Fig. 1: Hyperspectral THz nano-imaging of TI.
a Sketch of the hyperspectral THz nano-imaging experiment based on detection of the THz field forward scattered by the AFM tip of the s-SNOM with a TDS system based on two photoconductive antennas: the receiver and the transmitter. b Spectra of second-order demodulated signal s2 scattered by thick flakes of Bi2Se3 (d = 86 nm, lower panel, black circles and line) and Bi2Te2.2Se0.8 flakes (d = 76 nm, upper panel, red circles and line) as compared to the scattered spectra from a 140 nm thick Au reference marker (gray circles and line). c Contrast ηAu evaluated dividing the average of 60 spectra s2 scattered by Bi2Te2.2Se0.8 (red line) and Bi2Se3 (black line) by the average of 60 spectra measured on Au reference markers placed near the flakes after smoothing each curve with the average on 20 points window. The error bars are evaluated as standard deviation. d, e Near-field phase ϕ2 (d) and contrast ηAu (e) of the second-order demodulated signal s2 as a function of frequency for bulk Bi2Se3 (black line) and Bi2Te3 (red line) retrieved via numerical inversion51 of the finite-dipole model on a four-layer structure (see Supplementary Fig. 6). Gray dashed lines in panels c–e mark the probing frequencies of the detectorless near-field experiment.
