Richard K. Chang

We report on the design, fabrication, and testing of nozzles that we have used for single-particle sampling measurements of laser-induced fluorescence and two-dimensional angular optical scattering. Single-piece nozzles and a two-piece nozzle with a sheath flow are described.

Microspiral resonators allow for unidirectional lasing and the direct coupling of light to the device's microcavity-two characteristics that aren’t possible with conventional microresonators. These novel-shaped resonators are now finding promising applications as waveguide-coupled passive devices and multiple-element cascaded-resonator switches.

Using single-particle fluorescence spectroscopy, scientists are conducting widespread, real-time monitoring of bioaerosols that will deepen their understanding of the effects of airborne particles on human health and climate change.

In a proof-of-concept experiment, we demonstrate how selected airborne bacteria, detected in air by light scattering, can be deflected aerodynamically into the input well of a microfluidic flow cytometer and analyzed using fluorescein-labeled antibodies. This experiment demonstrates a new method for introducing airborne samples into a microfluidic cell. The results suggest that by deflecting only those airborne particles having the fluorescence spectra of bioaerosols and by depositing the individual bioaerosols into the smallest volume of reagent required for analysis, it should be possible to achieve a system that can continuously monitor the air for certain bioaerosols while minimizing reagent usage.

In the introduction a brief recollection is made of how one of us (RKC), accidentally, got into this field of linear and nonlinear spectroscopy of a dielectric micro-particle that can be treated as a micro-cavity or a micro-resonator. The basic principles of whispering gallery modes (WGMs) and their relationship with electromagnetic theory are presented. To simplify the mathematics, we only discuss an example from a 2-d case of light illumination perpendicular to the fiber axis. This 2-d example has relevance to semiconductor circular disk lasers, nonlinear optics in torroids, fibers and spheres at the tip of a fiber. The internal and near-field distribution of a WGM are graphically plotted to give the reader a chance to get a physical understanding of the spatial distribution as well as spectral distribution of WGMs. Several new techniques that enable the measurements of: (1) nanometer changes in the cladding diameter over a centimeter length of fiber; (2) some aspects of the morphology of micro-particles by elastic scattering; and (3) biochemical reactions at the interface of liquid media with a sphere at the end of a fiber. A few interesting nonlinear optical experimental results pertaining to stimulated Raman scattering (SRS) are touched upon. We present some preliminary results for promising applications in the area of bioaerosols. These include ambient aerosol characterization and identification with elastic scattering, fluorescence spectroscopy, and other optical and/or biochemical identifiers.

We present the lasing characteristics of a directly coupled spiral-shaped and semicircle-shaped 2-d microdisk device using a single quantum well AlGaAs heterostructure. Different degrees of coupling, as well as mode selection are achieved by altering the length along which the spiral and semicircle cavities are coupled and the curvature of the coupling junction. Because of the spiral's chirality, the high quality factor and low threshold of the spiral microdisk are preserved when coupled to a semicircle microdisk. A three-element device consisting of a spiral microdisk coupled to two semicircle microdisks is also considered and it is found to control the amount of gain to the spiral output.

A real-time aerosol-particle monitoring system is developed to measure size, concentration, and UV-laser-induced-fluorescence spectra. Data for atmospheric particles at New Haven, CT are measured over 48-hours and heirarchically cluster into various categories.

A compact sensor for physically sorting bioaerosols based on fluorescence spectra from single particles excited using arrays of ultraviolet light emitting diodes (UV LEDs) is presented. The optical system integrates electronics for real-time processing of spectral data and a miniaturized aerodynamic deflector for particle separation. Fluorescent polystyrene latex spheres are used to demonstrate fluorescence collection on-the-fly, operation of a real-time spectral algorithm, and physical separation of individual particles. This sensor illustrates the utility of recently developed UV LEDs, in conjunction with novel optical design and custom electronics, to shrink the size of aerosol fluorescence detection systems.

Uni-directional emission emerges perpendicularly to the notch of a 2-d spiral-shaped mu-disk and not with 30deg tilted, as is the case for poorer quality spiral-shaped mu-disks. While wanting to preserve the uni-directionality of the spiral, we also wanted to extract as much of the stored energy from the mu-disks at the expense of very high Q-value whispering gallery modes (WGMs). In this paper we present experimental results of directly coupling the flat edge of a spiral mu-cavity with the flat edge at one corner of a semicircle mu-cavity. The uni-directional emission appears normal to the flat edge of the opposite corner of a semicircle. The roles of the spiral as an oscillator and the semicircle as an amplifier are investigated with separate currents pumping the spiral and the semicircle. The competition between lasing modes (with narrow spectral peaks) and broadband amplified spontaneous emission (ASE) in the spiral for the inverted population in the semicircle limits the amplification process. The feedbacks provided by the Fresnel reflection on one side and the reflection from the channel on the coupled side cause the semicircle mu-disk to self oscillate above some current value. The directly coupled 2-element mu-disk provides an interesting system to explore as a means of achieving higher output power from mu-cavities