THz-TDS is based on electromagnetic transients excited by femtosecond (1 fs =10^-15s) laser pulses. The ultrafast transient currents radiate ultrashort bursts of THz pulses with a pulse width of typically less than 1 ps. The pulse covers a broad spectral range from below 100 GHz to more than 30 THz. The optically-gated detection technique allows the direct measurement of THz electric field with a femtosecond time resolution. From this coherent measurement, both the real and imaginary parts of the dielectric function of a sample medium can be extracted, even without resorting to the Kramers-Kronig (KK) relations. This is a great advantage over the conventional Fourier transform infrared (FTIR) spectroscopy, a frequency-domain spectroscopy (FDS) that usually needs certain forms of KK relations. In our lab, we have built a homemade THz-TDS system which is one of the best with the highest signal-to-ratio (SNR) in the world.

To determine the material characteristics of a sample, we measure two data scans: one with the sample in place (sample signal) and the other without sample (reference signal). The time-domain data is converted to the frequency domain by a numerical Fourier transform. By taking the ratio of sample and reference signals, we can remove the system response of the experiment. This ratio detects the small changes, in amplitude and phase spectra of the THz transmission or reflection coefficients, from which we can precisely determine the absorption coefficient and index of refraction for a given sample.

There are several methods available for the generation and detection of THz pulses.

A) Photoconductive Antenna

In brief, a femtosecond laser pulse generates a freely propagating THz pulse by photoexciting the small-gap dipole antenna located between biased coplanar transmission lines on a GaAs substrate. The THz pulses are collimated by silicon lenses and off-axis paraboloidal metal mirrors. Each laser pulse creates a large number of photocarriers in a biased antenna gap biased. The consequent acceleration of the carriers generates the THz pulse radiation.

B) Optical Rectification

Optical rectification is based on a second-order nonlinear property of electro-optic crystals, and produce THz pulses via difference frequency generation. THz generation using electro-optical rectification has been performed by using semi-insulating GaAs and recently more efficient nonlinear crystals.