| Pacific Rim Symposium on Surfaces, Coatings and Interfaces (PacSurf 2016) | |
| Thin Films | Tuesday Sessions |
| Session TF-TuP |
| Session: | Thin Films Poster Session |
| Presenter: | Hideo Takeuchi, Osaka City University, Japan |
| Authors: | H. Takeuchi, Osaka City University, Japan T. Sumioka, Osaka City University, Japan M. Nakayama, Osaka City University, Japan |
| Correspondent: | Click to Email |
Terahertz (THz) electromagnetic waves are generated by illumination of femtosecond (fs) laser pulses on semiconductor surfaces. The THz wave, therefore, supplies time-domain information on the dynamical phenomena around the surface. In the THz-wave emission from the coherent longitudinal optical (LO) phonons, initial polarization, which corresponds to the LO-phonon amplitude, is launched by a surge current of carriers generated by laser-pulse illumination. Subsequently, the coherent LO phonon, the dynamical polarization, starts oscillating and the THz wave is emitted. Here, we point out the possibility that the surge current scatters the coherent LO phonon in the launch stage. For clarifying the above possibility, we investigated the coherent LO phonon in an i-GaAs/n-GaAs epitaxial structure, focusing our attention on the decay time.
The sample used was grown by MOVPE. The i-GaAs (n-GaAs) layer thickness was 100 nm (300 μm). The doping density of the n-GaAs layer was 3x1018 cm-3. In the i-GaAs/n-GaAs structure, a uniform built-in electric field is produced in the i-GaAs layer, which induces the initial polarization. The THz wave was measured at room temperature and at the humidity below 10%. We used an optical gating method. The fs laser pulse had a photon energy of 1.55 eV and a pulse duration of 60 fs. The laser beam had a Gaussian shape. The pump powers were varied from 30 to 110 mW.
The THz waveforms show a monocycle signal from the surge current, which is followed by long-lived oscillations due to the coherent LO phonon. The Fourier power spectra of the waveforms show the surge current and coherent LO phonon bands. In addition, two bands, the frequencies of which depend on the pump power corresponding to the photogenerated carrier density, were observed. The frequencies of the latter two bands were evaluated using the Gaussian decomposition, and compared with the dispersion curve of the LO-phonon-plasmon coupled (LOPC) mode. We consider that the THz wave from the coherent LOPC mode is emitted from almost the center position of the Gaussian beam spot on the sample surface because of the presence of sufficient photogenerated carriers forming the plasmon. In contrast, the coherent LO phonon is generated around the beam-spot tail. The waveform shows only the coherent LO phonons in the time-delay range larger than 1.0 ps. The decay time of the coherent LO phonon, which is 3.0 ps at the pump power of 30 mW, is monotonically decreased with increasing the pump power: The decay time is 1.3 ps at 110 mW. Since the increase in the pump power enhances the surge current, we conclude that the surge current disturbs the coherent LO phonon through the scattering.