Terahertz time-domain spectroscopy

In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on both the amplitude and the phase of the terahertz radiation. By measuring in the time-domain, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude. In physics, terahertz time-domain spectroscopy (THz-TDS) is a spectroscopic technique in which the properties of matter are probed with short pulses of terahertz radiation. The generation and detection scheme is sensitive to the sample's effect on both the amplitude and the phase of the terahertz radiation. By measuring in the time-domain, the technique can provide more information than conventional Fourier-transform spectroscopy, which is only sensitive to the amplitude. Typically, an ultrashort pulsed laser is used in the terahertz pulse generation process. In the use of low-temperature grown GaAs as an antenna, the ultrashort pulse creates charge carriers which are accelerated to create the terahertz pulse. In the use of non-linear crystals as a source, a high-intensity ultrashort pulse is used to produce THz radiation from the crystal. A single terahertz pulse can contain frequency components covering much of the terahertz range, often from 0.05 to 4 THz, though the use of an air plasma can contain frequency components up to 40 THz. After THz pulse generation, the pulse is directed by optical techniques and can be focused through a sample and then measured. THz-TDS requires generation of an ultrafast (thus large bandwidth) terahertz pulse from an even faster femtosecond optical pulse typically from a Ti-Sapphire laser. That optical pulse has first been split to provide a probe pulse which undergoes an adjustable path length adjustment using an optical delay line. The probe pulse strobes the detector which is sensitive to the electric field of the resulting terahertz signal at the time of the optical probe pulse sent to it. By varying the path length traversed by the probe pulse, the test signal is thereby measured as a function of time, the same principle as a sampling oscilloscope (technically, the measurement obtains the convolution of the test signal and the time-domain response of the strobed detector). To obtain the resulting frequency domain response using the Fourier transform, that measurement must cover each point in time (delay-line offset) of the resulting test pulse. The response of a test sample can be calibrated by dividing its spectrum so obtained by the spectrum of the terahertz pulse obtained with the sample removed, for instance. Constructing a THz-TDS experiment using low temperature grown GaAs (LT-GaAs) based antennas requires a laser whose photon energy exceeds the band gap in that material. Ti:Sapphire lasers tuned to around 800nm, matching the energy gap in LT-GaAs, are ideal as they can generate optical pulses as short as 10 fs. These lasers are available as commercial, turnkey systems.