The development of nanophotonics is hindered by a fundamental trade-off between a material's refractive index (n) and its electronic bandgap (E g), which severely restricts the choice of materials for short-wavelength applications. This challenge is particularly acute in the visible and ultraviolet (UV) spectra, where high-performance devices require materials that are simultaneously highly refractive and transparent. Here, we report on the van der Waals (vdW) crystal cadmium phosphorus trisulfide (CdPS3) as a solution to this long-standing problem. Through comprehensive optical and structural characterization, we show that CdPS3 possesses an anomalously high in-plane refractive index across the visible spectrum approaching three in the near-UV, combined with a wide indirect bandgap. This combination of properties is validated by first-principles calculations and direct near-field imaging of highly confined waveguide modes. These findings establish CdPS3 as a leading material for UV-visible photonics, opening a new pathway for the development of high-density integrated circuits and metasurfaces.

The integration of high-refractive-index dielectrics into scalable photonic architectures is foundational to advancing integrated circuits and augmented reality displays. Van der Waals (vdW) materials offer exceptional optical properties, including high refractive indices and giant anisotropy, but their implementation is constrained by the small area and uncontrolled thickness of mechanically exfoliated flakes. Here, we demonstrate that atomic layer deposition grown gallium sulfide (GaS) films overcome the trade-off between high optical performance and manufacturability, emerging as a large-scale vdW dielectric platform. Through rigorous optical and structural benchmarking against pristine single crystals, we establish that the optical constants (n, k) of GaS films are virtually indistinguishable from those of single-crystal counterparts. By leveraging the out-of-plane anisotropy, we demonstrate that large-scale GaS enables superior suppression of crosstalk in densely integrated waveguides compared to conventional scalable high-index platforms. Our findings establish scalable GaS as a technologically viable pathway for implementing anisotropic vdW materials in visible-spectrum photonics.

Organic kainate is an emerging χ2 crystal for broadband terahertz (THz) emission. This study presents an experimental characterization of its THz performance using time-domain spectroscopy. We determined the complex refractive index and frequency-dependent coherence length. With a 17 fs pump pulse, the generated THz spectrum extends to ∼7 THz, while maintaining constant fluence (∼7.8 mJ/cm2) and increasing pulse duration to 100 fs reduces the peak field amplitude proportionally to 1/τ and narrows the spectral bandwidth. By combining the experimentally derived absorption coefficient with Raman spectroscopy and DFT-calculated phonon modes, we identify phonon-limited absorption notches between 2-5 THz and a low-loss transmission window around 1.5 THz. Comparative analysis reveals that aged crystals (∼6 months) exhibit reduced THz bandwidth (∼4 THz) compared to fresh crystals (∼7 THz), indicating that environmental degradation—particularly humidity—rather than intrinsic material limitations constitutes the immediate bandwidth constraint. These insights, combined with the material's favorable and scalable growth process, provide a clear pathway for optimizing kainate crystals toward their full potential for THz applications.

A thorough study of the wide-range (40-35000 cm-1) dynamic conductivity spectra of the rare-earth (RE) dodecaborides RB12 (R- Ho, Er and Tm) and Tm1-xYbxB12 substitutional solid solutions was carried out at room temperature. Both the Drude-type components and overdamped excitations have been separated and analyzed. An additional absorption band observed above 200 cm-1 in these RB12 with magnetic RE ions is attributed to the cooperative Jahn-Teller (JT) dynamics of the B12 complexes, which depends crucially on the RE-ion cage space and is compared with the same effect found in the non-magnetic LuB12 and ZrB12. It was shown that non-equilibrium (hot) electrons participating in the formation of the collective JT modes dominate in charge transport, and fraction of Drude-type carriers changes by 20-40% in these compounds with unstable boron lattice. Strong renormalization of the infrared response is observed in Tm1-xYbxB12 solid solutions with metal-insulator transition (MIT) and is discussed in terms of localized collective modes caused by Yb-ion valence instability. We demonstrate that even at room temperature the MIT is accompanied with simultaneous decrease in concentration of Drude-type electrons and redistribution of carriers to localized JT collective modes.

A thorough study of the wide-range (nu = 40-35000 cm-1) dynamic conductivity and dielectric permittivity spectra of the archetypal heavy fermion metal CeB6 and unusual superconductor ZrB12 was carried out at room temperature. Both the Drude-type components and overdamped excitations were separated and analyzed. An additional absorption band observed above 200 cm-1 was attributed to the cooperative Jahn-Teller (JT) dynamics of the boron complexes in CeB6 and ZrB12. It was shown that nonequilibrium electrons participating in the formation of the collective JT modes dominate in charge transport, and fraction of Drude-type electrons does not exceed 37% in CeB6 and 23% in ZrB12. We discuss also the additional Drude-type component in ZrB12 in terms of far-infrared conductivity from the sliding charge density wave, and suggest that the localized mode scenario reconciles the strong difference between the number of conduction electrons obtained from the Hall effect, and optical sum rule analysis in CeB6.

The broad-band optical spectroscopy was used to study the optical and the hidden transport properties of the Ba(Fe1-xNix)2As2 superconducting films with different Ni contents. The normal state data were analyzed using a Drude-Lorentz model with two Drude components: narrow and broad ones. In the superconducting state, two gaps with 2 4 (1) 0 / k B T c = 1.57 and 2 4 (2) 0 / k B T c = 3.48 are formed for the Ba(Fe 0.965 Ni 0.035 ) 2 As 2 films, while for the Ba(Fe0.95Ni0.05)2As2 films these characteristic ratios are 1.88-2.08 and 3.66-4.13. Both gaps are formed from the narrow Drude component, whereas the broad Drude component remains ungapped. The calculated from infrared data total dc resistivity of the films with Ni contents x = 0.05 and x = 0.08 as well as the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. On the contrary, the films with x = 0.035 manifest a hidden non-Fermi-liquid behavior. The Allen theory generalized to a multiband systems was applied to the analysis of the temperature dependences of a resistivity of the Ba(Fe1-xNix)2As2 films. The change of total electron-boson coupling and representative energy in the normal state versus the superconducting state for this system was shown to be typical of other iron-based superconducting materials as well as high-temperature superconducting cuprates.

Modern optoelectronic devices demand materials that can perform multiple, often conflicting functions, such as acting as metals for interconnects, dielectrics for waveguides, and semiconductors for light emission and detection. The integration of these materials is challenging, slowing industry progress and increasing costs. It inspired an intensive search for an optoelectronic response within a single material. Here, we reveal that palladium diselenide (PdSe2) provides an answer to this quest owing to its band structure. It exhibits a semimetallic band structure with a large bandgap for interband transitions responsible for semiconductor-metallic nature. This duality enables PdSe2 to function as both a photodetector and a waveguide, integrating two traditionally incompatible responses. As a result, our findings provide a full picture of PdSe2 optoelectronic properties, paving the way for its use in multifunctional optoelectronic applications.

The uncertainty of the nature of the normal state and superconducting condensate of unconventional superconductors continues to stimulate considerable speculation about the mechanism of superconductivity in these materials. Of particular interest are the type of symmetry of the order parameter and the basic electronic characteristics of the superconducting and normal states. We report the derivation of temperature dependences of the superconducting condensate plasma frequency, superfluid density, and London penetration depth by measuring terahertz spectra of conductivity and dielectric permittivity of the Ba(Fe1−xNix)2As2 thin films with different Ni concentrations. A comprehensive analysis of the experimental data was performed in the framework of the simple three-band Eliashberg model under the assumption that the superconducting coupling mechanism is mediated by antiferromagnetic spin fluctuations. The results of independent experiments support the choice of model parameters. Based on calculations of the temperature dependences of superconducting gaps, we may conclude that the obtained results are compatible with the scenario, in which Ba(Fe1−xNix)2As2 is a multiband superconductor with s±-wave pairing symmetry.

In this work, nanosized InGaZnO4 ceramics were obtained by metal-nitrate-glycol gel decomposition method. It was found that this method can produce a material with varying different of crystallinity (from amorphous to highly crystalline) as a result of sintering at different temperatures (500 ... 900 degrees & Scy;) without any crystalline impurities. The XRD method was used to study the kinetic patterns of particle growth. Isothermal and non-isothermal conditions of particle growth were studied. The activation energy of the InGaZnO4 crystallite growth process was calculated at low and high sintering times. SEM and TEM methods were also used to determine the morphology and crystallinity states of samples sintered at different temperatures. The EDS data confirm the homogeneous distribution of elements and the absence of amorphous agglomerates (impurities) in the material. The optical energy gap of InGaZnO4 was calculated from UV-vis spectroscopy data. In order to study the effect of crystallites size on the infrared-active excitations, room-temperature terahertz-infrared (frequencies 4-3500 cm-1) spectra of a series of ceramic samples with different crystallites size have been measured. 17 infrared absorption lines are observed and assigned to phonon and interatomic bonds vibrations. The parameters (frequency, damping, intensity) of the lines are found to be strongly dependent on the sintering temperature and thus on the crystallites size. The obtained data may be useful for further production of IGZO-based inks.

The study of the origin and transport of water in the universe is an important part of the scientific program of the Millimetron space observatory. This will be made possible by observations conducted in single-dish mode using an onboard instrument-the high-resolution spectrometer (HRS). This instrument incorporates heterodyne array receivers operating within the range 0.5 -2.7 THz, comprising 3-pixel arrays of superconductor-insulator-superconductor mixers operating at frequencies below 1.3 THz and 7-pixel matrix receivers based on NbN HEB mixers observing above 1.3 THz. This article presents the current status of development for a mixers planned for use in the HRS instrument of the Millimetron space observatory.

Van der Waals (vdW) materials, with their unique combination of electronic, optical, and magnetic properties, are emerging as promising platforms for exploring excitonic phenomena. Thus far, the choice of materials with exceptional excitonic response has been limited to two-dimensional (2D) configurations of vdW materials. At the same time, large interlayer distance and the possibility to create a variety of heterostructures offers an opportunity to control the dielectric screening in van der Waals heterostructures and van der Waals 3D materials, thus engineering the excitonic properties. Here, we reveal that bulk vdW crystal CrCl3 answers this quest with a record exciton binding energy of 1.64 eV owing to a delicate interplay of quasi-2D electronic confinement and local magnetic correlations. We also suggest that the non-local magnetic correlations play an important role in the temperature dependence of photoluminescence intensity. Furthermore, we observe colossal binding energies in vdW crystals NbOCl2 (0.66 eV) and MoCl3 (0.35 eV) and formulate a universal exciton binding energy dependence on bandgap for 2D and 3D vdW materials. Hence, our findings establish a fundamental link between the layered structure of vdW materials and their excitonic properties.

The uncertainty of the nature of the normal state and superconducting condensate of unconventional superconductors continues to stimulate considerable speculation about the mechanism of superconductivity in these materials. Of particular interest are the type of symmetry of the order parameter and the basic electronic characteristics of the superconducting and normal states. We report the derivation of temperature dependences of the superconducting condensate plasma frequency, superfluid density, and London penetration depth by measuring terahertz spectra of conductivity and dielectric permittivity of the Ba(Fe1-xNix)2As2 1- x Ni x ) 2 As 2 thin films with different Ni concentrations. A comprehensive analysis of the experimental data was performed in the framework of the simple three-band Eliashberg model under the assumption that the superconducting coupling mechanism is mediated by antiferromagnetic spin fluctuations. The results of independent experiments support the choice of model parameters. Based on calculations of the temperature dependences of superconducting gaps, we may conclude that the obtained results are compatible with the scenario, in which Ba(Fe1-xNix)2As2 1- x Ni x ) 2 As 2 is a multiband superconductor with s +/- +/--wave pairing symmetry.

van der Waals transition metal dichalcogenides, distinguished by a high refractive index and giant optical anisotropy, are promising materials for integrated photonic devices. However, their superior optical properties are nowadays limited to exfoliated samples with only a micrometer scale, whereas industrial integration requires at least cm-scale dimensions. Here, we resolve this problem for MoTe2 by demonstrating that chemical vapor deposition synthesis can provide an identical optical response to the benchmark exfoliated samples in a broad spectral range (250–5,000 nm). It allows us to show high-performance waveguiding properties of MoTe2 with a subwavelength footprint of ∼λ/8 for telecommunication wavelengths. Therefore, our findings reveal MoTe2 as an ideal platform for the next-generation nanophotonics.

The advent of a new era of flat optics is due to the optical metasurfaces, whose large number of degrees of freedom allowed one to fabricate various optical elements on a single technological platform. The use of van der Waals (vdW) layered materials, which have a remarkable combination of a high refractive index, record optical anisotropy and bright exciton resonances, can lead to operating regimes of metasurfaces that are unattainable for their dielectric counterparts. In this work, the degree of freedom related to the optical anisotropy is used for control of the balance and competition of modes of vdW resonators, leading to the observation of both the well-known hybrid anapole effect and recently predicted octupole quasi-trapped mode (OQTM) regime in the same metasurface. Using far-field and near-field analysis of the metasurface composed of MoS2${\rm MoS}_2$ disks, both the common and significantly different features of these two effects are demonstrated and it is found that the remarkable combination of narrow spectral features and strong energy localization makes OQTM-supported vdW metasurfaces a much more attractive alternative for creating flat nanophotonic devices, including narrowband converters, light concentrators, as well as surface-emitting lasers and nonlinear light converters.

Modern optoelectronic devices demand materials that can perform multiple, often conflicting functions, such as acting as metals for interconnects, dielectrics for waveguides, and semiconductors for light emission and detection. The integration of these materials is challenging, slowing industry progress and increasing costs. It inspired an intensive search for a universal optoelectronic response within a single material. Here, we reveal that palladium diselenide (PdSe2) provides an answer to this quest owing to its unique bandstructure. It exhibits a semimetallic band structure with an unusually large bandgap for interband transitions responsible for semiconductor-metallic nature. This duality enables PdSe2 to function as both a photodetector and a waveguide, integrating two traditionally incompatible responses. As a result, our findings provide a full picture of PdSe2 optoelectronic properties, paving the way for its use in multifunctional optoelectronic applications in one material.

Nature is abundant in material platforms with anisotropic permittivities arising from symmetry reduction that feature a variety of extraordinary optical effects. Principal optical axes are essential characteristics for these effects that define light-matter interaction. Their orientation – an orthogonal Cartesian basis that diagonalizes the permittivity tensor, is often assumed stationary. Here, we show that the low-symmetry triclinic crystalline structure of van der Waals rhenium disulfide and rhenium diselenide is characterized by wandering principal optical axes in the space-wavelength domain with above π/2 degree of rotation for in-plane components. In turn, this leads to wavelength-switchable propagation directions of their waveguide modes. The physical origin of wandering principal optical axes is explained using a multi-exciton phenomenological model and ab initio calculations. We envision that the wandering principal optical axes of the investigated low-symmetry triclinic van der Waals crystals offer a platform for unexplored anisotropic phenomena and nanophotonic applications.

In this article, we present the results of the research aimed at improving the fabrication process of a SIS-mixer for operation at frequencies close to 1 THz. We study the impact of buffer aluminum oxide layer and anodization effects on the properties of superconducting NbTiN films which form the electrodes of the transmission lines in THz-range devices. These layers are traditionally used in technological processes. The measurements of THz response are performed using terahertz time-domain spectrometer at frequencies from 0.3 to 2.0 THz. It was found that the critical temperature, normal-state conductivity just above the transition temperature, superconducting gap value and London penetration depth of the NbTiN film sputtered on aluminum oxide buffer layer are almost the same as of the film sputtered directly onto the substrate. The difference between the parameters is comparable to the measurement uncertainty for NbTiN films with and without additional surface layers of aluminum and anodization.

The broad-band optical spectroscopy was used to study the optical and the hidden transport properties of the Ba(Fe_1-xNi_x)_2As_2 superconducting films with different Ni contents. The normal state data were analyzed using a Drude-Lorentz model with two Drude components: narrow and broad ones. In the superconducting state, two gaps with 2Δ _0^(1)/k_BT_c=1.57 and 2Δ _0^(2)/k_BT_c=3.48 are formed for the Ba(Fe_0.965Ni_0.035)_2As_2 films, while for the Ba(Fe_0.95Ni_0.05)_2As_2 films these characteristic ratios are 1.88–2.08 and 3.66–4.13. Both gaps are formed from the narrow Drude component, whereas the broad Drude component remains ungapped. The calculated from infrared data total dc resistivity of the films with Ni contents x=0.05 and x=0.08 as well as the low-temperature scattering rate for the narrow Drude component show a hidden Fermi-liquid behavior. On the contrary, the films with x=0.035 manifest a hidden non-Fermi-liquid behavior. The Allen theory generalized to a multiband systems was applied to the analysis of the temperature dependences of a resistivity of the Ba(Fe_1-xNi_x)_2As_2 films. The change of total electron-boson coupling and representative energy in the normal state versus the superconducting state for this system was shown to be typical of other iron-based superconducting materials as well as high-temperature superconducting cuprates.

The uncertainty of the nature of the normal state and superconducting condensate of unconventional superconductors continues to stimulate considerable speculation about the mechanism of superconductivity in these materials. Of particular interest are the type of symmetry of the order parameter and the basic electronic characteristics of the superconducting and normal states. We report the derivation of temperature dependences of the superconducting condensate plasma frequency, superfluid density, and London penetration depth by measuring terahertz spectra of conductivity and dielectric permittivity of the Ba(Fe_1-xNi_x)_2As_2 thin films with different Ni concentrations. A comprehensive analysis of the experimental data was performed in the framework of the simple three-band Eliashberg model under the assumption that the superconducting coupling mechanism is mediated by antiferromagnetic spin fluctuations. The results of independent experiments support the choice of model parameters. Based on calculations of the temperature dependences of superconducting gaps, we may conclude that the obtained results are compatible with the scenario, in which Ba(Fe_1-xNi_x)_2As_2 is a multiband superconductor with s_±-wave pairing symmetry.

The effect of doping on the chemical and physical properties of semiconductors, alloys, ferroelectrics, glasses, and other substances has been a classic topic in materials science for centuries. Strontium titanate, SrTiO3, is an archetypal perovskite of interest for both fundamental science as quantum paraelectric and numerous outstanding physical properties and applications, including dielectrics, tunable microwave and photovoltaic devices, superconductors, thermoelectrics, potential multiferroics. Its chemical doping with transition metals leads to new functionalities, but intrinsic mechanisms of structural responses, activated by impurities, have not been systematically investigated. Herein, we present the results of a comparative study of the crystal structure, vibrational spectra, and dielectric properties of SrTiO3:M (M = Mn, Ni, and Fe, 2 at%) single crystals. It is shown that impurities constitute a different tendency to off-centering and the formation of dipoles: Mn and Fe atoms are shifted from the center of the oxygen octahedron, while Ni atoms remain on-centered. As a result, small chemical doping has a dramatic effect on the dielectric response through various structural mechanisms, including the pseudo Jahn-Teller effect, the first-order Jahn-Teller effect, and defect-induced distortion. These findings open up fundamentally new possibilities for the practical solution of a difficult problem: controlling the dielectric responses of quantum paraelectrics by choosing the type of chemical additive.