Terahertz refractometry of hard-to-access objects using the sapphire endoscope suitable for harsh environments

Abstract

While terahertz (THz) technology offers a variety of applications in medical diagnosis, nondestructive testing, and quality control, its acceptance in these practical fields is hampered by the absence of endoscopic systems, capable of sensing the complex refractive index of the hard-to-access objects. In this paper, we develop the THz endoscope based on the hollow-core antiresonant waveguide, formed by a polytetrafluoroethylene (PTFE)-coated sapphire tube with the outer end closed by a monolithic sapphire window. The endoscope is attached to the backward wave oscillator spectrometer to measure the sample reflectivity. By studying the well-known liquid and solid samples, we demonstrate that analysis of the Fabry–Pérot resonance in the measured reflection spectra makes it possible to quantify the complex refractive index of an analyte. Thanks to the advanced chemical inertness and thermal strength of sapphire and PTFE, the developed endoscope is capable of operation in harsh environments, which broadens the range of its applications. Our findings pave the way for the THz technology use in a number of demanding practical fields.