A terahertz scanning near-field optical microscope with an attenuated total internal reflection module

The first terahertz scanning near-field optical microscope with an attenuated total internal reflection module and a free-electron laser (FEL) as the radiation source was developed. A scanning system with positioning using a confocal sensor with chromatic coding and a surface-subwavelength probe touch sensor were developed and tested. A new technique for sensing the distance between the probe and a conducting surface via corona-discharge current measurement was developed. A specific lock-in system for detection of probe-scattered pulse-periodic radiation, which includes a hot-electron superconducting bolometer and an electronic signal-storage circuit, was developed to operate with the Novosibirsk terahertz FEL. All elements of the microscope were tested, and their working capacity was demonstrated. Experiments on the detection of microscope-probe-scattered terahertz radiation have been initiated.     

一种提高太赫兹波成像分辨力的方法

成像分辨力低是影响太赫兹波成像系统在质检、安全和医疗领域应用的重要因素之一。用返波管（BWO）作为太赫兹波辐射源搭建了一个连续太赫兹波点扫描透射成像系统。采用扫描步长小于系统聚焦光斑尺寸的方式获得样品物体的太赫兹透射图像,并利用增量维纳滤波法对该图像进行复原获得了超过系统衍射极限分辨力的太赫兹图像。实验结果表明该方法可以有效提高太赫兹波成像的空间分辨力。     

单层MoS2和WS2的太赫兹近场显微成像研究

采用太赫兹散射式扫描近场光学显微镜(THz s-SNOM)研究了化学气相沉积法制备的单层MoS₂和WS₂晶粒的太赫兹近场响应。在没有可见光激发时,未探测到可分辨的太赫兹近场响应,说明晶粒具有较低的掺杂载流子浓度。有可见光激发时,由于光生载流子的太赫兹近场响应,能够测得与晶粒轮廓完全吻合的太赫兹近场显微图。在相同的光激发条件下,MoS₂的太赫兹近场响应强于WS₂,反映了两者之间载流子浓度或迁移率的差异。研究结果表明,THz s-SNOM兼具超高的空间分辨率和对光生载流子的灵敏探测能力,对二维半导体材料和器件光电特性的微观机理研究具有独特的优势。     

基于磁杆直线电机的光学延迟线研究

在太赫兹时域波谱系统中,通过光学延迟线的时间延迟实现对太赫兹时域信号的采集,而传统的光学延迟线存在扫描速度慢、光学延迟分辨率低等问题。为此,提出了一种采用磁杆直线电机作为驱动和传动一体装置,并加入光栅测量系统的光学延迟线改进方案。测试结果表明,该方案可实现长距离内高速、精准往复运动,同时提供高达512 ps的延时范围和0.033 fs(位置定位精度0.01μm)的光学延迟分辨率,显著提高了太赫兹时域波形振幅的稳定性。     

Terahertz microlens array emitter based on the transverse Dember cross effect

A new method for generating terahertz radiation by exposure to femtosecond laser pulses on the semiconductor surface is proposed. The essence of this method is that the exciting radiation intensity is subjected to spatial modulation by using a microlens array and by shading a part of the semiconductor surface by metal stripes. This gives rise to a concentration gradient of photo carriers along the surface at the sharp boundary of the metallic coating in the semiconductor (transverse Dember photoelectric effect), and its relaxation for times of ～1 ps results in the emission of electromagnetic pulses of the terahertz range. A terahertz emitter model based on the proposed method was developed and designed, its efficiency was demonstrated, and methods for increasing its efficiency were considered.     

Two‐dimensional profiling of carriers in terahertz quantum cascade lasers using calibrated scanning spreading resistance microscopy and scanning capacitance microscopy

The distribution of charge carriers inside the active region of a terahertz (THz) quantum cascade laser (QCL) has been measured with scanning spreading resistance microscopy (SSRM) and scanning capacitance microscopy (SCM). Individual quantum well‐barrier modules with a 35.7‐nm single module thickness in the active region of the device have been resolved for the first time using high‐resolution SSRM and SCM techniques at room temperature. SSRM and SCM measurements on the quantum well‐barrier structure were calibrated utilizing known GaAs dopant staircase samples. Doping concentrations derived from SSRM and SCM measurements were found to be in quantitative agreement with the designed average doping values of the n‐type active region in the terahertz quantum cascade laser. The secondary ion mass spectroscopy provides a partial picture of internal device parameters, and we have demonstrated with our results the efficacy of uniting calibrated SSRM and SCM to delineate quantitatively the transverse cross‐sectional structure of complex two‐dimensional terahertz quantum cascade laser devices.     

散射式太赫兹扫描近场光学显微技术研究

基于散射式近场探测原理,设计并搭建了散射式太赫兹扫描近场光学显微系统(THz s-SNOM),实现了纳米量级空间分辨率的太赫兹近场显微成像测量。该系统以输出频率范围为0.1~0.3THz的太赫兹倍频模块为发射源,通过纳米探针的针尖产生纳米光源与样品相互作用,并将样品表面的倏逝波转化为可在远场测量的辐射波。通过探针逐点扫描样品表面,同时获得了样品表面的形貌图和太赫兹近场显微图。该系统的显微分辨率取决于探针针尖的曲率半径,而与太赫兹波的波长无关。使用该系统测量了金薄膜/硅衬底样品和石墨烯样品的近场显微图,结果表明,近场显微的空间分辨率优于60nm,波长与空间分辨率之比高达λ/26000。     

Small-size terahertz spectrometer using the second harmonic of a femtosecond fiber laser

Possible implementations of a small-size terahertz spectrometer with a subpicosecond resolution on the basis of a femtosecond fiber laser are considered. An experimental sample of a spectrometer is fabricated. This spectrometer employs the method of optical rectification in a ZnTe nonlinear crystal and the Dember (photodiffusion) effect on the InAs semiconductor surface to generate terahertz radiation and the polarization-optical method to detect radiation. System operation is demonstrated by an example of measuring the terahertz absorption spectrum of water and determining the refractive index of the β-BBO crystal. The basic spectrometer parameters are found: spectral range, spectral resolution, and dynamic range of the terahertz spectrum amplitude.     

Terahertz Response from a Silicon Surface with Deposited Nanosized Gold Particles

Terahertz emission spectra of the surface of silicon crystals with different types of conductivity were experimentally recorded upon excitation by femtosecond laser pulses at various temperatures. The observed features in the terahertz spectra of the silicon surface correspond to the energy structure of the impurity centers determining the type of conductivity of the sample. Comparison was made with the results obtained in the case of deposition of gold nanoparticles on a semiconductor surface. The spectral features of the surface with deposited nanoparticles are discussed using the terahertz re-emission mechanism in two-phonon absorption.

     

一种基于剪切力的距离控制方法在扫描近场光学显微镜中的应用

介绍了扫描近场光学显微镜中基于剪切力的样品、探针间距离控制的方法。当受振动激励的光纤探针由远处逐渐接近样品表面时,样品与针尖间的剪切力使针尖的振动振幅减小,通过检测探针振幅的变化从而控制针尖与样品间的距离。此种方法可以方便地将光纤探针导入工作区域内并在扫描过程中保持适当的高度。我们测量了探针系统的幅频特性和力曲线,并用该方法获得4μm×4μm的范围内光盘表面的形貌信息。     

Characteristics of the system of polarization-optical detection of a pulsed terahertz spectrometer

Characteristics of a system of polarization-optical detection of a pulsed terahertz spectrometer in nonlinear crystals of zinc telluride (ZnTe) are considered. The influence of the ZnTe crystal thickness (0.5, 1, and 2 mm) on the detection system sensitivity is studied experimentally; a comparison with model calculations is performed. The effect of the time step and the interval of scanning of the terahertz pulse on the spectral resolution and range of the spectrometer is demonstrated experimentally and theoretically. Terahertz transmission spectra of typical container materials are measured.     

一种基于FSS的太赫兹滤波器特性分析

基于太赫兹频段,为研究太赫兹滤波器的滤波特性,提出了一种基于频率选择表面(FSS)的反射型偏振太赫兹滤波器。通过分析不同旋转角度的滤波器透过性,得出该结构的滤波器具有中心反射率高,谐振频率拓展范围大等优点。优化计算结果表明,当FSS结构设计成不同的旋转角度时,可将中心频率控制在0.58～0.72 THz(动态可调范围约为24%),具有较强的频率拓展实用性,并且在0.58 THz处获得超过96%的最高反射率。     

ReS2薄膜的超快载流子动力学和太赫兹发射研究

基于超快时间分辨光谱实验手段,研究了化学气相沉积（chemical vapor deposition,CVD）生长的ReS_2薄膜的超快载流子动力学和太赫兹发射。分别利用光泵浦探测和光泵浦太赫兹发射两套系统对ReS_2薄膜进行了测试,结果表明：ReS_2薄膜具有超快的载流子热化过程和亚纳秒量级的复合过程;在飞秒激光泵浦下能够产生频谱宽度为2.5 THz的太赫兹辐射。通过分析太赫兹辐射随泵浦光入射角改变而出现极性相反的现象,得出ReS_2薄膜产生太赫兹辐射的主要机制为表面场效应。研究结果不仅有助于理解ReS_2薄膜对超快激光脉冲的瞬态响应,而且为太赫兹光子器件（如太赫兹发射器等）的研究设计提供了重要参考。     

Fast scanning of a pulsed terahertz signal using an oscillating optical delay line

We describe a fast measurement of a pulsed terahertz signal generated by a femtosecond laser and a photoconductive antenna using an oscillating optical delay line. The method to measure the amplitude of the retroreflector in the oscillating optical delay line is proposed and the displacement of the retroreflector is exactly calculated to acquire the optical delay time in the fast scan mode. With the different oscillation frequency and amplitude of the retroreflector, the pulsed terahertz signals are measured and analyzed. The comparison of the temporal waveform and frequency spectrum between the fast scan mode and the slow scan mode shows a good agreement with the decrease in the scanning time from 60 to 1 s at a signal to noise ratio of 430.     

Picosecond laser-induced nanopillar coverage of entire mirror-polished surfaces of Ti6Al4V alloy

The goal of this study was to investigate a study for the efficient generation of pillar-like nanostructure (nanopillar) on a material surface over a large area. In this research, a vertical cross-scanning (VCS) strategy using two linearly-polarized lasers with different laser conditions was proposed for the generation of nanopillars on a mirror-polished surface on a large scale. It found that the laser fluence and scanning speed of the second laser scanning should be controlled within a specific range to generate the nanopillars. Additionally, the distance between scan lines, which is defined as hatch distance, h, of the second scan, is also a non-negligible factor to induce nanopillars to cover the entire surface. This work demonstrated that the VCS method is a feasible strategy for the fabrication of nanopillars on the entire mirror-polished surface of Ti6Al4V alloy by linearly-polarized picosecond laser conveniently and efficiently.     

选区激光熔化医用316L多孔结构的多目标工艺优化

为解决选区激光熔化技术成形医用多孔结构成形质量较差的问题,研究了工艺参数对316L成形样件致密度、孔隙率差值和表面粗糙度的影响,结合灰色关联分析方法建立了多元非线性预测模型。结果表明,预测模型的拟合度可达96.85%,最优工艺参数分别为激光功率250 W、扫描速度800 mm/s、扫描间距0.08 mm。优化后的灰色关联度达0.9195,得到样件的致密度为99.25%,孔隙率差值为0.60%,表面粗糙度均值为4.39 μm。致密度与激光功率正相关,随着致密度的增大,扫描速度和扫描间距呈现先增大再减小的趋势,孔隙率差值和表面粗糙度与激光功率、扫描速度、扫描间距均保持先减小再增大的趋势。结合激光体能量密度概念,揭示了工艺参数影响成形质量的原因,体能量密度为109.65～145.83 J/mm3时粉末的熔化状态最好。     

飞秒激光精修面齿轮的扫描烧蚀特征控制

基于飞秒激光精修面齿轮时的能量累积效应,建立了光子-电子-晶格系统间耦合作用的三温传热模型,提出一种确定飞秒激光烧蚀齿面扫描路径间扫描间距的方法。仿真得出,能量密度为6.47 J/cm2时,平衡温度3200 K超过材料的熔化温度,热累积效应明显。根据不同的扫描间距计算激光累积强度,以控制凹坑烧蚀深度和轮廓。实验结果显示,能量密度为6.47 J/cm2时,随着扫描间距的减小,激光累积强度增大,齿面的烧蚀深度线性增大;扫描间距为25 μm时的齿轮表面加工质量良好。     

高分辨扫描显微热象原理的探讨

本文建立了间距小于分子运动平均自由程的两物体间电磁辐射导热模型，并通过对此模型的定量分析，给出了辐射传热与间距和温差之间的函数关系，证明了扫描近场热成象是一种高测温灵敏度，高扫描速率的热成象技术。同时，还提出一种非接触测量表面轮廓的原理。     

Distance measurement by circular scanning of the excitation beam in the two-photon microscope

We developed a method to measure relative distances with nanometer accuracy of fluorescent particles of different color in a two-photon scanning fluorescence microscope, with two-channel photon counting detection. The method can be used in the 10-500 nm range, for distances below the resolution limit of standard far field microscopy. The proposed technique is more efficient than the methods using raster scanning. To achieve maximum sensitivity in the radial direction, the excitation beam is moved periodically in a circular orbit with a radius of the size of the point spread function. The phase and the modulation of the periodic fluorescence signal, calculated by fast Fourier transform, gives the phase and the radial distance of the particle from the center of scanning. The coordinates of particles are recovered simultaneously in the two channels and the relative distance is calculated in real time. Particles can be tracked by moving the center of scanning to the recovered position, while measuring the distance from the second particle. Intensity data are saved and fitted later by a model accounting for light leakage between the channels. The total number of detected photons limited the accuracy of the position and distance measurement. Experiments demonstrating the advantages of the method were performed on fluorescent spheres and single dye molecules immobilized on quartz surface.     

高宝线研究进展

高宝线是一种涂覆介质的单金属传输线,主要应用于高频电磁波的传输与传感领域。结合超材料技术制作出的平面高宝线能够在微波与太赫兹波频段高效激发伪表面等离子体激元,具有高效传输与高灵敏传感特性。介绍了近些年来国内外高宝线的研究进展,综合概述其主要结构的设计方法,分析其测试与仿真结果,并且简要介绍了高宝线的未来发展趋势以及潜在应用。