THz chirped-pulse electro-optic dual comb generation

Dual electro-optic frequency comb (DOFC) methods are being developed to enhance detection sensitivity and speed for (bio)molecular sensing in THz region. Phase coherent chirped pulses are used to drive an electro-optic modulator in the near-IR and generate flat optical frequency combs in the THz region by mixing with an offset carrier in a photo-conductive switch. A second chirped pulse with a slightly different bandwidth is used to down-convert the THz comb to the radiofrequency (RF) region below 1 MHz by mixing with a slightly different offset carrier in a room-temperature photoconductive receiver. The optical crossover method effectively eliminates the phase noise between the two near-IR driving lasers in the RF domain, enabling highly coherent RF combs that uniquely map the THz spectral features and dynamics. We demonstrate the sensitivity of this system in studies of formic acid shown below. Significant enhancements in sensitivity are underway by replacing the two 850 nm lasers with lasers operating near 1570 nm to generate and detect THz radiation using new waveguide-based (WG) photomixers. The WG photomixer will increase the THz power generated by >100-fold (>200 μW at 500 GHz). For condensed phase studies, a new closed cycle cryostation with a multi-sample adapter is now available for studies from 4 K to 350 K.

Magnitude and phase spectra of formic acid (≈95% purity) around 12.1 cm⁻¹ in a 1 m long gas cell at 133 Pa pressure. THz spectra for different chirp directions (side-by-side) and with opposite bandwidths (right vs left pairs) are obtained in the temporally magnified frequency domain in blue (top two rows) and spectra in red (bottom two rows) are the corresponding back-transformed and unmagnified (normal) spectra in the time domain.