic ray experiments are aimed to find the anti-nuclei in cosmic rays. To support the experimental studies of anti-nuclei a Monte Carlo simulation of anti-nuclei interactions with matter is implemented in the Geant4 toolkit. The implementation combines practically all known theoretical approaches to the problem of antinucleon-nucleon interactions.     

Si–LiNbO3–air–metal structure for concentrated terahertz emission from ultrashort laser pulses

A planar Si–LiNbO₃–air–metal structure is proposed as a further development of the highly efficient optical-to-terahertz conversion scheme in sandwich structures that was recently demonstrated. The new structure allows one to collect the terahertz emission into one spatial direction and to control its spectrum by varying an air gap between the metal substrate and the LiNbO₃ layer. While the overall increase in the terahertz generation can reach a factor of 2, the spectral density in the interesting for practical application interval 0.5–1.5 THz can be increased by a factor of 3.5–4.     

Generation of femtosecond X-ray pulses via laser–electron beam interaction

The generation of femtosecond X-ray pulses will have important scientific applications by enabling the direct measurement of atomic motion and structural dynamics in condensed matter on the fundamental time scale of a vibrational period. Interaction of femtosecond laser pulses with relativistic electron beams is an effective approach to generating femtosecond pulses of X-rays. In this paper we present recent results from proof-of-principle experiments in which 300 fs pulses are generated from a synchrotron storage ring by using an ultrashort optical pulse to create femtosecond time structure on the stored electron bunch. A previously demonstrated approach for generating femtosecond X-rays via Thomson scattering between terawatt laser pulses and relativistic electrons is reviewed and compared with storage-ring based schemes.     

Harmonic generation at air–soil interface by femtosecond laser filament

We observe the third-harmonic generation and second-harmonic generation together with element fluorescence from the interaction of a femtosecond laser filament with a rough surface sample(sandy soil) in non-phasematched directions. The harmonics prove to originate from the phase-matched surface harmonics and air filament, then scatter in non-phase-matched directions due to the rough surface. These harmonics occurr when the sample is in the region before and after the laser filament, where the laser intensity is not high enough to excite the element fluorescence. The observed harmonics are related to the element spectroscopy, which will benefit the understanding of the interaction of the laser filament with a solid and be helpful for the application on filament induced breakdown spectroscopy.     

Blue light generation in photonic crystal fibers with 1-μm femtosecond laser pulses

Using 300-fs 1039-nm Yb-doped fiber laser, we experimentally demonstrate blue light generation in a high-△ and high nonlinear photonic crystal fiber (PCF). The zero dispersion wavelength of PCF is 793 nm, detuning 245.8 nm from the pump wavelength. PCF allows a frequency conversion exceeding the octave of pump wavelength. The visible component of the measured output spectrum occurs in the fundamental mode and spans from 391.3 to 492.3 nm. The peak wavelength of 441.8 nm has a frequency detuning of 390 THz from the pump wavelength of 1039 nm.     

Nonlinear refraction of silver nanowires from nanosecond to femtosecond laser excitation

In order to investigate the effect of pulse width and solvent on the nonlinear properties of metal nanostructures, silver nanowires were fabricated in a direct current electric field (DCEF) using a solid-state ionic method and characterized by transmission electron microscopy (TEM) and X-ray diffraction (XRD). The nonlinear refractive index (γ) of silver nanowires suspended in ethanol was measured using the Z-scan technique and laser radiation of various (femto-, pico-, and nanosecond) pulse durations. Experimental results indicated that silver nanowires have obvious positive refractive nonlinearities and γ (the Kerr-induced self-focusing) increases as the pulse duration increases from 7.4×10⁻⁸ cm²/GW at 110 fs to 1.6×10⁻⁴ cm²/GW at 8 ns, due to the additional influence of the atomic reorientational Kerr effect in the case of longer pulses. Due to the solvent dependence of the nonlinear behavior of the silver nanowires, the nonlinear absorption and refraction of silver nanowires suspended in de-ionized water are smaller than those of silver samples suspended in ethanol. The thermal nonlinearities are insignificant in our experimental conditions.     

High-order harmonics in static gas target by 795-nm Ti:sapphire femtosecond laser pulses

The 5th-23sd high-order harmonics generation in rare gases in static gas target with 120-fs, 85-mJ/pulse, 10-Hz laser system was investigated. Compared with the traditional gas target, static gas target is simple to be used in experiment, and the experimental parameters can be easily controlled. The effects on high-order harmonics due to laser intensities (energy), polarization, gas densities, confocal parameter, and phase mismatch were studied in this paper.     

X-ray diffraction from the ripple structures created by femtosecond laser pulses

In this paper, we present the investigation and characterization of the laser-induced surface structure on an asymmetrically cut InSb crystal. We describe diffraction from the ripple surface and present a theoretical model that can be used to simulate X-ray energy scans. The asymmetrically cut InSb sample was irradiated with short-pulse radiation centred at 800 nm, with fluences ranging from 10 to 80 mJ/cm². The irradiated sample surface profile was investigated using optical and atomic force microscopy. We have investigated how laser-induced ripples influence the possibility of studying repetitive melting of solids using X-ray diffraction. The main effects arise from variations in local asymmetry angles, which reduce the attenuation length and increase the X-ray diffraction efficiency.     

Study of white light emission from ZnS/PS composite system

ZnS films were deposited by pulsed laser deposition(PLD)on porous silicon(PS)substrates formed by electrochemical anodization of p-type(100)silicon wafer.The photoluminescence(PL)spectra of ZnS/PS composites were measured at room temperature.Under different excitation wavelengths,the relative integrated intensities of the red light emission from PS layers and the blue-green emission from ZnS films had different values.After samples were annealed in vacuum at different temperatures(200,300,and 400℃)for 30 min respectively,a new green emission located at around 550 nm appeared in the PL spectra of all ZnS/PS samples,and all of the ZnS/PS composites had a broad PL band(450-700 nm)in the visible region,exhibiting intensively white light emission.     

Ion–laser interactions: The most complete solution

Trapped ions are considered one of the best candidates to perform quantum information processing. By interacting them with laser beams they are, somehow, easy to manipulate, which makes them an excellent choice for the production of nonclassical states of their vibrational motion, the reconstruction of quasiprobability distribution functions, the production of quantum gates, etc. However, most of these effects have been produced in the so-called low intensity regime, this is, when the Rabi frequency (laser intensity) is much smaller than the trap frequency. Because of the possibility to produce faster quantum gates in other regimes it is of importance to study this system in a more complete manner, which is the motivation for this contribution. We start by studying the way ions are trapped in Paul traps and review the basic mechanisms of trapping. Then we show how the problem may be completely solved for trapping states; i.e., we find (exact) eigenstates of the full Hamiltonian. We show how, in the low intensity regime, Jaynes–Cummings and anti-Jaynes–Cummings interactions may be obtained, without using the rotating wave approximation and analyze the medium and high intensity regimes where dispersive Hamiltonians are produced. The traditional approach (low intensity regime) is also studied and used for the generation of non-classical states of the vibrational wavefunction. In particular, we show how to add and subtract vibrational quanta to an initial state, how to produce specific superpositions of number states and how to generate NOON states for the two-dimensional vibration of the ion. It is also shown how squeezing may be measured. The time dependent problem is studied by using Lewis–Ermakov methods. We give a solution to the problem when the time dependence of the trap is considered and also analyze a specific (artificial) time dependence that produces squeezing of the initial vibrational wave function. A way to mimic the ion–laser interaction via classical optics is also introduced.     

Wavelength and duration tunable soliton generation from a regeneratively mode-locked fiber laser

A 10-GHz soliton source with pulse duration between 4-8 ps and wavelength continuously tunable from 1530 to 1563 nm is presented. Using regeneratively mode-locking technology, the harmonically modelocked fiber ring laser could work without pulse dropout at room temperature when no cavity length or polarization maintaining mechanism is available. Applying only one 980-nm laser diode pump, the average output power reaches more than 4 mW.     

Prospects of “water-window” X-ray emission from subpicosecond laser plasmas

Soft-X-radiation in the “water-window” region (23.3–43.6 ?) mainly from carbon laser plasmas generated by subpicosecond (700 fs) 0.248-μm laser pulses is studied as a function of angle of incidence and intensity (up to 10¹⁸ W/cm²) for p-polarized laser light. Furthermore, comparison is made between plasmas generated from massive and foil targets. Numerical calculations are performed using a hydrocode coupled to X-ray line and continuum emission calculations including radiation transport. The optimized conditions to achieve maximum water-window X-ray emissivity and, in particular, carbon Lyman-α line emission are investigated. In addition, analytical scalings are presented. These theoretical results are essentially confirmed by previous experiments. It is found that at optimized conditions, picosecond or subpicosecond laser plasma X-ray sources with a power of the order of 1–10 GW in a spectral window of 1 ? could be developed. Received: 6 August 1998 / Final version: 6 August 1999 / Published online: 30 November 1999     

Multipulse feedback in self-organized ripples formation upon femtosecond laser ablation from silicon

The influence of positive feedback on self-organized nanostructure (ripples) formation is investigated for multipulse femtosecond laser ablation from silicon surface. We find an increase of the modified surface area and of complexity and feature size with accumulated dose, confirming the previously postulated feedback effect of dose accumulation. More interestingly, a variation of temporal pulse-to-pulse separation, at constant total incident irradiation dose, strongly affects the structure formation. Though the feedback becomes weaker with increasing time intervals between successive pulses, pulses do not act independently even for separations of up to one second. To account for this observation, a model of perturbation decay and outdiffusion from the excited volume is suggested and compared to the experimental results. Inspection by surface sensitive microscopy (AFM, SEM) and conventional and high-resolution transmission electron microscopy reveal complex structural modification upon the laser interaction: even well outside the irradiated area, the target surface exhibits fine ripple-like undulations, consisting of alternating crystalline and amorphous silicon. This is confirmed by photoluminescence studies on the band–band and the dislocation-related D1-line.     

Absorption of ultrashort intense lasers in laser–solid interactions

With the advent of ultrashort high intensity laser pulses,laser absorption during the laser–solid interactions has received significant attention over the last two decades since it is related to a variety of applications of high intensity lasers,including the hot electron production for fast ignition of fusion targets,table-top bright X-ray and gamma-ray sources,ion acceleration,compact neutron sources,and generally the creation of high energy density matters.Normally,some absorption mechanisms found for nanosecond long laser pulses also appear for ultrashort laser pulses.The peculiar aspects with ultrashort laser pulses are that their absorption depends significantly on the preplasma condition and the initial target structures.Meanwhile,relativistic nonlinearity and ponderomotive force associated with the laser pulses lead to new mechanisms or phenomena,which are usually not found with nanosecond long pulses.In this paper,we present an overview of the recent progress on the major absorption mechanisms in intense laser–solid interactions,where emphasis is paid to our related theory and simulation studies.     

Forward plasma emission through laser–foil interaction with nanosecond lasers

Fundamental investigations of plasma diagnostics of a forward laser plasma acceleration employing laser–foil interactions were conducted for an Al-foil target irradiated with an Nd:YAG laser of 1 J/pulse with pulse width of 10 ns. Temporal evolutions of electron temperatures and densities were evaluated with electrostatic probes and spectroscopic diagnostics. From the results, it was shown that an average speed of ions in a forward direction was about 40 km/s. Also, it was shown that the plasma temperature and density were about 2.5–8 eV and 10¹⁰ cm⁻³, respectively.     

Resonant ablation rules of femtosecond laser on Pr–Nd doped silicate glass

Resonant effect is found in femtosecond laser ablating Pr–Nd glass. When processed with resonant wavelength of807 nm, resonant ablation efficiency(RAE) with a single pulse can be improved by 45.22%. Furthermore, RAE closely relates to laser intensity. For resonant ablation, RAE is increased significantly when laser intensity0.556 × 1014 W∕cm2 at which multiphoton ionization dominates, while it fades away when laser intensity0.556 × 1014 W∕cm2 at which tunnel ionization dominates. Besides, it is also found that the ablation depth increases along with the wavelength rise when multiphoton ionization dominates, while the change rule is inversed when tunnel ionization dominates.