Search

Two-dimensional angle-resolved optical scattering (TAOS) is an experimental method which collects the intensity pattern of monochromatic light scattered by a single, micron-sized airborne particle. In general, the interpretation of these patterns and the retrieval of the particle refractive index, shape or size alone, are difficult problems. The solution proposed herewith relies on a learning machine ( LM ): rather than identifying airborne particles from their scattering patterns, TAOS patterns themselves are classified. The LM consists of two interacting modules: a feature extraction module and a linear classifier. Feature extraction relies on spectrum enhancement , which includes the discrete cosine Fourier transform and non-linear operations. Linear classification relies on multivariate statistical analysis. Interaction enables supervised training of the LM . The application described in this article aims at discriminating the TAOS patterns of single bacterial spores ( Bacillus subtilis ) from patterns of atmospheric aerosol and diesel soot particles. The latter are known to interfere with the detection of bacterial spores. Classification has been applied to a data set with more than 3000 TAOS patterns from various materials. Some classification experiments are described, where the size of training sets has been varied as well as many other parameters which control the classifier. By assuming all training and recognition patterns to come from the respective reference materials only, the most satisfactory classification result corresponds to ≈ 20% false negatives from Bacillus subtilis particles and ≤ 11% false positives from environmental and diesel particles.

We report the development of a Single-Particle Fluorescence Spectrometer (SPFS) system capable of measuring two UV-laser excited fluorescence spectra from a single particle on-the-fly. The two dispersed fluorescence spectra are obtained from excitation by two lasers at different wavelengths (263 nm and 351 nm). The SPFS samples single particles with sizes primarily in the 1–10 µm range. The fluorescence spectra are recorded from 280 nm to 600 nm (in 20 channels) for 263 nm excitation and from 370 nm to 700 nm (in 22 channels) for 351 nm excitation. The elastic scattering (channel 4 and 9) is also recorded for sizing each particle. A time stamp for single particles is marked with a variable time interval resolution from 10 ms to 10 minutes. The SPFS employs a virtual-impactor concentrator to concentrate respirable-sized particles with a resulting (size-dependent) effective flow rate of around 100 liters/min. The SPFS can measure single-particle spectra at a maximum rate of 90,000/sec, although the highest rates we have experienced for the ambient are only several hundred/sec. When the SPFS is combined with an aerodynamic deflector (puffer) to sort particles according to their fluorescence spectral characteristics, the SPFS/puffer system can selectively deflect and collect an enriched sample of targeted particles (at rates limited by the puffer) of 1200 particles/sec, for further examination. In laboratory tests, aerosol particles with similar UV-LIF spectra (e.g. B. subtilis and E.coli) are puffed into the reservoir of a micro-fluidic cell, where fluorescent-labeled antibodies bind to them and were classified by their labeled fluorescence. Measurements of the background ambient aerosol with the SPFS system made at sites with different regional climate (Connecticut, Maryland, and New Mexico) were clustered (unstructured hierarchical analysis) into 8-10 groups, with over 90% of all the fluorescent particles contained within these clusters (threshold dot product=0.9). However, the percentage of aerosols in each profile differed by sampling location. The unique features and performance of the SPFS/puffer system compared to other fluorescence-based bio-aerosol sensors will be discussed, with emphasis on reduction of false alarm rates.

We demonstrate a compact system incorporating a 32-element linear array of ultraviolet (UV) light-emitting diodes (LEDs) to the in-flight fluorescence detection of aerosolized particles. Custom electronics manage a standalone system and enable real-time processing of spectral data, which is used to cue a miniaturized aerodynamic deflector for physical particle separation. This front-end system improves the prospects for many second-stage analysis methods by reducing the background particle burden and providing a suspicious-particle enriched sample. The performance of UV LED arrays as an excitation source is established by the ability to detect emission from NADH and tryptophan in aerosol samples. On-the-fly fluorescence collection, operation of a real-time spectral algorithm, and aerosol concentration is demonstrated by separating particles that exhibit a specific spectral feature from a background of otherwise fluorescing particles.

An aerosol deflection technique based on the single-shot UV-laser-induced fluorescence spectrum from a flowing particle is presented as a possible front-end bio-aerosol/hazardous-aerosol sensor/identifier. Cued by the fluorescence spectra, individual flowing bio-aerosol particles (1-10 um in diameter) have been successfully deflected from a stream of ambient aerosols. The electronics needed to compare the fluorescence spectrum of a particular particle with that of a pre-determined fluorescence spectrum are presented in some detail. The deflected particles, with and without going through a funnel for pulse aerodynamic localization (PAL), were collected onto a substrate for further analyses. To demonstrate how hazardous materials can be deflected, TbCl 3 ⋅6H 2 O (a stimulant material for some chemical forms of Uranium Oxide) aerosol particles (2 μm in diameter) mixed with Arizona road dust was separated and deflected with our system.

The propagation of ultrashort, ultra-intense laser pulses gives rise to strongly nonlinear processes. In particular, filamentation is observed, yielding an ionized, conducting plasma channel where white-light supercontinuum due to self-phase modulation occurs. This supercontinuum, extending from the UV to the IR, is a suitable "white laser" source for atmospheric remote sensing, and especially Lidar (Light Detection and Ranging). Recent significant results in this regard are presented, as well as lightning control using ultrashort laser pulses. The application of ultrashort-pulse lidar to aerosol monitoring is also discussed.

Side-coupling between 2-D micropillars and waveguides that allows a gap spacing larger than the sub-micron range would open opportunities for integrated photonic devices. Two microcavity designs, square and oval, were investigated. The square allows longer interaction length and the oval allows refractive coupling thus larger gap spacing. In the elastic scattering of " 210-micron square microcavity in fused silica, multi-mode cavity resonances were imaged from the square sidewall. Based on a two-dimensional k-space representation, we accounted for the multi modes by different normal modes with trajectories confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. In oval microcavities, the ray trajectories are typically irregular. Light rays that are initially confined by total internal reflection could subsequently escape by refraction (according to Snell's law) in the vicinity of the higher-curvature region. Periodic filter spectrum in the elastic scattering of oval microcavity was observed in specific input and output coupling directions in the far-field. We interpreted the filter spectrum as the multiple-ray interference of the refractively output-coupled cavity modes at the detector.© (2001) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.

We extend our previous work on two-dimensional angular optical scattering (TAOS) patterns to water droplets with inclusions and absorption studies at two different wavelengths. Using an acoustic levitator, the TAOS from water droplets containing polystyrene latex sphere inclusions is observed as the droplet evaporates leaving only a dry aggregate of spheres. Dramatic changes in the TAOS patterns occur even when the droplet still has a large volume fraction of water. Two wavelengths (266 nm and 532 nm) are used to examine the TAOS patterns from droplets containing tryptophan. The effect absorption has on the angular scattering peaks is presented. Such dual wavelength measurements can provide useful information to those working on the inverse problem.

Our group has been developing a system for single-particle fluorescence detection of aerosolized agents. This paper describes the most recent steps in the evolution of this system. The effects of fluorophore concentrations, droplet size, and excitation power have also been investigated with microdroplets containing tryptophan in water to determine the effects of these parameters on our previous results. The vibrating orifice droplet generator was chosen for this study base don its ability to generate particles of well- known and reproducible size. The power levels required to reach saturation and photodegradation were determined. In addition, the collection of fluorescence emission was optimized through the use of a UV achromatic photographic lens. This arrangement permitted collection of images of the droplet stream. Finally, the use of a dual-beam, conditional firing scheme facilitated the collection of improved signal- to-noise single-shot spectra from individual biological particles.

The detection and characterization of micro-particles, particularly airborne biological particles, is currently of great interest. We present a novel technique for recording the 2D angular scattering pattern from a single airborne microparticle. Angular scattering measurements were performed in both the near-forward and near-backward regions for a variety of particles including for ethanol droplets, single polystyrene latex spheres, psl clusters, and clusters of Bacillus subtilis spores, all of various sizes. Because the angular scattering pattern is sensitive to size, shape and refractive index, the angular feature associate with clusters may be used to better characterize such airborne micro-particles. A watershed image processing routine has also been implemented. Through this routine, the number of intensity patches per solid angles is found to increase with cluster diameter.

We report the operation of an aerosol analyzer capable of measuring the fluorescence spectra ofsingle micrometer-size bioaerosol particles. Aerosol particles in an air stream initially traverse a cw488-nm "trigger" laser beam where their elastic scattering and total fluorescence is measured withphotomultipliers. When the elastic scattering and fluorescence signals meet certain criteria, a UV (266nm) "probe" laser is triggered and it illuminates the selected particles. The UV laser-excited spectraof particles are measured with the instrument's image-intensified CCD detecter, gated with signalsfrom the trigger laser. We demonstrate the ability of the instrument to capture the fluorescence ofsingle airborne biological particles. The results suggest that it may be possible to differentiate among biological particles based on their single particle fluorescence spectra. Keywords: Fluorescence, airborne detection, aerosols, spectra. 1 INTRODUCTION A variety of airborne bacteria, pollens, viruses, and proteins are of medical, agricultural, or ecologicalinterest.1'2 Some airborne pollens cause allergies in humans. Bacteria (e.g., Bacillus thuringensis) used

Spectrally and temporally resolved measurements of stimulated Raman scattering (SRS) from flowing ethanol droplets are presented. The equal frequency spacings of the SRS peaks are an order of magnitude smaller than the spacings for morphology-dependent resonances (MDRs) of a perfect sphere. The observed temporal oscillations of the SRS from two segments of the droplet rim are 180 degree(s) out-of-phase and dependent on the azimuthal mode number, m, of the MDR. The observed frequency splitting and precession of the MDR about the symmetry axis of an oblate droplet is consistent with perturbation predictions of the frequency splitting of a (2n + 1)-degenerate MDR of a perfect sphere.

Several nonlinear optical processes concurrently occur in single micrometer-sized droplets that are irradiated by the second-harmonic output of a single-mode Q-switched Nd:YAG laser. The authors review the following observations of stimulated Brillouin scattering (SBS) and stimulated Raman scattering (SRS) in droplets: (1) the image of the droplets that are emitting SBS and SRS; (2) the long delay and decay time of the SRS pulse; (3) the correlated temporal profiles of SBS and SRS pulses; (4) the high-resolution spectra of SBS; (5) the frequency splitting of a degenerate spherical normal mode in the SRS spectra; and (6) the fine structures in the angular distribution of SRS.© (1991) COPYRIGHT SPIE--The International Society for Optical Engineering. Downloading of the abstract is permitted for personal use only.