Surface phonon polaritons (SPhPs) are formed by hybridisation of photons with the coherent oscillations of a polar dielectric crystal. Like plasmons these modes allow for confinement of light on the nanoscale. Unlike plasmons, SPhPS typically have narrow linewidths and corresponding long lifetimes [1]. SPhPs are supported in the mid-infrared spectral region, specifically in the Reststrahlen region between the supporting polar dielectric's transverse and longitudinal optical phonon frequencies. Their large intrinsic nonlinearities [2,3] and ease of fabrication make SPhP systems an excellent platform for mid-infrared nonlinear optics.
Although the phonon branches of a polar dielectric crystal are dispersive the optical properties of the SPhP systems studied thusfar are dependent solely on the zone-centre phonon frequencies of the lattice. This is because the lengthscale of a typical SPhP nanoresonator is 100nm, many orders of magnitude larger than the supporting polar dielectric's lattice constant.
This talk will discuss a system where this zone-centre treatment of the lattice fails leading to hybridisation of longitudinal and transverse SPhPs as shown in the first figure. Exploiting the polymorphism of silicon carbide, whose polytypes unit cells are identical in two dimensions but extended to varying degree in the third, it is possible to fold the dispersive longitudinal optical phonon of the lattice back to zone-centre as illustrated in the second figure. We will discuss the implications of this folding for the optical response of SPhP nanoresonators, demonstrating theoretically and experimentally that this hybridisation can lead to strong-coupling, or formation of a mixed longitudinal-transverse mode. In addition we will discuss the possibility of exploiting these hybridised modes to create electrically pumped SPhP devices powered by the Fröhlich interaction.
[1] J. D. Caldwell et. al., Nano Letters 13, 3690 (2013).
[2] C. R. Gubbin and S. De Liberato, ACS Photonics 4, 1381 (2017).
[3] C. R. Gubbin and S. De Liberato, ACS Photonics 5, 284 (2017).
Optical properties of nanostructures , Strong light-matter interactions at the nanoscale
