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Abstract In this paper, we propose radial line slot array antenna for 5G fixed beam solutions aimed at integrating multiple frequencies of high gain aperture into a single antenna with a common feed. Thus generated shared aperture will facilitate multi-functional antenna aperture for 5G new radio applications in FR-2 band. With increasing diversity in paradigms of communication systems, especially in mm-wave frequency bands, antenna arrays with specialized, multifunctional radiation capabilities are being pursued. This includes RLSAs for multi-band, multi-polarization and multi-beam radiation. This paper will introduce a low-profile, shared-aperture Radial Line Slot Array (SA-RLSA) antenna for these applications in the mm-wave frequency band. The available design methods for these antennas require complex feeding structures, which could be inconvenient for mass production. This work uses a simple capacitive hole-assisted coaxial feed and a conventional substrate containing the radiating slots on the upper plane. A shared aperture approach is adopted to generate beams at multiple frequencies with pencil and conical shapes and to obtain dual polarization. The measured responses from the fabricated samples for dual-band (28 GHz and 47 GHz), dual-beam (pencil beam at 28 GHz and conical beam at 47 GHz), and dual-pol (28 GHz and 47 GHz with X-pol and 38 GHz with Y-pol) showed a very good agreement with the simulated result. At operating frequencies,$$\sim$$20+ dBi gain is obtained, which can be enhanced by increasing the size of the slotted aperture. 

ABSTRACT Hyperbolic phonon polaritons ‐ hybridized modes arising from the ultrastrong coupling of infrared light to strongly anisotropic lattice vibrations in uniaxial or biaxial polar crystals ‐ enable to confine light to the nanoscale with low losses and high directionality. In even lower symmetry materials, such as monoclinic ‐Ga₂O₃(bGO), hyperbolic shear polaritons (HShPs) further enhance the directionality. Yet, HShPs are intrinsically supported only within narrow frequency ranges defined by the phonon frequencies of the host material. Here, we report spectral tuning of HShPs in bGO by isotopic substitution. Employing near‐field optical microscopy to image HShPs in¹⁸O bGO films homoepitaxially grown on a¹⁶O bGO substrate, we demonstrate a spectral redshift of 40 cm⁻¹for the¹⁸O bGO, compared to¹⁶O bGO. The technique allows for direct observation and a model‐free estimation of the spectral shift driven by isotopic substitution without the need for knowledge of the dielectric tensor. Complementary far‐field measurements and ab initio calculations ‐ in good agreement with the near‐field data ‐ confirm the effectiveness of this estimation. This multifaceted study demonstrates a significant isotopic substitution induced spectral tuning of HShPs into a previously inaccessible frequency range, creating new avenues for technological applications of such highly directional polaritons. 

Abstract Nanostructures represent a frontier where meticulous attention to the control and assessment of structural dimensions becomes a linchpin for their seamless integration into diverse technological applications. However, determining the critical dimensions and optical properties of nanostructures with precision still remains a challenging task. In this study, by using an integrative and comprehensive methodical series of studies, the evolution of the depolarization factors in the anisotropic Bruggeman effective medium approximation (AB‐EMA) is investigated. It is found that these anisotropic factors are extremely sensitive to the changes in critical dimensions of the nanostructure platforms. In order to perform a systematic characterization of these parameters, spatially coherent, highly‐ordered slanted nanocolumns are fabricated from zirconia, silicon, titanium, and permalloy on silicon substrates with varying column lengths using glancing angle deposition (GLAD). In tandem, broad‐spectral range Mueller matrix spectroscopic ellipsometry data, spanning from the near‐infrared to the vacuum UV (0.72–6.5 eV), is analyzed with a best‐match model approach based on the anisotropic Bruggeman effective medium theory. The anisotropic optical properties, including complex dielectric function, birefringence, and dichroism, are thereby extracted. Most notably, the research unveils a generalized, material‐independent inverse relationship between depolarization factors and column length. It is envisioned that the presented scaling rules will permit accurate prediction of optical properties of nanocolumnar thin films improving their integration and optimization for optoelectronic and photonic device applications. As an outlook, the highly porous nature and extreme birefringence properties of the fabricated columnar metamaterial platforms are further explored in the detection of nanoparticles from the cross‐polarized integrated spectral color variations. 

A generalized approach derived from Bloch's equation of motion of nuclear magnetic moments is presented to model the frequency, magnetic field, spin density, and temperature dependencies in the electromagnetic permeability tensor for materials with magnetic resonances. The resulting tensor model predicts characteristic polarization signatures which can be observed, for example, in Mueller matrix element spectra measured. When augmented with thermodynamic considerations and suitable Hamiltonian description of the magnetic eigenvalue spectrum, important parameters such as density, spectral amplitude distribution, relaxation time constants, and geometrical orientation parameters of the magnetic moments can be obtained from comparing the generalized model approach to experimental data. We demonstrate our approach by comparing model calculations with full Mueller matrix element spectra measured at an oblique angle of incidence in the terahertz spectral range, across electron spin resonance quintuplet transitions observed in wurtzite-structure GaN doped with iron. Our model correctly predicts the complexity of the polarization signatures observed in the 15 independent elements of the normalized Mueller matrix for both positive and negative magnetic fields and will become useful for future analysis of frequency and magnetic field-dependent magnetic resonance measurements. Published by the American Physical Society2024 

Zirconia (ZrO2) thin films were fabricated using an ultra-high-vacuum electron-beam physical vapor deposition technique. Spectroscopic ellipsometry (SE) data from multiple samples with different thicknesses were acquired over a broad photon-energy range of 0.74–9.25 eV to determine the frequency-dependent complex dielectric function of ZrO2 thin films. Atomic force microscopy (AFM) analysis confirms the formation of highly uniform films with an average height variation below 2 nm across the scanned areas. In the SE-based optical analysis, a five-layer model consisting of the substrate, native oxide, ZrO2 thin film, surface roughness layer, and ambient was employed within a multi-sample analysis framework. The surface roughness layer was modeled using a 50:50 effective medium approximation of the film and ambient, yielding roughness thicknesses consistent with those obtained from AFM measurements. By parameterizing the intrinsic dielectric response using a set of oscillators, a critical-point model dielectric-function analysis was applied to identify band-to-band transition parameters. These results provide a comprehensive parameterization of the dielectric function of electron-beam-deposited ZrO2 thin films and establish a robust ellipsometry-based framework for correlating thin-film structure and optical properties in ultra-wide-bandgap oxide materials. 

Vector vortex modes (VVMs) in cylindrical waveguides provide a simple yet effective method for producing electromagnetic waves with both orbital and spin angular momentum. In this paper, the cylindrical waveguide-based vector vortex modes are examined in the far-field area by deriving and confirming their far-field properties. The field components have been defined using the equivalence principle and vector potential theory and then compared to the full-wave simulation findings from HFSS. The results reveal that the theoretical and simulated models align satisfactorily. This suggests the proposed design is perfectly capable of generating a higher-mode vector vortex beam backed by the theoretical validations. 

We present a model dielectric function composed of critical point functions in order to parameterize the temperature and wavelength dependencies of the dielectric function of InAs. This model is based on Adachi’s critical point model, with simple wavelength-dependent analytical functions whose parameters change linearly with temperature. The calculated dielectric function at room temperature is in excellent agreement with previously published data. We apply this model in the spectral range of 0.7–5 eV and in the temperature range of room temperature to 250°C with in situ spectroscopic ellipsometry measurements on an InAs substrate. Spectroscopic measurements were performed continuously while slowly ramping sample temperature in a stepwise manner in the controlled ambient environment of an atomic layer deposition system. We find that our model matches excellently with all experimental data with deviations less than 2% in pseudoepsilon. Our model permits smooth interpolation of the dielectric function of InAs for any intermediate temperature in the range studied and therefore can be used to monitor temperature, for example, during thin film deposition processes by in situ spectroscopic ellipsometry. We propose that this model can be applied to other semiconductors as well as wider temperature ranges. 

We investigate the temperature-dependent complex dielectric function of bulk single-crystal In2O3 over the spectral range of 1–6 eV and temperatures from room temperature to 600 °C under high-vacuum conditions using in situ spectroscopic ellipsometry. The dielectric function was modeled using wavelength-by-wavelength and critical-point model dielectric function analyses. The dielectric function exhibits pronounced alterations with increasing temperature, attributed to thermally induced changes in the band structure and carrier dynamics. We identify direct and indirect interband transitions and excitonic contributions associated with the direct bandgap near the onset of absorption. At elevated temperatures, features in the dielectric function due to indirect transitions emerge below the direct bandgap energy, which shift toward shorter photon energies with increasing temperature. Combining our results with low-temperature data from previous reports, both observed shifts of the direct and indirect transitions can be seamlessly explained with the Bose–Einstein model. The direct transition is coupled less strong to the phonon bath (average temperature θB=512 K), leading to a smaller high-temperature slope (γ=−0.2 meV/K) than for the indirect transition (θB=360 K, γ=−1.3 meV/K). The exciton contributions diminish toward higher temperatures reflected by the decrease in amplitude and increase in broadening model parameters. Our parameter set can be used to calculate the model dielectric function In2O3 at elevated temperatures.