Lasing and stimulated Raman scattering in spherical liquid droplets: time, irradiance, and wavelength dependence

Cavity-Enhanced Fluorescence in Colliding Droplets of Rhodamine 6G Aqueous Solutions

A liquid droplet can act as a microscale high-Q optical cavity via a whispering gallery mode, where light resonates with enhanced intensity. A collision of two droplets temporarily exhibits a unique morphology, which provides a more effective optical cavity than a single droplet. We investigated the mechanisms of cavity-enhanced fluorescence in colliding droplets of aqueous rhodamine 6G. Laser-excited spectra and fluorescence generation times were acquired. The fluorescence spectra had two peaks: one attributed to amplified spontaneous emission (ASE) and the other to lasing. The lasing generation time had a longer delay relative to that of ASE, which indicated that it required a longer propagation distance for the positive feedback. Overall, this provides a basis for the development of a highly efficient dye laser using colliding droplets.

Droplet lasers: a review of current progress

It is perhaps surprising that something as fragile as a microscopic droplet could possibly form a laser. In this article we will review some of the underpinning physics as to how this might be possible, and then examine the state of the art in the field. The technology to create and manipulate droplets will be examined, as will the different classes of droplet lasers. We discuss the rapidly developing fields of droplet biolasers, liquid crystal laser droplets and explore how droplet lasers could give rise to new bio and chemical sensing and analysis. The challenges that droplet lasers face in becoming robust devices, either as sensors or as photonic components in the lab on chip devices, is assessed.

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 (</=5 microm) circumferential ring near the cylinder surface. This ring can, depending on the pump intensity, be continuous or periodic in ø. We have observed rings in several liquids (ethanol, water, and carbon disulfide) and, for the case of ethanol, in cylinders ranging from ~14 to ~76 microm in radius. We conclude that the rings are composed of plasma generated by the intense stimulated Raman scattering fields in the cylinder.

Suppression and enhancement of dye lasing and stimulated Raman scattering from various dye-doped liquid spheres

The observation of suppression or enhancement of dye lasing and stimulated Raman scattering (SRS) from various dye-doped liquid droplets, in which SRS from the initial pumping wavelength appeared in shorter and longer wavelengths of various dye fluorescence regions, is reported; SRS from the dye-lasing wavelengths (double resonances) and stimulated resonance Raman scattering of dyes are included. Furthermore, the contribution to SRS of the dye fluorescence (depending on dye concentration and different dyes) and dye-lasing suppression that is due to stimulated resonance Raman scattering is also described.

Coherent stimulated Raman emission from a microcavity-excited optical resonator

We report the observation of coherent emission from a planar Fabry-Perot resonator driven by a single, coherently emitting microcavity. The microcavity, either a liquid microdroplet or microcylinder, emits coherent stimulated Raman scattering when irradiated by an intense pump laser; the Raman light, in turn, couples to modes of the resonator. The spatiotemporal characteristics of resonator emission are strongly influenced by the microcavity source within; notably, a significant excess time delay in the emission of stimulated Raman scattering (i.e., compared with that of the microcavity in free space) accompanies the appearance of a sharp resonator spatial interference pattern. This and other noteworthy features are described, and a qualitative discussion of their significance is given.

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.

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.

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.

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.

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.