Jay D. Eversole

Multiple UV wavelength excitation and fluorescence of bioaerosols

A two-wavelength excitation bioaerosol sensor has been developed and characterized for classifying various types of aerosols, including biological organisms and non-biological interferents. Single aerosols, smaller than 10 μm, are interrogated with 266 nm and 355 nm laser pulses separated in time by 400 ns. Fluorescence signals excited by these pulses are detected in three broad spectral bands centered at 350 nm, 450 nm and 550 nm. The results indicate that bacterial spores, vegetative bacterial cells and proteins can be differentiated based on the two wavelength excitation approach.

Vibrating orifice droplet generator for precision optical studies

A special purpose vibrating orifice droplet generator is described possessing improved short‐term monodispersity (instantaneous diameter fluctuations of 2×10−5 and a differential drift of 10−5/min.). We demonstrate that this is sufficient to allow cw laser excitation of specific morphology‐dependent resonances (MDRs). Improved performance results from (1) the liquid sample being direct pressure fed to the vibrating orifice from a closed pressure reservoir rather than by a conventional gear driven syringe pump, and (2) the vibrating orifice is driven by a periodic square wave voltage source having a frequency which is four orders of magnitude more constant (1 part in 108 per day) than sources normally used. A novel variational size spectroscopy is also described that is made possible by programmed frequency ramping of the voltage source driving the vibrating orifice. By monitoring elastic scattering during ramped size changes we have been able to infer precise values of droplet index of refraction (to ±0.0...

Size and Fluorescence Measurements for Field Detection of Biological Aerosols

This paper describes an instrument developed to monitor the biological fraction of an aerosol. The instrument simultaneously sizes individual particles in a flowing air stream and measures their total fluorescence following excitation at 266 nm. Laboratory data show that these two parameters enable discrimination between individuals of certain bacterial species. Field measurements are presented in which bacterial samples were aerosolized and subsequently detected 800 m downwind.

Absorption effects on microdroplet resonant emission structure.

The effect of absorption on microdroplet resonance emission line intensities was studied in 15-microm-diameter Rhodamine 6G/ethanol solution droplets. Absorption was controlled by varying the concentration of the additive nigrosin. Spectrally integrated intensities of resonant features are found to be proportional to a droplet cavity mode efficiency Q(a)/(Q(a)+Q(o)) expressed in terms of cavity output coupling and absorption factors Q(o) and Q(a), respectively. These Qs are determined from linewidths calculated from Lorenz-Mie theory by using combinations of the real and complex indices of refraction. An experimental upper limit of Q for first-order modes was determined to be 10(8) from the data.

Characterizing and monitoring respiratory aerosols by light scattering

The elastic-scattering intensity pattern from a single particle as a function of spherical coordinate angles theta and phi provides detailed information on the patterns morphology. By use of an ellipsoidal reflector and a CCD camera, a single-laser-shot intensity pattern from a large angular range (theta from 90 degrees to 168 degrees and phi from 0 degrees to 360 degrees) was detected from a single aerosol (e.g., a Bacillus subtilisspore, a 1-microm-diameter polystyrene latex sphere, or a cluster of either of these) flowing through the reflectors focal volume at 5 m/s. Noticeable difference 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.

Continuous-wave stimulated Raman scattering in microdroplets.

Continuous-wave stimulated Raman scattering was observed in 11-13-microm-diameter benzene and toluene microdroplets at pump intensities as low as 8 and 24 kW/cm(2), respectively. Low thresholds were achieved by exploiting simultaneous pump and Stokes wave resonance in the droplets and Raman gains that were cavity QED enhanced ~50 times with respect to bulk liquid values. Based on a photon-state conservation argument, the cavity gain enhancement factor may be approximated by the ratio of the spectral spacing between resonant modes of the same order to that of the homogeneous Raman linewidth. This relation appears to be consistent with the relative experimental behavior of benzene, ethanol, and toluene.

Aerosol Characteristics in a Subway Environment

This paper presents an attempt to characterize biological and nonbiological aerosols in a subway environment. This opportunity to study a subway station atmosphere was approached as a collaboration of different organizations within the Department of Defense (DoD) and a suite of instruments was assembled for real-time monitoring, sample collection, and subsequent sample analysis. Based on ultraviolet (UV) fluorescence, aerosols of a biological nature were found to comprise a small fraction of the total aerosols (typically <1%). The total number concentration of aerosols exhibits a diurnal cycle that depends on the station usage. Several bacterial species were identified using polymerase chain reaction (PCR) analysis. The most common element in the aerosols is iron. Sodium chloride is also prevalent in the aerosol mass.

Internal scattering effects on microdroplet resonant emission structure.

The effect of internal scattering on resonant emission structure was studied in 20-μm-diameter droplets. Internal scattering was controlled by varying the concentration of 87-nm-diameter polystyrene spheres in a Rhodamine 6G/water/ethanol solution. Results are interpreted in terms of a scattering-induced output coupling enhancement and consequent cavity Q degradation.

Spectral characterization of biological aerosol particles using two-wavelength excited laser-induced fluorescence and elastic scattering measurements

A two-wavelength laser-induced fluorescence (LIF) instrument has been developed and used to characterize individual biological aerosol particles, including biological warfare (BW) agent surrogates. Fluorescence in discrete spectral bands from widely different species, and also from similar species under different growth conditions were measured and compared. The two-wavelength excitation approach was found to increase discrimination among several biological materials, and especially with respect to diesel exhaust particles, a common interferent for LIF BW detection systems. The spectral characteristics of a variety of biological materials and ambient air components have been studied as a function of aerosol particle size and incident fluence.

Cavity-mode identification of fluorescence and lasing in dye-doped microdroplets

Recent advances in an aerosol-generation technique have permitted the accurate identification of optical resonance-mode features of micrometer-sized freely falling droplets for several different optical processes. Both input and output resonant features of fluorescence and lasing from dye-doped microdroplets were assigned to specific spherical cavity modes by using two independent procedures: (1) by matching observed fixed-angle elastic laser light scattering as a function of droplet size to calculated scattering intensities from the Lorenz-Mie theory, and (2) by matching observed resonance peaks to computed cavity-mode positions by automated correlation. Agreement between these two complementary techniques establishes high confidence in the resulting mode identifications. Assignments of observed emission peaks provide insight into droplet-emission mechanisms.