Optical terahertz wave generation using periodic GaAs structures
Photonic generation of THz waves via frequency down conversion in electro-optic crystals is an attractive (thanks to the progress in developing compact near-IR optical sources) but inefficient (because of the Ω-cubed scaling factor, where Ω is THz frequency) process. We develop photonic THz sources for imaging and spectroscopic application with much higher than state-of-the-art optical-to-THz conversion efficiency. These sources are based on (i) optical rectification of femtosecond pulses in a new material: periodically-inverted GaAs, where we produce monochromatic THz output tunable in the 0.5 – 4.5 THz range [1-3] and (ii) resonantly-enhanced difference frequency mixing in periodically-inverted GaAs with picosecond [4,5] and continuous-wave  pumping. Here we take advantage of the enhancement of the optical field inside the cavity of a doubly-resonant optical parametric oscillator (OPO) (Fig. 2). We observed red-shifted satellites in the optical spectrum, suggesting that overcoming of Manley-Rowe conversion limit is possible due to cascaded THz generation (similar to cascaded Raman effect). This approach allowed generating of 1 mW of average THz power, potentially scalable to 10 – 100 mW.