Nonlinear outcoupling mechanism in a cylindrical dielectric microcavity supporting stimulated Raman scattering.

We describe a previously unreported nonlinear scattering effect that occurs in cylindrical liquid jets irradiated by an intense laser pulse. The spatial characteristics of this feature are quite striking: light emanating from a thin (

Random occurrence of stimulated Raman scattering emission from liquid water microdroplets.

Stimulated Raman scattering (SRS) spectra from micrometer-sized water droplets have been obtained in the range 2100 < Δν < 5100 cm-(1). A number of Raman bands have been individually identified (to our knowledge, for the first time), corresponding to fundamental OH- and OD-stretching vibrations and to vibrations of hydrogen-bonded molecular complexes. All bands exhibit the intense morphologydependent resonance features that are characteristic of SRS emission from microdroplets. SRS emission is apparently random from all bands; however, the frequency of occurrence varies widely, from bands where emission is seen on practically every laser shot to bands where emission is seen only once in > 10(4) laser shots. Possible causes of these noteworthy emission features are discussed, including the difficulty of coupling weak spontaneous Raman emission to both the intense pump beam and the morphologydependent resonances within the droplet.

Effects of submicrometer-sized particles on microdroplet lasing.

We report the threshold emission characteristics of lasing from microdroplets that consist of Rhodamine 6G dye in an ethanol solution that contains undoped polystyrene latex spheres. The addition of latex particles to the droplets suppresses lasing. Our findings indicate that, for a fixed Rhodamine 6G concentration and fixed pump intensity, lasing ceases when a certain total-threshold latex particle surface area is reached in the droplet, independent of latex particle size. A possible explanation for these findings is the Förster-assisted annihilation of Rhodamine 6G dye lasing levels, facilitated by the adsorption of dye molecules on the surfaces of latex particles.

Suppression of stimulated Raman scattering from microdroplets by seeding with nanometer-sized latex particles.

Stimulated Raman scattering (SRS) in laser-irradiated microdroplets is suppressed by the addition of nanometersized latex particles. The microdroplets consist of either pure ethanol or a solution of Rhodamine 6G dye in ethanol, seeded with latex particles having diameters of 50 < d < 500 nm. SRS emission occurs at droplet morphology-dependent resonances (MDR's) after either direct pumping by the incident 532-nm laser or indirectly whereby the pump laser first initiates dye lasing, which in turn pumps SRS. For large latex, we observe SRS suppression at a near-coincident threshold concentration independent of the presence of dye, whereas, for small latex, adding dye reduces the threshold concentration by more than an order of magnitude. These findings are consistent with the interpretation that for large latex ~1 particle must occupy the MDR mode volume at threshold, whereas for small latex the addition of particles facilitates Förster-assisted annihilation of both 532-nm and dye-lasing MDR pump photons.

Energy-transfer-assisted lasing from microdroplets seeded with fluorescent sol.

We report observations of lasing emission from liquid microdroplets of Fluorescein 548 dye in ethanol, seeded with submicrometer-sized fluorescent sol. An incident pump laser excites fluorescein dye molecules, which in turn couple energy to sol-dye molecules, generating lasing in the sol. The pump laser can also generate fluorescein lasing in the droplets, which may excite lasing in the sol. Other noteworthy findings include the absence of sol emission for larger sol and the presence of sol emission, even without any observable fluorescein emission, for smaller sol. All emissions are at wavelengths corresponding to morphology-dependent resonances of the droplet. Studies of the dependence of these emissions on pump laser intensity and sol concentration suggest that they are driven either by Förster energy transfer between fluorescein dye molecules and dye within the sol or by enhanced radiative transfer that occurs when fluorescein emission couples to morphology-dependent resonances of the droplet microcavity.

Observations of stimulated Raman scattering and laser-induced breakdown in millimeter-sized droplets.

We report the first observations, to our knowledge, of nonlinear optical effects in large (millimeter-sized) droplets. Stimulated Raman scattering (SRS) and laser-induced breakdown (LIB) are simultaneously observed in acoustically levitated millimeter-sized glycerol droplets irradiated by either a frequency-doubled (532-nm) or a frequency-tripled (355-nm) Nd:YAG laser. The two processes, which occur above a nearly coincident irradiation threshold, are conjectured to arise from a common initiation mechanism: self-focusing. LIB generates vapor bubbles within the droplet, resulting in the quenching of SRS emission.

Multiple superheating thresholds of micrometer-sized droplets irradiated by pulsed CO(2) lasers.

Two distinct superheating fluence thresholds have been measured for micrometer-sized droplets of liquids irradiated by pulsed CO(2) lasers. The lower, deformation, threshold results in minimal droplet mass loss, whereas the higher, disintegration, threshold (which is well defined only if observed several tens of microseconds after the heating laser pulse) leads to droplet fragmentation into many microparticles and vapor. Deformation thresholds are nearly coincident for either long (10-micros) or short (0.4-micros) laser pulses. Disintegration thresholds are higher for long-pulse irradiation and increase with decreasing absorption. A qualitative explanation is given for these phenomena based on the effects of surface tension, thermal conduction, and thermally induced optical inhomogeneities.

Observations of Descartes ring stimulated Raman scattering in micrometer-sized water droplets.

Stimulated Raman scattering (SRS) from micrometer-sized droplets, which results from coupling of spontaneous Raman emission to droplet morphology-dependent resonances (MDR's), exhibits unique characteristics. Spatial patterns consist of bright SRS arcs on the droplet rim. A second source of SRS emission has recently been observed from a ringlike region encircling the droplet axis near the geometric focus (the Descartes ring). Investigation of the time and spectral characteristics of Descartes ring SRS and its suppression by the addition of absorptive dye to the droplet reveals it to be an additional manifestation of droplet MDR's. We conjecture that the Descartes ring results when the MDR light is scattered by refractive-index inhomogeneities produced by the intense pump field within the droplet.

Physical basis for Descartes ring scattering in laser-irradiated microdroplets.

Stimulated Raman scattering from laser-irradiated microdroplets is observed from two distinct spatial regions, the droplet rim and a ringlike region encircling the laser beam axis on the droplet shadow face (the Descartes ring). With the use of two pulsed laser beams, a perturbing beam and a delayed stimulated-Raman-scattering pump beam, the physical mechanism for Descartes ring scattering is investigated. Evidence is obtained of a persistent mechanism that continues even after the perturbing laser pulse is turned off. This mechanism is tentatively identified as electrostriction. The possible existence of an additional prompt mechanism that requires overlap between perturbing and pump laser pulses is also discussed.