Terahertz solid immersion microscopy: Recent achievements and challenges

Abstract

Unique effects of terahertz (THz)-wave–matter interaction push rapid progress in THz optoelectronics aimed at bridging the problematic THz gap. However, majority of modern methods of THz spectroscopy and imaging are still hampered by low spatial resolution. Common lens/mirror-based THz optics fails to overcome the Abbe barrier and usually provides resolution larger than a free-space wavelength λ (i.e., hundreds of micrometers or even few millimeters). To mitigate this difficulty, supperresolution THz imaging modalities were introduced recently, among which we particularly underline different methods of THz scanning-probe near-field microscopy. They not only rely on strong light confinement on sub-wavelength probes and provide resolution down to [Formula: see text]–[Formula: see text] but also suffer from small energy efficiency or presume an interplay among imaging resolution, signal-to-noise ratio, and performance. In this paper, we consider reflection-mode THz solid immersion (SI) microscopy that offers some compromise between the high imaging resolution of [Formula: see text] and high energy efficiency, which is due to the absence of any subwavelength probe in an optical scheme. Recent achievements, challenging problems, and prospects of SI microscopy are overviewed with an emphasis on resolving the inverse problem and applications in THz biophotonics.

Clinical Diagnosis of Gastric Cancer by High-Sensitivity THz Fiber-Based Fast-Scanning Near-Field Imaging

“…The parameter of the fiber is 600 μm in diameter and 40 cm in length. Behind the fiber output, we used a bow-tie filter to achieve high enhanced transmission power and near-field spatial resolution [ 23 , 24 , 25 ]. We also used a cryogenic-temperature-operated Schottky diode detector to improve the detection sensitivity.…”

Terahertz technology in intraoperative neurodiagnostics: A review

“…Modern THz spectroscopy and imaging systems suffer from low spatial resolution, since they usually rely on the lens-or mirror-based optics, the resolution of which obeys the Abbe diffraction barrier of free-space focusing 2,10,17 . Due to large THz wavelengths, resolution of such THz systems is a few hundreds of micrometers or few millimeters, even when working with a very wideaperture lenses 92,93 . Overcoming the Abbe limit is of particular importance in THz biophotonics and, particularly, neurodiagnosis 10,17,31 .…”

Hemispherical Rutile Solid Immersion Lens for Terahertz Microscopy with Superior 0.06–0.11λ Resolution

“…The rutile hemisphere serves as a resolution enhancer, with the subwavelength THz-beam caustic formed at its flat surface (i.e., an imaging plane). [2,25,29] The composite rutile hemisphere is comprised of a 4-mmthick rigidly-fixed hypohemisphere (Figure 1b) and a 1-mm-thick movable window (Figure 1c), which are kept in contact with no refraction between them. The movable rutile window serves as a sample holder and allows raster-scanning of the imaged objects with a focused THz beam.…”

“…For a detailed comparison of various subwavelength imaging modalities and a discussion of the comparative advantages of SI microscopy, refer to our recent review paper. [ 2 ]…”

“…To date, a variety of SI lenses have been developed exploiting different material platforms and fabrication strategies, geometries, and optical designs [ 2,4–9 ] from ultraviolet to terahertz (THz) spectral ranges. Moreover, SI microscopy was applied to solve many demanding fundamental and applied problems in super‐resolution visible, [ 10 ] infrared (thermal) [ 11 ] and Raman [ 12 ] imaging, condensed matter physics and superconductivity, [ 13–16 ] quantum sciences, [ 17,18 ] data storage, [ 19 ] non‐destructive evaluations, [ 20 ] biomedicine [ 21 ] and other disciplines.…”