Red-shift law of intense laser-induced electro-absorption in solids

A theoretical study on the red-shift of laser-induced electro-absorption is presented. It is found that laser-induced red-shift scales with the cube root of the pump laser intensity in the optical tunneling regime and has an obvious deviation from this scale in the multi-photon regime. Our results show that in the optical tunneling regime, the laser-induced red shift has the same law as that in the direct current （DC） approximation. Though the scales are the same in the optical tunneling regime, the physical pictures in the two cases are quite different. The electro-absorption in the DC case is a tunneling-assisted transition process, while the laser-induced electro-absorption is a mixed multi-photon process.     

Effect of Band Nonparabolicity on the Inter Band Tunneling in Semiconductors

A simple yet generalized theory is developed to study inter band tunneling property of narrow band gap III–V compound semiconductors. The band structures of these low band gap semiconductors with sufficiently separated split-off valance band are usually described by the three energy band model of Kane, so this has been adopted here for the analysis of interband tunneling property in the case of InAs, InSb, and In_1-xGa_xAs_yP_1-y lattice matched to InP as representative direct band gap semiconductors having varied split-off valence band compared to their bulk state band gap energy. It has been found that the magnitude of tunneling rate from heavy hole decreases with increasing band nonparabolicity and the impact is more significant at high electric field in the three-band model of Kane than those with simple parabolic energy band approximations reflecting the direct influence of energy band parameters on inter band tunneling transitions. With proper consideration of band nonparabolicity, the results of the analysis of tunneling rate of these narrow gap materials show significant deviations from the results when simple parabolic band approximation is considered. The exact physical basis of the sources of deviation in the nonparabolic case from the corresponding parabolic band approximations is discussed in association to band coupling effect, transverse energy dependence, and the interplay between them. Moreover, under certain limiting conditions, our results reduce to the well-known results of parabolic band approximation and thus providing an indirect test to the accuracy of our generalized formulations.     

一种纳米硅薄膜的传导机制

基于对实验和理论的分析，提出一种异质结量子点隧穿（HQD）模型，并导出了纳米硅薄膜电导率完整的表达式．其主要思想是，纳米硅薄膜中的微晶粒（几个纳米大小）具有量子点特征，在微晶粒与界面之间由于两者能隙的差异构成晶间势垒，这类似于多晶硅中经常使用的晶间势垒模型（GBT）．考虑到量子点中的单电子隧穿特征，认为纳米硅薄膜中的电传导是由微晶粒中电子弹道式输运与单电子越过势垒的隧穿构成的．这就是HQD模型的主要内容，理论结果与实验相符 关键词：     

Semiclassical Dirac theory of tunnel ionization

We present analytic tunnel ionization rates for hydrogenlike ions in ultrahigh intensity laser fields, as obtained from a semiclassical solution of the three-dimensional Dirac equation. This presents the first quantitative determination of tunneling in atomic ions in the relativistic regime. Our theory opens the possibility to study strong laser field processes with highly charged ions, where relativistic ionization plays a dominant role.     

Multiphoton and tunneling ionization of atoms in an intense laser field

We study the ionization probabilities of atoms by a short laser pulse with three different theoretical methods,i.e.,the numerical solution of the time-dependent Schro¨dinger equation(TDSE),the Perelomov-Popov-Terent’ev(PPT) theory,and the Ammosov-Delone-Krainov(ADK) theory.Our results show that laser intensity dependent ionization probabilities of several atoms(i.e.,H,He,and Ne) obtained from the PPT theory accord quite well with the TDSE results both in the multiphoton and tunneling ionization regimes,while the ADK results fit well to the TDSE data only in the tunneling ionization regime.Our calculations also show that laser intensity dependent ionization probabilities of a H atom at three different laser wavelengths of 600 nm,800 nm,and 1200 nm obtained from the PPT theory are also in good agreement with those from the TDSE,while the ADK theory fails to give the wavelength dependence of ionization probability.Only when the laser wavelength is long enough,will the results of ADK be close to those of TDSE.     

A study of light-hole electro-absorption in AlGaAs parabolic quantum wells,using a novel method

We report on polarization-resolved absorption measurements of parabolic quantum well structures embedded in an optical waveguide. This allows us to investigate in detail not only the heavy hole band structure, but also the light hole subbands whose contribution is suppressed in surface-normal incidence experiments. We present a novel experimental technique utilizing a segmented waveguide device that enables us to directly determine the modal absorption caused by the parabolic quantum well interband transitions. Experimental results for electro-absorption are given and compared to theoretical predictions.     

Dipole-dipole minimum energy configuration for Platonic,Archimedean and Catalan solid structures

Several magnetic materials consisting of dipoles owe their properties to the specific nature of the dipole-dipole interaction. In the present work, we study systems of dipoles where the particles are arranged on various types of three-dimensional structures. However, these solids are not arbitrary. They constitute the well-known Platonic, Archimedean and Catalan solids. We systematically study them in order to fill a gap in the literature that does not contemplate this interaction in the previous solids, despite the fact that they are encountered in many different physical systems. In particular, in the regime of strong dipole moments where a classical treatment is possible, we shall provide not only the minimum energy but also the precise orientations of all their dipoles. We will numerically obtain the minimum energy configuration where all vertices possess the same classic dipole, either electric or magnetic.     

基于光路追踪方法的激光交叉束能量转移模型

提出一种基于几何光路追踪方法并可在流体模拟程序中实现在线计算的激光交叉束能量转移(CBET)耦合模型。借助由激光逆轫致吸收公式引入的泵浦激光功率密度在流体网格尺度上的计算公式,该模型可计算探针激光束中每根光线所携带的能量经过与泵浦激光场相互作用带来的损失(或增加),从而实现激光能量在束间的转移。反复迭代的计算方法解决了由于激光束间能量转移与光线历史相关并且束间强耦合带来的方程求解困难。模型很容易推广到多束激光束两两能量交换的情形,也可用于研究逆轫致吸收和激光等离子体相互作用等物理内容。     

强激光场中模型氢原子和真实氢原子的高次谐波与电离特性研究

利用数值方法求解含时薛定谔方程，研究了一维、二维模型氢原子和真实的三维氢原子在强激光场中产生的高次谐波和电离特性.结果表明，在多光子电离区域和过垒电离区域，模型氢原子与真实的氢原子产生的高次谐波和电离概率差别很小；在隧道电离区域，它们产生的高次谐波的平台特征和截止位置相似，电离概率随时间变化的趋势相近，但其数值有明显的差异.对产生这种差异的原因进行了分析. 关键词： 强激光场 高次谐波 电离概率     

Redshift in the optical absorption of ZnO single crystals in the presence of an intense midinfrared laser field

We report time-resolved electroabsorption of a weak probe in a 500 μm thick zinc-oxide crystal in the presence of a strong midinfrared pump in the tunneling limit. We observe a substantial redshift in the absorption edge that scales with the cube root of intensity up to 1 TW/cm(2) (0.38 eV cm(2/3) TW(-1/3)) after which it increases more slowly to 0.4 eV at a maximum applied intensity of 5 TW/cm(2). The maximum shift corresponds to more than 10% of the band gap. The change in scaling occurs in a regime of nonperturbative high-order harmonic generation where electrons undergo periodic Bragg scattering from the Brillouin zone boundaries. It also coincides with the limit where the electric field becomes comparable to the ratio of the band gap to the lattice spacing.     

Laser energy deposition in relativistic interactions with underdense plasmas

It is well established that, at sub-relativistic intensities, the absorption of laser light by underdense plasmas decreases with increasing pulse intensity as interaction enters a non-linear regime. On the other hand, as the relativistic interaction regime is reached, further absorption mechanisms can be activated which can account for a substantial energy transfer. Using the particle code WAKE, we performed numerical simulations of the relativistic interaction of intense laser pulses with underdense plasmas in conditions that can be experimentally tested. Our simulations show that, while the relativistic laser intensity generates a population of fast electrons, a considerable fraction of the pulse energy goes into a population of thermal electrons. These findings open new possibilities for a direct observation of relativistic interaction processes using high resolution soft X-ray techniques.     

An all-optical buffer based on polarization rotation in an EAM

A theoretical model of the refractive index changes of the TE and TM modes in an electro-absorption modulator （EAM） is deduced. The photon absorption and refractive index changes are analyzed numerically. The influence of pump intensity on the phase difference between the TE and TM modes is studied. The polarization rotation effect is obtained in the EAM, and a novel all-optical fiber loop buffer is designed.     

Exciton susceptibility of semiconductors at high laser-excitation levels

The hysteresis behavior of the real and imaginary parts of the susceptibility of a semiconductor in the exciton region of the spectrum is investigated with consideration of the exciton-phonon and elastic exciton-exciton interactions in the pump-probe regime as a function of the intensity and frequency of the strong laser pulse and the frequency of the probe pulse. The conditions for the appearance of suppression of the damping and amplification of the probe pulse are determined. Abrupt red and blue shifts of the spectral position of the exciton absorption band as a function of the pump pulse intensity are predicted. Fiz. Tverd. Tela (St. Petersburg) 40, 934–935 (May 1998)     

强激光场原子隧道电离的研究新进展

光与物质的相互作用一直是科学的主旋律之一.随着超强超短激光技术的快速发展,如今人们可以研究单个原子的内部世界,并调控光与电子的相互作用,从而实现了对原子内电子的超快动力学过程的探索.强激光诱导的原子隧道电离是众多强场物理现象的基石,具有重要的研究意义,也是研究前沿的热点之一.综述了强场原子隧道电离的最新研究进展,基于隧...     

Multiwavelength GaN-Based Surface-Emitting Lasers and Their Design Principles

Dual-wavelength lasing operations are demonstrated in GaN-based vertical-cavity surface-emitting lasers (VCSELs) comprising ingeniously designed asymmetric InGaN quantum wells (AS-QWs). The dual laser modes show exact positive-correlated polarization dependences with a high degree of polarization of up to 98%. By simply tuning the pump energy, the components and intensity of the laser outputs can be continuously changed, making wavelength selection and switching available for the GaN-based VCSELs. Detailed theoretical analysis and experimental measurements show that the intensity of optical gain and the coupling between the active layer and optical field, namely the electron–photon interaction, as well as carrier tunneling and photon reabsorption play a crucial role in the multiwavelength lasing processes. Moreover, the design principles of the proposed AS-QWs and multistacked size-varied quantum dot (MS-QD) active regions are elaborated to provide guidelines for controllable multiwavelength emissions in GaN-based surface-emitting lasers. These results not only provide better understanding of lasing in nitride-based microcavity systems but also shed insight into the more fundamental issues of electron–photon coupling in such systems. Importantly, such controllable multiwavelength laser operations may extend nitride-based VCSELs to previously inaccessible areas, for example, flip-flop, ultrafast switches, and other functional devices such as Raman lasers and sensors.     

Global rate equation description of a laser

The global integro-differential rate equations describing a multimode laser are analyzed. Expressions for the relaxation oscillation frequencies and their damping rates in the single-mode and two-mode regimes are obtained without specifying either the cavity geometry or the longitudinal pump profile. On the same level of generality, we prove the existence of universal relations relating the peaks of the power spectra in the two-mode regime. For a Fabry-Perot with arbitrary longitudinal pump profile, series expansions of all the physical functions are derived in powers of the pump moments. These moments are averages of the pump profile over cavity modes at linear combinations of the lasing frequencies and their harmonics. These results apply to end-pumped and/or partially filled lasers. For a single mode Fabry-Perot laser, we prove that the contribution to the steady state intensity from the lasing mode varies from 75% close to the lasing threshold to zero at high intensity. The remainder comes from the harmonics of the lasing mode. Analyzing the steady state single mode intensity equation in terms of the pump gratings, we prove that close to the lasing threshold only the space average of the pump and its grating oscillating at twice the lasing wave number do not vanish. This provides a hint towards the justification of the usual modal rate equations which retain only these two functions in the dynamical evolution of a laser. For a Fabry-Perot with constant pump profile, an exact expression for the upper boundary of the stable single mode regime is derived. In that two-mode regime, we prove that there is a critical value of the pump at which the ratio of the two relaxation oscillation frequencies is 2, leading to an internal resonance.     

Investigation of the dependence of the coherent dynamics of a bismuth lattice on the crystal excitation level

Fully symmetric coherent phonons in bismuth have been investigated in a wide range of pumppulse energies by the pump-probe method using femtosecond laser pulses. It has been shown that in the linear regime, which is implemented only for low pump intensities, the coherent amplitude is proportional to the pump intensity, whereas the relaxation rate of the coherent state and its frequency remain unchanged. In the nonlinear regime, which can be divided into superlinear and sublinear regions, the relaxation rate of a photoinduced lattice state can be approximated by a two-component response, where only one component depends strongly on the pump intensity. The nature of the coherence of the phonon subsystem, which is created by an ultrashort laser pulse, is discussed on the basis of the analysis of the data.     

On the electron energy spectrum in heavily doped non-parabolic semiconductors

An attempt is made to present a simple theoretical analysis of the energy-wave vector dispersion relation of the conduction electrons in heavily doped non-parabolic semiconductors forming band tails. We observe that the complex energy spectrum in doped small-gap materials whose unperturbed conduction band is described by the three band model of Kane is due to the interaction of the impurity atoms in the tail with the spin-orbit splitting constant of the valence band (Δ), For band-gap (E_g)<Δ the imaginary part predominates which tails in to the conduction band. For the opposite inequality the real part comes in to play which tails in to the split-off band. In the absence of the band tailing effect, the imaginary part of the complex energy spectrum vanishes and the same is also true for doped two-band Kane-type and parabolic energy bands respectively. The present formulation helps us in investigating the Boltzmann transport equation dependent transport properties of degenerate semiconductors and are expected to agree better with experiments. The well-known results of unperturbed three and two band models of Kane together with wide-gap parabolic energy bands have been obtained as special cases of our generalized analysis under certain limiting conditions.     

Spin-induced forbidden evanescent states in III-V semiconductors

Within the band gap of a semiconductor no electronic propagating states are allowed, but there exist evanescent states which govern charge transport such as tunneling. In this Letter, we address the issue of their spin dependence in III-V semiconductors. Taking into account the spin-orbit interaction, we treat the problem using a k . p 14 x 14 Hamiltonian that we numerically compute for GaAs. Our results show that the removed spin degeneracy in the band gap can lead to giant energy splittings and induces forbidden zones in space where evanescent states are suppressed.