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van der Waals materials support numerous exotic polaritonic phenomena originating from their layered structures and associated vibrational and electronic properties. However, many van der Waals materials' unique properties are most prominent at cryogenic temperatures. This presents a particular challenge for polaritonics research, as reliable optical constant data are required for understanding light-matter coupling. This paper presents a cryogenic Fourier transform infrared microscope design constructed entirely from off-the-shelf components and associated fitting procedures for determining optical constants in the infrared. Data correction techniques were developed to directly quantify systematic errors in the fitting procedure. We use this microscope to present the first temperature-dependent characterization of the optical properties of hexagonal boron nitride enriched with isotopically pure boron. Our full analysis of the infrared dielectric function shows small but significant tuning of the optical constants, which is highly consistent with Raman data from the literature. We then use this dielectric data to perform and analyze the polariton propagation properties, which agree exceptionally well with published cryogenic scattering-type near-field microscopy results. In addition to the insights gained into hyperbolic polaritons in hBN, our paper represents a transferable framework for characterizing exfoliated infrared polaritonic materials and other infrared devices. This could accelerate discoveries in different material systems, especially those that are spatially inhomogeneous or cannot be prepared as large single crystals. 

Abstract One of the main bottlenecks in the development of terahertz (THz) and long-wave infrared (LWIR) technologies is the limited intrinsic response of traditional materials. Hyperbolic phonon polaritons (HPhPs) of van der Waals semiconductors couple strongly with THz and LWIR radiation. However, the mismatch of photon − polariton momentum makes far-field excitation of HPhPs challenging. Here, we propose an In-Plane Hyperbolic Polariton Tuner that is based on patterning van der Waals semiconductors, here α-MoO 3 , into ribbon arrays. We demonstrate that such tuners respond directly to far-field excitation and give rise to LWIR and THz resonances with high quality factors up to 300, which are strongly dependent on in-plane hyperbolic polariton of the patterned α-MoO 3 . We further show that with this tuner, intensity regulation of reflected and transmitted electromagnetic waves, as well as their wavelength and polarization selection can be achieved. Our results can help the development of THz and LWIR miniaturized devices. 

Abstract Wavelength‐selective absorbers (WS‐absorbers) are of interest for various applications, including chemical sensing and light sources. Lithography‐free fabrication of WS‐absorbers can be realized via Tamm plasmon polaritons (TPPs) supported by distributed Bragg reflectors (DBRs) on plasmonic materials. While multifrequency and nearly arbitrary spectra can be realized with TPPs via inverse design algorithms, demanding and thick DBRs are required for high quality‐factors (Q‐factors) and/or multiband TPP‐absorbers, increasing the cost and reducing fabrication error tolerance. Here, high Q‐factor multiband absorption with limited DBR layers (3 layers) is experimentally demonstrated by Tamm hybrid polaritons (THPs) formed by coupling TPPs and Tamm phonon polaritons when modal frequencies are overlapped. Compared to the TPP component, the Q‐factors of THPs are improved twofold, and the angular broadening is also reduced twofold, facilitating applications where narrow‐band and nondispersive WS‐absorbers are needed. Moreover, an open‐source algorithm is developed to inversely design THP‐absorbers consisting of anisotropic media and exemplify that the modal frequencies can be assigned to desirable positions. Furthermore, it is demonstrated that inversely designed THP‐absorbers can realize same spectral resonances with fewer DBR layers than a TPP‐absorber, thus reducing the fabrication complexity and enabling more cost‐effective, lithography‐free, wafer‐scale WS‐absorberss for applications such as free‐space communications and gas sensing. 

Abstract The lattice symmetry of a crystal is one of the most important factors in determining its physical properties. Particularly, low-symmetry crystals offer powerful opportunities to control light propagation, polarization and phase 1–4 . Materials featuring extreme optical anisotropy can support a hyperbolic response, enabling coupled light–matter interactions, also known as polaritons, with highly directional propagation and compression of light to deeply sub-wavelength scales 5 . Here we show that monoclinic crystals can support hyperbolic shear polaritons, a new polariton class arising in the mid-infrared to far-infrared due to shear phenomena in the dielectric response. This feature emerges in materials in which the dielectric tensor cannot be diagonalized, that is, in low-symmetry monoclinic and triclinic crystals in which several oscillators with non-orthogonal relative orientations contribute to the optical response 6,7 . Hyperbolic shear polaritons complement previous observations of hyperbolic phonon polaritons in orthorhombic 1,3,4 and hexagonal 8,9 crystal systems, unveiling new features, such as the continuous evolution of their propagation direction with frequency, tilted wavefronts and asymmetric responses. The interplay between diagonal loss and off-diagonal shear phenomena in the dielectric response of these materials has implications for new forms of non-Hermitian and topological photonic states. We anticipate that our results will motivate new directions for polariton physics in low-symmetry materials, which include geological minerals 10 , many common oxides 11 and organic crystals 12 , greatly expanding the material base and extending design opportunities for compact photonic devices. 

Abstract Exploiting polaritons in natural vdW materials has been successful in achieving extreme light confinement and low-loss optical devices and enabling simplified device integration. Recently, α-MoO 3 has been reported as a semiconducting biaxial vdW material capable of sustaining naturally orthogonal in-plane phonon polariton modes in IR. In this study, we investigate the polarization-dependent optical characteristics of cavities formed using α-MoO 3 to extend the degrees of freedom in the design of IR photonic components exploiting the in-plane anisotropy of this material. Polarization-dependent absorption over 80% in a multilayer Fabry-Perot structure with α-MoO 3 is reported without the need for nanoscale fabrication on the α-MoO 3 . We observe coupling between the α-MoO 3 optical phonons and the Fabry-Perot cavity resonances. Using cross-polarized reflectance spectroscopy we show that the strong birefringence results in 15% of the total power converted into the orthogonal polarization with respect to incident wave. These findings can open new avenues in the quest for polarization filters and low-loss, integrated planar IR photonics and in dictating polarization control.