Homogeneous linewidths of Rhodamine 6G at room temperature from cavity‐enhanced spontaneous emission rates

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.

Method for predicting whispering gallery mode spectra of spherical microresonators

A full three-dimensional Finite-Difference Time-Domain (FDTD)-based toolkit is developed to simulate the whispering gallery modes of a microsphere in the vicinity of a dipole source. This provides a guide for experiments that rely on efficient coupling to the modes of microspheres. The resultant spectra are compared to those of analytic models used in the field. In contrast to the analytic models, the FDTD method is able to collect flux from a variety of possible collection regions, such as a disk-shaped region. The customizability of the technique allows one to consider a variety of mode excitation scenarios, which are particularly useful for investigating novel properties of optical resonators, and are valuable in assessing the viability of a resonator for biosensing.

Quantum dot microdrop laser

We report single-mode and multimode lasing from isolated spherical liquid microcavities containing CdSe/ZnS nanocrystal quantum dots. Lasing is observed at densities more than 2 orders of magnitude lower than previously demonstrated or theoretically predicted, assuming a uniform nanocrystal quantum dot distribution. Charged droplets, between 10 and 40 microm in size, are electrodynamically levitated and optically pumped. Substantial laser signals at low thresholds are measured from the directional emission normal to the pump beam, owing to the high Q cavity modes.

Enhancement of Förster Energy Transfer within a Microspherical Cavity

Energy transfer from pyrene to perylene molecules co-doped within a poly(methyl methacrylate) latex microsphere was drastically accelerated relative to free space. Fluorescence spectra of the microspheres showed that the relative emission intensities of pyrene and perylene changed with the sphere diameter. Analyses of emission decay profiles clarified that Förster-type energy-transfer processes were induced and that the transfer rates increased within the microspherical cavity. This enhancement can be ascribed not only to the quantum electrodynamic effects on the pyrene emission rate, but also the cavity effect of increasing the overlapping factor between donor emission and acceptor absorption spectra.

Broadband fivefold reduction of vacuum fluctuations probed by dyes in photonic crystals

We observed for the first time a strong angle-independent modification of spontaneous emission spectra from laser dyes in photonic crystals, made of inverse opals in titania. Comparison with spectra from such crystals with much smaller lattice spacing, for which emission is in the long wavelength limit, reveals inhibition of emission up to a factor approximately 5 over a large bandwidth of 13% of the first order Bragg resonance frequency. The center frequency and bandwidth of the inhibition agree with calculated total density of states, while the measured inhibition of vacuum fluctuations is much larger. Because of the specific location of the dye molecules, we likely probe the strongly modulated local photonic density of states.

Fluorescence from airborne microparticles: dependence on size, concentration of fluorophores, and illumination intensity

We measured fluorescence from spherical water droplets containing tryptophan and from aggregates of bacterial cells and compared these measurements with calculations of fluorescence of dielectric spheres. The measured dependence of fluorescence on size, from both droplets and dry-particle aggregates of bacteria, is proportional to the absorption cross section calculated for homogeneous spheres containing the appropriate percentage of tryptophan. However, as the tryptophan concentration of the water droplets is increased, the measured fluorescence from droplets increases less than predicted, probably because of concentration quenching. We model the dependence of the fluorescence on input intensity by assuming that the average time between fluorescence emission events is the sum of the fluorescence lifetime and the excitation lifetime (the average time it takes for an illuminated molecule to be excited), which we calculated assuming that the intensity inside the particle is uniform. Even though the intensity inside the particles spatially varies, this assumption of uniform intensity still leads to results consistent with the measured intensity dependence.

Spatial photoselection of single molecules on the surface of spherical microcavities

We show that ultrasensitive microdroplet-stream fluorescence techniques combined with surfactant forms of Rhodamine dyes can be used to probe single molecules on the surfaces of spherical microcavities. Individual octadecyl Rhodamine B molecules, shown previously by ensemble measurements to be localized and oriented at the surfaces of liquid microspheres, were spatially photoselected primarily along great circles lying perpendicular or parallel to the detection axis by use of polarized laser excitation. A polarization dependence is observed in the distribution of single-molecule fluorescence amplitudes that can be interpreted qualitatively in terms of position-dependent fluorescence-collection efficiencies.

Collection of emission from an oscillating dipole inside a sphere: analytical integration over a circular aperture

We describe a method for integrating analytically, over a circular aperture, the emission from an oscillating dipole inside a dielectric sphere. The model is useful for investigating fluorescence, Raman, or other emission from molecules inside of spherical particles or droplets. The analysis is performed for two cases: (a) the dipole emits from a fixed orientation, and (b) the dipole emits from all orientations and the collected energy is summed. This second case models the collection of emission from a molecule that is excited repeatedly; after each excitation it rotates to a random orientation before emitting. These results are applicable to single-molecule detection techniques employing microdroplets and to other techniques for characterizing microparticles with luminescence or inelastic scattering.

Real-time observation of single-molecule fluorescence in microdroplet streams

We report real-time observation of fluorescence bursts from individual Rhodamine 6G molecules in streams of microdroplets (peak signal-to-noise ratios, approximately 30) whose trajectories are constrained with a linear electric quadrupole. This approach offers a reasonable dynamic range in droplet size (3- 12-microm diameter) with <1% shot-to-shot size fluctuations and sensitivity comparable with that of droplet levitation techniques with at least 10(3) higher analysis rates. Applications to the study of single-molecule microcavity effects and stimulated emission are discussed.

Collection of fluorescence from single molecules in microspheres: effects of illumination geometry

The collection of fluorescence from a molecule inside a sphere illuminated with single or counterpropagating plane waves is modeled. The results are applicable to microdroplet-based single molecule detection techniques and to some microparticle characterization techniques using inelastic emission. The large position-dependent variations in the fluorescence collection rate are primarily attributable to variations in the excitation intensity. With plane-wave illumination the collection from shadow regions is low because the incident energy is refracted by the droplet surface away from these regions. The average collection rate from molecules in shadow regions can be increased by illuminating with counterpropagating beams.

Effect of optical resonances on photochemical reactions in microdroplets

We have examined photochemical reactions in microdroplets under optical resonances for situations in which photoreactive molecules dissolve from a surrounding gas phase. The link among photochemical reaction, droplet-phase diffusion, and gas-phase mass transfer rate processes produces numerous concentration distributions of reactive molecules between two limiting distributions that are characterized by the absence of reactive molecules and by the saturation concentration level. Each distribution yields a unique intensity field. The analysis shows that the reaction rate is dependent on five dimensionless parameters and is significantly enhanced by a number of combinations of parameter values.

Modeling fluorescence collection from single molecules in microspheres: effects of position, orientation, and frequency

We present calculations of fluorescence from single molecules (modeled as damped oscillating dipoles) inside a dielectric sphere. For an excited molecule at an arbitrary position within the sphere we calculate the fluorescence intensity collected by an objective in some well-defined detection geometry. We find that, for the cases we model, integration over the emission linewidth of the molecule is essential for obtaining representative results. Effects such as dipole position and orientation, numerical aperture of the collection objective, sphere size, emission wavelength, and linewidth are examined. These results are applicable to single-molecule detection techniques employing microdroplets.

Aerosol-fluorescence spectrum analyzer: real-time measurement of emission spectra of airborne biological particles

We have assembled an aerosol-fluorescence spectrum analyzer (AFS), which can measure the fluorescence spectra and elastic scattering of airborne particles as they flow through a laser beam. The aerosols traverse a scattering cell where they are illuminated with intense (50 kW/cm(2)) light inside the cavity of an argon-ion laser operating at 488 nm. This AFS can obtain fluorescence spectra of individual dye-doped polystyrene microspheres as small as 0.5 µm in diameter. The spectra obtained from microspheres doped with pink and green-yellow dyes are clearly different. We have also detected the fluorescence spectra of airborne particles (although not single particles) made from various biological materials, e.g., Bacillus subtilis spores, B. anthrasis spores, riboflavin, and tree leaves. The AFS may be useful in detecting and characterizing airborne bacteria and other airborne particles of biological origin.