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Behaviour of Electromagnetic Waves in Different Media and Structures Part 7

Tham khảo tài liệu 'behaviour of electromagnetic waves in different media and structures part 7', kỹ thuật - công nghệ, cơ khí - chế tạo máy phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 168 Behaviour of Electromagnetic Waves in Different Media and Structures appears in the reststrahl region around 5.3 THz. This feature reflects the high absorption of the material and the large velocity mismatch near the lattice resonance. At the TO phonon frequency 6.2 THz in ZnTe the nonlinear coefficient v4i ij is resonantly enhanced. We can clearly see with decreasing the thickness of ZnTe crystal the upper limit of the frequency increases. However the sensitivity of the EO sensor is proportional to the thickness of the crystal as shown in Eq. 6 in section 2.1. Therefore a trade-off exists between broadband response and high sensitivity. 2.3 Time domain terahertz spectroscopy system Figure 6 shows the setup for free space THz EO sampling measurements. The ulrafast laser pulse is split by a beam splitter into two beams a pump beam strong and a probe beam weak . The pump beam illuminates the sample and generates the THz radiation. The generated THz radiation is a short electromagnetic pulse with a duration on the order of one picosecond so the frequency is in the THz range. The THz beam is focused by a pair of parabolic mirrors onto an EO crystal. The beam transiently modifies the index ellipsoid of the EO crystal via the Pockels effect as discussed in detail in section 2.1. The linearly polarized probe beam co-propagates inside the crystal with THz beam and its phase is modulated by the refractive index change induced by the THz electric field ETHz. This phase change is converted to an intensity change by a polarization analyzer Wollaston prism . A pair of balanced detectors is used to suppress the common laser noise. This also doubles the measured signal. A mechanical delay line is used to change the time delay between the THz pulse and the probe pulse. The waveform of ETHz can be obtained by scanning this time delay and performing a repetitive sampling measurement. The principle of THz signal sampled by the femtosecond pulse is shown in Fig. 7. By this .