Fig. 2. Directional launching of Bloch polaritons.

(A) Schematic of the device with the scanned region marked in the dashed box. The shades of blue colors represent different carrier densities in graphene. (B) Dependence of the polariton momentum k on average carrier density ${\overline{\mathit{n}}}_{\mathit{s}}$ at ω = 890 cm⁻¹. Dashed lines correspond to the experimental conditions in (C) to (E). Inset: BZ of the polaritonic crystal marked with symmetry points. (C) Near-field image acquired at T = 60 K and V_g = 47.5 V. Bloch polaritons propagate in $\hat{M}=\mathrm{\langle }-11\mathrm{\rangle }$direction, as marked by the black arrow. (D) and (E) same as (C) at V_g = 52.5 V and V_g = 72.5 V, respectively. (D) shows that Bloch polaritons propagate predominantly in $\widehat{\mathit{K}}=\langle 10\rangle$ direction, as marked by the cyan arrow. (E) reveals isotropic polariton propagation in all directions. (F) Averaged line profiles along $\widehat{\mathit{K}}$ and $\widehat{\mathit{M}}$ directions acquired from (C) to (E). Line profiles are normalized to gold and shifted vertically by multiples of 0.02 units for clarity.