Tham khảo tài liệu 'wave propagation 2010 part 5', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 112 Wave Propagation system for the terahertz wave is shown in Fig. 5 as a photograph. The sample by being illuminated by the pump beam emits a terahertz wave along the reflection direction of the pump beam as described in Section 2. The emitted terahertz wave was collected with use of two off-axis parabolic mirrors. The high resistivity silicon wafer was placed as a filter for the pump beam. The collected terahertz wave was focused on the bow-tie antenna with a gap of pm formed on a low-temperature-grown GaAs. The bow-tie antenna was optically gated with use of the laser-pulse beam gate beam which was controlled by the mechanical delay line the so-called stepper. Consequently the terahertz wave was detected only in the case where the bow-tie antenna was illuminated by the gate beam. The above-mentioned method for the detection of the terahertz wave is the so-called optically gating technique Nuss Orenstein 1999 Bolivar 1999 . In the present experiment the power of the gate beam was fixed to mW. For the reference samples a 001 n-GaAs about 2 X 1018 cm-3 crystal and a 001 r -InAs crystal were examined. Intense terahertz emission caused by the surge current in the i-GaAs n-GaAs structure Figure 6 a shows the terahertz waveforms of the r-GaAs n-Ga As solid line n-GaAs dotted line and r -InAs dashed line samples at the pump-beam energies of and eV. All the samples show a monocycle oscillation around the time delay of 0 ps the so-called first burst. It is obvious that the amplitude of the first-burst of the r-GaAs n-GaAs sample is larger by a factor of 10 than that of the n-GaAs crystal. It should be emphasized that the r -GaAs n-GaAs sample emits the more intense terahertz wave in spite of the fact that the built-in electric field is much weaker than the surface electric fields of the n-GaAs crystals shown in Figs. 3 a and 3 b . The above-mentioned results indicate that the presence of the relatively thick r-GaAs layer which is depleted .
