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We demonstrate an experimental approach that provides insight into how particle size and shape affect the scattering phase function of atmospheric aerosol particles. Central to our approach is the design of an apparatus that measures the forward and backward scattering hemispheres (scattering patterns) of individual atmospheric aerosol particles in the coarse mode range. We captured over 30 000 scattering patterns during winter (January 2007) at an urban site in Las Cruces, NM. The size and shape of each particle is discerned from the corresponding scattering pattern. In particular, autocorrelation analysis is used to differentiate between spherical and non-spherical particles, the calculated asphericity factor is used to characterize the morphology of non-spherical particles, and the integrated irradiance is used for particle sizing. We found that the fraction of spherical particles decays exponentially with particle size, decreasing from 11% for particles on the order of 1 μm to less than 1% for particles over 5 μm. The average phase functions of subpopulations of particles, grouped by size and morphology, are determined by averaging their corresponding scattering patterns. The phase functions of spherical and non-spherical atmospheric particles are shown to diverge with increasing size. In addition, the phase function of non-spherical particles is found to vary little as a function of the asphericity factor. Our results support the current remote sensing practice of characterizing atmospheric aerosol particles as a composition of spherical and non-spherical particles with less concern about the diversity of morphology within non-spherical particles. In addition, our results suggest that assuming a constant spherical fraction independent of particle size may not accurately reflect the real morphological distribution of atmospheric aerosol particles.

Fluorescence spectra from individual micron-sized atmospheric aerosol particles were measured by a Dual-wavelength-excitation Particle Fluorescence Spectrometer (DPFS). Particles were drawn into our laboratory at Adelphi, MD, an urban site in the Washington, DC, metroplex and within the Yale University campus at New Haven, CT. Two fluorescence spectra were obtained for every individual particle as it was moving through the DPFS system and excited sequentially by single laser pulses at 263 and 351 nm. There were around ten to a few hundred particles detected per second and up to a few million per day within the 1–10 μm particle size range. The majority of the particles have weak fluorescence, but 10–50% of the particles have fluorescence signals above the noise level at both sites at different time period. For the first time, these Ultra Violet laser-induced-fluorescence (UV-LIF) spectra from individual particles were integrated every 10 min, which forms a group of about a few thousand to a few tens of thousand particles, to provide the averaged background atmospheric fluorescence spectral profiles which may be helpful in the development of bioaerosol detection systems, particularly those systems based on integrated fluorescence from a group of aerosol particles, such as Light Detection And Rangeing (LIDAR) remotor biosensor and the point sensor based on collected particles on substrate. These integrated spectral profiles had small variations from time to time and were distinguishable from that of the bacterial simulant B. subtilis. Also for the first time, the individual spectra excited by a 351 nm laser were grouped using unstructured hierarchical cluster analysis, with parameters chosen so that spectra clustered into 8 main categories. They showed less spectral variations than that excited by a 263-nm laser. Over 98% of the spectra were able to be grouped into 8 clusters, and over 90% of the fluorescent particles were in clusters 3–5 with a fluorescence emission peak around 420–470 nm; these were mostly from biological and organic carbon-containing compounds. Integrated fluorescence spectral profiles and averaged spectra for each cluster show high similarity between New Haven, CT and Adelphi, MD.

We incorporate optics and an ICCD to record the two-dimensional angular optical scattering (TAOS) patterns retrieved from single aerosolized cells. We analyze these patterns by performing autocorrelations and demonstrate that we are able to retrieve cell size from the locations of the secondary maxima. Additional morphological information is contained in the autocorrelation functions and decay rate of the heights of the autocorrelation peaks. We demonstrate these techniques with C6 and Y79 cells, which are readily distinguishable. One key advantage of this methodology is that there is no requirement for antibody and fluorescent labeling molecules. Published 2011 Wiley-Liss, Inc.

Fluorescence spectra of aerosolized pollen (12 species or subspecies) and fungal (4 species) samples measured using a Dual-excitation-wavelength Particle Fluorescence Spectrometer (DPFS) are reported. The DPFS records two fluorescence spectra and two elastic scattering intensities from single airborne particles as they flow through an optical cell. Each particle is illuminated sequentially, first with a pulse from a 263-nm wavelength laser, followed by a second pulse from a 351-nm laser. The two dispersed fluorescence spectra (from 280 nme700 nm) and elastic scattering intensities are measured by a single 32-anode photomultiplier tube. Principal component analysis (PCA) of the spectra is used to examine differences in the spectra. Plots of several principal components show that samples of pollens and fungal materials can largely be differentiated. These preliminary results suggest that fluorescence spectra and elastic scattering may be useful for real-time discrimination among a variety of airborne pollens, fungal materials, and other airborne particles.

The relation between the surface roughness of aerosol particles and the appearance of island-like features in their angle-resolved elastic-light scattering patterns is investigated both experimentally and with numerical simulation. Elastic scattering patterns of polystyrene spheres, Bacillus subtilis spores and cells, and NaCl crystals are measured and statistical properties of the island-like intensity features in their patterns are presented. The island-like features for each class of particle are found to be similar; however, principal-component analysis applied to extracted features is able to differentiate between some of the particle classes. Numerically calculated scattering patterns of Chebyshev particles and aggregates of spheres are analyzed and show qualitative agreement with experimental results.

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.

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.

We demonstrate the use of linear arrays of LEDs in the 340 nm wavelength range as a source for fluorescence excitation of airborne particles containing a biological molecule NADH. Array design is tailored to maximize transfer efficiency to a moving particle stream and includes packaging details such as an integrated sapphire lens that increases backside light extraction by a factor of 3. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Elastic light scattering from a single non-spherical particle of various morphologies has been measured simultaneously with a large angular range (90° < θ < 165° and 0° < < 360°) and with high angular resolution (1024 pixels in θ and 512 pixels in ). Because the single-shot laser pulse is short (pulse duration of 70 ns), the tumbling and flowing particle can be treated as frozen in space. The large angle two-dimensional angular optical scattering (hereafter referred to as LA TAOS) intensity pattern, I(θ,), has been measured for a variety of particle morphology, such as the following: (1) single polystyrene latex (PSL) sphere; (2) cluster of PSL spheres; (3) single Bacillus subtilis (BG) spore; (4) cluster of BG spores; (5) dried aggregates of bio-aerosols as well as background clutter aerosols. All these measurements were made using the second harmonic of a Nd:YAG laser (0.532 μm). Islands structures in the LA TAOS patterns seem to be the prominent feature. Efforts are being made to extract metrics from these islands and compare them to theoretical results based on the T-matrix method.

Optical-based, real-time bioaerosol sensors are able to distinguish bioaerosols and nonbioaerosols but are unable to classify pathogens or even discriminate harmful from harmless bacteria. Biochemical techniques can identify genus and species of bacteria but cannot do so efficiently due to time limitation and the very high concentration of background aerosols. We report for the first time an efficient aerosol sorter consisting of an aerodynamic deflector that is cued from bioaerosol fluorescence. Bacillus subtilis (anthrax stimulant) aerosol was successfully separated from mixed aerosols. We achieved enrichment of the suspected bioaerosol concentration by at least 103, thus leading us new perspectives in pathogen identification. Although, at the present stage, a complete characterization of bioaerosols in real time is not yet possible, this new early warning/alarm sensor can supply high concentration of suspect bioaerosol particles for further specific analysis via bio-chemical assay technology or other s...

We measured laser-induced fluorescence spectra from individual supermicron-sized atmospheric particles drawn into our laboratory at Adelphi, MD, an urban site in the Washington, DC metroplex. A virtural impactor concentrator is used along with an aerodynamic-focusing-nozzle which forms, within an optical chamber, a focused aerosol jet where single aerosol particles can be interrogated on-the-fly with a pulsed 266-nm-wavelength laser. Sample rates are a few liter per minute, and are size dependent. Crossed-diode laser beams indicate when a particle is traversing the sample region and are used to trigger the UV laser to fire and the gated intensified CCD to record the fluorescence spectrum. Our breadboard fluorescence particle spectrometer measures particles in the 3–10 μm diameter size range. Typical trigger rates are a few per second. The usable spectral range is from about 295 to 605 nm. The majority of the particles have very weak fluorescence (on average 8 percent of particles have fluorescence signals above noise). The spectra were grouped using a heirarchical cluster analysis, with parameters chosen so that spectra typically cluster into 4–12 main categories. From the set of all cluster spectra we chose 8 template spectra for reanalyzing all the data. On average, 92 percent (81–94 percent) of the spectra were similar to these templates (using the same thresholds used for the cluster analysis). The major emission bands of the most commonly occurring spectra have peaks: near 460 nm (28 percent of fluorescent particles on average), a very broad hump, and may be humic acids or humic like substances; near 317 nm (on average 24 percent of fluorescent particles); near 321 and 460 nm (a double hump, 12 percent of fluorescent particles); and near 341 nm (8 percent of fluorescent particles). Some of the fluorescence in spectra peaking in the 317–341 nm range is probably from dicyclic aromatics and heterocyclics, including the amino acid tryptophan in biological particles such as bacteria and spores.

A fluorescence particle spectrometer (FPS) for real-time measurement of the fluorescence spectra of aerosol particles in the size range 1-10 w m diameter is reported. The prototype FPS has a sufficiently high sample rate (from 5 to 28 l/min for 3.5 w m to 11 w m diameter particles) to measure aerosol within buildings at practical rates (from 1 up to 600 particle fluorescence spectra per minute). Previously reported bioaerosol prototype detectors for measurement of single particle spectra (Pan et al., Opt. Lett ., 24, 116-118 (1999); Hill et al., Field Anal. Chem. Tech ., 3, 221-239 (1999)) were unable to sample the ambient environment; air containing particles had to be forced under pressure into a sample cell. In addition, sample rates were so small (less than 0.01 l/min) as to be impractical for most applications. The present design overcomes these deficiencies by the use of an airtight cell that highly concentrates micrometer-sized particles. A virtual impactor first concentrates aerosol particles, whi...

We have investigated, both theoretically and experimentally, multiphoton-induced processes in aerosol particles using femtosecond laser pulses. More specifically, we have demonstrated that both multiphoton (1, 2 and 3 photon)-induced fluorescence (MPEF) and laser-induced breakdown (LIB) emissions are strongly enhanced in the backward direction. The backward enhancement increases from 1.8 to 35 (emission ratio between the backward direction and 90°) with increasing non-linear process order n. Application to non-linear lidar of biological aerosols is discussed.

An unexpected blue-fluorescence band (around 420 nm) from both micrometer-sized dried particles and aqueous droplets of riboflavin [7,8-dimethyl-10-(D-1′-ribityl)-isoalloxazine] is observed when the microparticles are irradiated with a pulsed UV (355- or 351-nm) laser. The intensity of the band increases quadratically with input laser energy density (fluence) and is attributable to a one-photon-excited fluorescence of lumichrome (7,8-dimethyl-alloxazine) that is produced by photo-degradation of riboflavin. The well-known greenish-yellow fluorescence band (at 560 nm for dried particles and 535 nm for aqueous droplets) from riboflavin increases sublinearly with UV-laser fluence. With a laser input fluence above 5 J/cm2 the riboflavin fluorescence decays earlier and the lumichrome fluorescence reaches a maximum later than the peak of the input laser pulse. The temporal dynamics of the 420- and 535-nm fluorescence peaks are consistent with a rate-equation simulation of photon-induced conversion of riboflavin to lumichrome and the subsequent fluorescence of lumichrome.

Improved real-time methods for characterizing airborne biological particles are needed. Here we review our efforts in developing techniques for measuring the laser-induced fluorescence (total and spectrally dispersed) of individual airborne particles, and describe our present system, which can measure fluorescence spectra of single micrometer-sized bioaerosol particles with good signal-to-noise ratios. We demonstrate the capability of this system by showing measured spectra of a variety of airborne particles generated in the laboratory from road dust, ammonium sulfate, Bacillus subtilis and other bacteria prepared under various conditions, allergens, cigarette smoke, and chicken-house dust. These spectra illustrate the capability of the system to distinguish between some biological and nonbiological aerosols, and among several types of laboratory-generated biological aerosols. We suggest improvements needed to make our system field portable. © 1999 John Wiley & Sons, Inc.* Field Analyt Chem Technol 3: 221–239, 1999

We report the operation of an aerosol analyzer capable of measuring the fluorescence spectra of single micrometer-sized bioaerosol particles as they flow through the instrument. Particles entrained in an airstream initially traverse a cw (continuous wave) (488 nm) “trigger” laser beam where their elastic scattering and total fluorescence is measured with photomultipliers. When the elastic scattering and/or fluorescence signals exceed preset levels, a 266 nm UV “probe” laser is triggered to fire and illuminate preselected particles downstream from the trigger laser. The UV laser-excited spectra of particles are measured with the instrument's image-intensified CCD detector that is gated to be on when the probe laser fires. We demonstrate the ability of the instrument to capture the fluorescence spectra of single micrometer-sized airborne biological particles. Such spectra should be useful in differentiating between biological and nonbiological aerosols and in partially characterizing airborne biolo...

Second harmonic generation (SHG) has been observed for the first time from the surface of hanging water droplets coated with surfactant molecules. SHG from 4-(4-dimethylaminostyryl)-octadecylpyridinium bromide surfactants is observed in both the specularly reflected and refracted transmitted directions and suggests that the molecules are not lying completely flat at the water–air interface. A fluorescence imaging technique is used to verify that rod-like surfactants are lying approximately flat on the surface. Decreasing the size of the hanging droplet results in an increase in surfactant density and SHG, similar to Langmuir–Blodgett film techniques used on flat liquid surfaces.

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.

A micron-sized liquid droplet acts as a very high-quality factor optical cavity. The cavity modes are referred to as morphology-dependent resonances (MDRs), which are sensitive to the droplet size, shape, and inclusions. The MDRs greatly lower the input intensity needed to generate detectable amounts of linear and nonlinear optical radiation and exhibit sharp peaks in the spectra of elastic scattering, fluorescence, lasing, and stimulated Raman scattering. Novel spectroscopic-based techniques are now available for droplet diagnostics of its physical, chemical, and thermal properties. In particular, we will review how MDR related spectroscopy can be used to deduce the droplet size, shape, evaporation rate, surface tension, viscosity, near-surface temperature, species, and species concentration in multicomponent droplets.

A spiral-shaped microcavity heterojunction laser diode fabricated with InGaN multiple quantum wells is demonstrated to operate under current injection conditions and emit unidirectionally. Room-temperature laser operation was achieved for microcavity disk radii ranging from 50 to 350 μm and threshold current densities as low as 4.6 kA/cm2. Unidirectional laser emission is clearly revealed in the far-field pattern with the lateral divergence angle ranging from 60° to 75°. Output power of more than 25 mW was obtained for emission wavelengths near 400 nm.

Measurement of two-dimensional angle-resolved optical scattering (TAOS) patterns is an attractive technique for detecting and characterizing micron-sized airborne particles. In general, the interpretation of these patterns and the retrieval of the particle refractive index, shape or size alone, are difficult problems. By reformulating the problem in statistical learning terms, a solution is proposed herewith: rather than identifying airborne particles from their scattering patterns, TAOS patterns themselves are classified through a learning machine, where feature extraction interacts with multivariate statistical analysis.Feature extraction relies on spectrum enhancement, which includes the discrete cosine Fourier transform and non-linear operations. Multivariate statistical analysis includes computation of the principal components and supervised training, based on the maximization of a suitable figure of merit. All algorithms have been combined together to analyze TAOS patterns, organize feature vectors, design classification experiments, carry out supervised training, assign unknown patterns to classes, and fuse information from different training and recognition experiments. The algorithms have been tested on a data set with more than 3000 TAOS patterns. The parameters that control the algorithms at different stages have been allowed to vary within suitable bounds and are optimized to some extent.Classification has been targeted at discriminating aerosolized Bacillus subtilis particles, a simulant of anthrax, from atmospheric aerosol particles and interfering particles, like diesel soot. By assuming that all training and recognition patterns come from the respective reference materials only, the most satisfactory classification result corresponds to 20% false negatives from B. subtilis particles and

The highest Q optical resonators known are dielectric microspheres in which the high Q modes are created by total internal reflection of light circulating just inside the surface of the sphere[1, 2]. These high Q modes are referred to as “whispering gallery” (WG) modes or alternatively as “morphology-dependent resonances” (MDR’s) [2]. If the dielectric is a liquid droplet containing an appropriate dye then the droplet acts as a high Q micro-resonator to support lasing action of the dye when optically pumped [3]. The resonance properties of an ideal spherical dielectric, for which the wave equation separates, are described by Mie theory where the quasi-modes are the product of spherical Bessel functions jl(nkr) (n is the index of refraction) and vector spherical harmonics [4]. The radial equation then contains a repulsive term l(l + 1)/r2 which is the analogue of the angular momentum barrier for light rays and an effectively attractive term associated with the higher index of refraction in the liquid. The combination of the attractive “well” represented by the dielectric and the repulsive angular momentum barrier gives rise to quasi-bound states of the effective potential near the rim of the droplets[4] for certain ratios of l to kR (k is the wavevector in vacuum, R the radius of the spherical droplet). In the ideal sphere these resonances are only broadened by evanescent leakage (“tunneling”), hence their enormous Q values.

We have observed that in a binary mixture microdroplet, stimulated Raman scattering (SRS) from the highest-gain mode of the majority species pumps the SRS from the minority species and a weaker-gain mode of the majority species more efficiently than does the input laser. The spatial overlap between the pumping field and the SRS field is an important factor in determining the pumping efficiency. We have numerically studied the effect of spatial overlap. Qualitative discussions are made of the effects of some other factors, such as concentration and Raman gain of the majority species.

By using prism coupling to a square μ cavity, over 75% energy transfer to the whispering gallery modes (Q>106) by frustrated total internal reflection was observed. A good comparison was found between measured data and theoretical model (based on a Fabry–Perot resonator), for the dependence of the coupling and linewidth of the whispering gallery modes (WGMs) as a function of relative separation between the prism and the μ cavity. By selecting the incident angle at the prism interface, the coupling to all four different sets of WGMs can be changed to coupling to only one set of WGMs.

Two-dimension, angle-resolved optical scattering ( TAOS ) is an experimental technique by which patterns of LASER light intensity scattered by single (micrometer or sub-micrometer sized) airborne particles are collected. In the past 10 years TAOS instrumentation has evolved from laboratory prototypes to field-deployable equipment; patterns are collected by the thousands during indoor or outdoor sampling in short times. Although comparison between experimental and computed scattering patterns has been carried out extensively, there is no satisfactory way to relate a given pattern to the particle it comes from. This paper reports about the ongoing development and implementation of a method which is aimed at classifying patterns, rather than identifying original particles. A machine learning algorithm includes the extraction of morphological features and their multivariate statistical analysis. A classifier is trained and validated in a supervised mode, by relying on patterns from known materials. Then the tuned classifier is applied to the recognition of patterns of unknown origin.

Spectral broadening of the input-laser line and the stimulated Raman line have been observed in microdroplets of carbon disulfide, benzyl alcohol, toluene, chloroform and a binary mixture of carbon disulfide in ethanol. The broadening is asymmetrical and extends more than 500 cm −1 towards the low-frequency side of the input-laser line and the stimulated Raman lines. The broadening is proposed to arise from multiple-order stimulated low-frequency scattering associated with librational and dipole—induced-dipole translational motions.

Two-dimensional fluorescence, lasing, and stimulated Raman scattering images of a hollow-cone nozzle spray are observed. The various constituents of the spray, such as vapor, liquid ligaments, small droplets, and large droplets, are distinguished by selectively imaging different colors associated with the inelastic light-scattering processes.

An improved Dual-wavelength-excitation Particle Fluorescence Spectrometer (DPFS) has been reported. It measures two fluorescence spectra excited sequentially by lasers at 263 nm and 351 nm, from single atmospheric aerosol particles in the 1-10 mum diameter size range. Here we investigate the different levels of discrimination capability obtained when different numbers of excitation and fluorescence-emission wavelengths are used for analysis. We a) use the DPFS to measure fluorescence spectra of Bacillus subtilis and other aerosol particles, and a 25-hour sample of atmospheric aerosol at an urban site in Maryland, USA; b) analyze the data using six different algorithms that employ different levels of detail of the measured data; and c) show that when more of the data measured by the DPFS is used, the ability to discriminate among particle types is significantly increased.

Recent experiments on the laser-induced breakdown (LIB) of large transparent liquid droplets are reviewed. A physical model of LIB processes is presented with the aim of integrating the following recent results: (1) the internal and near-field distributions for large transparent spheres; (2) the location of LIB initiation based on spatially resolved plasma emission spectroscopic techniques; (3) spatially resolved but time-averaged density of the plasma plumes and temperature of the atomic species within the plasma; (4) the plasma front propagation velocities inside and outside the droplet; and (5) the fate of the remaining superheated droplet and the expelled material.

Spatial distributions of the near-field and internal electromagnetic intensities have been calculated and experimentally observed for dielectric cylinders and spheres which are large relative to the incident wavelength. Two prominent features of the calculated results are the high intensity peaks which exist in both the internal and near fields of these objects, even for nonresonant conditions, and the well-defined shadow behind the objects. Such intensity distributions were confirmed by using the fluorescence from iodine vapor to image the near-field intensity distribution and the fluorescence from ethanol droplets impregnated with rhodamine 590 to image the internal-intensity distribution.

External seeding of stimulated Raman scattering (SRS) in microdroplets is achieved by light injection into the droplet at the Raman-shifted frequency. Two implementations permit observation of the seeding effect. First, a broadband seed laser (∼2 nm FWHM) is used to spectrally discriminate the elastically scattered seed light from the narrow-linewidth SRS. Second, a crossed analyzer is used to reduce by several orders of magnitude the intensity of the detected elastically scattered seed laser, preventing saturation of the detector. Seeding of SRS in microdroplets is effective in enhancing the signal from weak-gain Raman modes but does not significantly increase the SRS intensity from strong-gain Raman modes.

We report the operation of an aerosol-fluorescence spectrum analyzer capable of selectively measuring the fluorescence spectra of single micrometer-sized aerosol particles as they flow through the instrument. As the particle first traverses a cw 488-nm probe laser beam, the total fluorescence and elastic scattering are measured with photomultipliers. When the photomultiplier output levels meet preset logic conditions, a UV laser (at 266 nm) is fired and the particle fluorescence spectrum is recorded. Fluorescence spectra of biological airborne particles are presented. The ability of the analyzer to capture the fluorescence spectrum of one type of particle while ignoring others, based on the particle characteristics, is also demonstrated.

The lasing spectra from a Rhodamine B–doped microdroplet (85% isopropanol and 15% water) with a thermochromic additive (CoCl2.6H2O) have been observed to shift progressively toward blue as the droplet temperature is gradually increased. The absorption of the thermochromic additive increases as the temperature of the solution is increased. The mechanism of temperature-dependent absorption is reviewed. We propose that the temperature-dependent shifting of the lasing spectra might be used for the determination of the droplet temperature.

A ray-optics model is developed to describe the spoiling of the high-Q (whispering gallery) modes of ring-shaped cavities as they are deformed from perfect circularity. A sharp threshold is found for the onset of Q spoiling as predicted by the Kolmogorov-Arnol'd-Moser (KAM) theorem of nonlinear dynamics. Beyond the critical deformation b(c), Q ~ (b - b(c))(-alpha), alpha asymptotically equal to 2.4-2.6. The escaping light emerges in certain specific directions, which may be predicted.

The frequencies of the normal azimuthal modes of a slightly deformed droplet that is axisymmetric about its flow direction are no longer degenerate but vary with position along the droplet rim. We measured the wavelength variation along the entire rim of a dye-lasing droplet with a spectrograph and a CCD array. We determined the amplitude and the shape of the droplet deformation from the observed and predicted parabolic dependence of wavelength variation with distance along the spectrograph slit.

Lasing is observed in laser-dye-doped ethanol droplets after two-photon absorption by the dye molecules. The two-photon-pumped lasing emission by the droplets is at a higher frequency than the input laser. Competitive nonlinear-optical effects that occur at high input-laser intensity are discussed.

When micrometer-sized CS2 and CCl4 droplets are illuminated with multiple 100-ps-duration green laser pulses, which are separated by 13.2 or 171.6 ns, the first few scattered green pulses are weaker than the subsequent pulses. Our data indicate that the Brillouin amplification during a given input-laser pulse is affected by the retention of acoustic phonon amplitude that is generated by the preceding input-laser pulses. Our results suggest the retention of long-lived acoustic phonons generated by the near-forward stimulated Brillouin scattering in droplets.

Fluorescence seeding by dissolving a fluorescent dye in a liquid droplet enhances the stimulated Raman scattering of a weaker-gain Raman mode. With the dye fluorescence at the Stokes wavelength of the weaker-gain Raman mode, the weaker-gain Raman mode can now build up from the much stronger fluorescence instead of from the weaker spontaneous Raman scattering. With fluorescence seeding, the weaker-gain C-C-O Raman mode of ethanol can be observed at a much lower input-laser intensity and can be more intense than the stronger-gain C-H Raman mode.

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.

Stimulated anti-Stokes Raman scattering (SARS) generated by one input beam is observed from CCI(4), ethanol, and water droplets. The first-order SARS intensity is approximately 10(4) times lower than the first-order stimulated-Raman-scattering (SRS) intensity for ethanol droplets. Simultaneous detection of SARS and SRS for water droplets shows an occasional lack of correlation between the SARS and SRS spectra.

The angular distribution of the stimulated Raman scattering (SRS) from ethanol droplets is observed to be sinusoidal with n peaks, which are consistent with the mode number n of the morphology-dependent resonances (MDR’s). Two distinct values of n depend on the illumination geometry and the linewidth of the input laser radiation. Angular fine structures of the SRS can be used to identify the n of the MDR.

The spectrum of the third-order sum-frequency generation (TSFG) that results from the electric fields of the laser E(omega(L)) and of the first-order Stokes stimulated Raman scattering E(omega(1s)) is dependent on the droplet radius. In addition to the amplitude and spatial overlap of the three generating E(omega)'s and of the generating and resultant waves, phase matching between the generating and TSFG waves is an important parameter. We discuss the range of phase velocities of the generating and TSFG waves, which must be tuned to coincide with spherical resonances in order to be guided waves circumnavigating the droplet rim.

Distortion, ejection, shattering, and propulsion of water and ethanol droplets containing Rhodamine 6G dye have been photographed at different time delays after initiation of a CO(2) laser pulse, whichcauses explosive vaporization of the droplets. We have developed a fluorescence imaging technique to photograph the liquid-phase portion of the ejected material and the parent droplet after irradiation by the CO(2) laser pulse.

The temporally and spatially resolved transmission is measured with a green probe beam through the plasma created by an intense near-IR beam that is incident upon a water droplet. We report the propagation speed of the expanding plasma and the evolution of the transmission decrease and recovery at various locations along a line in the direction of the IR beam.