Step-index sapphire fiber and its application in a terahertz near-field microscopy

Abstract

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.