We report a fast pump modulation scheme in a terahertz time domain spectroscopy (THz-TDS) system by utilizing the intensity modulation from the megahertz repetition rate of the pump pulse for lock-in detection. In conventional THz-TDS, the modulation required for the high signal-to-noise ratio lock-in detection is achieved through the use of an optical chopper or an AC bias. Here, we propose the use of an electro-optic modulator (EOM), operated as a pulse picker, to vary the repetition rate of the pump pulse, relative to the probe pulse, allowing us to directly use the megahertz laser repetition rate as the reference modulation frequency for lock-in detection. Our proposed scheme is applicable to all types of pulsed THz emitters, including those that cannot be electronically biased. Since the maximum allowable modulation frequency is limited only by the laser repetition rate and/or by the bandwidth of the EOM, megahertz modulation rates, and, consequently, rapid data acquisition times, become possible. Using our technique, we were able to detect an oscillating signal with frequencies up to 10 kHz, using ∼1 µs integration time per point, ∼100× faster than previously reported values for THz-TDS systems.

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