In terahertz (THz) spectroscopy water content correlates with the tissue pathological changes. At the same time, water reduces the depth of THz-wave penetration in tissues. In order to unmask cells’ and tissues’ biophysical properties and to increase the tissues probing depth, immersion optical clearing (IOC) was recently introduced in the THz range. For studying common IOC agent’s optical properties in the frequency range of 0.1–2.5 THz, pulsed spectroscopy was used. Diffusion coefficients of IOC agents in ex vivo rat brain tissue were studied using the collimated transmission spectroscopy in the visible range. Two-dimensional nomogram was used to objectively compare IOC agents, based on their THz-wave absorption coefficients and diffusion rates.
Since nanoporous three-dimensional structure based on SiO2 has been studied as a THz optical material [Optical Materials Express 10(9), 2100-2113 (2020)], in this work we study its fabrication properties. Porous SiO2 is represented by porous opal matrixes [Optical Materials 49, 208–212 (2015)], which are based on globules of amorphous SiO2 [Nano 8(4), 1350036 (2013)].
By the thermal treatment, the material is able to achieve materials with different stoichiometric composition and porosity, minimizing an amount of residual water, and obtaining pre-determined physics properties of material. We fabricated couple of cylindrical lenses and flat plates made of abovementioned nanoporous SiO2. We showed that simple-form components could be easily fabricated by grinding, while mechanical processing strategy depends on the annealing temperature used and the material strength.
In this article, we studied new porous material, which is based on artificial opals, made of 300-nm-diameter nanoporous SiO2 globules and annealed at different temperatures in the range of 200–1500◦C, as a prospective terahertz (THz) optical material. It was demonstrated experimentally that the THz optical properties of such material can be varied in a wide range (e.g. its refractive index varies from 1.65 to 1.95) by annealing, being a function of the total material porosity. Additionally, this material has rather low THz absorption coefficient (by field), which decreases from 10 to 1 cm−1 with increasing annealing temperature. Based on the Bruggeman effective medium theory, the practical model was introduced to predict the optical properties of the considered material as a function of the annealing temperature. A wide tunability of refractive index and a low-to-moderate THz-wave absorption, make the discussed nanoporous SiO2 a promising THz optical material.
We developed an optical cryostat with a sample-rotation unit for polarization-sensitive measurement in terahertz (THz) and infrared (IR) ranges. The cryostat, in combination with two metal-grid polarizers, provides full control of mutual orientation of the sample’s crystallographic axes and the light polarization plane. Importantly, this control is realized in-situ, i.e., during the sample cooling–heating cycle. To demonstrate the abilities of the developed cryostat, we used it in combination with a laboratory-made THz time-domain spectrometer, for polarization-sensitive measurements of an orthoferrite (YFeO3) in the range of 5 to 50 cm − 1. These measurements revealed strong angular dependence of the sample transmission. The developed cryostat is capable for solving numerous demanding problems of THz and IR spectroscopy in condensed matter physics and materials science, biophysics, chemical, and pharmaceutical sciences.
We developed a method for reconstructing the THz dielectric response of a thin liquid sample. A self-made sample cuvette was designed for the transmission-mode THz pulsed spectroscopy of liquids. Numerical simulations and theoretical studies of the proposed reconstruction procedure were performed in order to optimize the sample geometry and predict uncertainties in reconstructed dielectrical properties. A number of agents for immersion optical clearing of tissues was studied using the proposed method in the THz range. The developed method can be applied for all types of sufficiently transparent liquid samples.