Elucidating Energy Pathways through Simultaneous Measurement of Absorption and Transmission in a Coupled Plasmonic-Photonic Cavity.

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. However, in coupled systems which dissipate energy through multiple competing pathways, simultaneously resolving all parameters from a single experiment is challenging as typical observables such as absorption and scattering each probe only a particular path. In this work, we simultaneously measure both photothermal absorption and two-sided optical transmission in a coupled plasmonic-photonic resonator consisting of plasmonic gold nanorods deposited on a toroidal whispering-gallery-mode optical microresonator. We then present an analytical model which predicts and explains the distinct line shapes observed and quantifies the contribution of each system parameter. By combining this model with experiment, we extract all system parameters with a dynamic range spanning 9 orders of magnitude. Our combined approach provides a full description of plasmonic-photonic energy dynamics in a weakly coupled optical system, a necessary step for future applications that rely on tunability of dissipation and coupling.

Toward Real-Time Monitoring and Control of Single Nanoparticle Properties with a Microbubble Resonator Spectrometer.

Optical microresonators have widespread application at the frontiers of nanophotonic technology, driven by their ability to confine light to the nanoscale and enhance light-matter interactions. Microresonators form the heart of a recently developed method for single-particle photothermal absorption spectroscopy, whereby the microresonators act as microscale thermometers to detect the heat dissipated by optically pumped, nonluminescent nanoscopic targets. However, translation of this technology to chemically dynamic systems requires a platform that is mechanically stable, solution compatible, and visibly transparent. We report microbubble absorption spectrometers as a versatile platform that meets these requirements. Microbubbles integrate a two-port microfluidic device within a whispering gallery mode microresonator, allowing for the facile exchange of chemical reagents within the resonator's interior while maintaining a solution-free environment on its exterior. We first leverage these qualities to investigate the photoactivated etching of single gold nanorods by ferric chloride, providing a method for rapid acquisition of spatial and morphological information about nanoparticles as they undergo chemical reactions. We then demonstrate the ability to control nanorod orientation within a microbubble through optically exerted torque, a promising route toward the construction of hybrid photonic-plasmonic systems. Critically, the reported platform advances microresonator spectrometer technology by permitting room-temperature, aqueous experimental conditions, which may be used for time-resolved single-particle experiments on non-emissive, nanoscale analytes engaged in catalytically and biologically relevant chemical dynamics.

Single-particle photothermal imaging via inverted excitation through high-Q all-glass toroidal microresonators.

Whispering-gallery mode (WGM) microresonators have recently been employed as platforms for label-free single-molecule and single-particle detection, imaging, and spectroscopy. However, innovations in device geometry and integration are needed to make WGM microresonators more versatile for biological and chemical applications. Particularly, thick device substrates, originating from wafer-scale fabrication processing, prevent convenient optical interrogation. In this work, we fabricate all-glass toroidal microresonators on a coverslip thickness (~170 µm) substrate, enabling excitation delivery through the sample, simplifying optical integration. Further, we demonstrate the application of this new geometry for single-particle photothermal imaging. Finally, we discover and develop simulations to explain a non-trivial astigmatism in the point spread function (PSF) arising from the curvature of the resonator.

Mapping Forbidden Emission to Structure in Self-Assembled Organic Nanoparticles.

The interplay between micromorphology and electronic properties is an important theme in organic electronic materials. Here, we show that a spirofluorene-functionalized boron-dipyrromethene (BODIPY) with an alkyl norbornyl tail self-assembles into nanoparticles with qualitatively different properties as compared to the polymerized species. Further, the nanoparticles exhibit a host of unique emissive properties, including photobrightening, a blue satellite peak, and spectral diffusion. Extensive photophysical characterization, including single-particle imaging and spectroscopy, and time-resolved fluorescence, coupled with electronic structure calculations based on an experimentally determined crystal structure, allow a mechanism to be developed. Specifically, BODIPY chromophores are observed to form quasi-two-dimensional layers, where stacking of unit cells adds either J-aggregate character or H-aggregate character depending on the direction of the stacking. Particularly strongly H-coupled domains show the rare process of emission from an upper exciton state, in violation of Kasha's rule, and result in the blue satellite peak. The spatial heterogeneity of structure thus maps onto a gradient of photophysical behavior as seen in single-particle imaging, and the temporal evolution of structure maps onto fluctuating emissive behavior, as seen in single-particle spectroscopy. Taken together, this system provides a striking example of how physical structure and electronic properties are intertwined, and a rare opportunity to use one to chart the other.

Exploring Electronic Structure and Order in Polymers via Single-Particle Microresonator Spectroscopy.

PEDOT: PSS, a transparent electrically conductive polymer, finds widespread use in electronic devices. While empirical efforts have increased conductivity, a detailed understanding of the coupled electronic and morphological landscapes in PEDOT:PSS has lagged due to substantial structural heterogeneity on multiple length-scales. We use an optical microresonator-based absorption spectrometer to perform single-particle measurements, providing a bottom-up examination of electronic structure and morphology ranging from single PEDOT:PSS polymers to nascent films. Using single-particle spectroscopy with complementary theoretical calculations and ultrafast spectroscopy, we demonstrate that PEDOT:PSS displays bulk-like optical response even in single polymers. We find highly ordered PEDOT assemblies with long-range ordering mediated by the insulating PSS matrix and reveal a preferential surface orientation of PEDOT nanocrystallites absent in bulk films with implications for interfacial electronic communication. Our single-particle perspective provides a unique window into the microscopic structure and electronic properties of PEDOT:PSS.

Sculpting Fano Resonances To Control Photonic-Plasmonic Hybridization.

Hybrid photonic-plasmonic systems have tremendous potential as versatile platforms for the study and control of nanoscale light-matter interactions since their respective components have either high-quality factors or low mode volumes. Individual metallic nanoparticles deposited on optical microresonators provide an excellent example where ultrahigh-quality optical whispering-gallery modes can be combined with nanoscopic plasmonic mode volumes to maximize the system's photonic performance. Such optimization, however, is difficult in practice because of the inability to easily measure and tune critical system parameters. In this Letter, we present a general and practical method to determine the coupling strength and tailor the degree of hybridization in composite optical microresonator-plasmonic nanoparticle systems based on experimentally measured absorption spectra. Specifically, we use thermal annealing to control the detuning between a metal nanoparticle's localized surface plasmon resonance and the whispering-gallery modes of an optical microresonator cavity. We demonstrate the ability to sculpt Fano resonance lineshapes in the absorption spectrum and infer system parameters critical to elucidating the underlying photonic-plasmonic hybridization. We show that including decoherence processes is necessary to capture the evolution of the lineshapes. As a result, thermal annealing allows us to directly tune the degree of hybridization and various hybrid mode quantities such as the quality factor and mode volume and ultimately maximize the Purcell factor to be 104.

Tracking Lithium Ions via Widefield Fluorescence Microscopy for Battery Diagnostics.

Direct tracking of lithium ions with time and spatial resolution can provide an important diagnostic tool for understanding mechanisms in lithium ion batteries. A fluorescent indicator of lithium ions, 2-(2-hydroxyphenyl)naphthoxazole, was synthesized and used for real-time tracking of lithium ions via widefield fluorescence microscopy. The fluorophore can be excited with visible light and was shown to enable quantitative determination of the lithium ion diffusion constant in a microfluidic model system for a plasticized polymer electrolyte lithium battery. The use of widefield fluorescence microscopy for in situ tracking of lithium ions in batteries is discussed.

Optical Microresonators for Sensing and Transduction: A Materials Perspective.

Optical microresonators confine light to a particular microscale trajectory, are exquisitely sensitive to their microenvironment, and offer convenient readout of their optical properties. Taken together, this is an immensely attractive combination that makes optical microresonators highly effective as sensors and transducers. Meanwhile, advances in material science, fabrication techniques, and photonic sensing strategies endow optical microresonators with new functionalities, unique transduction mechanisms, and in some cases, unparalleled sensitivities. In this progress report, the operating principles of these sensors are reviewed, and different methods of signal transduction are evaluated. Examples are shown of how choice of materials must be suited to the analyte, and how innovations in fabrication and sensing are coupled together in a mutually reinforcing cycle. A tremendously broad range of capabilities of microresonator sensors is described, from electric and magnetic field sensing to mechanical sensing, from single-molecule detection to imaging and spectroscopy, from operation at high vacuum to in live cells. Emerging sensing capabilities are highlighted and put into context in the field. Future directions are imagined, where the diverse capabilities laid out are combined and advances in scalability and integration are implemented, leading to the creation of a sensor unparalleled in sensitivity and information content.

Chip-Scale Fabrication of High-Q All-Glass Toroidal Microresonators for Single-Particle Label-Free Imaging.

Whispering-gallery-mode microresonators enable materials for single-molecule label-free detection and imaging because of their high sensitivity to their micro-environment. However, fabrication and materials challenges prevent scalability and limit functionality. All-glass on-chip microresonators significantly reduce these difficulties. Construction of all-glass toroidal microresonators with high quality factor and low mode volume is reported and these are used as platforms for label-free single-particle imaging.

Photothermal Microscopy of Nonluminescent Single Particles Enabled by Optical Microresonators.

A powerful new paradigm for single-particle microscopy on nonluminescent targets is reported using ultrahigh-quality factor optical microresonators as the critical detecting element. The approach is photothermal in nature as the microresonators are used to detect heat dissipated from individual photoexcited nano-objects. The method potentially satisfies an outstanding need for single-particle microscopy on nonluminescent objects of increasingly smaller absorption cross section. Simultaneously, our approach couples the sensitivity of label-free detection using optical microresonators with a means of deriving chemical information on the target species, a significant benefit. As a demonstration, individual nonphotoluminescent multiwalled carbon nanotubes are spatially mapped, and the per-atom absorption cross section is determined. Finite-element simulations are employed to model the relevant thermal processes and elucidate the sensing mechanism. Finally, a direct pathway to the extension of this new technique to molecules is laid out, leading to a potent new method of performing measurements on individual molecules.