Gregory W. Faris

Spectrally resolved absolute fluorescence cross sections for bacillus spores.

Absolute fluorescence cross sections for Bacillus subtilis and B. cereus bacterial spores as both aqueous suspensions and aerosols were measured at a number of excitation wavelengths between 228 and 303 nm. The fluorescence was spectrally resolved at each excitation wavelength. We found that the optimum excitation wavelength for spore fluorescence is between 270 and 280 nm. The fluorescence cross section for aqueous suspensions is four times larger than for dry aerosols when measured under similar conditions. Measurements on wet aerosols showed an increase in fluorescence cross section over dry aerosols, indicating an enhancement of the fluorescence when the bacterial spores are wet. Mie scattering cross sections at 90 degrees to the direction of the incident radiation and extinction cross sections as a function of wavelength for B. subtilis suspensions and fluorescence cross sections for tryptophan are also reported.

Petri dish PCR: laser-heated reactions in nanoliter droplet arrays

We report high-speed real-time PCR performed on an unmodified disposable polystyrene Petri dish. The reaction cycle relies solely on an infrared laser for heating; no conventional heater is required. Nanoliter droplets of PCR mixture as water-in-oil emulsions printed in an array format served as individual PCR microreactors. A simple contact printing technique was developed to generate a large array of uniform sized nanoliter droplets using disposable pipette tips. Printed droplets showed variation of less than 10% in volume and the oil/water/polystyrene interface formed a compact droplet microreactor approximately spherical in shape. The uniform droplet array was used to optimize the laser power required for the two heating steps of PCR, annealing/extension and melting, while the ambient conditions were at room temperature. The optical heating allows for an extremely fast heating rate due to the selective absorption of the infrared laser by PCR buffer only and not the oil or polystyrene Petri dish, allowing completion of 40 amplification cycles in approximately 6 minutes. The quantitative assay capability of the system is also presented and discussed.

High-resolution stimulated Brillouin gain spectroscopy in glasses and crystals

Theory and measurements are presented for stimulated Brillouin gain spectroscopy in anisotropic media. The coupling between arbitrary pump and Stokes waves and the corresponding acoustic wave is formulated in terms of the elastic displacement vector and the photoelastic tensor. A propagation equation that fully includes transient effects is obtained for the Stokes wave. In the limit of small-signal gains this propagation equation can be solved analytically, yielding expressions that relate experimentally accessible quantities to material properties. Absolute Brillouin steady-state gain coefficients, linewidths, and frequency shifts are thereby determined in a number of optical materials at 532 nm. The Brillouin gain coefficient for fused silica is measured by three techniques, providing the reference for absolute gain measurements. Quantitative agreement is found between theory and experiment for transient effects on stimulated Brillouin scattering.

Upconverting time gate for imaging through highly scattering media.

We report the use of sum- and difference-frequency generation for two-dimensional upconversion time gating. Using difference-frequency generation, which can provide gain as well as upconversion (as an optical parametric amplifier), we have imaged through a scattering medium with an attenuation of exp(−25), using 1 mJ of energy from a single laser pulse.

High-precision frequency-domain measurements of the optical properties of turbid media

We report high-precision measurements of the absorption and the reduced scattering coefficients for turbid media. Using a frequency-domain measurement technique for a point-source infinite-medium geometry, we find that the standard deviations of multiple measurements of the absorption coefficient mu(a) and the reduced scattering coefficient mu(s)(?) are less than 0.6%, and three independently derived values for mu(a) and mu(s)(?) agree to better than 1%. Measurements of mu(a) agree with measurements of a nonscattering medium to within 1.2%. To obtain high precision requires attention to proper conditions for the spherical photon-density wave model, the detection linearity, and the ratio of the absorption rate to the source modulation rate. Frequency-domain amplitude and phase measurements deviate from fitted curves by 0.1% and 0.06 degrees rms, respectively.

Congener-specific detection of dioxins using jet-REMPI

Although 210 chemically different polychlorinated dibenzo-p-dioxin and dibenzofuran congeners can be produced during combustion, it is currently believed that fewer than 20 are toxic enough to warrant monitoring. SRI is developing a continuous emissions monitor to study the emission levels of these most toxic dioxins, leading eventually to an improved understanding of the formation of these molecules and to improved means of monitoring and control. We report here the first results of performing congener-specific detection for two dichloro dibenzo-p-dioxins present in low ppt concentrations in a mixture using the supersonic jet/resonantly enhanced multiphoton ionization time-of-flight mass spectrometer technique. In addition, we present preliminary data on the detection of chlorinated aromatic compounds using a two-color REMPI scheme with the same instrument.

Development of a jet–REMPI (resonantly enhanced multiphoton ionization) continuous monitor for environmental applications

The need for a continuous monitor for environmentally important pollutants at realistic [parts-per-trillion (parts in 10(12))] concentrations measured in real time (minutes) is widely recognized. We developed an instrument that is based on supersonic-jet expansion and cooling, followed by resonantly enhanced multiphoton ionization (REMPI) into a mass spectrometer. This approach furnishes the dual selectivity of narrow-band tuned laser absorption and mass analysis. We initiated a spectroscopic characterization of the jets collisional cooling behavior to optimize the instruments sensitivity and selectivity, made measurements of several aromatic compounds (including polychlorinated dioxins) by use of a one-color REMPI scheme, and demonstrated a two-color excitation scheme.

The O(1D) yield from O2 photodissociation near H Lyman-α (121.6 nm)

The solar H Lyman-α line is, through O2 photodissociation, an important source of O(1D) production throughout the mesosphere and lower thermosphere. To ascertain the energy balance in this altitude region, it is necessary to know the O(1D) yield across the solar H Lyman-α feature, since H Lyman-α absorption by O2 at ∼80 km accounts for a substantial fraction of the solar radiation absorbed in the mesosphere. An earlier laboratory study had provided a value of 0.44±0.05 for the O(1D) yield at the center of the solar H Lyman-α line, where the profile shows a minimum in intensity due to strong self-reversal of the line. Using tunable laser radiation, we have determined the O(1D) yield from O2 photodissociation across the entire H Lyman-α profile from 121.2 to 121.9 nm, at a spectral resolution of 0.0015 nm (1 cm−1). The results reveal a strongly wavelength-dependent window in the O(1D) yield, the origins of which are explained using calculations based on a coupled-channel Schrodinger-equations model of the O...

Observation of NO B 2Π(v=3)←X 2Π(v=0) absorptions with 1+1 multiphoton ionization: Precision line position measurements and parity assignment of the B 2Π state

Absorptions are observed in the β(3,0) band of NO, B 2Π(v=3)←X 2Π(v=0) using 1+1 multiphoton ionization at photon energies in the range 47 801–48 542 cm−1. Line positions are reported in 10 of the 12 possible branches with an accuracy of 0.05 cm−1, directly traceable to the visible I2 absorption spectrum. Λ doubling in the β(3,0) band is explicitly resolved, allowing a definitive parity assignment of the B 2Π(v=3) rotational levels. Accurate term energies and molecular constants are presented for B 2Π(v=3). Comparison with previous measurements reveals significant discrepancies. Line positions are also reported in the γ(2,0) band, A 2Σ+(v=2)←X 2Π(v=0), of the 14N18O and 15N16O isotopes.

Upconversion from aqueous phase lanthanide chelates

We have prepared and characterized several lanthanide ion complexes of multidentate ligands or chelates in an effort to develop new upconverting luminescent labels that can be immune to autofluorescence and photobleaching. This study has involved the characterization of various chelates of Nd, Er, and Tm with respect to relative luminescent efficiency and excited-state lifetimes and explored various two-photon stepwise excitation mechanisms. Using peak laser powers near 100 kW, the upconversion emissions of Nd in Nd(EDTA)2(5-) at 386 nm, Er in Er(DPA)3(3-) at 550 nm, and Tm in Tm(DPA)3(3-) at 480 nm, at levels of approximately 10(-12) moles can be detected.