Fig. 2.

Gate-tunable plasmonic response of a graphene-based photonic crystal. a Schematic of the photonic crystal structure with an engineered domain wall in the middle, highlighted in orange. Color contrast represents carrier density modulation n_1,2 in graphene. A gold antenna in the left serves as a plasmon launcher. The scanned area in panel c is marked with a dashed box. K/M directions in BZ are marked with arrows. b Simulated LDOS maps for upper and lower plasmonic bands. c Experimental near-field images s(r,ω) acquired at different gate voltages at T = 60 K and laser frequency ω = 904 cm⁻¹. At V_g = −40 V, only faint plasmonic fringes are observed. At V_g = −60 V, a hexagonal pattern of dark spots emerges. At V_g = −70 V, a 1D domain wall state appears in the middle of the image. At V_g = −90 V, propagating plasmons are launched by the gold antenna on the left. This latter image also reveals a hexagonal pattern of bright spots. Black solid line marks the location of the line profiles in panel d. Scale bar: 400 nm. d Line profiles s(r,ω) taken in the photonic crystal region away from the domain wall. Dotted lines display the corresponding line profiles in un-patterned region. The systematic increase of SPP wavelength λ_p with carrier density n_s matches the expected scaling: ${\lambda }_{\mathrm{p}}\propto \frac{v_{\mathrm{F}}\sqrt{n_{\mathrm{s}}}}{ϵ{\omega }^2}$