Real-time data acquisition in THz-TDS using MHz laser repetition rates for lock-in detection

Abstract

Terahertz time domain spectroscopy (THz-TDS), which relies on the coherent detection of terahertz radiation using a pump-probe scheme, is one of the most common techniques to measure optical constants at THz frequencies [1]. Standard lock-in detection is often employed to measure weak THz signals with high signal-to-noise ratio (SNR), For lock-in detection, the signal is modulated at a certain frequency fmod, which is different from the frequencies of unwanted signals. The lock-in amplifier detects and amplifies the signal only at fmod resulting in high SNR. However, data acquisition time is limited by fmod. In particular, the minimum data acquisition time is given by 2/fmod, as dictated by the Nyquist criterion. Previous reports to modulate the THz signals in THz-TDS mostly utilized optical choppers or AC biased photoconductive antenna (PCA) emitters, with reported modulation frequencies varying from few kHz to few hundreds of kHz, respectively [2,3]. Consequently, the data acquisition times need to be about few milliseconds to few hundreds of microseconds, to get acceptable SNR.