A method for local evaluation of optical properties of heterogeneous media of various nature using a pulsed terahertz solid immersion microscope

Abstract

This research proposes a method to solving the inverse problem in terahertz (THz) pulsed solid immersion microscopy with a spatial resolution of 0.15λ, where λ denotes the free-space wavelength. The method makes it possible to recover, in a quantitative manner, the local optical properties of an studied object from its observed multispectral images. The spectral complex refractive index is retrieved over the 0.35–1.1 THz range by minimizing a vector error functional, which quantifies the mismatch between experimental measurements and an analytical model. The strategy was examined using several model media with contrasting THz absorption: weakly absorbing materials (polymethylpentene, high-density polyethylene, and crystalline quartz) as well as strongly absorbing media, represented by aqueous solutions of propylene glycol at different concentrations within a microfluidic chip. Across the specified frequency range, the root-mean-square error of the retrieved refractive index remained below 3.9%, while that of the absorption coefficient was below 14.1%. These results indicate that the proposed method can provide reliable quantitative insight into the THz dielectric response of samples. More broadly, it creates new opportunities for investigating sub-wavelength and heterogeneous objects of diverse origin, which are often challenging to characterize quantitatively. By combining high spatial resolution with model-based data analysis, the technique may help bridge the gap between THz imaging and quantitative material characterization, thereby expanding the practical scope of THz pulsed solid immersion microscopy.

Speaker

Vladislav Zhelnov
Prokhorov General Physics Institute of the Russian Academy of Sciences
Russia

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