Comparison of unit cell coupling for grating‐gate and high electron mobilitytransistor array THz resonant absorbers

We report experimental studies on the excitation of synchronized plasmon resonances inAlGaN/GaN High Electron Mobility Transistor (HEMT) arrays. In contrast to the commonlyemployed grating-gate configurations, the analyzed structure contains periodicallypatterned ohmic contacts to the two-dimensional electron gas, which are laid-out parallelto the gate fingers. In this structure, the terahertz to plasmon coupling mechanism isfundamentally different from that in grating-gate configurations. Whereas the grating-gateconfiguration constitutes a coupled resonant system in whichthe resonance frequency depends on the grating periodicity, when periodical ohmic contactsare incorporated, the system behaves as a synchronizedresonant system in which each unit cell is effectively independent. As a result, in aHEMT-array, the resonance is no longer set by the periodicity but rather by the gate andthe ungated region length. Experimental results of fabricated samples compare well withnumerical simulations and theoretical expectations. Our work demonstrates that theproposed approach allows: (i) more efficient excitation ofhigh order plasmon modes and (ii) superior overall terahertzto plasmon coupling, even in configurations having less number of devices per unit area.From this perspective, our results reveal a simple way to enhance the terahertz to plasmoncoupling and thus improve the performance of electron plasma wave-based devices; thiseffect can be exploited, for example, to improve the response of HEMT-based terahertzdetectors.