Mid-infrared nonlinear polaritonics using surface phonon polaritons

Introduction Recently the potential of surface phonon polaritons for light localisation in tailored geometries has been demonstrated. By hybridising photons to coherent oscillations of a polar dielectric crystals ionic... [ view full abstract ]

Introduction

Recently the potential of surface phonon polaritons for light localisation in tailored geometries has been demonstrated. By hybridising photons to coherent oscillations of a polar dielectric crystals ionic lattice, field confinements beyond those theoretically achievable by plasmonic means are possible, with the boon of circumventing Ohmic losses characteristic in metal-based systems. The amalgamation of these features presents a potential platform for mid-infrared nanophotonics. One area where SPhPs present exceptional promise is as a platform for mid-infrared nonlinear optics, where deep-subdiffraction localisation can be exploited to drive fast scattering mediated by the intrinsic anharmonicity of the dielectric. A description of nonlinear optics in these confined systems necessitates theory capable of accounting for real-space variation in the dielectric constant.

Methods

In this work we build from the foundations of a microscopic Hopfield picture, where coupled light-matter modes of the system are linear, polaritonic, superpositions of the bare modes, to develop a quantum theory of perturbative second[1] and third[2] -order optical nonlinearities in polar dielectric systems.

Results

This theory, validated by fits to recent experiments of second harmonic generation in reflection from planar beta-SiC substrates [3] as illustrated in Fig. 1a, provides a methodology to describe nonlinear optical processes in systems with analytically or numerically derived linear solutions. We apply our theory to the describe excitation of SPhPs by difference frequency generation, showing the process to be orders of magnitude more efficient than analogous surface plasmon excitation by four-wave mixing [1]. Furthermore our theory is developed to describe third-order processes, showing that SPhP based parametric oscillators lie within experimental reach [2].

Discussion

The frame for nonlinear polaritonics presented can be expected to find general application as a predictive tool in the emergent field of surface phonon polariton nanophotonics. Our specific results regarding SPhP difference frequency generation and parametric oscillation illustrate the specific potential of polar dielectric systems as a platform for mid-infrared nonlinear optics.

[1] 10.1021/acsphotonics.7b00020

[2] arXiv:1707.06585

[3] 10.1103/PhysRevB.94.134312