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

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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