Light-induced difference Terahertz spectroscopy and its biomedical applications

Laser-induced difference THz spectroscopy has been used to investigate three samples with different lifetimes. The spectroscopy system is based on a 10 nJ titanium sapphire laser with a pulse duration of 12 fs and a centre wavelength of 790 nm. For semi-insulting (SI) GaAs and high-resistivity (HR) silicon samples, absorption in the THz range is mainly a result of mobile electrons. A lifetime of about 50 ps has been determined for the SI-GaAs sample, whereas the lifetime of the HR-Si samples was found to be much larger than the time interval between two successive laser pulses (12 ns). As a result, the differential THz signal is about twenty times larger than that for SI-GaAs. We also observed that the THz pulse arrives at the detector 100 fs earlier when it transmitted through an optically excited HR-Si wafer. For copper pathancyonine (CuPc) pellet samples, the excited state remains for at least 1 ms. The absorption peak at 1.08 THz changes significantly under 790 nm laser excitation, suggesting that we have observed the first evidence of light-induced vibrational mode changes, in the THz range.