2.2. Terahertz Spectroscopy

Figure 1 shows a schematic diagram of the THz experimental setup. THz radiation is produced by optical rectification (OR) method.^9,14,15 The key parts of the THZ setup are numbered in Fig. 1. A Ti-Sapphire 100-fs pulse laser (#1) is used as the pump generator for THz source. The laser source is a Coherent Mira-Seed, a RegA 9000 amplifier pumped by a Veri-V18 laser (solid-state, single-frequency, CW 532 nm, Coherent Inc., Santa Clara, California). The average output power of the laser source is 700 mW at λ=800nm, with the pulse duration 100 fs and repetition rate 250 kHz. Both the emitter and detector are ZnTe crystals with 110 orientation. The focal length for both THz TPX-lenses is 100 mm; the spot size of the THz beam on the samples is around 1.5 mm. The linear polarized light output from the pump laser is divided into two parts by a beam splitter (#4), (a) the higher portion (pump optical arm 70%) of the beam power is focused onto the emitter (#6) (ZnTe crystal, 110-orientation, thickness 2 mm) by a short-focus lens (#5); and (b) the lower portion (30%) of the beam goes into a receiver optical arm. The emitted beams are collimated by a parabolic mirror (#7) and focused into a sample (#9) by a TPX (polymethylpentene) THz-lens (#8, focal length 100 mm). The THz-wave, refracted and partially absorbed by the sample, is collimated by a second THz-lens (#8) and focused by a second parabolic mirror (#10) onto a ZnTe detector (#13). This ZnTe detector is the same as the emitter (thickness 2 mm, 110 orientation). Both the ZnTe emitter and detector are placed at the focal points of the offset parabolic mirrors (#7, #10), respectively. The polarization (E-vector direction) of infrared pump beam (800 nm) is parallel to the ZnTe emitter crystal 110-orientation, and parallel to the receiver optical arm beam polarization. The polarization of emitted THz wave has the same orientation as the polarization of the pump beam. The pump optical arm and receiver optical arm must have the same optical length. For precise equalizing of both arm lengths, back-reflector prism-mirrors (#2, #12) are installed on the translated stages. The receiver optical arm beam is usually named the probe beam. This method of THz-TDS was called pump-probe method.^9,14,15 The probe beam is focused by a long-focus lens (#11), goes through the hole in the center of the parabolic mirror (#10) collinearly to the focused THz beam, and shoots at the same point on the detector as the focused THz-beam.

Setup of experiment. 1. Ti-Sapphire laser (800 nm); 2. translated-stage prism-mirror; 3. mirror, 4. beam-splitter (T/R=30/70); 5. short-focus lens; 6. THz (ZnTe) emitter; 7. parabolic mirror; 8. THz-lens; 9. tissue sample; 10. parabolic mirror with a hole; 11. long focus lens; 12. soft-driven translated-stage prism mirror; 13. ZnTe detector; 14. λ/4-waveplate; 15. Wollaston-prism; 16. balanced detector; 17. lock-in-amplifier; 18. PC; 19. chopper.

The THz pulse transmitted through a sample goes into the detector, which has a high birefringence. The electric field component of the THz-pulse modulates the amplitude of the birefringence. The infrared probe beam pulse goes through the detector and changes its own polarization (from linear to elliptical), which is synchronized to variation of intensity generated by THz-pulse electric field. This change in polarization of infrared probe beam is analyzed by a λ/4-waveplate (#14) and a Wollaston prism (#15). The THz-signal appears as a derivative curve at the output of a balance detector (#16), goes into lock-in-amplifier (#17), and is analyzed by a PC (#18). A THz-box filled with nitrogen is used to eliminate water absorption which may influence the THz spectra obtained from the samples.

The profiles of absorption and refraction index were obtained from measured THz signals by performing fast Fourier transform (FFT).^9,14