Switchable-coupling terahertz directional coupler operating at 100  GHz

In the frequency range of 90-110 GHz, we show a dynamically tunable terahertz directional coupler based on the phase transition properties of vanadium dioxide, allowing for a continuous coupling ration adjustment from -3 to -5dB. To validate the concept experimentally, we created two metallic-dielectric hybrid counterparts that mimic the coupling effects of the VO₂ film in both its metallic (shielded branches) and extremely insulating (fully conductive branches) phase states. For -3 and -5dB coupler configurations, respectively, simulation results in the 90-110 GHz range show a coupling degree of -2.1 (0.7 dB imbalance) and -3.8dB (1.0 dB imbalance). The mode coupling dynamics in terahertz directional couplers were investigated through the branch-line waveguide theory, revealing the underlying transmission mechanism. The experimental results for the coupler's S-parameters were in relatively good agreement with the theoretical predictions. The switchable directional coupler proposed in this paper has applications in dynamic terahertz power allocation, terahertz imaging, and electromagnetic stealth.