Fig. 3: Characterization and tuning of plasmon modes.
a Microwave frequency-dependent transport map. Transport measurements are performed by sweeping Vch to control the electron density in the central channel while simultaneously applying a microwave signal ω/2π to the gate electrodes. For Vch ≲ 0.29V, the channel is empty (I). The above \({V}_{{{{\rm{ch}}}}}^{{{{\rm{th}}}}}\) electrons can enter the microchannel and form a low-density, high-conductivity state (II) and a high-density low low-conductivity Wigner solid (III). The transport signal Vs reveals a family of density and frequency-dependent plasmon resonances that manifest as local minima in regime II. Measurements were performed at P = − 15 dBm, with Vgt = 0 V, Vac = 8 mV, fac = 3 MHz, \({V}_{{{{\rm{res}}}}}\,=\,0.29\) V, and T = 26 mK. b Blue dots are the extracted local minima from the first seven resonances in a. Red dashed lines correspond to the first seven longitudinal plasmon modes along the channel, calculated using the dispersion relation given in Equation (1) with our device design parameters. The grey shaded region indicates the empty channel (I), and the grey dashed line indicates \({V}_{{{{\rm{ch}}}}}^{{{{\rm{th}}}}}\,=\,0.29\) V.
