THzTDS

Therz time-domain spectroscopy, or THz-TDS, is a spectroscopic technique that uses ultrashort optical pulses to generate and detect terahertz radiation. It measures the time-domain electric field of a THz pulse after it interacts with a sample in transmission or reflection geometry. By scanning the relative delay between the THz generation and detection, a time-domain waveform is obtained, which is then converted to frequency-domain information via Fourier transform to yield amplitude and phase across the THz range.

In a typical THz-TDS setup, a femtosecond laser drives a THz emitter, such as a photoconductive antenna or a nonlinear crystal for optical rectification, and a THz detector based on electro-optic sampling or another antenna. The sample can be a solid, liquid, or thin film, and measurements can be performed in transmission, reflection, or scanning imaging modes. Analysis of the measured complex transmission or reflection coefficients allows extraction of the sample’s complex refractive index and dielectric function, from which conductivity and other material properties can be derived.

THz-TDS offers several advantages, including broad spectral coverage, phase sensitivity, and a non-contact, non-destructive measurement approach. It is particularly effective for characterizing low-energy excitations, thin films, polymers, ceramics, semiconductors, and biological materials. Limitations include strong water absorption that complicates measurements of hydrated samples, sensitivity to alignment and timing stability, and reliance on numerical data processing and calibration.

Applications span materials science, pharmaceuticals, coatings, and fundamental research on vibrational and rotational modes in various substances. Since its development in the late 1990s, THz-TDS has become a standard tool in both research and industry for quantitative, spectroscopic analysis in the terahertz regime.