Non-Local, Non-Linear and Non-Hermitian Nanophotonics

Miniaturing optical resonators into meta-atoms and packing them together in meta-devices have revolutionized nanophotonics. Building various meta-atoms have been extensively studied in the past. However, packing them together into meta-devices remains less explored. Quantum many-body effects offer a novel direction in this regard. Here, we discuss two approaches – emulating and integrating quantum many-body systems – to deliver meta-devices exhibiting quantum many-body effects.<br/><br/>Emulating quantum many-body effects in nanophotonics is difficult because nanophotonic resonators only weakly confine light in them. Radiative and often non-radiative losses are not negligible in nanophotonics necessitating non-Hermitian physics. We demonstrated a non-Hermitian metasurface based on passive parity-time symmetry. We coupled a lossy plasmonic resonator with a lossless dielectric resonator to achieve the best of plasmonics and photonics, i.e., enhanced absorption as in plasmonics and high Q-factor as in dielectric photonics. Further, we demonstrated non-trivial topology in this system resulting in robust directional thermal emission. Here, we will discuss the directionality of thermal emission from such metasurfaces. We show that a highly asymmetric thermal emission is possible from a transparent metasurface held at 900 K. Such directional, bright, and spectrally selective thermal light sources could be revolutionary for thermal imaging and efficient thermophotovoltaic energy conversion.<br/><br/>In another approach, we combine quantum materials with nanophotonics to leverage powerful design tools. We study the optical properties of a layered charge-density-wave (CDW) material, 1T-TaS₂ under illumination and electrical bias. We observed a unity-order index change in 1T-TaS₂ under both stimuli. Our investigation showed that charge density wave (CDW) domains reorganize into a different stacking order under a stimulus. As a result, the optical response of this quantum material is not only non-linear but also non-local. We build a non-local model to capture the light-matter interaction in 1T-TaS₂ and other quantum materials and understand the energy landscape of strong correlations in this material. Measuring non-locality could be a non-invasive probe of the quality of a quantum material and thus could catalyze the discovery of functional quantum materials. Further, we demonstrate a tunable 1T-TaS₂-based metasurface with 100% modulation depth, MHz bandwidth, and ultralow-power operation. Such tunable optical devices could be disruptive for emerging imaging and display applications.<br/><br/>In summary, we demonstrate that quantum many-body effects in meta-devices lead to non-local, nonlinear, and non-Hermitian light-matter interaction. Understanding this unique regime of light-matter interaction would unlock novel nanophotonic phenomena leading to meta-devices with unprecedented functionalities.