Tuning of ungated plasmons by a gate in the field-effect transistor with two-dimensional electron channel

We show that voltage variations at a short gate can effectively tune higher-order ungated plasmon resonances in field-effect transistors (FETs). These higher-order ungated plasmon resonances may be excited by incoming terahertz radiation with much greater efficiency than the gated plasmon resonances. We calculate the spectra of the terahertz plasmon absorption in the frame of a rigorous electromagnetic approach, which allows us to estimate the radiation resistance. Based on the calculation results, we explain the behavior of different plasmon resonances in terms of the alternating-current FET equivalent circuit. The results may help design high performance plasmonic devices operating in the terahertz frequency range.