Picosecond spectroscopy of semiconductors

The time-resolved reflectivity of picosecond pulses from optically excited carrier distributions can provide important information about the energy relaxation rates of hot electrons and holes in semiconductors. The basic optical properties of non-equilibrium carrier distributions are discussed, and in the specific case of GaAs, a semi-empirical analysis of the reflectivity spectrum is described which estimates the contributions from the principal critical points of the band structure. Using Boltzmann factors to approximate the hot carrier distributions, it is found that the non-equilibrium reflectivity spectrum is a sensitive function of carrier temperature and that it can reverse its sign as the distribution relaxes. These results are in good qualitative agreement with recent experiments employing a mode-locked cw dye laser.     

基于热效应飞秒激光诱导LiNbO3表面结构的研究

为了研究飞秒激光与铌酸锂表面作用过程中热效应对其表面的影响,采用重复频率为25MHz、脉宽为100fs、平均输出功率为500mW的紧聚焦飞秒激光对铌酸锂表面进行刻蚀.通过建立热扩散理论模型,模拟重复频率25MHz的飞秒激光在刻蚀点的温度场分布,并通过电子显微镜对刻蚀点的形貌进行分析,发现刻蚀点从内至外形成了3种大小不同、排列疏密不同的颗粒状结构区域,且中心区域被高温烧蚀并伴有裂缝产生.通过能量色散谱仪对刻蚀点的3种不同结构区域中Nb和O元素的相对含量进行测量,结合刻蚀点的温度场分布、形貌和元素含量进行了分析.结果表明,Nb和O元素在温度场的驱动下向外扩散;两者的相对含量对刻蚀点的表面结构具有极大影响.这一结果对高重频飞秒激光刻蚀铌酸锂物理机制及应用的研究是有帮助的.     

IR thermography and FEM simulation analysis of on-chip temperature during thermal-cycling power-metal reliability testing using in situ heated structures

A calibrated system for power metal reliability analysis in smart power technology chips is presented. This system is mainly designed for temperature evaluation during temperature-cycling experiments. Infrared camera measurements under single shot high energy pulses are correlated with electro-thermal finite element simulation and failure analysis. A special test structure, containing poly-silicon heaters, is used to produce thermal stress. The location of a hot spot agrees well with the position of degraded power metal.     

飞秒激光扫描不同温度下的硅片诱导形成微结构的差别

不同温度下利用钛宝石激光器输出的飞秒激光脉冲(脉宽42fs,中心波长800nm,最大单脉冲能量3.6mJ),通过扫描方式在硅表面诱导产生表面微结构。采用光学显微镜和扫描电镜观察飞秒激光诱导硅表面微结构的形貌,发现不同温度下硅片表面形成的微结构区域和形貌出现明显的差异。根据观测结果,分析比较了不同温度条件下硅材料微结构形成的能量阈值。随着温度升高,形成的微结构区域减小,飞秒激光诱导形成硅表面微结构的能量阈值升高。这对于研究飞秒激光与物质的相互作用有一定的参考价值,也能对将来实现硅表面微结构的制作提供参考。     

Hot carrier internal thermionic photoemission in pulsed CO2-laser-excited n+-p silicon junctions

Experimental and theoretical studies were carried out on the electrical response of a forward biased n⁺-p silicon junction under photoexcitation with high-intensity 60-ns laser pulses at λ=10.6 μm. The observed strong bias-dependent electrical pulses are associated to changes in the junction forward current due to the carriers of the high energy tail within the photoexcited hot-electron distribution. A thermionic emission model of hot electrons over the junction barrier account for the experimental results. According to this model and the measured electrical response, the temperature of the carriers is nearly 100 K higher than the lattice temperature, which essentially remains constant at room temperature     

Structural and electronic properties of Si1 − x Ge x binary semiconducting alloys under the effect of temperature and pressure

Based on the empirical pseudo-potential method which incorporates compositional disorder as an effective potential, the band structure of Si_{1 ? x} Ge _x alloy are calculated for different alloy composition x. The effect of temperature and pressure on the electronic band structure of the considered alloy has been studied. Monotonic decreasing and increasing functions are obtained for the temperature and pressure dependent form factors respectively. Some physical quantities as band gaps, bowing parameters, and the refractive index of the considered alloy with different Ge concentration and under the effect of temperature and pressure are calculated. The results obtained are found in good agreement with the experimental and published data.     

Intersubband absorption of light in selectively doped asymmetric double tunnel-coupled quantum wells

The spectrum of equilibrium intersubband absorption has been studied in selectively doped asymmetrical double tunnel-coupled quantum wells designed for research into the modulation of IR light in a longitudinal electric field. The comparison of calculated and experimental spectra at different temperatures is carried out. In calculations, the influence of the space charge on the energy spectra of electrons and the difference in the electron effective mass in different subbands are taken into account. The data obtained on the intersubband absorption spectra in equilibrium conditions and under electron excitation by high-power picosecond pulses of light in the mid-IR range allow us to refine the energy spectrum of electrons in the actual structure.     

Modification of quantum dots in Ge/Si nanostructures by pulsed laser irradiation

The goal of this study was the development of a method for the modification of a quantum dot (QD) structure in Ge/Si nanostructures by pulsed laser irradiation. The Ge_xSi_1?x QD structures were analyzed using data furnished by Raman spectroscopy. Frequency-dependent admittance measurements were used to study the energy spectrum of holes in the Ge/Si heterostructures with Ge_xSi_1?x QDs before and after the laser treatment. The obtained experimental data show that laser treatment makes it possible to reduce the sheet density of QDs, modify their composition, and increase the average size. The most important result is that the QD parameters become more uniform after the treatment with nanosecond laser pulses. In a sample with ODs of 8-nm average lateral size (six monolayers of Ge), the scatter of energy levels in the QD array is reduced by half after the treatment with 10 laser pulses.     

Fast Microwave Detectors Based on the Interaction of Holes with Phonons

Experimental results and a proposed model are discussed in this paper on a new microwave detector which has subnanosecond response times and a pulsed power measuring capability between 0.5 W and >10 kW for a frequency band larger than 0-50 GHz. Our model suggests that electromagnetic energy is absorbed by holes in p-type germanium (Ge). This absorption increases the mobile hole temperature above the lattice temperature. The absorbed energy is determined by measuring the change in average mobility of the holes in the nonequilbrium state. We experimentally observed greater than > 50-V output pulses for kilowatt microwave input pulses to the detector and a bias current of 1 A. The detector exhibited a linear response between 0.5 W and 1 kW. We propose that the voltage pulses in the p-type Ge detector are caused by nonequilibrium holes exchanging energy with the phonons in the crystal lattice. This energy exchange modulates the hole mobility.     

The role of phonons and plasmons in describing the pulsewidth dependence of the transmission of ultrashort optical pulses through germanium

Recently, Elci et al. developed a first principles model that accounts for the generation and transient behavior of dense electron-hole plasmas produced in germanium by picosecond optical pulses. The agreement between this model and early experiments is substantial. However, recent transmission measurements, involving optical pulses of varying width, are in disagreement with initial predictions of this model. Here, we emphasize the dependence of the optical properties of the Ge plasma on the electron-phonon coupling constant, the broadening in the plasmon-assisted recombination, and the energy band structure, and we suggest adjustments to the original model that should produce agreement with these more recent experiments. The pulsewidth dependence in the transmission is shown to be sensitive to the relative strengths of the electron-phonon relaxation and the plasmon recombination during the temporal evolution of the hot electron-hole plasma.     

Effect of number of laser pulses on p+/n silicon ultra-shallow junction formation during non-melt ultra-violet laser thermal annealing

We investigate the effect of the number of laser pulses on the formation of p⁺/n silicon ultra-shallow junctions during non-melt ultra-violet laser (wavelength, 355 nm) annealing. Through surface peak temperature calculating by COMSOL Multiphysics, the non-melt laser thermal annealing is performed under the energy density of 130 mJ/cm². We demonstrate that increasing the number of laser pulses without additional pre-annealing is an effective annealing method for achieving good electrical properties and shallow junction depth by analyzing sheet resistance and junction depth profiles. The optimal number of laser pulses is eight for achieving a high degree of activation of dopant without further increase of junction depth. We have also explained the improved electrical characteristics of the samples on the basis of fully recovered crystallinity as revealed by Raman spectroscopy. Thus, it is suggested that controlling the number of laser pulses with moderate energy density is a promising laser annealing method without additional pre-annealing.     

Impurity spectroscopy on tellurium by means of magnetoresistance measurements under nonohmic conditions

Low temperature magnetoresistance measurements on weakly doped tellurium samples (p < 10¹⁵ cm^?3) show a large number of structures which appear only under nonohmic conditions. Depending on the electric field strength and the chemical nature of the impurities, different types of structures in the magnetoresistance are visible. The most remarkable effect is the appearance of sharp maxima in the magnetoresistance (halfwidth ΔB ～ 0.07 T) which show up at relatively low electric fields of about 1 V/cm. The magnetic field positions of these peaks are correlated with the chemical nature of the impurities. The resonance appears for the different impurities at B = 3.40 T (Bi), B = 3.61 T (Sb), and B = 3.77 T (As)(B ⊥ c). Further structures in the magnetoresistance under hot carrier conditions are correlated with a resonance between the lowest impurity excitation energy and the cyclotron resonance energy. From hot magnetophonon measurements, impurities with quite different binding energies could be identified.     

Generation of bottle beam using low-density channel in air

Cylindrical density depressions generated by femtosecond laser pulses filamenting in air for different energy depositions are investigated numerically, by using a set of hydrodynamic equations. The evolution of density profile is calculated for different temperature elevations. The results indicate that the gas density hole is getting shallower and wider with the increasing temperature elevations. A simulation of the propagation inside low-density channel implies a new way to generate a type of bottle beam.     

Topology-Based Prediction of Pathway Dysregulation Induced by Intense Terahertz Pulses in Human Skin Tissue Models

The strong interaction between terahertz (THz) radiation and biological systems has motivated the development of several biomedical technologies, including imaging and spectroscopy applications with promising potential for improved disease diagnosis. This interaction mechanism also implies that external excitation with intense pulses of THz energy could couple to important biological structures and induce significant downstream phenotypic effects. In this study, we expose human skin tissue models to a prolonged train of high-intensity THz pulses and measure the resulting global differential gene expression. From these data, signal pathway perturbation analysis identified pathways that are predicted to be significantly dysregulated, including the cytokine-cytokine receptor interaction and glioma pathways, and further identified the gene-level mechanisms predominantly responsible. These results indicate that induction of an inflammatory-like response and suppression of division/differentiation in cancer are possible. These effects could be further explored and characterized in different types of normal and cancerous tissues to determine potential novel clinical applicability of intense THz pulses.     

Computational Investigation of the Electronic and Thermoelectric Properties of Strained Bulk Mg2Si

The purpose of this work was to investigate, by numerical simulation, the effect of isotropic and anisotropic strain on the transport properties of Mg₂Si. Analysis of the effects of temperature and charge-carrier concentration on evolution of the energy gap and on the thermoelectric properties of strained Mg₂Si is also reported in this paper. Gap evolution is highly dependent on the type of strain applied to the structure. The Seebeck coefficient (S) and power factor (PF) are strongly modified; a gain of up to 40% can be obtained for S and up to 100% for PF under specific conditions of strain. In most cases the temperature corresponding to the maximum value of PF was found to shift downward under the effect of strain.     

拉曼效应下飞秒光脉冲间的相互作用及其抑制

数值模拟了Raman自频移效应下fs光脉冲间的相互作用及其抑制,结果表明：Raman效应的存在使得fs脉冲的相互作用与常规的ps脉冲间的相互作用不同,在初始阶段孤子相互吸引,而后一直处于排斥状态,脉冲间距随传输距离增加而增大,脉冲在传输过程中发生漂移,孤子产生能量交换,传输距离显著减小,严重影响了通信质量;同步幅度调制、非线性增益与滤波器相结合可有效抑制Raman效应下fs光脉冲间的相互作用。     

Analysis of Linewidth Tunable Terahertz Wave Generation in Periodically Poled Lithium Niobate

A new scheme of optical rectification of femtosecond laser pulses in a periodically poled lithium niobate crystal, which generates high energy and linewidth tunable multicycle terahertz (THz) pulses, is analyzed. The developed simple theoretical model allows investigating the generated THz spectrum and its dependence on spot size of the pump beam. It is shown that the transformation of THz radiation from narrowband to broadband is possible by simply reducing the pump beam size. The temporal waveform and energy of the multicycle THz pulses were calculated as well. It is shown that THz energy is inversely proportional to the pump beam size r _y , whereas energy spectral density is independent of r _y . The efficiency of optical-to-THz energy conversion for pump pulse energy of 1?mJ is estimated to be 0.8?×?10^?4. The possibility of tuning the generation frequency by changing the direction of the pump beam propagation is analyzed.     

Effect of high-power nanosecond and femtosecond laser pulses on silicon nanostructures

The effect of high-power nanosecond (20 ns) and femtosecond (120 fs) laser pulses on silicon nanostructures produced by ion-beam-assisted synthesis in SiO₂ layers or by deposition onto glassy substrates is studied. Nanosecond annealing brings about a photoluminescence band at about 500 mn, with the intensity increasing with the energy and number of laser pulses. The source of the emission is thought to be the clusters of Si atoms segregated from the oxide. In addition, the nanosecond pulses allow crystallization of amorphous silicon nanoprecipitates in SiO₂. Heavy doping promotes crystallization. The duration of femtosecond pulses is too short for excess Si to be segregated from SiO₂. At the same time, such short pulses induce crystallization of Thin a-Si films on glassy substrates. The energy region in which crystallization is observed for both types of pulses allows short-term melting of the surface layer.     

多个短脉冲辐照下光学元件的温升分布

从傅里叶热传导方程出发,推导了在多个幅度、间隔不等的短脉冲辐照下,无源光学元件温升三维分布的含有时间的解析表达式,针对具有周期性的重复脉冲照射的情况进行了具体的计算分析,讨论了入射脉冲数目、脉冲间隔及脉冲通量密度等与光学元件的温升分布的关系,并估计了温升引起的光波相位畸变。     

Shaping pulse of faster-than-Nyquist signaling with truncated optimal detector

In this paper, we investigate the shaping pulse of faster-than-Nyquist (FTN) signaling by making use of the reduced complexity truncated optimal maximum-likelihood sequence detection. Specifically, the nonorthogonal Gaussian shaping pulse which owns the approximate optimal energy concentration in time-frequency domains is exploited. Moreover, for fair of comparisons, a general benchmark for different shaping pulses is adopted, and based on which, the Euclidean distance (or Mazo limit) and practical information rate performance of FTN signaling with Gaussian pulse and conventional T-orthogonal shaping pulses such as \({\mathrm{sinc}}\) pulse and root raised cosine pulse are evaluated. Theoretical analyses and numerical results demonstrate that when employed with truncated optimal detector and small channel memory at the receiver, the Gaussian pulse could achieve better BER and information rate performance than conventional T-orthogonal pulses.