Search

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.

We demonstrate output wavelength and intensity switching in a three-element directly coupled microdisk device consisting of one spiral microdisk coupled to two semicircle microdisks. The gapless coupling mechanism used allows individual elements to achieve lasing while achieving optimal transfer of optical power between adjacent microdisks. By controlling the transparency of the center element via injection current, the edge elements can be allowed to exchange their amplified spontaneous emission. In this manner, on-off-on switching of the output intensity, as well as discontinuous shifts in the output wavelength, can be achieved as a function of increasing injection current.

Angularly resolved elastic light scattering patterns from individual aerosolized Bacillus subtilis spores were qualitatively compared with simulations. Two-dimensional angular optical scattering patterns of the spores were collected for polar scattering angles varying from approximately 77 degrees to 130 degrees and azimuthal angles varying from 0 degrees to 360 degrees . Computations were performed with single T-matrix formalism by simulating a spore with three different particle shapes: (1) a finite-length cylinder with spherical end caps, (2) a spheroid, and (3) two spheres in contact. Excellent agreement between computation and measurement was found for the finite-length cylinder with spherical end caps, poorer agreement was found for the spheroids, and the poorest agreement was for the two spheres in contact.

Two-dimensional light-scattering patterns from aggregates have undergone feature extraction followed by multivariate statistical analysis. The aggregates are comprised of primary particles of varying shape and size. Morphological descriptors (features) were extracted by a nonlinear filtering algorithm (spectrum enhancement) and then processed by principal component analysis and discriminant function analysis. The analysis was performed on two data sets, one in which the aggregates had a fixed primary particle size but varied in overall dimension and another in which the aggregate size was fixed but the primary particle size varied. Classification of the samples was performed adequately, providing some distinction among the limited classes that were analyzed.

Two-dimensional angular optical scattering (TAOS) patterns of droplets composed of a mixture of H2O and D2O are detected in the mid infrared. First, a lens is used in the Abbé sine condition to collect a small solid angle of light, where the scattering pattern matches well numerical simulations based on Mie theory. Next, TAOS patterns from droplets spanning a large (approximately 27pi sr) solid angle are captured simultaneously at two wavelengths. The effects of absorption are evident in the patterns and are discernible without the need for curve matching by Mie theory.

Lasing has been observed in optically pumped 4-dicyanomethylene-2-methyl-6-( p-dimethylaminostyrl)-4H-pyran-doped poly(methyl methacrylate) square-shaped micropillars that allow four-bounce closed and open ray orbits with internal incident angle theta(inc)theta(c) (the critical angle for total internal reflection) and with the associated surface waves that emit at the four corners. We also detect strongly TE-polarized and spatially varying emission from the square sidewalls that is due to leaky open ray orbits with theta(inc) near but less than theta(c) for two of the four bounces. By selectively pumping the square microcavity with a stripe-shaped beam, we excite different four-bounce ray orbits.

Recent experiments on similarly shaped polymer micro-cavity lasers show a dramatic difference in the far-field emission patterns. We show for different deformations of the ellipse, quadrupole and hexadecapole that the large differences in the far-field emission patterns is explained by the differing ray dynamics corresponding to each shape. Analyzing the differences in the appropriate phase space for ray motion, it is shown that the differing geometries of the unstable manifolds of periodic orbits are the decisive factors in determining the far-field pattern. Surprisingly, we find that strongly chaotic ray dynamics is compatible with highly directional emission in the far-field., Comment: 14 pages, 16 figures (eps), RevTeX 4, submitted to JOSA B

We demonstrate a proof-of-concept optical spectroscopic system for bioaerosol-particle fluorescence detection, in which a pulsed high-power laser is replaced by a highly compact linear array of sequentially fired light from blue light-emitting diodes. The results suggest that low-cost, compact optical aerosol detection may be feasible with the contemporary emergence of efficient UV light-emitting diodes.

±± ± to 360 ± ) was detected from a single aerosol (e.g., a Bacillus subtilis spore, a 1-mm-diameter polystyrene latex sphere, or a cluster of either of these) f lowing through the ref lector’s focal volume at 5 ms. Noticeable differences in the large-angle-range two-dimensional angular optical scattering (LATAOS) suggest that the LATAOS pattern could be useful in differentiating and classifying life-threatening aerosols from normal background aerosols. © 2003 Optical Society of America OCIS codes: 000.1430, 010.1100, 290.5820, 290.5850. Protection from threats of bioterrorism by dispersal of pathogenic aerosols requires advanced detection systems. It is highly desirable to be able to detect and distinguish in situ, continually, and in real time, potentially life-threatening bioaerosols from normal background conditions, especially in the respirable size range of 1 10-mm diameter. 1,2 Currently proposed sensors for such detection are based on laser-induced f luorescence 3,4 ; mass spectrometry 5 ; microchip-based mass, capacitance, and calorimetric transducers 6 ; lidar 7 ; and elastic light scattering. 8–1 4 Among these processes, elastic scattering is the most sensitive technique and has potential for providing size, shape, and surface-texture information on individual particles. Extracting information about aerosols by elastic scattering started as early as the 17th century when Descartes and others explained the rainbow. 15 – 17 In the past century, most research in this area was devoted to the development of computational methods, such as Lorenz–Mie theory, 18 and to one-dimensional angular scattering measurements (see, e.g., Refs. 8 and 9). Such one-dimensional data can hardly supply adequate information with which to characterize nonspherical aerosols with random orientations. Recently attention has necessarily shifted toward natural aerosols, 10 – 12,14 and controlled particles such as clusters of multispheres 14 that model aerosols found in the environment (e.g., pollens, soot aggregates, paper and wood f ibers, sand, and animal dander). The elastic-scattering distribution in the nearforward direction is dominated by diffraction and provides information about particle size and shape, whereas in the near-backward direction it is affected by more subtle particle characteristics such as surface texture, homogeneity, and refractive index. Thus, potentially useful information is contained in the

Measured fluorescence from single-particle clusters of dye-doped polystyrene microspheres, dried nonspherical particles of tryptophan, and single polystyrene microspheres is enhanced in the backward direction (180 degrees from the incident laser). This enhancement (a factor of 2-3 compared to 90 degrees), which can be interpreted as a consequence of the reciprocity principle, increases with the particle refractive index.

Square-shaped two-dimensional optical microcavities (micro-cavities) were investigated for possible applications as filters for dense wavelength-division multiplexing. Multimode cavity resonances were observed in the elastic scattering of approximately 200-microm square-shaped micro-cavities in fused silica. Based on a two-dimensional k-space representation, we accounted for the multimode spectrum by different normal modes with rays confined by total internal reflection. The cavity-mode trajectories need not be closed after each round trip. Single-mode spectra are expected from smaller square-shaped micro-cavities.

Two-dimensional angular optical scattering (TAOS) patterns from clusters of polystyrene latex spheres are measured in the near-forward and near-backward directions. In both cases, the scattering pattern contains a rich and complicated structure that is the result of the interaction and interference of light among the primary particles. Calculations are made for aggregates that are similar to those generated experimentally and also demonstrate the rich structure in the scattering pattern. A comparison of the experimental and theoretical TAOS patterns gives good qualitative agreement.

We demonstrate lasing in liquid pendant droplets through a chemiluminescence process, which uniquely provides spatially uniform pumping throughout the sample. Pendant droplets of 2-mm equatorial radius are formed at the tip of a capillary tube through which the chemiluminescence material is injected. The chemiluminescence spectra along the highlighted rim of the droplet show redshifted intensity enhancement in the wavelength region where the absorption is low. The lasing threshold is found by addition of different amounts of absorbers. The observed nonuniform laser-emission intensity distribution along the droplet rim is caused by a spatially varying rate of diffractive-light leakage related to the droplet surface curvature. Using WKB approximation, we express the diffractive-light leakage rate on a curved surface as an exponentially decreasing function of angle of incidence. The standard laser rate equation with distributed leakage loss is employed to express the laser-emission output intensity from the pendant droplet. The light leakage from the surface was further investigated by localized perturbations formed by poking of the surface with a sharply tipped fiber. The Q of cavity modes as high as 3.5×108 in the pendant droplet was determined from the cavity-lifetime measurement.

Two-dimensional angular optical scattering (TAOS) is recorded for several particle shapes and configurations. A lens is used to collect a large solid angle of the light and transform the angular profile into a planar distribution according to the Abbé sine condition. Qualitative agreement is found between experiment and theory for the TAOS from spheroids having the same aspect ratio but different sizes. A distinctive irregular island structure is observed in the TAOS from clusters of Bacillus subtilis spores and polystyrene latex spheres. The density per solid angle of these islands is found to increase with cluster diameter.

Localized laser-induced electrostrictive distortion to liquid droplets is shown to increase the input coupling of mode-locked laser pulses to optical microcavity resonances. The increase in input coupling for a given pulse is dependent on the number of preceding laser pulses, as well as on their intensities. The electrostrictive distortion locally increases the leakage rate of light from the droplet-cavity modes. For lower input intensities, the localized distortion does not significantly degrade the quality factors of those cavity modes that spatially overlap the distortion. The increased input coupling is demonstrated by a decrease in the input intensity required to generate stimulated Raman scattering in the double-resonance condition with a train of mode-locked laser pulses. The cumulative effect of too many input pulses of too great an intensity ruins the quality factors of the resonances such that there is not sufficient feedback to support the stimulated Raman scattering.

Laser-induced modification of cavity Q's has been achieved in a microdroplet containing a saturable absorber. The elastic-scattering spectra from such droplets for higher incident intensities show that cavity Q's are increased when the absorption is bleached. The lasing spectra from a droplet containing a saturable absorber and laser dye are modified when an intense bleaching field is injected into the droplet cavity after the pump field has initiated the lasing.

Injection seeding of lasing has been achieved in both small (~20-mu; m radius) and larger (~35-mu; m radius) dye-doped microdroplets. Seeding is effective in enhancing the intensity from one lasing mode that normally occurs because of the droplet morphology. Competition between the seeded mode and unseeded lasing modes that have good spatial overlap with the seeded mode prevents lasing on some unseeded lasing modes.

Asymmetric resonant cavities (ARCs) with highly non-circular but convex cross-sections are predicted theoretically to have high-Q whispering gallery modes with very anisotropic emission. We develop a ray dynamics model for the emission pattern and present numerical and experimental confirmation of the theory., Comment: 7 pages LaTeX, 3 postscript figures

We report the design and operation of a prototype conditional-sampling spectrograph detection system that can record the fluorescence spectra of individual, micrometer-sized aerosols as they traverse an intense 488-nm intracavity laser beam. The instrument's image-intensified CCD detector is gated by elastic scattering or by undispersed fluorescence from particles that enter the spectrograph's field of view. It records spectra only from particles with preselected scattering-fluorescence levels (a fiber-optic-photomultiplier subsystem provides the gating signal). This conditional-sampling procedure reduces data-handling rates and increases the signal-to-noise ratio by restricting the system's exposures to brief periods when aerosols traverse the beam. We demonstrate these advantages by reliably capturing spectra from individual fluorescent microspheres dispersed in an airstream. The conditional-sampling procedure also permits some discrimination among different types of particles, so that spectra may be recorded from the few interesting particles present in a cloud of background aerosol. We demonstrate such discrimination by measuring spectra from selected fluorescent microspheres in a mixture of two types of microspheres, and from bacterial spores in a mixture of spores and nonfluorescent kaolin 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.

Spatially resolved nonlinear scattering of a train of picosecond pulses from ethanol microdroplets is investigated. Different time behavior is observed for light with the same and the opposite sense of circulation as the input-coupled light. The forward-circulating light exhibits a smooth, delayed pulse-to-pulse growth, attributed to increased input coupling with time induced by the laser pulses. The backward-circulating light has an irregular time behavior, attributed to near-backward stimulated Brillouin scattering. Increased scattering as a result of CO(2) (compared with He) dissolved gas is demonstrated by an intensity decrease of the spatially precessed radiation on a cavity resonance.

With addition of an absorber to dye-doped microdroplets, an absorption-dependent blue shift in the lasing spectra has been observed. The better the spectral overlap between the absorption and the fluorescence spectra of the dye, the larger the blue shift. The experimental observations are explained by a simple model of dye lasing in an optical cavity that includes losses because of the absorber and radiation leakage.

Dispersive optical bistability in a dielectric sphere is modeled. The analysis is applicable to cases in which the incident frequency is near a morphology-dependent resonance of the sphere. The refractive index of the sphere is assumed to vary as m(r) = m0 + m2I(r), where I(r) is the internal intensity at position r. In general I(r) contains all the modes of the sphere. However, we first obtain a simplified analytical expression for bistability in which we assume that I(r) consists of a single near-resonant mode. We also analyze the bistability problem; in the analysis we include all the modes of the sphere in computing I(r). The agreement between the all-mode and the single-mode results is good when the incident frequency is within a few linewidths of ahigh-Q (>104) mode. We compare bistability in a dielectric sphere with that in a Fabry–Perot cavity. We use a quasi-steady-state approximation to calculate the time-dependent backscattering from a CS2 sphere near a resonance. The computed backscattered intensity has bistable characteristics.

In the geometrical-optics framework, the internal fields near morphology-dependent resonances (MDR) of dielectric spheres are represented by rays undergoing total internal reflection at the sphere surface. The round-trip path length of rays circumnavigating the sphere is used to compute the mode spacing of MDR’s. The Goos–Hanchen shift of the total internally reflected rays at the sphere surface is included in the ray picture to explain the qualitative behavior of the MDR frequency spacing in the Lorentz-Mie formalism for the entire size-parameter (circumference/wavelength) range. The MDR’s are characterized by a radial distance rm′. A connection between the ray picture and the Mie theory is established, based on rm′.

Stimulated resonance Raman scattering (SRRS) of Rhodamine 6G (R6G) has been observed in microdroplets. By increasing the R6G concentration and input-laser intensity, SRRS from R6G can be distinguished from the R6G lasing. SRRS can be further amplified by the lasing gain when the SRRS is near the peak of the lasing gain profile. When the R6G Raman emission is at wavelengths longer than the lasing envelope, SRRS can still be enhanced by seeding from residual R6G fluorescence.

Partial suppression of lasing by intense stimulated Raman scattering of ethanol in a 10−4 M Rhodamine 6G-doped ethanol droplet can be observed at high input-laser intensity when the Raman signal overlaps with the lasing spectrum. Extra stimulated emission of the Rhodamine 6G caused by stimulated Raman scattering of ethanol can saturate the Rhodamine 6G lasing gain to below threshold.

Intensity distributions on the droplet surface and within the droplet are calculated and found helpful in explaining the observed laser-induced radiation leakage from various locations on or within the droplet. In an attempt to distinguish various possible mechanisms for laser-induced perturbation of micrometer-sized droplets, two types of experiment are conducted, and the measured intensity distribution is compared with that predicted by theoretical computation. The observed extra-light leakage of dye-laser radiation trapped in the droplet can be grouped into quasi-instantaneous and cumulative effects. One set of experiments uses a variable number of 100-ps laser pulses because of the laser’s low fluence at 0.1 GW/cm2. Quasi-instantaneous perturbations associated with an intensity-dependent index of refraction change are found to be important. Cumulative perturbations become evident as the number of 100-ps pulses increases. The second set of experiments uses one green laser beam and one near-IR laser beam, both with 7-ns duration, which because of the high fluence emphasize the electrostrictive- and temperature-related perturbations. Extra leakage from surface perturbations caused by the laser-induced temperature rise cannot be isolated but is found not to be dominant.

Temporally and spatially resolved measurements of stimulated Raman scattering from flowing ethanol droplets are presented. The observed temporal oscillations of stimulated Raman scattering from two segments of the droplet rim are 180° out of phase and dependent on the azimuthal mode number of the morphology-dependent resonance (MDR). The observed precession of the MDR about the symmetry axis of an oblate droplet is consistent with the angular momentum of the MDR, n, and with perturbation predictions of the frequency splitting of a (2n + 1)-degenerate MDR of a perfect sphere.

A steady-state model of third-order sum-frequency generation (TSFG) in liquid droplets is presented. A third-order polarization is generated in a droplet by stimulated Raman scattering (SRS) or by SRS in combination with fields at the incident laser frequency. This third-order polarization radiates inside the sphere as described in the general model of Chew et al. [ Phys. Rev. A19, 396 ( 1976)]. The frequency of the third-order polarization will only rarely be within a few linewidths of a high-Q output morphology-dependent resonance (MDR). The TSFG intensity is proportional to the spatial and frequency overlap between the generating polarization and the fields of the output modes. For a distribution of particle sizes, the probability of detecting TSFG increases with the density of output MDR’s. Numerical results are presented for the specific case of the generation of the third harmonic of the first-order SRS. Although the comparison with experiment is only qualitative, the theory is consistent with experimental observations that TSFG in droplets is detectable at only a small fraction of the droplet sizes and is 4–6 orders of magnitude weaker than SRS.

New experimental third-order sum-frequency generation (TSFG) spectra are presented for CCl4 droplets. The droplet radii are discretely tuned over a size range in increments of 1 part in 6000. For each droplet radius only one or a few of the many possible TSFG peaks are dominant. Both low- and high-resolution TSFG spectra of D2O droplets are measured. The possible generating waves are the stimulated Raman radiation [known to occur at the morphology-dependent resonance (MDR) frequencies] and the internal fields at the incident laser frequency, which is usually not on a MDR. The experimental findings suggest that the resultant TSFG frequency must be near an output MDR. A physical description of the third-harmonic generation (THG) process in droplets is presented. MDR’s are treated as traveling waves that rotate in the azimuthal direction. Their azimuthal phase velocities are derived, and the concept of phase matching among the MDR’s is presented. The relation between phase matching and spatial overlap of MDR’s is discussed. Reasons are presented for the low probability of having the THG or TSFG frequency on or near a MDR.

Standard one-dimensional nonlinear-wave equations are modified to accommodate the growth and coupling of nonlinear waves in droplets. The propagation direction of the nonlinear waves along the length of an optical cell is changed so that it is along the droplet rim. The model includes radiation losses because of nonzero absorption, leakage from the droplet, and depletion in generating other nonlinear waves. For multimode-laser input, the growth and decay of the first- through fourth-order Stokes stimulated Raman scattering (SRS) are calculated as a function of the phase matching of the four-wave mixing process and the model-dependent Raman gain coefficient. The Raman gain coefficient determines the delay time of the first-order SRS, while the phase matching determines the correlated temporal profiles of the multiorder SRS. Both the Raman gain and the phase matching are found to be enhanced in the droplet. The spatial distribution of the internal input-laser intensity is calculated by using the Lorenz–Mie formalism. The temporal profile of the input-laser intensity used in the calculations is identical to the experimentally observed laser time profile. The delay time and the correlated growth and decay of nonlinear waves resulting from the numerical simulation compare favorably with those of the experimental observations. Similar calculations are made for single-mode-laser input for which the stimulated Brillouin scattering (SBS) achieves its threshold before the SRS does and subsequently pumps the SRS.

High-resolution interferometric spectra of the stimulated Raman scattering (SRS) spectra from flowing ethanol droplets are presented. The linewidths of the SRS peaks are less than 0.005 cm(-1), and the equal frequency spacings of the SRS peaks are an order of magnitude smaller than the spacings for morphology-dependent resonances of a perfect sphere. The observed results from droplets that are deformed by inertial effects are consistent with T-matrix and perturbation predictions of frequency splitting into the various azimuthal modes of a (2n + 1)-degenerate morphology-dependent resonance with angular momentum n in a perfect sphere.

Temporally and spatially resolved plasma emission and stimulated Raman-scattering profiles of water droplets are simultaneously recorded at two selectable wavelengths corresponding to the emission of the resonant and nonresonant sodium, the plasma continuum, the Balmer hydrogen, and the stimulated Raman scattering of water. The propagation speed of the breakout plasma from the droplet shadow face (in the form of a packet) and from the illuminated face is determined. The relative heating of the atoms in the plasma is extracted from the differences between the emission profiles of sodium and hydrogen, which have different electronic energy levels. The plasma response to the multipulse structure of the input laser radiation provides information on plasma shedding and heating for the subsequent pulses and an additional check of the validity of the one-dimensional electrohydrodynamic model, which includes a multipulse input.

The intensity threshold for stimulated Raman scattering (SRS) with a single-mode laser beam is noted to be ≈3 times lower than that with a multimode beam. The intensity threshold for stimulated Brillouin scattering (SBS) from droplets is lower than that for SRS. The temporal profiles of the laser pulse, SRS, and SBS are simultaneously measured with a streak camera (100-psec resolution). The first SBS pulse always occurs earlier than the first SRS pulse. In addition, the subsequent series of SBS and SRS pulses is temporally correlated; i.e., the minimum of the (n + 1)th SBS pulse occurs when the nth SRS pulse reaches a maximum. The second-harmonic beam of a single-mode or multimode Q-switched Nd:YAG laser is tightly focused at the center of the droplet’s illuminated face in order to avoid excitation of any morphology-dependent resonances of a droplet. We conclude that, for single-mode laser excitation of droplets, the internal SBS pumps the SRS.

Carbon dioxide laser-induced explosive vaporization of water and ethanol droplets at high laser fluence has been observed with time-resolved shadowgraphs. The asymmetry seen in the droplet vaporization can be qualitatively explained by comparison to the internal-field intensity distribution. A central green spot observed in the shadowgraph is attributed to the near-field distribution just outside the shadow face of the droplet when the droplet is illuminated by a visible laser. This spot can be used to probe the shape deformation and optical inhomogeneity of the droplet. The energy dependence of the explosive vaporization of water was also studied. Increasing the CO(2) laser fluence increases the rate of explosive vaporization.

Time profiles of the elastically scattered and stimulated Raman scattered radiation from single ethanol droplets illuminated by 100-psec mode-locked pulses were measured with a streak camera. The Q factor of the droplet, which acts as an optical cavity, was deduced from the decay time of the internally trapped radiation. Based on the intensity dependence of time profiles, it is also deduced that the photon lifetime is limited by the depletion of the internal intensity in generating nonlinear-optical radiation.

Electrostrictive distortions of a flowing water droplet are photographed with a framing camera after the laser pulse is shut off. With medium-energy laser illumination, large asymmetric bulging from the droplet shadow face precedes shape oscillation of the entire droplet. With high-energy laser illumination, the bulge acquires a cylindrical shape that can eject a small droplet. The laser-induced electrostrictive force exerted on a single transparent droplet is compared with the surface-tension force.

Coherent anti-Stokes Raman scattering (CARS) from the pure rotational Raman lines of N(2) gas is employed to measure the instantaneous (10-nsec) rotational temperature of the gas at room temperature and below. An entire rotational CARS spectrum is generated by a single laser pulse using a broad-bandwidth dye laser and is recorded on an optical multichannel analyzer. A best-fit temperature is obtained for individual experimental spectra by comparison with calculated spectra. Good agreement between CARS temperatures and thermocouple temperatures is observed.

Calculations of the electric field distribution within and around an infinite circular dielectric cylinder, having a relative index of refraction of 1.53 and illuminated with plane-wave radiation of wavelength lambda, have been performed for size parameters ranging from 39 to 51. Results for size parameters that satisfy resonance conditions for various atural modes of oscillation of the cylinder (mode number n = 53; order numbers l = 1-5) clearly illustrate the physical significance of n and l and show that the internally reflected circumferential waves are localized near, but not confined to, the cylinder surface.

A new diagnostic technique provides temporally and spatially resolved information at two settable wavelengths within the plasma-emission profile of a single transparent water droplet (40 μm in radius and containing 4 M NaCl) irradiated by a high-intensity laser beam. Data from this technique are compared with those from conventional techniques that separately provide time-averaged spatially resolved spectra or spatially averaged temporally resolved spectra of the plasma emission from droplets.

Angular- and spectral-light-scattering measurements have been made for a glass fiber parallel to and at varying distances from a mirror. Scattering calculations have been made for two parallel fibers. The multiple-scattering effects between a fiber and its mirror image or between two fibers are shown to be quite similar, even for an imperfectly conducting mirror. The multiple scattering becomes less important when one fiber is not in the shadow of the other and when the separation is large. Also, the morphology-dependent resonances shift, and the line shapes change as the separation decreases. In the fiber–mirror configuration, the evanescent fields of the fiber couple to propagating surface plasmons of the metal mirror for TE polarization, greatly damping the scattered intensity.

A new optical technique, based on morphology-dependent peaks in the fluorescence spectra, is-used to determine the evaporation and condensation rates of a linear stream of ethanol droplets. The droplets are monodispersed, in close proximity to one another, and impregnated with fluorescent dye molecules. On irradiation of the droplets with a single N(2) laser pulse, the evaporation or condensation rates can be deduced from the wavelength shift (to the blue or to the red, respectively) of the spectrally narrow (0.1-nm) structure-resonance peaks in the fluorescence spectra.

The electromagnetic intensity is known to be greatly enhanced at the surface of small Ag particles and to be less enhanced for larger Ag particles. Electrodynamic results show that the enhancement for other large metal particles (Au, Pt, Ni, Pd, Rh, and Ir) can be comparable with that for large Ag particles. Specifically, the calculated enhancement for Ag and Au prolate spheroids with a:b = 2 confirms earlier experimental results that show similar enhancements in spite of large differences in the Im ∊(ω). Results for catalytic metals in air and water show that significant enhancements can exist. These electrodynamic results indicate that surface-enhanced Raman scattering should be an effective approach for in situ monitoring of catalytic reactions at the surface of appropriately shaped metallic particles.

Shadowgraph studies of the explosive vaporization of a transparent water droplet after irradiation by a high intensity beam show that dielectric breakdown occurs within the droplet shadow face and generates a dense plasma, which absorbs the laser pulse. The convective forces expel the vapor from the droplet shadow face. We have deduced (1) the vapor propagation velocities, (2) the recoil velocity of the remaining droplet, and (3) the deformation rate of the illuminated face. Droplets are noted to eject fingerlike material from the surface facing the single laser-vaporized droplet when the asymmetrical vapor intercepts the neighboring droplets.

Experimental data on the angular distribution of fluorescence from thick liquid dye layers excited by evanescent waves are found to agree well with Fresnel theory and with an effective thickness formulation. Qualitative agreement of theory with fluorescence data obtained from monodispersed spherical particles having diameters comparable to the wavelength of the incident evanescent radiation and impregnated with dye molecules is also attained. From our results, the optimum incident and observation angles and polarizations to enhance the contrast and SNR for inelastic ATR reemission spectroscopy applied to studying micron or submicron layers of particulates can be predicted.

Inelastic emission characteristics from individual ethanol droplets (60-microm diameter) containing Rhodamine 6G dye and pumped by a cw laser (514.5 nm) were investigated. Laser emission was confirmed by noting the spectral, temporal, and output-versus-input intensity behavior. The liquid-air boundary of the droplets provides the optical feedback at selected wavelengths corresponding to the morphology-dependent resonances of a spherical droplet.