Quantitative Nano-Infrared Spectroscopy of Anisotropic van der Waals Materials

F. L. Ruta, A. J. Sternbach, A. B. Dieng, A. S. McLeod and D. N. Basov

Nano Letters 20, 7933 (2020)
Anisotropic dielectric tensors of uniaxial van der Waals (vdW) materials are difficult to investigate at infrared frequencies. The small dimensions of high-quality exfoliated crystals prevent the use of diffraction-limited spectroscopies. Near-field microscopes coupled to broadband lasers can function as Fourier transform infrared spectrometers with nanometric spatial resolution (nano-FTIR). While dielectric functions of isotropic materials can be readily extracted from nano-FTIR spectra, the in- and out-of-plane permittivities of anisotropic vdW crystals cannot be easily distinguished. For thin vdW crystals residing on a substrate, nano-FTIR spectroscopy probes a combination of sample and substrate responses. We exploit the information in the screening of substrate resonances by vdW crystals to demonstrate that both the in- and out-of-plane dielectric permittivities are identifiable for realistic spectra. This novel method for the quantitative nano-resolved characterization of optical anisotropy was used to determine the dielectric tensor of a bulk 2H-WSe2 microcrystal in the mid-infrared.