Abstract
Launching and manipulation of polaritons in van der Waals materials offers novel opportunities for field-enhanced molecular spectroscopy and photodetection, among other applications. Particularly, the highly confined hyperbolic phonon polaritons (HPhPs) in h-BN slabs attract growing interest for their capability of guiding light at the nanoscale. An efficient coupling between free space photons and HPhPs is, however, hampered by their large momentum mismatch. Here, we show —by far-field infrared spectroscopy, infrared nanoimaging and numerical simulations— that resonant metallic antennas can efficiently launch HPhPs in thin h-BN slabs. Despite the strong hybridization of HPhPs in the h-BN slab and Fabry-Pérot plasmonic resonances in the metal antenna, the efficiency of launching propagating HPhPs in h-BN by resonant antennas exceeds significantly that of the non-resonant ones. Our results provide fundamental insights into the launching of HPhPs in thin polar slabs by resonant plasmonic antennas, which will be crucial for phonon-polariton based nanophotonic devices.
Highlights
Launching and manipulation of polaritons in van der Waals materials offers novel opportunities for field-enhanced molecular spectroscopy and photodetection, among other applications
Low-dimensional van der Waals materials have recently been attracting a substantial interest regarding photonic and optoelectronic applications since they support a variety of polaritons—oscillating dipolar excitations coupled to electromagnetic fields—that exhibit intriguing properties[1,2]
By means of Fourier-transform infrared spectroscopy (FTIR), scattering-type scanning near-field optical microscopy and numerical simulations we study the launching of hyperbolic phonon polaritons (HPhPs) in thin hexagonal BN (h-BN) slabs with resonant gold rod antennas
Summary
Launching and manipulation of polaritons in van der Waals materials offers novel opportunities for field-enhanced molecular spectroscopy and photodetection, among other applications. In hexagonal BN (h-BN), hyperbolic phonon polaritons (HPhPs) can propagate as ultra-confined rays within the mid-IR Reststrahlen bands (the spectral intervals between the transversal and longitudinal optical h-BN phonons, in which the transmission through the sample is strongly suppressed due to the negative real part of the dielectric permittivity), owing to the strongly anisotropic permittivity of h-BN Due to their remarkably long lifetimes[3], HPhPs in h-BN (and especially, in isotopically enriched h-BN4) offer a strong potential for fieldenhanced molecular vibrational sensing and strong coupling with molecular vibrations[5]. In spite of the suppression of the far-field antenna response in the h-BN Reststrahlen bands, we find that the HPhPs launching efficiency by the resonant gold rod antenna is significantly higher than that of non-resonant metallic launchers (e.g., non-resonant rods, small discs, or long stripes). Both numerical simulations and near-field experiments evidence that the launching efficiency has an optimum as a function of frequency, clearly related to the antenna resonance
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