A coherent coupling between graphene plasmon and molecular vibration in strong coupling regime

Abstract

Manipulating the energy states of atoms or molecules is of great interest due to its unlimited potential in spectroscopy, analytical chemistry, atomic structural analysis, imaging, chemical and biological sensing, and many other fields of micro-nanotechnology. Various ways of atomic or molecular energy modulation have been employed to date, such as electric or magnetic field excitation, molecular vibrational or rotational excitation, quantum tunneling, and laser excitation. When electromagnetic wave interacts with materials, it may fall into either a weak, i.e., Fano resonance and Purcell effect, or strong coupling regime depending on the coupling strength. The formation of hybridized polaritonic states in the strong coupling regime opens up a new way to modify materials' physical and chemical properties by altering energy levels of the material excitation. While the formation of polaritonic states has been demonstrated for excitonic transitions, the same phenomenon can also take place with molecular vibrations. In general, a Fabry-Pérot (FP) cavity having two parallel metal or dielectric mirrors is used to induce strong coupling as a cavity optical mode interacts with molecular vibrations of a material placed in the cavity. However, a planar optical cavity has a limited spatial resolution for monitoring polaritonic states. Moreover, strong coupling in the FP cavity represents an ensemble-average due to spatially-varied optical fields inside the FP cavity. To address some of the limitations of the FP cavity, I used a deep metal grating as a new optical resonator to demonstrate vibrational strong coupling (VSC). First, I simulated the optical properties of the deep metal grating and found that the hybridization between localized surface plasmon resonances and magnetic polaritons is responsible for generating strong mid-infrared optical resonances. When single-layer graphene was integrated into the grating, the grating resonances showed Fano line shapes as a result of a weak coupling between discrete graphene plasmon modes and continuum grating resonances. In addition, with the addition of a molecular absorber to the deep metal grating, VSC was achieved with tunability in polaritonic energy states by changing the system parameters including the grating height and widths, as well as the chemical potential applied to the graphene. In parallel with simulations, I generated a large area of double-layer graphene by chemical vapor deposition and fabricated deep metal grating structures by electron-beam lithography and inductively-coupled plasma etching.