Efficient Photoconductive Terahertz Generation with Tunable Pulse Compression and High-frequency Modulation

Abstract

Photoconductive antennas (PCAs) have proven to be an excellent platform for efficient broadband terahertz (THz) generation and detection, especially in THz spectroscopy and sensing applications. However, since sub-picosecond optical pulses are required for pumping the PCAs, dispersion-induced pulse broadening hampers a full integration of such THz systems to optical fiber links and limits their compactness and portability [1]. In addition, modulation of PCAs is routinely done at 10-100 kHz, while MHz frequencies have not been well investigated. In this work, we demonstrate high-frequency modulation of THz pulses up to 75 MHz with a Mach-Zehnder modulator (MZM) by controlling the intensity of each single optical pump pulse of a 100 MHz femtosecond laser, while biasing the InGaAs PCA with V = 10 V DC. We compare the generated THz signals to direct PCA modulation at the same frequencies. To compensate the dispersion of fiber and free-space components, we implement a tunable Si prism pair and 1.2 m-long dispersion compensating fiber (DCF) in order to suppress the second-order dispersion of the complete system. As shown in the setup in Fig. 1 (a), one prism can be translated to fine-tune the pre-chirp induced into the 1550 nm pump pulse. In contrast to conventional prism setups and grating mirrors, which exploit angular dispersion, we utilize material dispersion since it has the opposite chirp sign and compensates the fiber dispersion at 1550 nm. The prisms exhibit nearly unitary transmission thanks to incidence at Brewster angle.