Metasurfaces control light propagation at the nanoscale for applications in both free-space and surface-confined geometries. However, all recent designs have exhibited concepts using geometrically fixed structures, or used materials with excessive propagation losses, thereby limiting potential applications. Here we show how to overcome these limitations using a reconfigurable hyperbolic metasurface comprising a heterostructure of isotopically enriched hexagonal boron nitride (hBN) in direct contact with a phase-change material (PCM), single crystal vanadium dioxide (VO2). Metallic and dielectric domains in VO2 provide spatially localized changes in the local dielectric environment to tune the wavelength of hyperbolic phonon polaritons (HPhPs) supported in hBN by a factor of 1.6. This contrasts with earlier work using surface phonon polaritons, where propagation could only be observed above a low-loss dielectric phase. We demonstrate the first realization of in-plane HPhP refraction, which obeys Snell’s law and the means for launching, reflecting and transmitting HPhPs at the PCM domain boundaries. To demonstrate practical applications of this platform, we show how hBN could be combined with either VO2 or GeSbTe glasses to make refractive nanophotonic waveguides and lenses. This approach offers control of in-plane HPhP propagation at the nanoscale and exemplifies a reconfigurable framework combining hyperbolic media and PCMs to design new optical functionalities including resonant cavities, beam steering and waveguiding.
Polaritonic materials that support epsilon-near-zero (ENZ) modes offer the
opportunity to design light-matter interactions at the nanoscale through phenomena like resonant perfect absorption and extreme sub-wavelength light concentration. To date, the utility of ENZ modes is limited in propagating polaritonic systems by a relatively flat spectral dispersion, which gives ENZ modes small group velocities and therefore short propagation lengths. Here we overcome this constraint by coupling ENZ modes to surface plasmon polariton (SPP) modes in doped cadmium oxide ENZ-on-SPP bilayers. What results is a strongly coupled hybrid mode, characterized by strong anti-crossing and a large spectral splitting on the order of 1/3 of the mode frequency. The resonant frequencies, dispersion, and coupling of these polaritonic-hybrid-epsilon-near-zero (PH-ENZ) modes are controlled by tailoring the modal oscillator strength and the ENZ-SPP spectral overlap. As cadmium oxide supports polaritons over a wide range of carrier concentrations without excessive losses, strong coupling effects can potentially be utilized for actively tunable strong coupling at the nanoscale. PH-ENZ modes ultimately leverage the most desirable characteristics of both ENZ and SPP modes through simultaneous strong interior field confinement and mode propagation. As a result, this system could see applications in sub-diffraction modulators using carrier injection schemes, or narrow linewidth thermal emitters working in the 3-5µm spectral window.
Localized surface phonon-polariton (SPhP) resonances in polar semiconductor nanostructures can provide highly sub-diffractional electromagnetic fields. Furthermore, SPhP resonances offer enhanced resonant quality factors when compared to plasmon-polariton based systems. The various material platforms and nanostructure geometries achievable in polar semiconductors suggest they would be ideal platforms for tunable, long-wavelength photonics applications. Moreover, the constituent atomic basis defines the operating frequency regime for SPhP resonances; tunable from the mid-infrared to THz. Here, we investigate Raman active aspects of SPhP modes in GaN nanowire arrays that are grown via selective area molecular beam epitaxy. We detect strong Raman peaks within the Reststrahlen band of GaN that are not found in the bulk GaN Raman spectrum. These SPhP modes occur around 700 cm^-1 (~ 14.3 microns), offering a spectral region for device applications which is currently not accessible by plasmonic based systems or other SPhP enabled materials. Utilizing selective area epitaxy, we created GaN nanowire arrays with various diameters and pitches, from which the Raman spectra showed tuning of the apparent SPhP resonances. Infrared reflectance measurements were also performed with an FTIR microscope to further establish the physical properties of the resonances. Finally, computational studies of the structures’ reflectance were used to solidify our understanding of the geometry/SPhP-resonance-tuning relationship.
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