Shape of ion energy spectra in ultra-short and intense laser–matter interaction

Structured proton spectra and a two-temperature electron distribution were simulated using a 1D3V particle-in-cell code for a plasma created by 35-fs laser pulses with an intensity of I10¹⁹ W/cm². Such a two-temperature electron distribution was used as a prerequisite for a fluid dynamical model allowing us to describe the experimentally measured deep dips in the proton energy spectra of laser-produced plasmas from water-droplet targets. PACS 52.50.Jm; 52.38.Kd; 41.75.Jv     

Nonlinear propagation of an intense Laguerre–Gaussian laser pulse in a plasma channel

The nonlinear propagation of an intense Laguerre–Gaussian(LG) laser pulse in a parabolic preformed plasma channel is analyzed by means of the variational method. The evolution equation of the spot size is derived including the effects of relativistic self-focusing, preformed channel focusing, and ponderomotive self-channeling. The parametric conditions of the LG laser pulse and plasma channel for propagating with constant spot size, periodically focusing and defocusing oscillation,catastrophic focusing, and solitary waves are obtained. Compared with the laser pulse with fundamental Gaussian(FG)mode, it is found that the effect of vacuum diffraction is reduced by half and the effects of relativistic and wakefield focusing are decreased by a quarter due to the hollow transverse intensity profile of the LG laser pulse, while the effect of channel focusing is the same order of magnitude with that of the FG laser pulse. Thus, the matched condition for the intense LG laser pulse with constant spot size is released obviously, while the parameters of the laser and plasma for the existence of solitary waves nearly coincide with those of the FG laser pulse.     

High-resolution observation of basal-plane C-core edge dislocations in 4H–SiC crystal by transmission electron microscopy

High-resolution transmission electron microscopy was used to analyze basal-plane dislocations, which display very characteristic contrasts in grazing incidence monochromatic X-ray topographic images, on the Si-face of 4H–SiC. Grazing incidence monochromatic synchrotron X-ray topography, which is a lattice defect observational technique, has been used in power devices made from 4H–SiC. This technique is useful in analyzing lattice defects near the surface but without the contrast of high-density lattice defects inside the wafer. Basal-plane dislocations exhibit several distinct types of contrast: dark, bright, asymmetric dark/bright and intermediate contrast. Dark and bright contrast areas have been reported to be the edge dislocation regions of basal-plane dislocations. Nevertheless, it remains unclear whether the dark contrast regions are edge dislocations with extra half-planes on the surface side, i.e. Si-core edge dislocations, or those with extra half planes on the deeper crystal side, namely C-core edge dislocations on the Si-face. In this paper, basal-plane dislocations with dark contrast edge dislocations in grazing incidence X-ray topographic images around the (0001) surface were observed via high-resolution transmission electron microscopy, and it was determined that the extra half planes are located on the deeper side against the Si-face. This indicates that the dark contrast edge dislocations are those with a C-core structure on the Si-face. This conclusion is important in establishing the analytical procedure for dislocation contrast in grazing incidence monochromatic X-ray topography on Si face images.     

Morphology of single Shockley-type stacking faults generated by recombination enhanced dislocation glide in 4H–SiC

Morphology of single Shockley-type stacking faults (SFs) generated by recombination enhanced dislocation glide (REDG) in 4H–SiC are discussed and analysed. A complete set of the 12 different dissociated states of basal-plane dislocation loops is obtained using the crystallographic space group operations. From this set, six different double rhombic-shaped SFs are derived. These tables indicate the rules that connect shapes of SFs with the locations of partial dislocations having different core structures, the positions of slip planes in a unit cell, and the Burgers vectors of partial dislocations. We applied these tables for the analysis of SFs generated by the REDG effect reported in the past articles. Shapes, growing process of SFs and perfect dislocations for origins of SFs were well analysed systematically.     

Polarons in magnesium-doped lithium niobate crystals induced by femtosecond light pulses

Strong light-induced absorption has been observed in lithium niobate crystals doped with magnesium after application of femtosecond illumination. In this material there are no Nb-on-Li-site defects and hence no antisite polarons occur, but small free polarons close to the conduction band can be generated. The light-induced absorption observed is attributed to these polarons. For LiNbO₃:Mg, their decay times are about two orders of magnitude smaller than those of the Nb-on-Li-site polarons in undoped material. The results are relevant for a better understanding of the suppression of the so-called optical damage in these crystals and for their use in femtosecond applications.     

Sodium profiles in β″-alumina crystals: Modifications induced by argon bombardment

Abstract

Sodium depth profiles in implanted sodium β″-alumina single crystals have been measured by the nuclear resonance technique. A systematic investigation of the depth profile modifications as function of the implanted ion energy has been done using argon-ion (E = 50–600 keV) irradiation at fixed dose (Φ = 4 × 10¹⁶ions/cm²) and beam current (I = 1 μA/cm²). Argon doses were checked by Rutherford backscattering spectrometry. The changes in the sodium profiles are discussed in terms of transport equations which include three main processes: radiation enhanced transport, electric field assisted migration, and preferential surface sputtering of the alkali element. Special attention is devoted to the discussion of sputtering processes.     

Investigation of V-shaped extended defects in a 4H–SiC epitaxial film

Abstract

We investigated two types of V-shaped extended defects on the basal plane in epitaxial 4H-SiC by synchrotron X-ray topography, photoluminescence imaging/spectroscopy and transmission electron microscopy (TEM). One is the (2, 5) stacking fault (in Zhdanov notation) bounded by two partial dislocations with the Burgers vector b ± 1/4[0?0?0?1]; the other is the (2, 3, 3, 5) stacking fault bounded by partial dislocations with b = ±1/4[0?0?0?1]. The core of the partial dislocations associated with the (2, 3, 3, 5) fault has an out-of-plane component (Frank component) and three in-plane components (Shockley components); the three Shockley components are cancelled out in total. The electronic structures of the (2, 5) and (2, 3, 3, 5) stacking faults were further examined by photoluminescence spectroscopy and first-principles calculations. It is suggested that the (2, 5) and (2, 3, 3, 5) stacking faults both have an interband state at a similar energy level, although they differ structurally.     

Comprehensive study of a 4H–SiC MES–MOSFET

In this paper, we studied the enhancement of the breakdown voltage in the 4H–SiC MESFET–MOSFET (MES–MOSFET) structure which we have proposed in our previous work. We compared this structure with Conventional Bulk-MOSFET (CB-MOSFET) and Field plated Conventional Bulk-MOSFET (FCB-MOSFET) structures. The 4H–SiC MES–MOSFET structure consists of two additional schottky buried gates which behave like a Metal on Semiconductor (MES) at the interface of the active region and substrate. The motivation for this structure was to enhance the breakdown voltage by introducing a new technique of utilizing the reduced surface field (RESURF) concept. In our comparison and investigation we used a two-dimensional device simulator. Our simulation results show that the breakdown voltage of the proposed structure is 3.7 and 2.9 times larger than CB-MOSFET and FCB-MOSFET structures, respectively. We also showed that the threshold voltage and the slope of drain current (I_D) as a function of drain–source voltage (V_DS) for all the structures is the same.     

Spectral broadening and electron–photon coupling in III–V infrared detectors of low dimensional quantum confined system

Present work explores the mid-IR photodetection mechanism in III–V quantum confined system in twofold ways. Firstly, it models the extent of spectral linewidth broadening of photo-detector. Secondly, it investigates whether a strong perturbation of light can modulate the electronic bandstructure. Photo-absorption mechanism in the detector correlated to reduced carrier lifetime in ground state leading to homogeneous spectral widening is calculated. Besides, contribution of non-uniform size and composition of quantum dots towards spectral broadening is modeled in order to get the envelop of inhomogeneously broadened photocurrent spectrum. Our model generates photocurrent spectrum with 1.4 μm broadening centered at 3.5 μm at 77 K for a DWELL-IP, which agrees with the experimental result. The calculated photocurrent spectral width of 1.3 μm for GaAs/AlGaAs Quantum Well (QW) centered at 8.31 μm at 77 K also supports experimental data. In addition, our calculation reveals the emergence of a broad resonant peak in the spectrum of QW-IP in far infrared region (20–50 μm) as the photon volume density increases up to 0.1% of carrier density inside the active region. We introduce a hybrid density-of-states for strongly coupled electron–photon system to explain both mid and far IR peak.     

Microdipole electromagnetic radiation induced by a high-power γ-ray pulse

The microdipole mechanism of electromagnetic emission induced by a high-power γ-ray pulse is theoretically studied. The model of electromagnetic field generation has been developed and investigated. It is shown that conductivity affects the microdipole component of radiation only slightly, and the amplitude and waveform of the detected signal do not depend on the duration of the γ-ray pulse.     

Variation of electron density in spectral broadening process in solid thin plates at 400 nm

The generation of continuous spectrum centered at 400 nm from solid thin plates is demonstrated in this work. A continuum covering 365 nm to 445 nm is obtained when 125-μJ frequency-doubled Ti:sapphire laser pulses are applied to six thin fused silica plates at 1-k Hz repetition rate. The generalized nonlinear Schr ¨odinger equation simplified for forward propagation is solved numerically, the spectral broadening with the experimental parameters is simulated, and good agreement between simulated result and experimental measurement is achieved. The variation of electron density in the thin plate and the advantage of a low electron density in the spectral broadening process are discussed.     

块状介质超连续谱光谱稳定性的理论优化

用变分法讨论了飞秒脉冲在块状透明介质中超连续谱的产生，结合频谱展宽与非线性相移的关系，讨论了块状介质的长度、抽运光的功率、光束口径、透镜的焦距以及介质与透镜的相对位置对超连续谱光谱宽度和光谱稳定性的影响，结果表明入射功率为几倍自聚焦阈值功率时，超连续谱光谱稳定性最好。为实验上获得稳定的超连续谱光谱输出提供了理论指导。     

二阶偏振模色散矢量致脉冲展宽的理论分析

我们首次提出了消偏矢量的概念，并用它来描述二阶偏振模色散中偏振模色散矢量方向的变化。本文导出了由于二阶偏振模色散所引起的脉冲展宽的解析表达式，结论指出，偏振相关的本征色散总是使脉冲展宽加剧；而偏振模色散矢量方向的变化（消偏矢量），却是使脉冲展宽减弱。二阶偏振模色散对脉冲的展宽，不仅与偏振相关的本征色散和消偏矢量有关，而且还与信号的传输速率以及初始一阶偏振模色散的大小有关。信号速率的提高，将明显的使二阶偏振模色散的影响增强。在偏振相关的本征色散不为零的情况下，通过调整初始偏振态矢量、初始一阶偏振模色散矢量以及消偏矢量三者的方向，使它们互相平行，可以获得最佳的色散补偿。但是却不能获得完全的色散补偿。在最佳色散补偿时的最小脉冲展宽为σ= (2^1/2/4)( DCF/T₀).     

向列相液晶中弱光引致各向异性衍射图样的研究

研究了弱线偏振光(≈0.16 W/cm²)通过垂直排列C₆₀掺杂的向列相液晶(5CB)薄膜的远场衍射图样.基于取向光折变机理,二波耦合使液晶分子进行二次取向之后,强度为高斯分布的光束通过样品时将形成高斯分布的空间电荷场,偏振光束通过样品时将产生偏振衍射图样.衍射图样的轮廓是同心圆环,在垂直于光的偏振方向有对称缺口.改变入射光的偏振方向可以看到衍射图样也随之改变, 有效非线性折射率系数n₂≈0.3cm²/W 关键词： 向列相液晶 取向光折变效应 自相位调制 衍射图样     

Continuum generation in water and carbon tetrachloride using a picosecond Nd-YAG laser pulse

We have measured the continuum generated in two non-linear media, distilled water and carbon tetrachloride, pumped by a 36 ps Neodymium Yttrium Aluminium Garnet laser pulse operating at 1.06 μm. We show that the induced spectral broadening extends from 400 nm to 850 nm in both media. However, we find that continuum spectra of carbon tetrachloride exhibit an oscillatory structure with an energy range of about 445 cm⁻¹ which corresponds to the fundamental Raman component. This behaviour is due to a spectral non-overlapping of the Raman anti-Stokes components photoinduced in this medium.     

高功率超短脉冲激光系统中用AOPDF实现增益窄化补偿的实验研究

超短脉冲激光系统中，足够宽的带宽是获得超短脉冲的先决条件.高增益放大器，特别是再 生放大器的增益窄化效应严重制约着输出激光脉冲的时间特性.将可编程声光色散滤波器(AO PDF)应用到高功率激光系统中，能有效地克服增益窄化效应.实验表明，再生放大器的光谱 宽度由未应用前的27?nm增加到了44?nm，压缩脉冲的宽度减小了一半，从而也使峰值功率 提高了一倍. 关键词： 超短脉冲 增益窄化 光谱整形 可编程声光色散滤波器     

Spectral diffusion broadening by soft local modes in Sm-doped sodium borate glass

An explanation for the anomalous temperature broadening of zero-phonon transition corresponding to ⁷F₀↔⁵D₀ electronic transition in Sm²⁺-doped sodium borate glasses is given. The main broadening mechanism is spectral diffusion induced by thermal fluctuation of long-living soft localized modes over the whole studied temperature region 5-293 K.     

飞秒激光在不同位置温度梯度的惰性气体中成丝及光谱展宽的差异

通过对密闭气体的前端、中端和后端不同位置进行加热处理,使密闭气体内部不同位置呈现温度梯度,对比了不同位置温度梯度对成丝及光谱展宽的影响.实验结果表明对气体不同位置加热都可以实现成丝状态的转变,但相对来讲最佳方案是加热中心位置位于焦点处,从而使温度梯度方案效果得到最大实现.同时,随着温度的增加光谱有逐渐变窄的趋势. 关键词： 温度梯度 成丝 光谱展宽 飞秒激光     

拉曼光谱研究n型4H-和6H-SiC晶体的载流子浓度

半导体材料的纵光学声子与等离子体激元耦合模(LOPC模)能够提供材料电学方面的相关信息。本文在室温下测得了n型4H-和6H-SiC的拉曼光谱,分析了掺入的杂质对于SiC晶体拉曼光谱的影响,通过拟合n型4H-和6H-SiC晶体的LOPC模的线型得到等离子体频率,并由此从理论上计算了载流子浓度。载流子浓度的理论计算值与霍尔测量的结果符合得很好。研究结果进一步证实了对于n型4H-和6H-SiC晶体,可以通过分析LOPC模的线形来较准确地给出相关材料的载流子浓度。     

Analysis of contrasts and identifications of Burgers vectors for basal-plane dislocations and threading edge dislocations in 4H-SiC crystals observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry

Contrasts of dislocations in the sub-surface region of the Si-face of a 4H-SiC wafer were observed by monochromatic synchrotron X-ray topography in grazing-incidence Bragg-case geometry. Basal-plane dislocations show very characteristic contrast depending on their Burgers vectors, running directions, and types of dislocations, whether they are screw dislocations, C-core edge dislocations, or Si-core edge dislocations. The rules for contrasts of basal-plane dislocations are summarized. It is shown that by observing those contrasts at fixed diffraction conditions, Burgers vectors of the basal-plane dislocation can be identified without performing a g?·?b analysis in some cases. Threading edge dislocations also have very characteristic contrasts depending on the angles between the projected g and their Burgers vectors. It is shown that Burgers vectors of threading edge dislocations can be determined uniquely by observing their characteristic contrasts without performing g?·?b analysis. Contrast mechanisms for these dislocations in grazing-incidence X-ray topography are discussed.