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

东北大学最近的研究表明，高能电子脉冲通过磁场或薄金属箔时会产生强烈的远红外辐射。该辐射为30-40 cm~(-1)~(-1)的强连续辐射，时间宽度小于几ps，可作为瞬变吸收光谱的探测光。在碱卤化物晶体中，电子激发由于自陷激子的复合而导致缺陷的形成。最近一项利用PS激光光解的研究表明，在氯化钠中的STE可能具有红外激活的低频振动。本项目的目的是(1)建立一个远红外瞬变光谱系统，(2)检测低频模式，(3)检测激光激发的半导体表面的THz辐射。在该系统中，电子脉冲激励晶体，并利用远红外脉冲探测诱导的瞬态态，远红外脉冲属于…更多地受到电子脉冲的影响。为此，我们研制了十通道远红外多色仪探测器，可以用单电子脉冲测量诱导吸收光谱。研制了由10个InSb热电子测辐射热计和凹面光栅组成的10通道多色仪探测器。该多色探测器可探测4~30厘米的远红外辐射。用东北大学的300 MeV直线加速器验证了探测器对脉冲辐射的性能。针对连续波远红外辐射，本课题研制了十通道锁相放大器。在LHeT下对几种氯化钠单晶进行了瞬时吸收测量，但是由于低频模式的原因，测量的瞬时光谱没有显示出明确的吸收带。利用该系统还测量了飞秒激光脉冲照射的砷化镓晶体表面的远红外辐射。确定了飞秒激光脉冲的入射角、偏振和强度对辐射的依赖关系。较少

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