Fig. 3: Conversion of graphene plasmon-polariton into thermoelectric photocurrent.

a Schematic illustration of hBN/graphene/hBN transferred onto the SiO₂/Si dual-gate field-effect device. The AFM-tip launches the SPP on the device. The distance between graphene (G) and Au is d, where the charge distribution is equivalent to the double-layer G. b Optical microscopic image of the dual-gate device, where the gate voltages V_L and V_R changes the Fermi energy. The demodulated signal I₂ is measured at the right side of the electrode. c Photocurrent microscopy image of the dual-gate device. Inset: six-fold changes in I₂ sign are shown at different V_L and V_R. d An enlarged image of the box in (c) with carrier densities n_L = −0.2 × 10¹² cm⁻² (left) and n_R = −7.4 × 10¹² cm⁻² (right). e Photocurrent I_PC microscope image at different carrier densities on the left (n₁ = 0.77 × 10¹² cm⁻²) and right side (n₂ = −0.71 × 10¹² cm⁻²) of the dual-gate device. f Photocurrent fringes are measured along with the vertical dashed line indicated in (e). The frequency f is shifted from 2.52 THz to 5.67 THz. One period of the fringe corresponds to λ_p/2. g Experimental results (red dots) at n₂ = −1.11 × 10¹² cm⁻² are shown with the acoustic plasmon dispersion (blue color curve) and its first-order expansion at λ_p → ∞ (black dashed line). The plasmon dispersion of monolayer graphene (solid blue line) is displayed with the photon dispersion (blue dashed line). h‒j The plasmon phase velocity v_p (black dots) is obtained with d = 27 nm (h), d = 14.5 nm (i), and d = 5.5 nm (j) as a function of the carrier-density (n_s). The red color maps are obtained from the non-local RPA with electron-electron interactions. The local approximations (dashed lines) are deviated from the measured data. a‒d Adapted with permission from ref. ²¹, Springer Nature. e‒g Adapted with permission from ref. ³⁰, Springer Nature. h‒j Adapted with permission from ref. ³¹, American Association for the Advancement of Science