Time dependence of multiorder stimulated Raman scattering from single droplets

Stimulated Raman Scattering of an Organic Liquid in a Spherical-Shell Optical Cavity around an Aqueous Pendant Drop

Stimulated Raman scattering (SRS) in a liquid droplet enhances the Raman scattering intensity through optical resonances. We have previously used a pendant drop at the tip of a capillary as the Raman-enhancing medium. In this study, we develop a new optical cavity for SRS measurement that consists of a spherical shell of organic liquid. This enables us to extend the applicability of the pendant-drop SRS method to liquids with low viscosity or low interfacial tension. This method is used to observe low-frequency modes of liquid benzene. The results indicate that the SRS emerges locally with respect to the drop size. The developed method extends the study of liquid structures based on vibrational spectroscopy.

Multiorder Stimulated Raman Scattering in Colliding Droplets

Nonlinear Raman spectroscopy is a versatile method to enhance the intensities of Raman scattering. It requires an intense light field that can be provided by a liquid droplet acting as a high-quality optical cavity. Here, colliding droplets were used as a novel optical cavity to enhance the intensity of Raman scattering. Specifically, multiorder stimulated Raman-scattered light was generated with significant intensity from colliding droplets of carbon tetrachloride (CCl₄). The intensities of the Raman bands were analyzed with a simple theory that roughly reproduced the experimental spectrum. Overall, the method facilitates Raman spectroscopy of molecules in liquids because of its high sensitivity and resolution.

High stability breakdown of noble gases with femtosecond laser pulses

In the past, laser-induced breakdown spectroscopy (LIBS) signals have been reported to have a stability independent of the pulse length in solids. In this Letter, we perform the first stability study of femtosecond LIBS in gases (to our best knowledge) and show a significant improvement in signal stability over those achieved with longer pulses. Our study shows that ultrashort-pulse LIBS has an intrinsically higher stability in gas compared to nanosecond-pulse LIBS because of a deterministic ionization process at work in the femtosecond pulse. Relative standard deviations below 1% are demonstrated and are likely only limited by our laser output fluctuations. This enhanced emission stability may open up possibilities for a range of applications, from monitoring rapid gas dynamics to high-quality broadband light sources.

Determining the composition of aqueous microdroplets with broad-band cavity enhanced Raman scattering

We demonstrate that broad-band cavity enhanced Raman scattering (CERS) can be used to determine the composition of binary alcohol-water aerosol droplets over a wide compositional range from 10% v/v to 90% v/v. In contrast to conventional CERS using narrow-band laser excitation, the excitation is provided by a broad-band Nd:YAG pumped dye laser. A change in the spontaneous spectrum resulting from the change of the linewidth of the excitation laser permits tuning of the sensitivity range over which the droplet composition can be determined by CERS. We demonstrate that this change in sensitivity can be estimated using a simulation of the change in the sensitivity to the species in spontaneous bulk phase measurements. We further show that the compositional calibration is independent of droplet radius in the range 33-56 microm. The compositional range over which CERS is sensitive can be controlled and optimised for any particular application by exploiting the dependence of the stimulated Raman scattering on the laser linewidth and wavelength. Thus, quantitative measurements of droplet composition can be made in situ with high accuracy, providing a valuable new tool for analysing aerosol composition.

Stimulated light scattering in transparent liquid particles: effect of the descartes ring

We consider some peculiarities of the evolution of processes of stimulated scattering in liquid transparent particles in the presence of ponderomotive action of a light field. We observed the occurrence of sharp deformations in the Descartes ring zone, which exceeds by more than 1 order of magnitude the deformations of the remaining particle surface. Investigation of the dynamics of the evolution of droplet deformations has made it possible to consider these deformations as the main cause of suppression of stimulated scattering indicated in the experiments. An analytical expression was derived to evaluate the decrease of the Q factor of droplet quasi-normal eigenmodes caused by perturbation of the particle shape from the spherical. Our study revealed that the larger the Q factor of the droplet eigenmodes, the greater the influence of surface deformation.

Lasing behavior in a liquid spherical dye laser containing highly scattering nanoparticles

With the addition of scattering nanoparticles to dye-doped spherical droplets, lasing has been observed with well-defined thresholds in input-output data. One-order or more magnitude enhancement of peak intensity from droplet emission has been obtained with certain (optimum) additive scattering particles compared with nonadditive scattering particles (neat dye-doped droplets). Characteristics of input-output intensities, emission spectra (with wavelength shifts), and spectral linewidths are reported experimentally, depending on additive quantities of scattering nanoparticles.

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.

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.

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.

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.

Aerosol-induced laser breakdown thresholds: effect of resonant particles

Laser intensity thresholds for the onset of stimulated Raman scattering and the breakdown in resonant micro-meter-sized droplets are reduced to below those for nonresonant droplets by a factor of ~ 3. This reduction is most likely caused by the enhancement of electromagnetic energy (photon) densities within the droplets over and above that in nonresonant droplets. The magnitude of the threshold reduction for breakdown is consistent with the assertion that (1) input (pump) wavelength resonances that initiate plasma have cavity Q's of ~ 10(4) and (2) finite regions of high-electromagnetic-energy density within the droplet, with dimensions of the order of the Debye length, are required to initiate plasma.

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

Measurement of lasing from dye doped micron sized ethanol droplets irradiated by a pulsed Nd:YAG laser shows evidence of lifetime broadening and relaxation oscillations. Observations of stimulated Raman scattering (SRS) and lasing generated by the same laser pulse in a dye doped ethanol droplet and coupled to the same morphology dependent resonance exhibit longer delays in the initiation of SRS than for pure ethanol droplets where lasing is absent. SRS emission above the plasma breakdown threshold is not measurably delayed and is quenched by the developing plasma.

Stimulated Raman-scattering threshold behavior of binary mixture micrometer-sized droplets

Stimulated Raman scattering (SRS) from binary liquid mixture micrometer-sized droplets irradiated by nanosecond laser pulses exhibits previously unreported features. SRS emission at wavelength shifts corresponding to combination frequencies of the individual component Stokes shifts are observed in droplets but not in bulk mixtures. Furthermore, droplet SRS thresholds are determined by component refractive indices and concentrations, in contrast to bulk thresholds, for which self-focusing likely plays a dominant role.

Frequency pulling of stimulated Raman scattering in microdroplets

Spectral features observed in stimulated Raman scattering from 20-microm methanol droplets at 1-GW/cm(2) pumping levels appeared locked in wavelength at the center of their respective gain profiles. Typically, a feature's wavelength remained constant while the droplet size varied by as much as 0.3%. This is attributed to frequency-pulling effects caused by the presence of Raman gain. Wavelength shifts of up to +/-0.5 nm in the positions of morphologydependent resonances would explain the observations and are consistent with calculations.

Time-resolved spectroscopy of laser emission from dye-doped droplets

Micrometer-sized droplets of Rhodamine 6G solution in water and ethanol are irradiated by high-intensity nanosecond pulses from a frequency-doubled Nd:YAG laser. Coupling of the spontaneous fluorescence emission with natural resonant modes of the spherical droplets results in stimulated emission, with each droplet behaving like a laser cavity. Spectral observations suggest that droplet lasing emission is supported by resonances of a single mode order. The emission exhibits faster rise times and is shorter lived than corresponding bulk-liquid fluorescence. Lasing in droplets is generally initiated almost simultaneously with elastic scattering, unlike stimulated Raman scattering, which is significantly delayed.

Stimulated Raman scattering in micrometer-sized droplets: measurements of angular scattering characteristics

Measurements of the angular scattering characteristics of elastic and stimulated Raman scattering (SRS) in single (nominal 20-microm-radius) water, ethanol, and CC1(4) droplets irradiated with 0.532-microm-wavelength radiation from a pulsed laser demonstrate that SRS is more isotropic than elastic scattering yet qualitatively mimics angularly smoothed elastic scattering patterns. The angular fine structure characteristic of the coherent elastic scattering process is lacking in SRS, regardless of whether the SRS derives from multiple orders of Stokes shifts, multiple resonances within a single Stokes shift, or single resonances within a single Stokes shift.