Nonlinear terahertz gain estimated from multiphoton-assisted tunneling in resonant tunneling diodes

The power-dependent terahertz (THz) gain due to subband transitions between adjacent quantum wells is estimated from the current change of triple-barrier resonant tunneling diodes (RTDs) under THz irradiation. To induce high THz voltage across the RTDs, patch antennas with low conduction loss are integrated. Due to high THz voltages induced, photon-assisted tunneling by the multiphoton process is observed. Components of tunneling currents with photon emission and absorption are extracted from the observed results, and the gain coefficient in the multiphoton process is estimated. It is shown that the gain coefficient is almost constant for incident power in the low incident power region in which one- or two-photon processes are dominant, and starts to rapidly decrease when the incident power exceeds this region. Reasonable agreement with theoretical calculation is obtained.