The solid immersion (SI) effect is widely used to increase the spatial resolution of optical focusing systems and even overcome the Abbe diffraction limit. Resolution enhancement offered by a SI lens is mostly a function of its geometry and refractive index nSI. While SI lenses are relatively well understood, the scaling of the resolution enhancement by such lenses is still a subject of debate, with some works reporting <^>nSI and <^>n2SI dependencies for the hemispherical and hyperhemispherical SI lens configurations, respectively. In this paper, we offer a general argument for a resolution limit for SI optics and, then, verify it via the numerical analysis of the hemispherical and hyperhemispherical silicon SI lenses designed for the terahertz (THz) range. In fact, we find that there is no contradiction in the reported resolution enhancements <^>nSI and <^>n2SI; however, they happen in different operation regimes. We then demonstrate that the resolution values reported for the different SI lens arrangements in the visible (VIS), near-, and middle-infrared (NIR and MIR), as well as THz bands obey the derived limit. Our findings will be useful for the further design and applications of SI optics. (c) 2024 Optica Publishing Group. All rights, including for text and data mining (TDM), Artificial Intelligence (AI) training, and similar technologies, are reserved.

Artificial opals fabricated by sedimentation and self-assembly of colloidal SiO2 nanoparticles and annealed at different temperatures were recently considered favorable terahertz (THz) optical materials with manageable optical properties. However, interactions between such a porous material and water vapour in a humid atmosphere can hamper their THz applications due to the related changes in the material parameters and additional power loss. To quantify such an effect, in this paper, moisture adsorption by artificial SiO2 opals is studied using THz pulsed spectroscopy. Particularly, opals of two kinds were sedimented from the colloidal suspension of 300-nm-diameter SiO2 nanoparticles with different intraglobular structures and porosity. They were annealed at temperatures of 200–800°C aimed at changing their internal structure, porosity, and THz optical properties. Opals were dehydrated in a vacuum and then exposed to a humid atmosphere with 82.0 ± 2.0% relative humidity, while their THz complex dielectric permittivity was evaluated in situ in the 0.5–2.5 THz range. The observed changes in the THz dielectric curves were analyzed using the sum rule and the adsorption kinetics models. Our findings reveal a strong dependence of the THz dielectric response, amount of adsorbed water, and adsorption time constant on the opal type and annealing conditions. This effect has a general character: it can hamper real-live applications of a variety of porous THz optical materials and, thus, should be taken into account during their synthesis.

Continuously tunable middle-infrared bandpass filters have been developed based on gradient metal-hole arrays with two distinct geometries. The rotation filter relies on an array of metal holes with gradually changing periods and hole sizes in the azimuthal direction, while the translation filter exploits a metal-hole array with a linear gradient. The filters are fabricated in a Ti film on a ZnSe substrate using electron-beam nanolithography. They are characterized experimentally using Fourier-transform infrared spectroscopy, and the observed results are compared with numerical predictions of the finite element method. The developed filters offer wide spectral tunability when operating with a focused beam. Particularly, the central wavelength of the transmission band is tunable in the λc∈(9,15)μm range, for the rotation filter, and in the λc∈(8,13)μm range for the translation one, as a linear function of the filter angular or linear displacement. The filters feature relatively broad bandwidths of Δλ≃0.2λc, while their spectral contrast and energy efficiency depend on the gradient type. The filter spectral response function shape and the extent of its spectra tunability can be further optimized by judicious design of the hole geometry and the metal-hole array gradient, respectively. The developed filters hold strong potential in the infrared multispectral sensing and imaging, thanks to their conceptual simplicity. Considering the linearity of Maxwell’s equations and availability of appropriate technologies for the fabrication of gradient arrays of sub-wavelength metal holes, the developed concept can be translated to other spectral ranges.

Opal-like structures are proposed as a promising new material for terahertz (THz) optics. Opal matrices deposited from SiO2 globules with a diameter of 300 nm with subsequent annealing in the temperature range 200–1500°C are investigated. It is experimentally shown that the THz optical properties of such a material can vary over a wide range with an increase in the annealing temperature (refractive index from 1.6 to 1.95, amplitude absorption coefficient of the material from 7 to 0.4 cm–1). On the basis of effective medium theory, namely the Bruggemann equation, a model is proposed that makes it possible to predict the optical properties of the material under study depending on the annealing temperature. To demonstrate the possibility of manufacturing an optical THz component, a plane-convex cylindrical lens is fabricated from the material under study. The possibility of varying the optical properties of the opal matrix in a wide range, a low absorption coefficient, and the absence of dispersion in the THz spectral region show the prospects of opal-like materials for THz applications.

In this paper, we study artificial opals as a promising material platform for terahertz (THz) optics. Materials were synthesized using self-assembly of porous SiO 2 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 SiO 2 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.

We study a new method in fabrication of THz optical components from previously studied porous three-dimensional nanostructures based on SiO2 [Optical Materials Express 10, 2100-2113 (2020)]. Porous SiO2 is represented by artificial opals [Optical Materials 49, 208–212 (2015)]. By the thermal treatment, it is able to achieve materials with different stoichiometric composition and porosity, and obtaining pre-determined physical and optical properties of thematerial. In this paper we produced the THz conical lens (axicone) by direct sedimentation of aqueous colloidal suspension of the SiO2 nanoparticles onto a shaped mold, followed by annealing and finished machining. The THz field transformation behind the axicone was studied by THz imaging system. The observed results were then compared with numerical predictions obtained by using the methods of computational electrodynamics. Finally, we discuss the potential of the described fabrication method in the THz optics.

Sapphire shaped crystals are considered as a favorable material platform of the terahertz (THz) waveguide and fiber optics. Unique physical properties of sapphire, along with advantages of the Edge-defined Film-fed Growth (EFG) technique, yield fabrication of the THz waveguides and fibers with a complex cross-section geometry directly from the Al2O3-melt, where no labour-intensive mechanical processing is required. Wide variability of the as-grown sapphire shaped crystal geometries yields different physical mechanisms of electromagnetic waveguidance. In this review, recent advantages in the THz waveguides and fibers based on the EFG-grown sapphire shaped crystals are discussed. While possessing moderate THz-wave absorbtion and quite high dispersion, flexible sapphire fibers with a simple step-index cross-section geometry yield strong confinement of guided modes in a fiber core due to a high refractive index of sapphire in the THz range. This effect opens novel opportunities of sapphire fibers in high-resolution THz imaging, using the principles of either scanning-probe near-field optical microscopy or optical fiber bundles. In turn, antiresonant and photonic crystal hard hollow-core waveguides demonstrate advanced optical performance, along with wide capabilities in THz endoscopy and sensing in harsh environments. This review highlights that the EFG-grown sapphire shaped crystals hold strong potential in different branches of THz optics.

Since nanoporous three-dimensional structure based on SiO2 has been studied as a THz optical material [Optical Materials Express 10(9), 2100-2113 (2020)], in this work we study its fabrication properties. Porous SiO2 is represented by porous opal matrixes [Optical Materials 49, 208–212 (2015)], which are based on globules of amorphous SiO2 [Nano 8(4), 1350036 (2013)]. By the thermal treatment, the material is able to achieve materials with different stoichiometric composition and porosity, minimizing an amount of residual water, and obtaining pre-determined physics properties of material. We fabricated couple of cylindrical lenses and flat plates made of abovementioned nanoporous SiO2. We showed that simple-form components could be easily fabricated by grinding, while mechanical processing strategy depends on the annealing temperature used and the material strength.

In this paper, artificial opals, made of 300-nm-diameter nanoporous SiO2 globules by sedimentation of a colloidal suspension and annealing at different temperatures in the range of 200–1500 °C, are studied as a promising material platform for terahertz (THz) optics. Our findings reveal that THz optical properties of such materials can be predictably varied in a wide range by annealing, while being a deterministic function of the material porosity. Thus, when increasing annealing temperature, the resultant material refractive index increases from 1.65 to 1.95 at 1.0 THz, while the material absorption coefficient (by field) reduces from 10 to 1 cm−1. The Bruggeman effective medium theory was then successfully applied to model optical properties of the nanoporous SiO2 at THz frequencies as a function of the material porosity and the annealing temperature. Finally, bulk nanoporous SiO2 were shaped using conventional grinding techniques into plates and cylindrical lenses to demonstrate robustness of the novel THz optical materials. A wide range of the nanoporous SiO2 refractive indices, their low-to-moderate THz-wave absorption, as well as their mechanical robustness make such materials a promising platform for THz optics.

In this work, a thorough analysis of hyperosmotic agents for the immersion optical clearing (IOC) in terahertz (THz) range was performed. It was aimed at the selection of agents for the efficient enhancement of penetration depth of THz waves into biological tissues. Pulsed spectroscopy in the frequency range of 0.1 to 2.5 THz was applied for investigation of the optical properties of common IOC agents. Using the collimated transmission spectroscopy in visible range, binary diffusion coefficients of tissue water and agent in ex vivo rat brain tissue were measured. IOC agents were objectively compared using two‐dimensional nomogram, accounting for their THz‐wave absorption coefficients and binary diffusion coefficients. The results of this study demonstrate an interplay between the penetration depth enhancement and the diffusion rate and allow for pointing out glycerol as an optimal agent among the considered ones for particular applications in THz biophotonics.

We developed an optical cryostat with a sample-rotation unit for polarization-sensitive measurement in terahertz (THz) and infrared (IR) ranges. The cryostat, in combination with two metal-grid polarizers, provides full control of mutual orientation of the sample's crystallographic axes and the light polarization plane. Importantly, this control is realized in-situ, i.e., during the sample cooling-heating cycle. To demonstrate the abilities of the developed cryostat, we used it in combination with a laboratory-made THz time-domain spectrometer, for polarization-sensitive measurements of an orthoferrite (YFeO3) in the range of 5 to 50 cm(-1). These measurements revealed strong angular dependence of the sample transmission. The developed cryostat is capable for solving numerous demanding problems of THz and IR spectroscopy in condensed matter physics and materials science, biophysics, chemical, and pharmaceutical sciences. (C) 2019 Society of Photo-Optical Instrumentation Engineers (SPIE)

In this article, we studied new porous material, which is based on artificial opals, made of 300-nm-diameter nanoporous SiO2 globules and annealed at different temperatures in the range of 200–1500◦C, as a prospective terahertz (THz) optical material. It was demonstrated experimentally that the THz optical properties of such material can be varied in a wide range (e.g. its refractive index varies from 1.65 to 1.95) by annealing, being a function of the total material porosity. Additionally, this material has rather low THz absorption coefficient (by field), which decreases from 10 to 1 cm−1 with increasing annealing temperature. Based on the Bruggeman effective medium theory, the practical model was introduced to predict the optical properties of the considered material as a function of the annealing temperature. A wide tunability of refractive index and a low-to-moderate THz-wave absorption, make the discussed nanoporous SiO2 a promising THz optical material.

We measured the transmission spectra of the most common hyperosmotic agents, such as pure glycerol, propylene glycol (PG), dimethyl sulfoxide (DMSO), polyethylene glycol (PEG) with molecular weights of 200, 300, 400, and 600 Da, their aqueous solutions, and aqueous solutions of sucrose, glucose, fructose, and dextran 40 and 70. The experiments were carried out using a THz pulsed spectrometer with an evacuated measuring compartment to eliminate the effect of water vapor on spectral measurements. We reconstructed the dielectric characteristics of hyperosmotic agents in the spectral range from 0.1 to 2.5 THz and plotted a dependence of the amplitude absorption coefficient on the concentration of the considered agents at a frequency of 0.5 THz. The results are useful for selecting optimal agents for immersion optical clearing in the THz range.

The transmission spectra of the most common hyperosmotic agents, such as pure glycerol, propylene glycol (PG), dimethyl sulfoxide (DMSO), polyethylene glycol (PEG) with 200, 300, 400 and 600 Da molecular weights, and their aqueous solutions, as well as aqueous solutions of sucrose, glucose, fructose, dextran 40 and 70 were measured. The experiments were carried out using a THz pulsed spectrometer with a vacuum measuring compartment to reduce the effect of water vapor on spectral measurements. The dielectric properties of hyperosmotic agents were restored in the spectral range from 0.1 to 2.5 THz and the dependence of the amplitude absorption coefficient on the concentration of considered agents at 0.5 THz frequency was constructed. The obtained results make it possible to choose the optimal agents for immersion optical clearing in the THz range.

Atmospheric water adsorbed by porous materials may significantly change its electrodynamic response in a wide frequency range. Mechanical and chemical stabilities of silicon dioxide along with a proven manufacturing method of porous SiO2 samples made it convenient for studying the effects of moisture adsorption on various parameters of the porous media. We report the dielectric properties of SiO2-based nanoporous glass in the frequency range of 20 Hz–400 THz at ambient atmospheric conditions and at a low residual pressure of ≤1 mbar. We observed a significant low-frequency dispersion of the complex dielectric permittivity and enhancement of dielectric loss for the porous sample exposed to the ambient moisture. In the terahertz range, the change in dielectric response is smaller and correlates with the moisture saturation of the sample.

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.

The dispersion relations of exciton polariton waves in terbium nitrate hexahydrate are determined based on the model of the interaction of electromagnetic waves with resonant electronic states of Tb3+ ions at the 5D4–7F6 transition. Frequencies of unitary polaritons characterized by refractive indices equal to unity are determined. It is shown that the group velocity of electromagnetic waves in the region of unitary polaritons is lower than the speed of light in vacuum by several orders of magnitude. A sharp increase in the efficiency of photoluminescence and Raman scattering is predicted in the case where the excitation radiation frequency approaches the unitary polaritons frequency.