THz spectroscopy is a growing area of research due to its applications in many areas of science, security and product testing. THz radiation sits between the infra-red and microwave regions of the electromagnetic spectrum, but unlike UV/Vis or IR spectroscopy, there are currently no competitive straight forward methods capable of measuring absorption in the conventional way. The answer is to use time domain based THz spectroscopy (THz-TDS), where the frequency spectrum is derived from Fast Fourier Transformation from a transient in the time-domain. Typically, THz-TDS is based on ultrashort laser pulses from a modelocked laser for the generation of an ultrashort THz pulse and the delayed probe pulse for phase-sensitive detection of the radiation. THz-TDS systems can cover a large frequency range from a few tens of GHz into the near infrared.
One drawback of conventional THz-TDS systems is the use of mechanical delay stages to realize the time delay between the THz pulse and the optical detection pulse. This leads to an inherently low acquisition rate for THz transients. For the analysis of molecular absorption spectra in the gas phase, a high spectral resolution in the 1 GHz range and thus a time delay of 1 ns is required. To this end, a conventional THz-TDS system needs a mechanical delay stage with a travel distance of 15 cm. The time required for accelerating and decelerating the stage between approximately 10,000 data points and to average out technical laser noise typically leads to a total acquisition time in the range of a few tens of minutes. Hence, applications, where the acquisition of a THz trace should be completed within a few seconds or even milliseconds, are impossible.
Asynchronous optical sampling (ASOPS) is a time-domain spectroscopy technique that does not require a mechanical delay stage, thus avoiding the potential drawbacks described above. Laser Quantum’s high-speed implementation of ASOPS operates with two taccor lasers with offset repetition rates.
The generation of THz radiation is either done by nonlinear processes in special media or by a photoconductive switch made of semiconductors. Both methods convert the optical pulse train into a train of THz pulses. Photoconductive switches allow multiplexing for increased conversion efficiency which can reach levels up to 2x 10-3.
Laser Quantum is an expert in the field of THz-TDS and offers a broad range of applicable products.