H.-B. Lin

Some characteristics of a droplet whispering-gallery-mode laser

We report lasing characteristics of 40-60-microm-diameter Rhodamine 590/water solution droplets pumped by a 20-nsec-duration Q-switched laser. The Rhodamine/water solution provides a useful model system for studying the properties of oscillators based on whispering-gallery-wave spherical cavities. The low threshold for lasing, 10(4) W/cm(2) for 10(-4) M solutions, is consistent with particle size and a cavity Q factor of 10(4). Portions of the droplet lase purely in transverse electric (TE) modes, while other portions contain both TE and lower-Q transverse magnetic modes. In the far field, the lasing droplet approximates a coherent point source emitting in all directions.

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...

High-precision identification of morphology-dependent resonances in optical processes in microdroplets

The spectroscopy of the morphology-dependent resonances of a microdroplet has been studied at high precision. The line positions are shown to reveal optical dispersion and permit the refractive index to be determined with sufficient accuracy to provide an estimate of the droplet cooling that is due to evaporation. Comparison of the remaining discrepancies in mode positions for different radial modes indicates a small temperature gradient near the surface. Both the cooling and the temperature gradient are compatible with thermodynamic estimates. The mode quantum numbers are identified with high confidence, and the systematics of the line intensities permit an estimate of the extra radiative loss 1/QL over and above that predicted by Lorenz–Mie theory for a perfect homogeneous microsphere, for example, that which is due to internal scattering, with QL ≈ 2 × 108 for first-order modes.

Spectral properties of lasing microdroplets

The spectral behavior of 15.3-μm-diameter Rhodamine 6G in water solution droplets was studied. Microdroplet lasing is known to occur simultaneously at many discrete wavelengths, each corresponding to one of many possible spherical cavity resonances. We show that lasing takes place on several mode orders at once. Modes of a given order were found to form a bell-shaped spectral cluster of typically 4–6 resonance lines having consecutive principal mode numbers. Clusters of different mode orders appear somewhat displaced spectrally from one another, with lowest-order clusters shifted to the red. This multiplicity of lasing modes is accounted for by spatial hole-burning effects. The relative lasing intensities of the differing mode orders are explained by an output coupling theory that considers the gain enhancement that is due to cavity quantum electrodynamic effects. An upper limit of 108 for the Q of a nondegenerate cavity mode was estimated from the data.

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.

Double-resonance stimulated Raman scattering in micrometer-sized droplets

Programmed size ramping of an aerosol stream permits a Q-switched mode-locked 532-nm laser to satisfy periodically the input resonance condition of various low-order (l = 2 to l = 4) TE and TM morphology-dependent resonances in 23.0–23.7-μm-diameter ethanol droplets. The resulting size-versus-amplitude spectra of stimulated Raman scattered light revealed high-Q modes that are not normally observed in elastic scattering. Simultaneous elastic scattering measurements permit unambiguous identification of these input resonances. The relative output intensities of stimulated Raman scattering and time histories imply that the Q of the l = 2 modes is degraded to a value of 107 by the departure of the droplet from an ideal homogeneous sphere.

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.

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.

TWO-DIMENSIONAL PHOTONIC BANDGAP OPTICAL LIMITER IN THE VISIBLE

Operation of a two-dimensional photonic bandgap optical limiter was studied at 514.5 nm for pulse durations of 0.1 to 4 ms . Photonic crystals consisted of 180- 230-nm spatial-period nanochannel glasses containing a thermal nonlinear liquid. A dynamic range in excess of 130 was observed in a single-element device.

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.