Probing the Ultrafast Dynamics of Phonon Polariton Active Tuning

One current limitation of many nanophotonic devices is that once fabricated, the device’s optical response is fixed. Surface phonon polaritons (SPhPs), hybrid excitations of light strongly coupled to the vibrational modes of a polar dielectric, offer a path to create nanophotonic devices whose infrared response can be modulated on the fly. For instance, spectral tuning in nanophotonic materials can be achieved by coupling the free carrier plasma to localized SPhP modes. The photoinjection of charge carriers perturbs the dielectric function, causing the SPhP resonances to shift. Thus, spectral tuning can be achieved by modulating the free carrier density. Resonance shifts are defined by a tuning figure of merit (FOM) that is anticorrelated to the resonance bandwidth, which depends on the free carrier lifetime. Therefore, determining the mechanisms that dictate the free carrier relaxation kinetics in these nanophotonic materials is crucial to extending the tuning range and optimizing the tuning FOM. This presentation details using a pump-probe spectroscopy technique to extract carrier lifetimes and image ambipolar diffusion in nanopillar arrays that support SPhP modes. The results of these experiments provide insight on the impacts of surface defects and diffusion into the substrate that potentially limit SPhP resonance tuning.