Mid-IR photothermal beam deflection technique for fast measurement of thermal diffusivity and highly sensitive subsurface imaging

Abstract

The resonances of phonon and plasmon modes make the absorbance coefficient of materials tremendously high in the mid-infrared spectral range, which allows for a mid-infrared excitation laser to heat the surface layers of these materials with high efficiency. Furthermore, phonon scattering by defects and defect-induced localized vibrational modes affect the local infrared dielectric function and, hence, the local infrared absorption coefficient. In this paper, we present a mid-infrared photothermal beam deflection technique that takes advantage of the strong interaction between infrared optical excitations and vibrational modes to measure the thermal diffusivity of materials without any sample preparation and takes advantage of the strong dependence of the infrared complex dielectric function on defects to detect subsurface defects with high sensitivity. We demonstrate the importance of the developed technique by measuring the thermal properties of highly transparent and reflective samples and detecting defects undetectable with any of the existing optical methods. Namely, using the developed technique, we find that the thermal diffusivities of high-quality Si, crystalline AlN, and crystalline α-SiO2 substrates are 1.00 ± 0.05, 0.67 ± 0.02, and 0.09 ± 0.01, respectively, and we record highly resolved images of structural subsurface defects as well as defects produced by ion-implantations at a depth of 62 μm under the surfaces of 4H-SiC substrates. © 2020 Oxford University Press. All rights reserved.