Development of a Millimeter-Long Travelling Wave THz Photomixer

THz sources based on the optical-heterodyne (photo)mixing in an ultrafast photodetector are very promising since they operate at room temperature, are potentially compact, cost-efficient and, above all, are widely frequency-tunable. However, their widespread use is currently hampered by available power levels in the μW range at THz frequencies. We present here a travelling wave structure, with millimeter level coherence length at THz frequencies opening the way to large active area (~4000 μm
²
) photomixing devices capable of handling optical pump power beyond 1 W well beyond the capabilities of standard lumped-element devices using small active areas (<; 50 μm
²
) needed to maintain a capacitance level (<; 10 fF) compatible with THz operation. It is based on a silicon nitride waveguide coupled to a membrane-supported low-temperature-grown GaAs photoconductor embedded in a coplanar waveguide. Milliwatt power levels up to 1 THz and still above 1 μW up to 4 THz are expected according to the optoelectronics model of this device elaborated in this study. Experimentally, the frequency response of a 1-mm-long structure, measured up to 100 GHz by using the beatnote produced by two 780-nm-DFB lasers, shows clearly the expected travelling wave signature consisting in a 6-dB-decrease ending at ~50 GHz when the contribution of the backward travelling wave is fully cancelled, following by a constant level up to ~100 GHz. The experimental demonstration of operation in the travelling wave regime is a first step towards the fulfillment of the original promises of this concept in terms of power level and frequency bandwidth.