In this paper, we present the step-index sapphire fiber, applied as a THz probe. The low THz attenuation of sapphire makes it attractive for fabrication of THz optical components. Moreover, it has a high refractive index in THz range, which guarantees a strong modal confinement in a fiber core. The advantages of the edge-defined film-fed growth (EFG) technique allow for fabrication of fibers with close-to-cylindrical shape, the length of 1 m and longer, and the subwavelength diameter of 150 − 400 μm. In order to improve the coupling efficiency, the fiber has polished flat ends. We apply the fabricated 300-μm-diameter sapphire fiber for the THz near-field scanning-probe microscopy. The spatial resolution of our experimental setup is defined by the fiber diameter, thus, it reaches ~ λ/4 for the radiation wavelength λ = 1200 μm. The obtained images of the test objects demonstrate the advanced resolution, which is close to the theoretical limit and beyond the Abbe diffraction limit.
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.