Interband phototransitions involving free electrons: Part II. Crystals with an indirect band gap

Nonlinear absorption of laser radiation with a photon energy lower than the indirect band gap of crystal has been considered. It is shown that, as in the case of direct-gap materials, single-photon resonance at the transitions between two conduction bands even at radiation intensities j ?? 10⁵?C10⁶ W/cm² yields a range of j values where optical absorption and concentration of nonequilibrium electron-hole pairs increase very sharply with an increase in j.     

Interband phototransitions involving free electrons: I. Crystals with a direct band gap

Nonlinear absorption of laser radiation with a photon energy exceeding the half-width of the direct band gap of crystal but lower than its width has been considered. It is shown that, in the case of singlephoton resonance at transitions between two conduction bands, even at radiation intensities j ??10⁵?C10⁶ W/cm², there is a range of j values where the optical absorption and concentration of nonequilibrium electron-hole pairs sharply increase with an increase in j. The transition of the material between states with different optical and electric properties occurs for few nanoseconds.     

Nonlinear Compton scattering of strong laser radiation on channeled particles in a crystal

A version for intense γ-ray radiation based on the multiphoton scattering of strong laser radiation on relativistic particle beam channeled in a crystal is proposed. The incident laser beam and charged particles beam are counter-propagating and the laser radiation is resonant to the energy levels of transversal motion of channeled particles.     

On the Calculation of an X-Ray Natural Circular Dichroism Signal

The sequential derivation of an expression that describes an X-ray natural circular dichroism (XNCD) signal in the most general case is performed. The obtained expression is used as a basis to consider XNCD signals and their structural origins in copper metaborate CuB₂O₄ crystals at an incident radiation energy near the K absorption edge of copper and lanthanum–gallium silicate La₃Ga₅SiO₁₄ crystals at an incident radiation energy near the K absorption edge of gallium. It has been demonstrated that the measurements of circularly polarized X-ray absorption spectra in combination with corresponding first-principle calculations provide an efficient method for studying the contributions of crystallographically non-equivalent atoms to the X-ray optical activity of crystals.     

Effect of fullerenes on spectral and nonlinear optical properties of <Emphasis Type="Italic">N</Emphasis>-(4-nitrophenyl)-(<Emphasis Type="Italic">L</Emphasis>)-prolinol structure

The influence of fullerenes C₆₀ and C₇₀ on the optical characteristics of a promising organic system, N-(4-nitrophenyl)-(L)-prolinol (NPP), is studied. The UV and blue absorption bands are found that can be controlled by introducing fullerenes. These bands are caused by the change in the electronic structure of the organic system induced by fullerenes and indicate the presence of an ordering in the arrangement of macromolecules. The observed changes in the IR absorption result from the structural rearrangement of the system and may be related to a weak complexing between a donor fragment of the NPP molecule and fullerene. The effect of restriction of the nanosecond laser radiation at 532 nm is considered and characteristics of nonlinear transmission of laser radiation at 337 nm are presented. The levels of attenuation and incident energy are estimated, which determines the possibility of using results of these studies in the development of optical gates in a broad spectral range.     

Femtosecond laser writing of subwave one-dimensional quasiperiodic nanostructures on a titanium surface

One-dimensional quasiperiodic structures whose period is much smaller than the wavelength of exciting radiation have been obtained on a titanium surface under the multipulse action of linearly polarized femtosecond laser radiation with various surface energy densities. As the radiation energy density increases, the one-dimensional surface nanorelief oriented perpendicularly to the radiation polarization evolves from quasiperiodic ablation nanogrooves to regular lattices with subwave periods (100–400 nm). In contrast to the preceding works for various metals, the period of lattices for titanium decreases with increasing energy density. The formation of the indicated surface nanostructures is explained by the interference of the electric fields of incident laser radiation and a surface electromagnetic wave excited by this radiation, because the length of the surface electromagnetic wave for titanium with significant interband absorption decreases with an increase in the electron excitation of the material.     

硅的间接跃迁双光子吸收系数谱

利用皮秒Nd:YAG脉冲激光器作为激发光源,测量出光子能量介于1.36 μm (0.912 eV)—1.80 μm (0.689 eV)之间的硅间接跃迁双光子吸收系数谱.尽管此波段范围内的激光光子能量小于硅间接带隙,但当激光辐照在硅基光电二极管受光面时,在二极管两电极端仍然探测到了显著的脉冲光伏信号.光伏信号峰值强度与入射光强呈二次幂函数关系,表明其是双光子吸收过程.采用pn结等效结电容充放电模型,将光伏响应信号峰值与入射光强相关联,从中提取出硅的间接跃迁双光子吸收系数,改变入射波长得到系数谱.研究表明:     

Nonlinear optical properties of a hydrogenic donor quantum dot

A investigation of the nonlinear optical properties of a hydrogenic donor in a disc-like parabolic quantum dot has been performed by using the matrix diagonalization method. The optical absorption coefficient between the ground (L=0) and the first excited state (L=1) have been examined based on the computed energies and wave functions. The results are presented as a function of the incident photon energy for the different values of the incident optical intensity and the confinement strength. We found the total optical absorption coefficient is strongly affected by the incident optical intensity and the confinement strength.     

激光驻波中运动电子的谐波自发辐射

采用单电子模型分析了电子在线极化激光驻波中的动力学及谐波自发辐射谱,数值计算了电子在驻波中的运动情况及辐射谱。结果表明:电子在波节和波腹处入射后,其辐射谱出现不同的特征;电子在波节处垂直磁场入射后,在洛伦兹力作用下快速振动并向前运动,其向后辐射的光谱发生红移,向前辐射的光谱发生蓝移,谱线出现展宽;当激光强度或者电子初始能量增大时,这些效应更加突出,以至于产生更高阶谐波,形成连续谱;而电子在波腹处以平行电场的方向入射后,仅在电场作用下作直线运动,其自发辐射谱线没有发生移动和展宽。     

Properties of optical limitation of radiation in π-conjugated organic systems doped with fullerenes (The COANP-C70 system)

The effect of optical limitation of laser radiation in 2-cyclooctylamino-5-nitropyridine (COANP-C₇₀) doped with fullerene is experimentally studied. The energy density of the incident beam is measured to be ～4–4.5 J cm^?2 and the attenuation coefficients and the absorption spectrum of a new structure are determined. The photoinduced variation of the refractive index of the COANP-fullerene system and its influence on the optical limitation is studied in the visible range upon irradiation by nanosecond laser pulses. The mechanisms of optical limitation of radiation in these structures are specified. The nonlinear optical coefficients n ₂ and χ⁽³⁾ of the fullerene-doped π-conjugated COANP system are measured for the first time by the methods of dynamic holography.     

Diploe-allowed optical absorption of an exciton in a spherical parabolic quantum dot

A investigation of the linear and nonlinear optical properties of an exciton in a spherical parabolic quantum dot has been performed by using the matrix diagonalization method. The optical absorption coefficients between the ground state (L=0,π=+1) and the first excited state (L=1,π=-1) have been examined based on the computed energies and wave functions. The results are presented as a function of the incident photon energy for the different values of the incident optical intensity and the confinement strength. We found the optical absorption coefficient is strongly affected by the incident optical intensity and the confinement strength.     

High X-ray conversion efficiency with target irradiation by a frequency tripled Nd : Glass laser

High conversion efficiency of laser energy into X-rays from a laser irradiated target is of great interest for a variety of dynamical (pulsed) studies, e.g.: radiography of laser-imploded targets, structure determination by diffraction and absorption fine-structure, and X-ray laser pumping. We report here on a frequency tripled Nd : glass laser used to irradiate targets of various materials at ～5 x 10¹⁴W/cm². We find conversion efficiencies of between 1% and 0.1% (with respect to the incident laser energy) for individual X-ray lines between 1.8 and 7.8 keV. These efficiencies are more than an order of magnitude higher than whose achieved with 1.06 μm lasers.     

Studying the energy dependence of intrinsic conversion efficiency of single crystal scintillators under X-ray excitation

Single crystal scintilators are used in various radiation detectors applications. The efficiency of the crystal can be determined by the Detector Optical Gain (DOG) defined as the ratio of the emitted optical photon flux over the incident radiation photons flux. A parameter affecting DOG is the intrinsic conversion efficiency (n _C ) giving the percentage of the X-ray photon power converted to optical photon power. n _C is considered a constant value for X-ray energies in the order of keV although a non-proportional behavior has been reported. In this work an analytical model, has been utilized to single crystals scintillators GSO:Ce, LSO:Ce and LYSO:Ce to examine whether the intrinsic conversion efficiency shows non proportional behavior under X-ray excitation. DOG was theoretically calculated as a function of the incident X-ray spectrum, the X-ray absorption efficiency, the energy of the produced optical photons and the light transmission efficiency. The theoretical DOG values were compared with experimental data obtained by irradiating the crystals with X-rays at tube voltages from 50 to 140 kV and by measuring the light energy flux emitted from the irradiated screen. An initial value for n _C (calculated from literature data) was assumed for the X-ray tube voltage of 50 kV. For higher X-ray tube voltages the optical photon propagation phenomena was assumed constant and any deviations between experimental and theoretical data were associated with changes in the intrinsic conversion efficiency. The experimental errors were below 7% for each experimental setup. The behavior of n _C values for LSO:Ce and LYSO:Ce were found very similar, i.e., ranging with values from 0.089 at 50 kV to 0.015 at 140 kV, while for GSO:Ce, n _C demonstrated a peak at 80 kV.     

Ejected electron spectrum of sodium autoionizing levels obtained by electron impact excitation 'at 500 eV incident energy

The ejected electron spectrum of sodium vapour has been observed at 90° to the direction af an incident electron beam with kinetic energy 500 eV. Comparisons are made with the ultraviolet absorption data.     

Effect of laser incident energy on the structural,morphological and optical properties of Cu2ZnSnS4 (CZTS) thin films

The polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been prepared by pulsed laser deposition (PLD) method at room temperature. The laser incident energy was varied from 1.0 at the interval of 0.5–3.0 J/cm². The effect of laser incident energy on the structural, morphological and optical properties of CZTS thin films was studied by means of X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM), and optical absorption. The studies reveal that an improvement in the structural, morphological and optical properties of CZTS thin films with increasing laser incident energy up to 2.5 J/cm². However, when the laser incident energy was further increased to 3.0 J/cm², leads to degrade the structural, morphological and optical properties of the CZTS thin films.     

Optoacoustic detection of multiple photon molecular absorption in a strong IR field

A new method based on the optoacoustic effect has been proposed to measure multiple photon absorption at vibrational molecular transitions in a strong IR laser field. Comparison measurements of the average absorbed energy have been done by this method for molecules with different dissociation limits in a strong CO₂-laser field, the field intensity changed therewith by four orders. For the poly-atomic molecules C₂H₄ and SF₆ having a comparatively low dissociation limit, the absorption increases monotonously as the power density of laser radiation P increases up to the dissociation limit. For three-atom molecules, such as D₂O and OCS, absorption saturation takes place with P ? 10 MW/cm².     

Intense laser effects on nonlinear optical absorption and optical rectification in single quantum wells under applied electric and magnetic field

In this work the effects of intense laser on the electron-related nonlinear optical absorption and nonlinear optical rectification in GaAs-Ga_1−xAl_xAs quantum wells are studied under, applied electric and magnetic field. The electric field is applied along the growth direction of the quantum well whereas the magnetic field has been considered to be in-plane. The calculations were performed within the density matrix formalism with the use of the effective mass and parabolic band approximations. The intense laser effects are included through the Floquet method, by modifying the confining potential associated to the heterostructure. Results are presented for the nonlinear optical absorption, the nonlinear optical rectification and the resonant peak of these two optical processes. Several configurations of the dimensions of the quantum well, the applied electric and magnetic fields, and the incident intense laser radiation have been considered. The outcome of the calculation suggests that the nonlinear optical absorption and optical rectification are non-monotonic functions of the dimensions of the heterostructure and of the external perturbations considered in this work.     

电子在激光驻波场中运动产生的太赫兹及X射线辐射研究

采用单电子模型和经典辐射理论分别对低能和高能电子在线偏振激光驻波场中的运动和辐射过程进行了研究. 结果表明: 垂直于激光电场方向入射的低速电子在激光驻波场中随着光强的增大, 逐渐从一维近周期运动演变为二维折叠运动, 并产生强的微米量级波长的太赫兹辐射; 高能电子垂直或者平行于激光电场方向入射到激光驻波场中, 都会产生波长在几个纳米的高频辐射; 低能电子与激光驻波场作用中, 激光强度影响着电子的运动形式、辐射频率以及辐射强度; 高能电子入射时, 激光强度影响了电子高频辐射的强度, 电子初始能量影响着辐射的频率; 电子能量越高, 产生的辐射频率越大. 研究表明可以由激光加速电子的方式得到不同能量的电子束, 并利用电子束在激光驻波场的辐射使之成为太赫兹和X射线波段的小型辐射源. 研究结果可以为实验研究和利用激光驻波场中的电子辐射提供依据.     

A novel “sandwich” method for observation of the keyhole in deep penetration laser welding

A sandwich method was used to observe the keyhole in deep penetration laser welding, which provided an effective way to analyze both the Fresnel and inverse Bremsstrahlung absorption. In the transparent metal-analog system, different densities of metal vapor, ionized atoms, and free electrons in the keyhole can be simulated by changing the thickness of aluminum films. The research results show that inverse Bremsstrahlung absorption exerts a tremendous influence on the energy absorption of the laser beam for CO₂ laser welding. Low density of keyhole plasma benefits the incident laser energy coupling to the materials. However, excess density of keyhole plasma baffles the transmission of the incident laser beam to the interior material. By comparing inflow energy and outflow energy, there exits an energy balance on the keyhole wall by balancing the absorbed laser intensity and heat flux on the wall.     

Fluorescent X-ray transport in microchannel plates

The X-ray absorption spectra of microchannel plates have been measured at grazing incidence of the primary radiation. The Si L components of X-ray absorption near-edge structure (XANES) spectra exhibit specific features for incident angles below a certain critical value. In order to explain the experimental data, a mechanism of selective X-ray fluorescence transport via microchannels is proposed, which is based on the notion of anomalous dispersion in the vicinity of the Si L absorption edge.