Lorcan M. Folan

Fluorescence spectrometer for a single electrodynamically levitated microparticle

A spectrometer for measuring the fluorescence emission from a single trapped aerosol particle is described. This device consists of a particle generator, an electrodynamic levitator, an excitation source, and detection optics and electronics. Provision is made to cool the sample chamber, and spectra can be obtained in real time. Typical spectra from particles containing organic laser dyes are presented.

Theory of enhanced energy transfer between molecules embedded in spherical dielectric particles

The effects of morphological resonances on radiative emission rates and on dipole–dipole energy transfer rates between molecules embedded in aerosol droplets are studied by developing and applying a theory based on electromagnetic solutions for a dielectric sphere. Expressions for the Mie‐resonance contributions to the transfer and emission rates are obtained in terms of the experimentally measurable spectral shape functions for the donor and acceptor molecules in the bulk host medium. Mie resonances in droplets of about 10 μm diameter are found to cause only minor changes in the integrated radiative rate, while strongly enhancing the rate and quantum efficiency of energy transfer. Estimates of the enhancement effect and calculations of its dependence on donor/acceptor location yield results consistent with experiment, supporting our interpretation that the observed energy–transfer enhancement occurs via Mie resonances of the nearly spherical droplets, and providing further insight into the Mie‐enhanced e...

Energy transfer and the photon lifetime within an aerosol particle

The theory of enhanced energy transfer within an aerosol particle based on donor coupling to morphological dependent resonances is tested for the first time to our knowledge. The correspondence between theory and experiment is good and allows one to obtain the photon lifetime within the longest-lived resonances. A comparison between this lifetime and the lifetime as obtained by Mie theory is discussed.

Spherical void electrodynamic levitator

We report a new design for an electrodynamic levitator for individual microparticles based on a spherical void. This configuration with its interior surface covered with a thin dielectric coating having high diffuse reflectivity is expected to prove ideal for the integration of inelastic scattered radiation such as fluorescence or Raman scattering.

Optimal long term imaging of a charged microparticle at the center of a Paul trap in an atmosphere near standard temperature and pressure: Experiment and stochastic model

The thermal rms fluctuation in position of a levitated microparticle in a Paul trap in N2 near standard temperature and pressure is calculated numerically from a stochastic equation of motion and is measured experimentally by long term imaging. The calculations reveal loci of minima within the lowest stability region and enhanced fluctuations as the first instability is approached. Pseudopotential results deviate below our calculations progressively with increased drive potential. This disparity grows by more than an order of magnitude near instabilities, although the pseudopotential approach provides a good asymptotic approximation for small drive potentials. Long term imaging experiments are performed in a trap modified to eliminate stray static fields at its ac ‘‘null’’ point. These experiments are well‐described by the stochastic model and suggest the use of such a trap as a stage for long term fluorescence and Raman microphotography experiments on individual microparticles in a gaseous environment.

Electric-field-modulated light scattering near a morphological resonance of a trapped aerosol particle.

We describe electric-field-induced fluctuations in the elastic scattering near optical resonances of a spherical aerosol particle trapped at the null point of an electrodynamic levitator. The particle is apparently driven into a quadrupolar distortion by the electromechanical stress at its surface. Our results coupled with recent theory indicate that the effect is principally due to a modulation in the frequencies of individual optical resonances, that a coherent distortion smaller than one part in 10(5) ( approximately 0.1 nm) is detected, and that the effect may be used for a continuous probe of surface tension and bulk viscosity.

Optical bistability of an aqueous aerosol particle detected through light scattering: theory and experiment

We present a catastrophe-based graphic model for understanding the optical bistability (OB) of an aqueous aerosol particle detected in light scattering. The model is shown to be in good agreement with hidden resonance experiments and indicates that OB in such particles can occur at an incident power level of <50 x 10(-9) W. This threshold is controlled principally by the quality factor of morphological resonances and thermophysical properties of the particle. A catastrophe scheme for the bistability of solid particles is anticipated.

Determination of molecular orientation at the surface of an aerosol particle by morphology-dependent photoselection

The orientation of fluorescent molecules on the surface of an aerosol particle has been determined by using a new technique, morphology-dependent photoselection. The excitation spectrum of the angularly averaged fluorescence from a single levitated particle is measured by using an integrating sphere levitator. By utilizing the relative intensities of resonant peaks in this spectrum and knowledge of the polarization properties of different resonances, we show that the orientation of molecules relative to the surface normal may be obtained.

Characterization of the accretion of material by microparticles using resonant ellipsometry

An optical technique to characterize the accretion of material by microparticles is described. Experiments on the absorption of water vapor by single levitated polystyrene microparticles are reported as examples of an application of the technique. The optical resonant frequencies of the microparticles are perturbed by the accretion of material and the observed shifts are used to characterize the growth. This technique, resonant ellipsometry, makes use of the polarization character of optical resonant modes to distinguish particle swelling from surface layer formation. The experimental results indicate that water vapor absorbed by polystyrene microparticles diffuses primarily into the particle bulk.

Electrometer for repeated charge exchange measurements between a microparticle and a surface: Effect of water adsorption

Abstract We report a new electrometer which allows the study of contact charging between a single microparticle and a metal surface. The electrometer is based on an electrodynamic levitator-trap and has a leakage current of about 10 −23 A. The electrometer is applied to the study of the contact charging of spherical polystyrene-divinyl benzene copolymer particles against a Ni surface under ambient and a “dry” N 2 atmosphere. Significantly reduced charge exchange after eliminating water molecules from the particle would be strong evidence for an ionic charge exchange mechanism. Through a combination of gravimetric and light scattering measurements we have determined both the amount of water accreted in a humid atmosphere and the maximum amount of water that could be on the surface of the particle. Our experiments indicate approximately constant charging, independent of the amount of water on the particle.