P. Rubino, J. Keeley, N. Sulollari, A. D. Burnett, A. Valavanis, I. Kundu, M. C. Rosamond, L. Li, E. H. Linfield, A. G. Davies, J. E. Cunningham and P. Dean
ACS Photonics 8, 1001 (2021)
We report all-electronic coherent scattering-type scanning near-field microscopy (s-SNOM) using a terahertz-frequency quantum cascade laser. By exploiting the coherent self-mixing effect in these lasers, in conjunction with electronic frequency tuning of the laser, we demonstrate spatial mapping of both the amplitude and the phase of the scattered field with deeply subwavelength resolution. We apply our technique for coherent microscopy of a phonon-resonant crystal. The extracted amplitude and phase parameters reveal clear contrast when compared to both metallic and nonresonant dielectric materials and show excellent agreement with those calculated using a finite-dipole model of the near-field interaction between the s-SNOM tip and the resonant sample in the Reststrahlen band. Our technique paves the way for fast nanoscale-resolved mapping of the dielectric function of solid state systems and optoelectronic nanodevices at terahertz frequencies.