Terahertz time domain spectroscopy - Terahertz Time-Domain-Spektroskopie

Single pulses of terahertz (THz) light lasting only picoseconds (ps) or femtoseconds (fs) can be generated by using a fs laser exciting a semiconductor or a non-linear optical crystal of ZnTe. The fourier-transform of such a pulse contains a wide range of frequencies. For our 20 fs pulses a potential spectral coverage from 0 to 50 THz can be achieved and a time resolution in the sub-ps region.

In summer 2003 we started to set up a THz time-domain spectometer (THz-TDS) using a mode-locked 20 fs titan:sapphire laser emitting in the wavelength range from 600-1100 nm. Since the beginning of 2004 we have an operating THz-TDS using low-temperature-grown (LTG) GaAs emitters and detectors (in collaboration with Prof. M. Tani, Osaka University, Japan) and ZnTe emitters. Electro-optic sampling is possible by using a ZnTe crystal as a detector.

Measured ps pulse	Fourier-Transform of ps pulse

The T-ray spectrometer uses a pump-probe technique to measure the single THz pulse waveform. The high power green laser at 532 nm pumps the titan:sapphire (Ti:Sa) mode-locked laser which generates 20 fs pulses with a repetition rate of 80 MHz in the visible and near-infrared spectral range from 600 to 1100 nm. This beam is separated by the beam splitter BS into a strong pump beam guided to the emitter and a weak probe beam guided to the detector. The pump pulse generates a single THz pulse in the LTG GaAs semiconductor if a voltage of up to 20 V is applied. The THz pulse radiates via the metal dipole antenna on the GaAs chip into free space to the sample. In the detector the reverse process takes place.

10 Watt green laser	T-ray spectromter using a pump-probe method

The fs probe pulse induces in the detector chip electron and hole pairs - free carriers. If the THz pulse reaches the antenna of the detector it induces an electric field varying in time, accelerating the carriers, leading to a measurable current which is positive or negative depending on the impinging electric field. In order to obtain the full spectral information the set-up allows recording with different delay times similar to a Fourier-Transform spectrometer. Delaying the optical probe pulse in respect to the THz pulse, we can record the spectrum as a function of the delay path. Subsequent Fourier transformation leads to a frequency resolution of 500 MHz due to our translation stage with a maximum delay path of 60 cm corresponding to a delay time of 2000 ps.