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1. Highly confined epsilon-near-zero and surface phonon polaritons in SrTiO3 membranes.

   Xu, Ruijuan; Crassee, Iris; Bechtel, Hans A; Zhou, Yixi; Bercher, Adrien; Korosec, Lukas; Rischau, Carl_Willem; Teyssier, Jérémie T; Crust, Kevin J; Lee, Yonghun; et al (June 2024, Nature communications)

   Recent theoretical studies have suggested that transition metal perovskite oxide membranes can enable surface phonon polaritons in the infrared range with low loss and much stronger subwavelength confinement than bulk crystals. Such modes, however, have not been experimentally observed so far. Here, using a combination of far-field Fourier-transform infrared (FTIR) spectroscopy and near-field synchrotron infrared nanospectroscopy (SINS) imaging, we study the phonon polaritons in a 100 nm thick freestanding crystalline membrane of SrTiO3 transferred on metallic and dielectric substrates. We observe a symmetric-antisymmetric mode splitting giving rise to epsilon-near-zero and Berreman modes as well as highly confined (by a factor of 10) propagating phonon polaritons, both of which result from the deep-subwavelength thickness of the membranes. Theoretical modeling based on the analytical finite-dipole model and numerical finite-difference methods fully corroborate the experimental results. Our work reveals the potential of oxide membranes as a promising platform for infrared photonics and polaritonics. 

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2. Low‐Loss Far‐Infrared Surface Phonon Polaritons in Suspended SrTiO ₃ Nanomembranes

   https://doi.org/10.1002/adfm.202501041  

   Koons, Konnor; Bechtel, Hans A; Taboada‐Gutiérrez, Javier; Ghanbari, Reza; Wang, Yueyin; Corder, Stephanie_N Gilbert; Kuzmenko, Alexey B; Xu, Ruijuan; Liu, Yin (May 2025, Advanced Functional Materials)

   Phonon polaritons (PhPs), excitations arising from the coupling of light with lattice vibrations, enable light confinement and local field enhancement, which is essential for various photonic and thermal applications. To date, PhPs with high confinement and low loss are mainly observed in the mid‐infrared regime and mostly in manually exfoliated flakes of van der Waals (vdW) materials. In this work, the existence of low‐loss, thickness‐tunable phonon polaritons in the far‐infrared regime within transferable freestanding SrTiO₃membranes synthesized through a scalable approach, achieving high figures of merit is demonstrated, which are comparable to the previous record values from the vdW materials. Leveraging atomic precision in thickness control, large dimensions, and compatibility with mature oxide electronics, functional oxide membranes present a promising large‐scale 2D platform alternative to vdW materials for on‐chip polaritonic technologies in the infrared regime. 

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3. Tunable Phonon Polariton Hybridization in a Van der Waals Hetero‐Bicrystal

   https://doi.org/10.1002/adma.202401349  

   Wehmeier, Lukas; Yu, Shang‐Jie; Chen, Xinzhong; Mayer, Rafael A; Xiong, Langlang; Yao, Helen; Jiang, Yue; Hu, Jenny; Janzen, Eli; Edgar, James H; et al (August 2024, Advanced Materials)

   Abstract Phonon polaritons, the hybrid quasiparticles resulting from the coupling of photons and lattice vibrations, have gained significant attention in the field of layered van der Waals heterostructures. Particular interest has been paid to hetero‐bicrystals composed of molybdenum oxide (MoO₃) and hexagonal boron nitride (hBN), which feature polariton dispersion tailorable via avoided polariton mode crossings. In this work, the polariton eigenmodes in MoO₃‐hBN hetero‐bicrystals self‐assembled on ultrasmooth gold are systematically studied using synchrotron infrared nanospectroscopy. It is experimentally demonstrated that the spectral gap in bicrystal dispersion and corresponding regimes of negative refraction can be tuned by material layer thickness, and these results are quantitatively matched with a simple analytic model. Polaritonic cavity modes and polariton propagation along “forbidden” directions are also investigated in microscale bicrystals, which arise from the finite in‐plane dimension of the synthesized MoO₃micro‐ribbons. The findings shed light on the unique dispersion properties of polaritons in van der Waals heterostructures and pave the way for applications leveraging deeply sub‐wavelength mid‐infrared light‐matter interactions. 

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4. Twist-engineered phonon polaritons in α − V ₂ O ₅

   https://doi.org/10.1364/ol.557267  

   Shiravi, H; Zheng, W; He, Y; Ran, S; Rhodes, D A; Balicas, L; Zhou, H D; Ni, G X (January 2025, Optics Letters)

   The advent of layered materials has unveiled new opportunities for tailoring electromagnetic waves at the subwavelength scale, particularly through the study of polaritons, a hybrid light–matter excitation. In this context, twist-optics, which investigates the optical properties of twisted stacks of van der Waals (vdW) layered specimens, has emerged as a powerful tool. Here, we explore the tunability of phonon polaritons in α-V₂O₅via interlayer twisting using scanning nano-infrared (IR) imaging. We show that the polaritonic response can be finely adjusted by varying their interlayer electromagnetic coupling, allowing for precise control over the propagation direction and phase transition from open unidirectional iso-frequency contours to closed elliptic geometries. Our experimental results, in conjugate with theoretical modeling, reveal the mechanisms underpinning this tunability, highlighting the role of twist-induced nano-light modifications for advanced nanophotonic control at the nanoscale. 

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5. The role of IR inactive mode in W(CO)6 polariton relaxation process

   https://doi.org/10.1515/nanoph-2023-0589  

   Hirschmann, Oliver; Bhakta, Harsh H; Xiong, Wei (January 2024, Nanophotonics)

   Abstract Vibrational polaritons have shown potential in influencing chemical reactions, but the exact mechanism by which they impact vibrational energy redistribution, crucial for rational polariton chemistry design, remains unclear. In this work, we shed light on this aspect by revealing the role of solvent phonon modes in facilitating the energy relaxation process from the polaritons formed of aT_1umode of W(CO)₆to an IR inactiveE_gmode. Ultrafast dynamic measurements indicate that along with the direct relaxation to the darkT_1umodes, lower polaritons also transition to an intermediate state, which then subsequently relaxes to theT_1umode. We reason that the intermediate state could correspond to the near-in-energy Raman activeE_gmode, which is populated through a phonon scattering process. This proposed mechanism finds support in the observed dependence of the IR-inactive state’s population on the factors influencing phonon density of states, e.g., solvents. The significance of the Raman mode’s involvement emphasizes the importance of non-IR active modes in modifying chemical reactions and ultrafast molecular dynamics. 

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