Speaker
Description
Surface Phonon Polaritons (SPhP) are recently investigated as an alternative building block for mid-infrared (MIR) nanophotonic applications, promising to possibly solve the intrinsic loss problem of plasmonics [1]. SPhPs arise in polar dielectrics due to IR-active phonon resonances leading to negative permittivity between transverse and longitudinal optical phonon frequencies, a region called the Reststrahlen band. SPhPs exhibit tremendous field enhancements [2,3], driving the lattice ions into a strongly non-linear regime. Hence, SPhPs might grant a frequency-tunable access to vibrational-driven transient material phases.
In contrast to surface plasmon polaritons in metals, the strong dispersion of the SPhPs in the Reststrahlen region provides a natural way for tailoring SPhP resonances. Multilayers composed of different polar dielectrics with overlapping Reststrahlen bands exhibit a variety of novel phenomena such as mode-splitting, index-sensing, and wave-guiding [4], thus allowing for the engineering of novel hybrid materials with custom-designed polaritonic response.
Here, we use linear and nonlinear MIR spectroscopy [5] for studying SPhPs in SiC and AlN, employing Otto-type prism coupling. The air gap in the Otto geometry is actively steered, as monitored by the white light interferometry, granting extrinsic control over the critical conditions of the SPhP excitation [6]. Employing intense, tunable and narrowband MIR pulses from the FHI free-electron laser, our experiments reveal prominent increase of the resonant second harmonic generation (SHG), arising from the optical field enhancement that is associated with propagating SPhPs at the SiC/air interface [3].
Further, a nanoscale thin layer of AlN is grown on SiC, leading to the strong coupling and mode-splitting of the SiC SPhP and the AlN ultrathin film polariton. Specifically, we show that the coupling strength can be tuned both intrinsically (using different AlN layer thicknesses) and extrinsically, by modulation of the SPhP radiative losses through variation of the air gap width. These experimental observations are corroborated by a specifically developed matrix formalism for anisotropic multilayer wave propagation [4], in order to achieve precise understanding and predictability of the linear and non-linear properties of SPhPs in polar dielectric heterostructures.
References
[1] Caldwell et al., Nanophotonics 4 (1), 44-68 (2015).
[2] Razdolski et al., Nano Lett. 16 (11), 6954-6959 (2016).
[3] Passler et al., ACS Photonics 4 (5), 1048-1053 (2017).
[4] Passler and Paarmann, J. Opt. Soc. Am. B 34 (10), 2128-2139 (2017).
[5] Paarmann et al., Appl. Phys. Lett. 107, 081101 (2015).
[6] Pufahl et al., in preparation (2018).
