In the interaction of light and matter, strong coupling occurs when exchange between a photon and electronic transitions exceeds the relative loss rate leading to hybridization of the optical and electronic states. The behavior is well known in cavity quantum electrodynamics (QED), and is a fundamental ingredient in single photon quantum logic gates. In solid-state systems, many strong coupling phenomena have been explored between different material excitations. Plasmons in particular have attracted great interest owing to their small mode volumes, allowing for strong coupling of a plasmon and single quasi-particles excitations such as excitons, potentially recreating in the solid-state, at the nanoscale, and at elevated temperatures many of the phenomena previously studied in traditional trapped atom QED.

Here, I describe our investigations of strong coupling between TMD excitons and a tunable plasmonic nanocavity formed between a plasmonic tip and gold substrate. Strong coupling between plasmons and excitons has been observed in excitons systems: e.g., J-aggregates, and colloidal quantum-dots. While these systems offer large coupling strengths, the exciton transition energies are largely fixed, and vary randomly depending on variations in growth conditions. By contrast, TMDs offer strong exciton emission and large tunability of exciton energy by applied strain. We utilized this tunability to control coupling strengths in nanocavities in a proof-of-principle nano-electro-mechanical system (NEMS) platform, demonstrating the ability to continuously tune, between weak and strong coupling conditions where we observe both upper and lower polariton bands.

Modern integrated circuits have active elements on the order of nanometers; however, optical devices are limited by diffraction effects with dimensions measured in wavelengths. Nanoscale photodetectors capable of converting light into electrical signals are necessary for the miniaturization of optoelectronic applications. Strong coupling of light and free electrons in plasmonic nanostructures efficiently confines light into sub-wavelength volumes with intense local electric fields. Localized electric fields are amplified in nanogap regions between nanostructures where enhancements can reach over 1000. Hot carriers generated within these high field regions from nonradiative decay of surface plasmons can be injected into the conduction band of semiconductors at room temperature, enabling sub-bandgap photodetection. The optical properties of these plasmonic photodetectors can be tuned by modifying antenna materials and geometric parameters like size, thickness, and shape. Electrical interconnects provide connectivity to convert light into electrical signals. In this paper, we will describe the optical properties of plasmonic nanostructures with electrical interconnects and compare experiment and theory using finite-difference time-domain (FDTD) simulations. We will present experimental extinction data and FDTD simulations to elucidate how geometric structure and dielectric properties influence optical properties. We will present sub bandgap photodetection for nanostructures integrated with ALD deposited TiO₂, and investigate both wavelength and polarization dependence. We will also discuss how plasmonic heating effects contribute to photocurrent generation. These plasmonic nanogap antennas are subwavelength, tunable photodetectors with sub-bandgap responsivity for a broad spectral range.