Super-resolution THz endoscope based on a hollow-core sapphire waveguide and a solid immersion lens.

To address a challenging problem of super-resolution terahertz (THz) endoscopy, in this paper, an antiresonant hollow-core waveguide was coupled with a sapphire solid immersion lens (SIL), aimed at subwavelength confinement of guided mode. The waveguide is formed by a polytetrafluoroethylene (PTFE)-coated sapphire tube, the geometry of which was optimized to ensure high optical performance. SIL was judiciously designed, fabricated of bulk sapphire crystal, and then mounted at the output waveguide end. Study of the field intensity distributions at the shadow side of the waveguide-SIL system revealed the focal spot diameter of ≃0.2λ at the wavelength of λ = 500 µm. It agrees with numerical predictions, overcomes the Abbe diffraction limit, and justifies super-resolution capabilities of our endoscope.

THz generation by two-color laser air plasma coupled to antiresonance hollow-core sapphire waveguides: THz-wave delivery and angular distribution management.

In this paper, hollow-core antiresonance sapphire waveguides were applied to guide the THz radiation emitted by the two-color laser air plasma, as well as to manage the THz source angular distribution. For this aim, three distinct waveguides were developed. Each of them is based on a cylindrical sapphire tube, either suspended in free space or coated by a polymer. The waveguides were first studied numerically, using the finite-difference eigenmode method, and experimentally, using the in-house THz pulsed spectrometer. The observed data uncovered the antiresonance regime of their operation, as well as their ability to guide broadband THz pulses over tens of centimeters with a high optical performance. The waveguides were then used to couple and guide (over the considerable distance) of THz radiation from the in-house two-color laser air plasma emitter, that exploits the mJ-energy-level femtosecond pulses of a Ti-sapphire laser. Small dispersion of a THz pulse and low-to-moderate propagation loss in the developed waveguide were observed, along with a considerable narrowing of the THz radiation angular distribution after passing the waveguide. Our findings revealed that such technologically-reliable hollow-core sapphire waveguides can boost the performance of laser air plasma-based THz emitters and make them more suitable for applications in the vigorously-explored THz sensing and exposure technologies.

Boosting photoconductive large-area THz emitter via optical light confinement behind a highly refractive sapphire-fiber lens.

We report on a sapphire-fiber-based lens that can be used to enhance the emitted THz power of a large-area photoconductive antenna (PCA). Using numerical simulations, we demonstrate that the lens provides a spatial redistribution of the photocarriers density in the PCA's gap. By optimizing the diameter of the sapphire-fiber, one could reach efficient confinement of the photocarriers in the vicinity of the PCA electrodes with a 10-µm gap size for a 220-µm-thick sapphire-fiber. This allows enhancing the coupling of the incident electromagnetic waves at the interface between the sapphire fiber and the semiconductor with the antenna terminals by ∼40 times for a single PCA element, as well as boosting the total efficiency of the large-area PCA-emitter up to ∼7-10 times. To validate our approach, we propose a step-by-step process that can be used for the precise and controllable placement of the sapphire-fiber on the surface of a single PCA.

Object-dependent spatial resolution of the reflection-mode terahertz solid immersion microscopy.

Terahertz (THz) solid immersion microscopy is a novel promising THz imaging modality that overcomes the Abbe diffraction limit. In our prior work, an original reflection-mode THz solid immersion microscope system with the resolution of 0.15λ (in free space) was demonstrated and used for imaging of soft biological tissues. In this paper, a numerical analysis, using the finite-difference time-domain technique, and an experimental study, using a set of objects with distinct refractive indexes, were performed in order to uncover, for the first time, the object-dependent spatial resolution of the THz solid immersion microscopy. Our findings revealed that the system resolution remains strongly sub-wavelength 0.15-0.4λ for the wide range of sample refractive indices n = 1.0-5.0 and absorption coefficients α = 0-400 cm-1 (by power). Considering these findings, two distinct regimes of the THz solid immersion microscopy were identified. First is the total internal reflection regime that takes place when the sample refractive index is relatively low, while the sub-wavelength resolution is enabled by both the evanescent and ordinary reflected waves at the interface between a high-refractive-index material and an imaged object. Second is the ordinary reflection regime that occurs when the sample refractive index is high enough, so that there is no more total internal reflection at the interface, while only the ordinary reflected waves inside a high-refractive-index material are responsible for the sub-wavelength resolution. The resultant conclusions are general and can be applied for analysis of solid immersion lenses operating in other spectral ranges, such as visible and infrared, given linear nature of the Maxwell's equations.

Opal-based terahertz optical elements fabricated by self-assembly of porous SiO2 nanoparticles.

In this paper, we study artificial opals as a promising material platform for terahertz (THz) optics. Materials were synthesized using self-assembly of porous SiO2 nanoparticles and annealing at different temperatures to further tune their optical properties. Two distinct approaches for the fabrication of bulk THz optics from these novel materials were considered. First, THz cylindrical lenses of identical geometry but different refractive indices and focal lengths were produced using standard mechanical processing of opals, in order to highlight their compatibility with conventional technologies of bulk optics fabrication. Second, a THz axicone was made via direct sedimentation of aqueous colloidal suspension of SiO2 nanoparticles in the mold of geometry inverse to that of a desired optical shape, followed by annealing and polishing. The second approach has an advantage of being considerably less labor intensive, while capable of obtaining optical elements of complex geometries. Thus fabricated bulk THz optical elements were studied experimentally using continuous-wave THz imaging, and the results were compared with 2D and 3D numerical predictions based on the finite-difference time-domain and finite-element frequency-domain methods. Our findings highlight technological robustness of the developed THz optical material platform and, thus, open the door for creating a variety of bulk THz optical elements of complex shapes and widely-tunable optical performance.

Proof of concept for continuously-tunable terahertz bandpass filter based on a gradient metal-hole array.

A continuously-tunable terahertz (THz) bandpass filter based on the resonant electromagnetic-wave transmission through a metal-hole array featuring a gradually changing period was developed and fabricated on a silicon substrate using optical lithography. A gradient geometry of the metal-hole array yields a wide tunability of the filter transmission, when operating with a focussed THz beam. The filter was studied numerically, using the finite element method, and experimentally, using the THz pulsed spectroscopy. We find that the central wavelength of the filter transmission band can be tuned in the wide range of λc = 400-800 µm with the relative bandwidth of Δλ/λc ≃ ~0.4. Finally, Kapton-based anti-reflection coating was applied to the filter flat side, in order to suppress an interference pattern in the filter transmission spectrum. We believe that the developed filter holds strong potential for multispectral THz imaging and sensing due to its conceptual simplicity and case of operation. Moreover, the presented filter concept can be translated to other spectral ranges, where appropriate technologies are available for the fabrication of gradient sub-wavelength metal-hole arrays.

Terahertz dielectric spectroscopy of human brain gliomas and intact tissues ex vivo: double-Debye and double-overdamped-oscillator models of dielectric response.

Terahertz (THz) technology offers novel opportunities in the intraoperative neurodiagnosis. Recently, the significant progress was achieved in the study of brain gliomas and intact tissues, highlighting a potential for THz technology in the intraoperative delineation of tumor margins. However, a lack of physical models describing the THz dielectric permittivity of healthy and pathological brain tissues restrains the further progress in this field. In the present work, the ex vivo THz dielectric response of human brain tissues was analyzed using relaxation models of complex dielectric permittivity. Dielectric response of tissues was parametrized by a pair of the Debye relaxators and a pair of the overdamped-oscillators - namely, the double-Debye (DD) and double-overdamped-oscillator (DO) models. Both models accurately reproduce the experimental curves for the intact tissues and the WHO Grades I-IV gliomas. While the DD model is more common for THz biophotonics, the DO model is more physically rigorous, since it satisfies the sum rule. In this way, the DO model and the sum rule were, then, applied to estimate the content of water in intact tissues and gliomas ex vivo. The observed results agreed well with the earlier-reported data, justifying water as a main endogenous label of brain tumors in the THz range. The developed models can be used to describe completely the THz-wave - human brain tissues interactions in the frameworks of classical electrodynamics, being quite important for further research and developments in THz neurodiagnosis of tumors.