Abstract
Enhanced light-matter interactions are the basis of surface-enhanced infrared absorption (SEIRA) spectroscopy, and conventionally rely on plasmonic materials and their capability to focus light to nanoscale spot sizes. Phonon polariton nanoresonators made of polar crystals could represent an interesting alternative, since they exhibit large quality factors, which go far beyond those of their plasmonic counterparts. The recent emergence of van der Waals crystals enables the fabrication of high-quality nanophotonic resonators based on phonon polaritons, as reported for the prototypical infrared-phononic material hexagonal boron nitride (h-BN). In this work we use, for the first time, phonon-polariton-resonant h-BN ribbons for SEIRA spectroscopy of small amounts of organic molecules in Fourier transform infrared spectroscopy. Strikingly, the interaction between phonon polaritons and molecular vibrations reaches experimentally the onset of the strong coupling regime, while numerical simulations predict that vibrational strong coupling can be fully achieved. Phonon polariton nanoresonators thus could become a viable platform for sensing, local control of chemical reactivity and infrared quantum cavity optics experiments.
Highlights
Infrared spectroscopy is a powerful tool for label-free and non-destructive characterization of materials via their specific vibrational fingerprints[1]
We first demonstrate the potential of hyperbolic phonon polaritons (HPhPs) nanoresonators for nanophotonics and sensing applications by comparing the spectral response of linear Au and hexagonal boron nitride (h-BN) antennas
Note that the h-BN antenna is much shorter because of the extremely small wavelengths of the HPhP modes in the h-BN rod compared to the surface plasmon polariton modes in the Au rod[32]
Summary
Infrared spectroscopy is a powerful tool for label-free and non-destructive characterization of materials via their specific vibrational fingerprints[1]. SEIRA relies on enhancing the interaction of infrared light with molecules via the strongly confined near fields at the surface of plasmon-resonant metallic structures, such as gratings[6], nanoparticles or antennas[4, 7, 8, 9]. SEIRA experiments might allow for exploring strong light-matter coupling. Molecular vibrational strong coupling (VSC) in the mid-IR spectral range is challenging because of the weak infrared dipole moment of molecular vibrations. It has been achieved essentially with infrared micro-cavities[18, 19], and only very recently with mid-IR surface plasmons coupled to a micrometer-thick molecular layer 20. Molecular VSC at the nanoscale employing plasmonic nanostructures has not been realized yet, owing to the high losses of plasmonic materials in the mid-IR range
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