Terahertz transmission-mode near-field scanning-probe microscope based on a flexible sapphire fiber

We present the terahertz (THz) near-field microscope, in which a flexible sapphire fiber serves as a scanning probe. High refractive index of sapphire at THz frequencies allows for a strong confinement of guided modes in a fiber core, and, thus, for a sub-wavelength resolution THz imaging. In turn, low THz-wave absorption in sapphire allows for guiding the THz waves in a fiber over tens of centimeters with rather high energy efficiency. The developed THz microscope operates in a transmission mode and uses a backward wave oscillator, as a continuous-wave THz source with the output wavelength of λ = 1200 µm, a Golay cell, as a detector of THzwave intensity, and a 300 µm-diameter flexible sapphire fiber with at input and output ends, as a scanning probe. In our THz microscope arrangement, the input end of a sapphire fiber is mounted on a motorized translation stage, which yield two- or even three-dimensional imaging of electromagnetic field formed at the shadow side of an object; while the output fiber end is rigidly fixed in front of the detector aperture. The experimental setup was applied for imaging of representative test objects, and the observed results demonstrated its advanced spatial resolution of ~ λ=4, which is beyond the Abbe diffraction limit. In our opinion, the sub-wavelength spatial resolution, along with a high energy throughout, open a wide range of the developed THz microscope applications in material science, non-destructive testing, and biophotonics.