Detection of transient low frequency mode by time-resolved far-infrared spectroscopy with a coherent radiation.

日本东北大学最近的研究表明，当高能电子脉冲穿过磁场或薄金属箔时，会产生强烈的远红外辐射。该辐射是一种强连续波辐射，范围为30 -40 cm^<-1>，时间宽度小于几个ps，表明它可以用作瞬态吸收光谱的探测光。在碱卤化物晶体中，由于自捕获激子STE的重组，电子激发诱导缺陷形成。最近一项利用ps激光光解的研究表明，STE在NaCl中可能具有红外活性的低频振动。本课题的目标是：(1)建立一个远红外瞬态光谱系统；(2)检测低频模式；(3)检测激光激发半导体表面的太赫兹辐射。该系统利用电子脉冲激发晶体，利用电子脉冲产生的远红外脉冲探测晶体的诱导瞬态。为此，我们研制了一种十通道远红外多色探测器，该探测器可以用单电子脉冲测量诱导吸收光谱。结果:(1)。建立了一个由10个InSb热电子辐射热计和凹光栅组成的10通道多色探测器。这种多色探测器可以探测范围从4到30 cm^<-1>的远红外辐射。利用日本东北大学的300 MeV直线加速器对脉冲辐射探测器的性能进行了验证。对于连续波远红外辐射，本课题研制了一种十通道锁相放大器。对几种NaCl单晶在高温下的瞬态吸收进行了测量。然而，测量的瞬态光谱由于低频模式没有显示出明确的吸收带。利用该系统还测量了激光脉冲辐照砷化镓晶体表面的远红外辐射。测定了激光入射角度、偏振度和激光脉冲强度与辐射的关系。少

Recent research in Tohoku University revealed that a high energy electron pulse generates intense far-infrared radiation when it passes through magnetic field or thin metal foils. This radiation is an intense CW radiation ranging 30 -40 cm^<-1> with a time width of less than several ps, suggesting that it can be used as a probe light for the transient absorption spectroscopy. In alkali halide crystals, electronic excitation induces defect formation due to recombination of a self-trapped exciton, STE.A recent research using ps laser photolysis has suggested that the STE in NaCl may have the low-frequency vibration which is infrared active.The aim of this project is (1) to build a system for the far-infrared transient spectroscopy, (2) to detect the low-frequency mode, and (3) to detect THz radiation from laser-excited semiconductor suefaces. In this system, a crystal is excited by the electron pulses and the induced transient states are probed by the far-infrared pulses which was genera … More ted by the electron pulses. For this purpose we make a ten-channel far-infrared polychromator-detector by which the induced absorption spectrum can be measured with single electron pulse.Results :(1). A 10-channel polychromator-detector consists of ten InSb hot-electron bolometers and concave gratings were built. This polychromator-detector can detect far-infrared radiation ranging from 4 to 30 cm^<-1>. The detector performance for the pulsed radiation was confirmed by using 300 MeV linac in Tohoku University. For cw far-infrared radiation, a ten-channel lock-in amplifier was made by this project.(2). The transient absorption measurements were made on several NaCl single crystals at LHeT.However, measured transient spectra showed no definite absorption band due to the low-frequency mode.(3). By using this system, far-infrared radiation from a fs-laser pulse irradiated GaAs crystal surface was also measured. The dependence of the radiation on the incident angle, polarization, and the intensity of the fs laser pulses were determined. Less