Toshiji Suzuki

Observation of ultrafast all‐optical modulation based on intersubband transition in n‐doped quantum wells by using free electron laser

A picosecond‐range all‐optical modulation based on the nonlinear interaction between interband‐ and intersubband‐resonant light in GaAs/AlGaAs multiple quantum wells was demonstrated. A free electron laser (FEL) with a very short pulse width (<10 ps) was used for the intersubband excitation. The dependence of the modulation depth on the wavelength of the intersubband‐resonant light was investigated by utilizing the wide tunability of the FEL. The result is in good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by FT‐IR spectroscopy, which indicates that the observed modulation is indeed based on the intersubband transition induced by the FEL.

Evaluation of the CdS/CdTe interface using free-electron laser internal photoemission technique

The CdS/CdTe interface was investigated by the free-electron laser (FEL) internal photoemission technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found two thresholds in the photocurrent spectrum, which can be identified as steplike band discontinuities. It is demonstrated that there is a mixed crystal layer of CdS1−xTex at the CdS/CdTe interface.

DETERMINING THE BAND DISCONTINUITIES OF ZNSE/GAAS AND ZNMGSSE/GAAS HETEROJUNCTIONS USING FREE ELECTRON LASER

The conduction band discontinuities of ZnSe/GaAs and ZnMgSSe/GaAs heterojunctions were investigated using the free electron laser (FEL) internal photoemission technique. This technique is based on the photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found the conduction band discontinuities of 113 (ZnSe/GaAs) and 180 meV (ZnMgSSe/GaAs). It is suggested that the band gap discontinuity between ZnSe and ZnMgSSe is δEc (conduction band): δEv (valence band)=0.45:0.55 at 77 K.

Performance of Prototype Hard X-ray Polarimeter Utilizing Compton Scattering

In X-ray astronomy, various kinds of information on stellar objects have been obtained from energy spectrum, time variabilities, and images. Although the observation of polarization is also important, it has not been carried out for 25 years, because of the difficulty of developing a polarimeter with a high sensitivity. Thus, we have been developing a Compton-scattering-type hard X-ray polarimeter sensitive to the energy range from 40 to 300 keV, employing segmented plastic and CsI(Tl) scintillators mounted on a flat-panel multianode photomutiplier (MAPMT). Constructing the prototype polarimeter and using polarized hard X rays of 80 keV, we carried out basic experiments to investigate the performance of our polarimeter. Moreover, we carried out computer simulations and confirmed that the results obtained by computer simulation are in good agreement with those obtained by experiment. Finally, it was found that the prototype polarimeter can obtain a modulation factor and a detection efficiency of ~40 and ~20%, respectively.

Luminescence from ZnSe excited by picosecond mid-infrared FEL pulses

Abstract We have observed blue band-edge emission from a ZnSe crystal under irradiation of mid-infrared picosecond free electron laser (FEL) pulses. The emission characteristics including spectrum, excitation power dependence, excitation wavelength dependence, and decay time have been investigated. The experimental results have indicated that it is difficult to understand the excitation process by multiphoton excitation, thermal excitation, or excitation through mid-gap levels.

ULTRAFAST INTERBAND-LIGHT MODULATION BY INTERSUBBAND LIGHT (FREE-ELECTRON LASER) IN QUANTUM WELLS

Abstract Ultrafast interband-resonant light modulation induced by an intersubband transition in GaAs/AlGaAs multiple quantum wells is observed by using a free-electron laser (FEL) for intersubband excitation. The FEL operates with a picosecond pulse width and a wide wavelength tunability. The dependence of the modulation depth on the wavelength of the intersubband-resonant light shows good agreement with the intersubband absorption spectrum of the multiple quantum wells measured by Fourier transform infrared (FTIR) spectroscopy. We also investigated the relaxation time of the intersubband transition by a pump–probe experiment and found that it was less than the FEL micropulse width (∼3 ps).

Free electron laser annealing of N-ion-implanted 3C-SiC films

Infrared laser annealing at a variety of wavelengths (10.0–13.0 μm) was performed under room temperature for cubic silicon carbide (3C-SiC) films after N-ion implantation by using a free electron laser with features of wide-range tunability, ultrashort pulse operation (∼10 ps), and intense peak power (∼MW). Infrared absorption spectroscopy indicated that the annealing at 12.6 μm, which corresponds to the absorption peak of Si–C stretch mode, was effective for removing the crystalline damage induced by the ion implantation. On the other hand, Hall effect measurements showed an increase of carrier density for samples annealed at around 10.4 μm, whereas the absorption was weak at this wavelength. This important result is most likely attributed to the activation of N donors caused by the direct excitation of a local vibration mode associated with N atoms in the SiC matrix.

Measurement of Band Discontinuity at ZnSe/GaAs Boundary Using Free Electron Laser

We investigated the band discontinuity at the ZnSe/GaAs boundary using the free electron laser internal photoemission (FEL-IPE) technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We have found the threshold of the photocurrent near the photon energy of 110 meV which can be identified as the band discontinuity in the ZnSe/GaAs heterojunction.

Measurement of Semiconductor Heterojunction Band Discontinuities Using Free Electron Laser.

Measurements of the band discontinuity of semiconductor heterojunctions was carried out using the free electron laser internal photoemission (FEL-IPE) technique. This technique is based on photocurrent spectroscopy utilizing the tunability and intense peak power of the FEL operative in the infrared range. We found that there is a photocurrent threshold in the IPE spectrum of the ZnSe/GaAs and GaAlAs/GaAs heterojunctions which can be identified as the band discontinuity.

Free Electron Laser Annealing of Amorphous Silicon Carbide

We have applied a free electron laser (FEL) to crystallize amorphous silicon carbide (a-SiC) at room temperature. The FEL has two main characteristics, wavelength tunability and ultrashort-pulse operation (~10 ps) with intense peak power ( ~MW). The wavelength was selected to be 12.6 µ m which corresponds to the energy of the Si–C stretch mode in order to excite the lattice vibration directly. FT-IR measurements of the a-SiC film showed that the broad peak in the absorption spectrum became sharp after irradiation with the FEL. X-ray diffraction measurements of the irradiated sample revealed a peak related to SiC crystals. These measurements suggest that crystallization of a-SiC occurs at room temperature when irradiation with a FEL.