Two-photon interaction of coherent radiation with a thin film of resonant atoms

The problem of the interaction of an ultrashort optical pulse and a thin film of resonant atoms under the conditions of two-photon absorption, third-harmonic generation, and the inverse effect of the latter on the pump pulse via Raman scattering is studied. The fact that the field acting on an atom differs from the macroscopic field in the film is also taken into consideration. It is shown that the polarization of the film undergoes dynamic relaxation even in the absence of irreversible relaxation, suppressing Rabi oscillations and establishing stationary values of the populations of the resonant energy levels and of the polarization of the film at the pump and the third-harmonic frequencies. Zh. éksp. Teor. Fiz. 115, 30–42 (January 1999)     

Effect of the inner structure of domain walls on the stability of an isolated stripe domain in a pulsed field

This paper studies the effect of the inner structure of domain walls on the stability of an isolated stripe domain localized in a thin ferromagnetic film against a pulse of magnetic field applied perpendicularly to the film surface. It is found that the value of the critical amplitude of the pulsed signal strongly depends on the value of the magnetizing field in which the system was initially placed. It is also established that the difference on stability of domains with unipolar and bipolar walls in pulsed fields diminishes as the amplitude of the magnetizing field decreases. Finally, the dependence of the region of stability in a pulse field on the parameters of the system is determined for various domain types. Zh. éksp. Teor. Fiz. 116, 1694–1705 (November 1999)     

Escape into vacuum of fast electrons generated by oblique incidence of an ultrashort, high-power laser pulse on a solid target

The possibility that fast electrons can escape in a direction close to the trajectory of a reflected ultrashort laser pulse at extremely high laser radiation fluxes is examined analytically and numerically. Analytic estimates are made of the feasibility of forming electron bursts in the plasma and of their subsequent motion. The self-consistent, collisionless motion of a plasma acted on by specified incident and reflected ultrashort laser pulses is modeled in two dimensions by the particle-in-cell method. It is shown that a substantial number of electrons located in the subcritical region are gathered into bunches by the resultant forces and escape to the vacuum in a direction different from the normal to the target surface within a narrow range of solid angles. This demonstrates the feasibility of laser acceleration of an electron burst during reflection of an ultrashort laser pulse from a solid target. Zh. éksp. Teor. Fiz. 116, 1184–1197 (October 1999)     

MoS2纳微薄膜激光非线性透射的调控研究

超短飞秒脉冲激光(脉冲时间<40 fs)有独特的热效应机理, 但尚无针对其设计的光限幅保护膜. 本文采用易于产业化的离散-旋涂法制备了MoS₂纳微薄膜(厚度150–200 nm). 光限幅测试结果表明, 针对超短脉冲激光, 此纳微薄膜在低光强下增透, 高光强下减透(光限幅); 且能通过改变入射波长, 调控其光限幅阈值, 可用于聚光太阳能电池的效率增强和损伤保护.利用此方法, 对已商用的砷化镓太阳能电池进行涂膜, 发现转换效率降低<3%, 但损伤阈值提高>50%.     

Characterization and Optimization of a Resonant Cavity Enhanced P-i-N Photodiode Response

This paper presents linear pulse response of a Resonant Cavity Enhanced (RCE) P-i-N fotodiode. The RCE P-i-N photodiode designed for high-speed aplication is analysed for various submicron thicknesses of absorption layer, bias voltages, active areas and incident pulse optical excitations. The results are obtained by numerical simulation of the complete phenomenological model for two valley semiconductor. Great enhancement of the quantum efficiency and the product bandwidth-quantum efficiency, is obvious from obtained results for this photodiode type.     

Forward light reflection from a moving inhomogeneity

The reflection of monochromatic and quasi-monochromatic pulsed light incident on a moving inhomogeneity in the optical characteristics of a medium having plasma-type dispersion has been analyzed. The velocity V of the inhomogeneity, induced in the medium by an intense laser pulse, has been changed by varying its carrier frequency. It has been shown that the usual back-reflection mode, when the reflected radiation pulse moves in the direction opposite the direction of incident radiation, is implemented only if the velocity V is less than the critical value V _min, which depends on the carrier frequency of the incident radiation pulse. It has been found that reflected radiation moves in the same direction as the incident radiation in a certain range of the velocity V _min < V < V _max (forward reflection). In this case, the reflected radiation pulse begins to lag behind a fast-moving inhomogeneity. When V _max < V < c, where c is the speed of light in vacuum, the group velocity of the incident radiation pulse is less than the speed of inhomogeneity, and there is no reflection. Analytical treatment is supported by numerical simulation.     

Statistical modelling of the backscattered field in a one-dimensional non-stationary stochastic problem

Abstract

Within the framework of an exact wave approach in the spatial-time domain, the one-dimensional stochastic problem of sound pulse scattering by a layered random medium is considered. On the basis of a unification of methods which has been developed by the authors, previously applied to the investigation of non-stationary deterministic wave problems and stochastic stationary wave problems, an analytical-numerical simulation of the behaviour of the backscattered field stochastic characteristics was carried out. Several forms of incident pulses and signals are analysed. We assume that random fluctuations of a medium are described by virtue of the Gaussian Markov process with an exponential correlation function. The most important parameters appearing in the problem are discussed; namely, the time scales of diffusion, pulse durations, the medium layer thickness or the largest observation time scale in comparison with the time scale of one correlation length for the case of a half-space. An exact pattern of the pulse backscattering processes is obtained. It is illustrated by the behaviour of the backscattered field statistical moments for all observation times which are of interest. It is shown that during the time interval when the main part of the pulse energy leaves the medium, the backscattered field is a substantially non-stationary process, having a non-zero mean value and an average intensity that decays according to a power law. There are various power indices for the different duration incident pulses, however, they are not the same as those of previous papers, which were obtained on the basis of an approximate and asymptotic analysis. We have also verified that the Gaussian law is valid for the probability density function of the backscattered field in the case of any incident pulse duration.     

Q-Switched Thulium-Doped Fiber Laser with Pure Titanium-Film-Based Saturable Absorber

ABSTRACT

We demonstrate the generation of Q-switching pulse train in thulium-doped fiber laser (TDFL) cavity by employing titanium-based saturable absorber (Ti-SA). The Ti-SA was fabricated by depositing titanium particles molecules using electron beam evaporation on the surface of a polyvinyl alcohol (PVA) film. Subsequently, stable Q-switched pulses were obtained within the 1,552 nm pump power range from 272.1 to 467 mW, with repetition rate tuned from 21.8 to 39.1 kHz. At the maximum pump power, the TDFL showed that the pulse duration of 2.22 μs and the maximum pulse energy of 124 nJ.     

Nonlinear transmission of ultrashort laser pulses by a thin semiconductor film under conditions of a two-photon two-pulse excitation of biexcitons

Specific features of the nonstationary transmission of two pulses exciting biexcitons from the ground state in a thin semiconductor film are studied. It is shown that one of the incident rectangular pulses is totally reflected from the film, whereas the second pulse passes through it as through an absolutely transparent medium. Criteria for appearance of a stationary bistable transmission are determined.     

Laser ablation of liquid surface in air induced by laser irradiation through liquid medium

The pulse laser ablation of a liquid surface in air when induced by laser irradiation through a liquid medium has been experimentally investigated. A supersonic liquid jet is observed at the liquid–air interface. The liquid surface layer is driven by a plasma plume that is produced by laser ablation at the layer, resulting in a liquid jet. This phenomenon occurs only when an Nd:YAG laser pulse (wavelength: 1064 nm) is focused from the liquid onto air at a low fluence of 20 J/cm². In this case, as Fresnel’s law shows, the incident and reflected electric fields near the liquid surface layer are superposed constructively. In contrast, when the incident laser is focused from air onto the liquid, a liquid jet is produced only at an extremely high fluence, several times larger than that in the former case. The similarities and differences in the liquid jets and atomization processes are studied for several liquid samples, including water, ethanol, and vacuum oil. The laser ablation of the liquid surface is found to depend on the incident laser energy and laser fluence. A pulse laser light source and high-resolution film are required to observe the detailed structure of a liquid jet.     

Fabrication of Au thin film gratings by pulsed laser interference

We report that one-dimensional (1D) and two-dimensional (2D) metal thin film gratings can be directly fabricated by interfering Nd-YAG pulsed laser beams (wavelength = 1064 nm, pulse width = 6 ns) incident from the backside of glass substrate. This process utilizes a laser-induced thermo-elastic force which plays a role to detach the film from the substrate. Micro-scale Au transmission gratings with a minimum feature size of 1 μm could be generated by interference-driven periodic detachment. The fabrication of tube-structured patterns as well as stripes was also possible by adjusting the pulse power and this is explained with the effect of film cohesion.     

Interaction of light with a semiconductor thin film, taking into account concentration gain of the dipole moment of the excitonic transition

The peculiarities of nonlinear transmission of ultrashort pulses of resonance laser radiation by a semiconductor thin film in the excitonic spectral region have been studied, taking into account the exciton-phonon interaction and concentration gain of the dipole moment of the excitonic transition. It has been shown that under exact resonance conditions the film transmits only the leading edge of the incident rectangular pulse and completely reflects the remaining part. For a nonzero off-resonance, a residual transmission occurs. A series of possibilities for transforming Gaussian pulses is predicted. The state equations for steady-state bistable transmission (reflection) have been derived, and the stability of the solutions has been studied. A theorem of areas, which predicts area restriction of the transmitted pulses, has been stated; namely, the transmitted pulse area cannot exceed π/2.     

Formation of amorphous carbon on melting of microcrystalline graphite by picosecond laser pulses

Observations of microcrystalline graphite subjected to picosecond laser pulses reveal the formation of a liquid phase with a subsequent transition to a uniform amorphous state of a surface layer upon solidification. This phenomenon is observed on a definite type of graphite and with the radiation incident on a plane parallel to the sixfold symmetry axis, and only for certain parameters of the laser pulse. A structural analysis of the amorphous phase is performed by electron microscopy and Raman scattering spectroscopy. A periodic structure with a period of the order of the wavelength of the heating pulse is formed in the heating region. The “rulings” of this periodic structure are oriented in the direction of polarization of the heating pulse. A study of the reflection kinetics of the probe laser pulse showed that the characteristic existence time of the liquid phase and of the solidification process is ∼10⁻¹⁰ s. Pis’ma Zh. éksp. Teor. Fiz. 66, No. 10, 661–665 (25 November 1997)     

相对论效应对激光在等离子体中的共振吸收的影响

采用一维粒子模拟(PIC)方法，研究了相对论效应对P偏振激光斜入射非均匀等离子体时产生的共振吸收的影响. 计算表明，弱相对论情况下，在临界面附近产生的电子等离子体波的相对论非线性效应占主要作用；随着入射光场的逐渐增大，吸收率逐渐降低. 当入射光强超过3.7×10¹⁷W/cm²时，由于超短激光脉冲本身在等离子体中产生相对论效应、等离子体波破裂效应，以及参量不稳定过程激发等，吸收系数随着激光强度又开始增加. 固定等离子体密度标长，取不同的激光入射角、电子初始温度，相对论效应对吸收系数的影响是一致的. 关键词： 激光等离子体 相对论效应 共振吸收 粒子模拟     

The formation of different structures in the interaction between a single femtosecond laser pulse and a thin Au film

The interactions between femtosecond (fs) laser pulses and a thin Au film deposited on a silica glass substrate were systematically investigated based on experimental data. Different structures, including microholes, nanoholes, and nanobumps, are obtained when pulses with different energies are incident on the surface of a gold film. The experimental results are discussed according to specific experimental parameters. Two physical models were constructed in order to explain the experimental results. The formation of nanoholes in a silica substrate is attributed to etching by higher order harmonic generations (HHG) when the femtosecond laser pulse interacts with the generated plasma layer, while the formation of nanobumps on the surface of an Au film is attributed to the elastic and plastic characteristics of the metal film under laser pulse irradiation.     

Misinterpretation yields supervelocities during transmission of wave packets through a barrier

This paper is concerned with the transmission time of an incident Gaussian wave packet through a symmetric rectangular barrier. Following Hartman (J. Appl. Phys. 33, 3427 (1962)), the transmission time is usually taken as the difference between the time at which the peak of the transmitted packet leaves the barrier of thickness and the time at which the peak of the incident Gaussian wave packet arrives at the barrier. This yields a corresponding transmission velocity which appears under certain conditions as a supervelocity, i.e. becomes larger than the corresponding propagation velocity in free space which is the group velocity for electrons or the velocity of light for photons, respectively. By analysing the propagation of a broadband wave packet (which leads in free space to an extremely concentrated wave packet at a certain time) we obtain the pulse response function of the barrier and show that the insertion of the barrier is physically unable to produce a supervelocity. Therefore, the peak of an incident Gaussian wave packet and the peak of the transmitted wave packet are in no causal relationship. The shape of the transmitted wave packet is produced from the incident wave by convolution with the pulse response of the barrier. This yields a distortion of the shape of the wave packet which includes also the observed negative time shift of the peak. We demonstrate further that the phenomenon of Hartman's supervelocities is not restricted to barriers with their exponentially decaying fields but occurs for instance also in transmission lines with an inserted LCR circuit. Received 7 January 1999 and Received in final form 22 April 1999     

On the ability of resonant diffraction gratings to differentiate a pulsed optical signal

The passage of an optical pulse through a resonant grating is considered. The conditions under which the resonant grating differentiates the envelope of the incident pulse are determined. It is shown that the necessary condition for computing the k-order derivative is the presence of k resonances in the transmission spectrum of the grating in the vicinity of the central frequency of the incident pulse. A method is described for constructing the stacked structure for computing the kth derivative on the basis of repetition of the structure for computing the first derivative. The results of numerical simulation of diffraction of the pulse from the analyzed structure for computing the first, second, and third derivative are presented.     

Second harmonic generation of light in the Kretschmann attenuated total reflection geometry in the presence of surface roughness

Abstract

Using a computer simulation approach we study the generation of second harmonic light in reflection and in transmission in the Kretschmann attenuated total reflection geometry. In this geometry a thin metal film is deposited on the planar base of a dielectric prism, through which p-polarized light is incident on the film. The back surface of the film, which separates the film from vacuum, is a one-dimensional, randomly rough surface, whose generators are normal to the plane of incidence of the light. The nonlinearity responsible for the second harmonic generation is assumed to arise at the prism-metal and metal–vacuum interfaces, and thus enters the problem only through the boundary conditions at these interfaces at the harmonic frequency. The source terms entering these boundary conditions are obtained from the solutions of the corresponding scattering and transmission problems at the fundamental frequency. It is found that a peak in the angular dependence of the intensity of both the transmitted and reflected second harmonic light occurs in the directions normal to the mean scattering surface, in addition to an enhanced backscattering peak in the retroreflection direction. The enhanced transmission peak occurs in the non-radiative region, and therefore cannot be observed in the far field.     

Pulse broadening of enhanced backscattering from rough surfaces

Abstract

Recently, we presented a study of pulse scattering by rough surfaces based on the first-order Kirchhoff approximation which is applicable to rough surfaces with RMS slope less than 0.5 and correlation distance l?λ. However, there has been an increased interest in enhanced backscattering from rough surfaces, study of which requires inclusion of the second-order Kirchhoff approximation with shadowing corrections. This paper presents a theory for the two-frequency mutual coherence function in this region and shows that the multiple scattering on the surface gives rise to an additional pulse tail in the direction of enhanced backscattering. The theory predicts pulse broadening approximately 20% greater than that caused by single scattering alone for a delta-function incident pulse and typical surface parameters. Analytical results are compared with Monte Carlo simulations and millimetre-wave experiments for the one-dimensional rough surface with RMS height 1λ and correlation distance 1λ, showing good agreement.