A. J. Campillo

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...

Broadband thermal optical limiter

The limiting behavior of nigrosin dye dissolved in carbon disulfide was investigated in an f/5 defocusing geometry using 6 ns duration 532 nm laser excitation. Nigrosin dye is a broadband visible light absorber that is used here in conjunction with the large thermal nonlinearity of carbon disulfide solvent to defocus intense incident visible light. A limiting threshold energy of 40 nJ, corresponding to a fluence of only 100 mJ/cm2 in the solution, was observed with device absorption adjusted to 53%.

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.

Photothermal aerosol absorption spectroscopy

In situ aerosol absorption spectroscopy was performed using three photothermal detection schemes: phase fluctuation optical heterodyne spectroscopy, photothermal modulation of Mie scattering, and photophoresis. Particle specific absorption data of ammonium sulfate aerosols were obtained. The specific absorption coefficient for the sulfate ion was measured to be 0.5 m2/g at 1087 cm−1. Each of the three techniques allows single-particle and subnanogram per cubic centimeter detection sensitivity, and in all cases the relative absorption spectrum can be obtained directly from the data without the need for complex inverse scattering calculations.

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.

Excitonic optical nonlinearity in quantum‐confined CuCl‐doped borosilicate glass

The nonlinear refractive index n2 is reported for quantum‐confined CuCl microcrystallites in borosilicate glass near the Z3 excitonic resonance. Induced index changes were estimated using Kramers–Kronig analyses of absorption bleaching data from 370 to 388 nm. The nonlinearity was found to increase with increasing particle radius over the range 22 to 34 A, in agreement with theoretical predictions.

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.

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.