The transmission spectra of the most common hyperosmotic agents, such as pure glycerol, propylene glycol (PG), dimethyl sulfoxide (DMSO), polyethylene glycol (PEG) with 200, 300, 400 and 600 Da molecular weights, and their aqueous solutions, as well as aqueous solutions of sucrose, glucose, fructose, dextran 40 and 70 were measured. The experiments were carried out using a THz pulsed spectrometer with a vacuum measuring compartment to reduce the effect of water vapor on spectral measurements. The dielectric properties of hyperosmotic agents were restored in the spectral range from 0.1 to 2.5 THz and the dependence of the amplitude absorption coefficient on the concentration of considered agents at 0.5 THz frequency was constructed. The obtained results make it possible to choose the optimal agents for immersion optical clearing in the THz range.

The development of terahertz (THz) imaging methods is hampered by the low spatial resolution of traditional diffraction-limited imaging systems, mainly due to the large wavelength of used radiation (from a few of mm to tens of µm). To solve this problem, we have proposed a new method of THz endoscopy with subwavelength spatial resolution, which is designed to study hard-to-reach areas of living organisms in vivo. A hollow-core sapphire tube with polytetrafluoroethylene outer coating is used as a waveguide, in which the antiresonant principle of radiation transmission is implemented. The waveguide and the immersion lens are optimized to provide high optical characteristics in a given wavelength range to ensure the best focusing. Two immersion lenses made of sapphire and silicon were developed and fabricated, which were then mounted on plane-parallel windows fixed on the rear end of the waveguide. The study of the field intensity distribution on the shadow side of the “waveguide–lens” system revealed a focal spot diameter of ≃0.2λ in the case of a lens made of sapphire and ≃0.3λ in the case of a lens made of crystal silicon at a wavelength λ = 500 µm, which significantly exceeds the Abbe diffraction limit. This agrees with our numerical predictions and demonstrates the promise of using the proposed endoscope for measurements with subwavelength resolution.

With a change in temperature, a-lactose monohydrate crystals undergo changes in the molecular structure due to dehydration and decay of intramolecular bonds. The transmission spectra of the pressed microcrystalline samples of a-lactose monohydrate were measured using terahertz pulsed spectroscopy in the temperature range of the existence of the solid phase 10 – 475 K. An analysis of the observed absorption lines using the classical oscillator model made it possible to reveal the complex temperature evolution of the eigenfreuencies of the resonances, as well as to determine the region of existence of response phase of a-lactose monohydrate. Data obtained can find practical application in various fields of terahertz optics, including pharmacology, food industry, analytical chemistry and biophotonics.