Extended Data Fig. 1: Wavelength dependent SNOM contrast of ABCA and ABAB regions.

a, Schematic diagram of a scattering-type scanning near-field optical microscope (s-SNOM). The wavelength of quantum cascade laser is tunable from 900 to 1675 cm⁻¹, allowing us to find a best wavelength for the phonon-polariton assisted infrared imaging of graphene under hBN. The beam splitter and lens are both made of ZnSe, and the scattered light signal is collected by an HgCdTe detector working at liquid nitrogen temperature in the far field. b–m, IR-SNOM images of the same tetra-layer graphene sample with that in Fig. 2c–e at different incident wavelength ranging from 1000 to 1600 cm⁻¹. Only when the incident wavelength lies in the hBN Reststrahlen (1370 ~ 1610 cm⁻¹), ABAB and ABCA regions can be clearly distinguished. Between 1500 ~ 1600 cm⁻¹, ABCA is always darker than ABAB (j ~ m), between 1400 ~ 1500 cm⁻¹, ABAB should be darker (Fig. h, i). Below 1350 cm⁻¹ there is no obvious boundary between ABCA and ABAB and the contrast is weak. The hBN thickness is about 35 nm.