Fig. 1.

Sample structure and device characterization. a Scanning electron microscope photograph of a typical sample. An ~1-μm-wide graphene ribbon was suspended over four contacts, labeled as S, D₁, D₂, and D₃, respectively. These contacts divide the ribbon into three sections, each with a gate of ~150 nm beneath the ribbon. A driving microwave with frequency ω_d + δω is applied to contact S and is detected at contact D₃ after mixing with another driving tone with frequency ω_d applied to one or more of the control gates. Scale bar is 1 μm. b The differentiation of the mixed current dI_x/dω_d as a function of driving frequency ω_d and gate voltage $V_{\mathrm{g}3}^{\mathrm{DC}}$ with $V_{\mathrm{g}1}^{\mathrm{DC}}=V_{\mathrm{g}2}^{\mathrm{DC}}=0$ V. Here, the frequencies of all resonators can be tuned from several tens of MHz to ~100 MHz by adjusting the dc gate voltages. c The mixing current as a function of the driving frequency ω_d at voltage $V_{\mathrm{g}3}^{\mathrm{DC}}=10\mathrm{V}$. Using a fitting process (see Supplementary Fig. 8), we extract the linewidth of the mechanical mode. The data were obtained at a driving power of −5 dBm. d, e Spectra of coupled modes R₁ and R₂ (d, where $V_{\mathrm{g}1}^{\mathrm{DC}}=10.5$ V and $V_{\mathrm{g}3}^{\mathrm{DC}}=0$ V) and R₂ and R₃ (e, where $V_{\mathrm{g}1}^{\mathrm{DC}}=0$ V and $V_{\mathrm{g}3}^{\mathrm{DC}}=10.5$ V). Strong couplings between these modes are manifested as avoided level crossings in the plots. Coupling strengths Ω₁₂/2π ~ 240 kHz and Ω₂₃/2π ~ 200 kHz are extracted from the plots. f The spectrum of R₂ coupled to both R₁ and R₃. In this case, the gate voltages $V_{\mathrm{g}1}^{\mathrm{DC}}=10.45$ V and $V_{\mathrm{g}3}^{\mathrm{DC}}=8.35$ V are fixed