Broadband terahertz spectroscopy and its application to the characterization of thin films

Recent progresses of the terahertz time-domain spectroscopy are reviewed in view of broadening the detection bandwidth that can be measured. Using an ultra-short pulsed laser with the pulse duration of 15 fs and photo-conducting antennas in the reflection geometry, we can realize useful broadband terahertz spectrometers that are capable of measuring real and imaginary parts of the dielectric constants in materials over a wide frequency range. The detection bandwidth expands up to 20 THz with the continuous phase information with only a small discontinuity around 8 THz. Even the higher frequencies up to 170 THz can be generated and detected using a 5-fs pulsed laser with on organic nonlinear crystal (DAST) and a photo-conducting antenna. These methods have a wide range of the applications and the several examples such as characterization of dielectric thin films and light-pump terahertz-probe spectroscopy are reviewed.