Microresonator Enabled Single-Molecule Spectroscopy

An image of the microtoroid resonators the group uses to perform some of our single-particle spectroscopy experiments.

One part of our group’s research focuses on using microresonators as single-molecule spectroscopy instruments. The microresonators act as tiny thermometers that allow us to sense the energy released from a molecule (in the form of heat) after it relaxes from absorbing light. To increase our thermometer sensitivity, several aspects of the resonator can be modified. Our research therefore focuses on developing new microresonators to increase our sensitivity in measuring single-molecule’s interactions with light.

Vibrational spectroscopy via Raman scattering

Vibrational spectroscopy via Raman scattering can probe different molecular bonds to provides chemical and structural insights to materials, including dielectric nanospheres and biological samples. The position, amplitude, and width of the Raman peaks in the vibrational spectrum all carry information about the molecules and nanoscale objects. Raman spectroscopy can probe different vibrational modes of a specific chemical bond using visible and near-infrared light. The Raman spectrum provides the fingerprint to identify the molecular structure of materials. Particularly, nitrile and alkyne vibrational modes are of special interest to biological application. However, Raman scattering signal from nitrile bonds are often weak, making detecting and characterizing this type of bonds difficult, especially at the single-particle level at nanometer scale.

We are using the toroidal cavity and micro Fabry-Perot cavity to amplify the scattered Raman signal through the Purcell enhancement. When the Raman scattered light from the sample meet the resonance condition of the cavity, the increased density of the resonant states inside the cavity increased the transition rate into that state and thus increased the number of Raman photons produced through the inelastic Raman scattering process.

Hybrid Photonic-Plasmonic Systems

Schematic of dissipative coupled AuNR-microresonator system with all parameters as well as experimental observables (inset spectra). Energy enters the coupled system via pump laser excitation of the dipolar LSP of the AuNR and is dissipated through various pathways. Once excited, the LSP decays through both radiative and nonradiative means, and in addition may exchange energy with the microresonator via LSP-WGM coupling. The WGM likewise may exchange energy with the LSP or decay via outcoupling to the waveguide in addition to radiative and nonradiative dissipation channels. The conservation of energy through these various pathways in the steady state is reflected by the equality between the extinction cross-section, which is a measure of the rate at which energy enters the system, and the sum of the absorption, scattering, and transmission cross sections (inset equation), each of which probes a particular dissipative pathway.

Control of light–matter interactions is central to numerous advances in quantum communication, information, and sensing. The relative ease with which interactions can be tailored in coupled plasmonic–photonic systems makes them ideal candidates for investigation. To exert control over the interaction between photons and plasmons, it is essential to identify the underlying energy pathways which influence the system’s dynamics and determine the critical system parameters, such as the coupling strength and dissipation rates. We have demonstrated that we can simultaneously measure both photothermal absorption and two-sided optical transmission in a coupled plasmonic–photonic resonator consisting of plasmonic gold nanorods (AuNRs) deposited on a toroidal whispering-gallery-mode optical microresonator. With the aid of an analytical model which predicts and explains the distinct line shapes observed, we quantify the contribution of each system parameter. Based on this work, to tailor the coupling strength from weak to intermediate and possibly to strong regime, we use an index-matching polymer to embed AuNRs within the polymer hybrid microresonator and thus modulate plasmonic-photonic interaction. The resulting spectral observables informs us to what extent we dynamically control this interaction.

Solution Phase Dynamics with Microresonators

Another frontier of instrument development within our group is adapting our microresonator spectrometers to solution-phase dynamics. One promising platform for such studies is the microbubble resonator, which incorporates a microfluidic inside of the resonator, allowing for easy-exchange of chemical reagents. We have leveraged this technology to study the chemical and rotational dynamics of single gold nanorods (DOI: 10.1021/acsnano.9b04702). In the future, microbubbles and other solution-compatible resonator platforms will be further developed for studying single nanoparticles and molecules undergoing dynamic processes.

Topological Photonic Crystals

Topological photonic crystals provide an exciting platform for molecular spectroscopy and exotic light-matter interactions. The unique bulk dispersions near Weyl points and nodal lines in 3D topological photonic crystals can amplify resonant scattering from embedded molecules, and mediate long-range dipole-dipole interactions which play a central role in molecular energy and information transfer. The edge states supported by Weyl and nodal line crystals, as well as 2D topological insulators can act as backscatter-free unidirectional waveguides, and exotic optical cavities and resonators.

We fabricate and characterize 2D and 3D topological photonic crystals designed for visible to microwave frequencies, and everywhere in between. A variety of additive manufacturing techniques provide us with flexible 3D design capabilities for nanometer to centimeter scale features. We can then explore the interactions of these unique photonic environments with embedded molecules and defects – a rich and unexplored frontier in molecular spectroscopy.