Terahertz Time-Domain Spectroscopy by Asynchronous Sampling with Modelocked Semiconductor Lasers

Broadband terahertz spectroscopy measurements are commonly recorded with time domain systems that require a mechanical delay stage to obtain sampling of the terahertz transient. The principle limitation in measurement time for such detection schemes arises from the necessary stage movement to acquire the terahertz signal. The fastest systems use two ultrafast fiber or solid-state lasers for asynchronous sampling, which results in a fast system at the drawback of high cost. In our proof of principle study, we synchronize two hybridly modelocked semiconductor lasers to perform terahertz time domain spectroscopy by asynchronous sampling. The two external cavity lasers operate at repetition frequencies of 392.8 MHz with an offset of 125 Hz and emit in the wavelength range of 830 nm. We obtain Terahertz transients with spectral components of up to 250 GHz which is mainly limited by the detection system. This enables very compact and highly cost effective terahertz spectroscopy systems and may prepare the way for industrial applications.