A strong light-matter interaction is one of the most exploited features provided by plasmonic systems.1 To extend this capability beyond the visible and near-infrared regimes, using low-loss materials is a major goal in current nanophotonics research.2 Polar dielectric crystals, such as silicon carbide (SiC), can provide sub-diffraction confinement of mid-infrared and terahertz radiation with mode volumes and quality factors exceeding the best case scenario attained by plasmonic counterparts.3, 4 This makes these materials extremely sensitive to minute changes in the ambient environment and also strong candidates for resonant surface-enhanced spectroscopy in the infrared (SEIRA).5, 6 We report on the behaviour of surface phonon polariton (SPhPs) resonances of SiC nanopillar arrays upon their coverage with sub-nanometric and nanometric alumina (Al2O3) and zirconia (ZrO2) thin films. Highly conformal and uniform oxide layers were obtained through atomic layer deposition (ALD) and measurements of SPhP modes were performed using Fourier transform infrared spectroscopy (FTIR) in reflectance mode. Concurrent anomalous red and blue shifts of SPhP resonances were observed upon Al2O3 deposition, with shift direction being dictated by their relative position to the ordinary longitudinal optic (LO) phonon the thin film. The concurrent shifts, attributed to the coupling to the Berreman mode of the dielectric layer, persisted for thicker films and are shown to be correctly predicted by numerical calculations using the measured permittivity of the deposited oxide. On the other hand, the deposition of sub-nanometric layers of ZrO2 lead to anomalous blue-shifts of transverse and longitudinal SPhP resonances around 900cm-1, a behaviour which persisted for layers up to ≈1.5nm in thickness, and reversed to the canonical red-shift expected for a dielectric screening of resonances when the pillars were covered with thicker layers. These anomalous shifts could not be reproduced numerically by employing the measured permittivity of the films and provide evidence for a localized surface state, which when modelled as a simple Lorentz oscillator, provide semi-quantitative agreement with experimental results. In addition, the predictive red-shifts obtained for thicker films may provide a tool for real-time monitoring of thin-film growth. This work demonstrates the potential of low-loss phonon polariton systems and offers new prospects to near-field based ultra-sensitive chemical sensing and to the field of nanophotonics.
1. Maier, S. A., [Plasmonics: Fundamentals and Applications.] 1st ed.; Springer US: NY, (2007).
2. Caldwell, J. D.; Lindsay, L.; Giannini, V.; Vurgaftman, I.; Reinecke, T. L.; Maier, S. A.; Glembocki, O. J., "Low-loss, infrared and terahertz nanophotonics using surface phonon polaritons." Nanophotonics, 4 (1), 44-68, (2015).
3. Caldwell, J. D.; Glembocki, O. J.; Francescato, Y.; Sharac, N.; Giannini, V.; Bezares, F. J.; Long, J. P.; Owrutsky, J. C.; Vurgaftman, I.; Tischler, J. G.; Wheeler, V. D.; Bassim, N. D.; Shirey, L. M.; Kasica, R.; Maier, S. A., "Low-Loss, Extreme Subdiffraction Photon Confinement via Silicon Carbide Localized Surface Phonon Polariton Resonators." Nano Letters, 13 (8), 3690-3697 (2013).
4. Gubbin, C. R.; Maier, S. A.; De Liberato, S., "Theoretical investigation of phonon polaritons in SiC micropillar resonators." Physical Review B, 95 (3), (2017).
5. Anderson, M. S., "Enhanced infrared absorption with dielectric nanoparticles." Applied Physics Letters, 83 (14), 2964-2966, (2003).
6. Neubrech, F.; Huck, C.; Weber, K.; Pucci, A.; Giessen, H., "Surface-Enhanced Infrared Spectroscopy Using Resonant Nanoantennas." Chemical Reviews, 117 (7), 5110-5145, (2017).