Figure 1: Few-layer BP samples and polarization dependent infrared spectroscopy.

(a) Lattice structure of few-layer BP, showing the puckered hexagonal crystal with two nonequivalent directions: AC (x direction) and ZZ (y direction). (b) Optical image of a representative 6L sample. Scale bar, 20 μm. (c) Schematic illustrations of optical transitions between quantized subbands, solid and dotted arrows indicate the main transitions (Δn=0) and hybrid transitions (Δn=±1), respectively. (d) Real part of the optical conductivity σ for the 6L sample in b, in the unit of the universal optical conductivity σ₀=πe²/2 h, with polarization angles of incident light from 15° to 90°, in steps of 15°. The marks E₁₁ and E₂₂ denote the first and second subband transitions, asterisks (*) denote hybrid transitions. (e) Real part of the optical conductivity σ at peaks E₁₁ and E₂₂, as a function of the polarization angle θ. The solid lines are cos²θ fits.