Gang Chen

DIRECTIONAL EMISSION FROM ASYMMETRIC RESONANT CAVITIES

Asymmetric resonant cavities with highly noncircular but convex cross sections are predicted theoretically to have high-Q whispering gallery modes with highly anisotropic emission. We develop a ray dynamics model for the emission pattern and present numerical and experimental confirmation of the theory.

Aerosol Fluorescence Spectrum Analyzer for Rapid Measurement of Single Micrometer-Sized Airborne Biological Particles

ABSTRACT 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 instruments 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...

Aerosol-fluorescence spectrum analyzer: real-time measurement of emission spectra of airborne biological 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.

Laser diagnostics for droplet characterization: Application of morphology dependent resonances

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.

Wavelength variation of laser emission along the entire rim of slightly deformed microdroplets

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.

Frequency splitting of degenerate spherical cavity modes: stimulated Raman scattering spectrum of deformed droplets

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.

CONDITIONAL-FIRING AEROSOL-FLUORESCENCE SPECTRUM ANALYZER FOR INDIVIDUAL AIRBORNE PARTICLES WITH PULSED 266-NM LASER EXCITATION

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.

Conditional-sampling spectrograph detection system for fluorescence measurements of individual airborne biological particles

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 instruments image-intensified CCD detector is gated by elastic scattering or by undispersed fluorescence from particles that enter the spectrographs 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 systems 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.

Pumping of stimulated Raman scattering by stimulated Brillouin scattering within a single liquid droplet: input laser linewidth effects

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.

Fine structures in the angular distribution of stimulated Raman scattering from single droplets

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 (MDRs). 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.