Session Y05: Polaritonic Metamaterials

Active and Passive Tuning of Mid-Far Infrared Surface Phonon Polariton Resonances

Resonant nanocavities designed for the manipulation of localized and propagating surface phonon polaritons on polar dielectrics hold great promise as an essential platform for mid-far infrared metasurfaces. Passive nanocavity arrays are constructed with deep sub-wavelength grooves of a metal-insulator-polar dielectric layered structure, which acts as a surface waveguide that supports coupled plasmon-phonon polariton hybrid modes. For active nanocavity arrays, vanadium dioxide (VO₂) is a promising candidate as a spacer layer in the surface waveguide because VO₂ undergoes a reversible insulator-to-metal phase transition near room temperature. Here, we employ SiO₂ films as the passive and VO₂ films for the active platform. These materials are applied on two polar dielectric substrates: sapphire and gallium arsenide (GaAs), which Reststrahlen band corresponding to the deep mid-infrared (10 - 20 microns) and far-infrared (28 – 33 microns) regions, respectively. we numerically and experimentally demonstrate passive and active tunable surface phonon polaritonic devices working within the Reststrahlen band. The devices consist of 40 nm thick gold and 100 nm thick spacer on sapphire and GaAs substrates. The cavity resonance shows redshifts as temperature increases for active tuning.   

Mapping Phonon Polaritons with Visible Light

Phonon polaritons (PhPs) are hybrid light-matter waves which enable strong light-matter interactions and subdiffractional confinement, potentially empowering nanophotonic applications in sensing, nonlinear optics and nanoscale energy manipulation. Proposals to electrically inject PhPs are limited by the presumed weak coupling between the PhP modes and bulk phonons. Further, the multiple interactions between phonons, electrons, and PhPs have a dramatic impact on the electromagnetic response in nanostructures, but remain difficult to investigate due to a lack of experimental techniques to map electromagnetic eigenstates in all three dimensions which are inexpensive, easy, and non-destructive. In this work, we use confocal Raman microscopy combined with a geometric eigenmode expansion to map the full 3D eigenstates of PhP modes in Indium Phosphide (InP) nanopillars and 4H-Silicon Carbide (SiC) gratings. Our results indicate that, contrary to expectation, the bulk phonon modes of InP and 4H-SiC couple strongly to the localized PhP modes without the need for zone folding. Further, we confirm that polarizability selection rules form the predominant coupling method between phonons and PhP modes, with electron-phonon coupling playing an important role only for certain phonon modes (A1(LO) and E1(TO) in 4H-SiC). These observations further provide a method for extending Raman studies of PhP modes to achieve full 3D reconstruction of the PhP eigenmodes and thus enable applications in nanophotonics.   

Far- and near-field heat transfer in transdimensional plasmonic film systems

Radiative heat transfer in transdimensional plasmonic film systems is analyzed using the confinement-induced nonlocal electromagnetic response model built on the Keldysh-Rytova electron interaction potential [1]. Results are compared to the local Drude model routinely used in plasmonics. The former predicts greater Woltersdorff length in the far-field and larger film thickness at which heat transfer is dominated by surface plasmons in the near-field, than the latter. Analysis performed suggests that the theoretical treatment and experimental data interpretation for thin and ultrathin metallic film systems must incorporate the confinement-induced nonlocal effect to provide reliable results in radiative heat transfer studies. The fact that the enhanced far- and near-field radiative heat transfer occurs for much thicker films than the standard Drude model predicts is crucial for thermal management applications and in general for the development of new quantum photonics materials based on ultrathin metallic films and metasurfaces of controlled thickness. The latest experiments to confirm this fact will also be reported [2]. – [1] S.-A.Biehs and I.V.Bondarev, Adv. Optical Mater. 11, 2202712 (2023); [2] H.Salihoglu, et al., Phys. Rev. Lett. 131, 086901 (2023).   

Low-Loss Infrared Ultrawide Type I Hyperbolic Metamaterial Based on III-V Semiconductors

While ionic crystals provide natural low-loss infrared hyperbolic resonances through the excitation of phonon polaritons (PhPs), the operational bandwidth of these materials is limited to a few hundred wavenumbers (cm^-1) or tens of millielectronvolts. Additionally, the integration of these materials with large-scale infrared optoelectronic devices presents its own challenges. In this work, we implement an ultrawide low-loss Type I hyperbolic metamaterial covering a spectral bandwidth of 2000 cm^-1 for wavelengths above 5.3 μm. We produced the hyperbolic metamaterial with a stack of intercalated heavily-doped InAs and undoped InAs epilayers grown by molecular beam epitaxy (MBE). The InAs epilayer was heavily doped with Tellurium to obtain electron concentrations of 10¹⁹ cm^-3 and the optical properties of this stack were measured by infrared ellipsometry. These materials were then dry etched to form one-dimensional (1D) square gratings (with periods from 2 to 10 μm) and modeled by finite element electromagnetic calculations (COMSOL). The models agree with measurements, showing the formation of hyperbolic plasmon polaritons at the same frequencies where experimental features were observed. Additionally, we have identified an Epsilon Near Zero (ENZ) mode associated with long-range surface plasmon polaritons contained in the dielectric layers. This work demonstrates that highly subdiffractional light confinement can be achieved with a III-V metamaterial that can be integrated with III-V semiconductor infrared devices such as photodetectors and emitters at a large scale.   

van der Waals isotope heterostructuring showcased in engineered light-matter waves

Element isotopes exist universally. They possess distinct atomic masses and nuclear spins and significantly influence material properties. Notably, isotopes distribute evenly in space. To explore isotope spatial heterogeneity, we propose a new materials engineering method—van der Waals (vdW) isotope heterostructuring—to configure material properties by repositioning isotopes in engineered isotopic heterostructures. We showcase vdW isotope heterostructuring in engineering confined photon-lattice waves—hyperbolic phonon polaritons—in hexagonal boron nitride (hBN) isotopic heterostructures. By varying the composition, stacking, and thicknesses of h¹⁰BN and h¹¹BN building blocks, hyperbolic phonon polaritons can be engineered into a variety of new energy-momentum dispersions. These confined and tailored polaritons are promising for various nanophotonic and thermal functionalities. Due to the universality and importance of isotopes, the method of vdW isotope heterostructuring showcased here can apply to a broad range of materials and properties.   

Localized and Traveling Surface Phonon Polariton Modes in Triangular 4H-Silicon Carbide One-Dimensional Gratings

Surface phonon polaritons (SPhPs) are a low-loss alternative to surface plasmon polaritons with the potential for mid-infrared to terahertz nanophotonic devices including biochemical sensing, imaging, and optical logic.

In this study, the exploration of SPhP modes on 4H-SiC substrates featuring one-dimensional triangular gratings is undertaken. FTIR reflection spectrum revealed the presence of two distinct SPhP modes, each with their unique characteristics. Through a careful analysis of their group velocities determined through modeling and experiment, it was discerned that one of these modes is localized, while the other exhibits traveling behavior. Finite element calculations using COMSOL were employed to better understand the origin of these SPhP resonances. These calculations yielded results that closely matched the experimental observations.

To further enhance comprehension of SPhP modes and their behaviors within the material, Raman scattering depth scans were conducted using a confocal Raman microscopy system. The results of these scans clarify the coupling between the SPhP modes and bulk Raman modes, mediated by the material's polarization properties. In particular, it’s found that the localized modes are concentrated at the top of triangular gratings while the traveling modes are localized on the substrate within the triangular gratings. This study provides a mechanism to control the group velocity of the traveling modes by engineering the geometric parameters of triangular gratins. Moreover, these investigations revealed the presence of electron-phonon coupling in the LO modes, adding a layer of complexity to the intricate interplay of phenomena at the nanoscale.   

Surface Phonon Polaritons from a Sapphire Nanocone Array

Ionic crystals have attracted much attention due to their ability to achieve efficient light confinement with minimal loss in the infrared (IR) range. This is primarily based on interactions between the incident light and the ionic lattice vibrations. Sapphire (Al₂O₃) is a material widely used as a substrate in multi-layered structures and optics such as windows. Sapphire is also a hyperbolic material with multiple Reststrahlen bands (RBs) in the IR. While the optical properties of sapphire have been measured and studied from the visible to the IR, there is still a need for further investigations into the material’s unexplored functionalities based on both positive and negative permittivities. In this study, Raman scattering and infrared (10 to 30 µm) reflectivity of a sapphire nanocones array sample have been investigated with special attention to its in-plane and out-of-plane permittivity components.  Three confined modes have been observed in the sapphire nanocones array sample. The high positive in-plane refractive index of the material close to the hyperbolic transition point (at 17.5 µm) results in a dielectric resonance. Additionally, two surface phonon polariton excitations have been observed at wavelengths of 12.6 µm and 13.5 µm where the material exhibits metallic behavior and the permittivity is negative in all directions. Finite element modeling of the electromagnetic fields has been performed and is in excellent agreement with reflection spectroscopy results. This study demonstrates that the sapphire nanostructures are a promising platform for nanophotonic applications within the IR spectral range.