Droplet Lasing Research Articles

Spectral tuning of lasing emission from optofluidic droplet microlasers using optical stretching

We introduce tunable optofluidic microlasers based on active optical resonant cavities formed by optically stretched, dye-doped emulsion droplets confined in a dual-beam optical trap. To achieve tunable dye lasing, optically pumped droplets of oil dispersed in water are stretched by light in the dual-beam trap. Subsequently, resonant path lengths of whispering gallery modes (WGMs) propagating in the droplet are modified, leading to shifts in the microlaser emission wavelengths. Using this technique, we present all-optical, almost reversible spectral tuning of the lasing WGMs and show that the direction of tuning depends on the position of the pump beam focus on the droplet. In addition, we study the effects of temperature changes on the spectral position of lasing WGMs and demonstrate that droplet heating leads to red-tuning of the droplet lasing wavelength.

Droplet lasing spectroscopy applied to droplet stream flames

Droplet lasing spectroscopy (DLS) has been applied to the measurement of droplet size and vaporization rates in both reacting and non-reacting rectilinear droplet streams. A Berglund-Liu droplet generator was used to generate a stream of droplets, approximately 63 microns in diameter and 6.5 droplet diameters apart. Ethanol, methanol, and a pentane/ethanol mixture were doped with Rhodamine 6G. Lasing spectra were examined in the steady-state combustion regime. In the pentane/ethanol case the measurements were carried out in a sooting region of the flame. In some cases, vaporization rates were high enough to measure the rate from consecutive droplets, yielding a quasi-instantaneous measurement. In all cases, the D 2 law of droplet vaporization was evident. In addition, photographs of the flames yielded measurements of flame height and thickness.

Demonstration of Droplet size and vaporization rate measurements in the near field of a two phase jet using droplet lasing spectroscopy

Demonstration of Droplet size and vaporization rate measurements in the near field of a two phase jet using droplet lasing spectroscopy

Demonstration of droplet size and vaporization rate measurements in the near field of a two-phase jet with droplet lasing spectroscopy

Droplet lasing spectroscopy has been applied to the measurement of droplet size and evaporation rate in a spray. A single droplet, doped with laser dye, was injected along the centerline of a liquid spray. Filters were used to block the strong elastic-scattering signal. The lasing emission from the doped droplet could be detected against the background with mass loadings of liquid in the spray as high as 20%. An analysis of the spectrum of droplet lasing was used to evaluate the droplet diameter. The evaporation rate of the droplet was obtained from consecutive lasing spectra that were obtained from the same droplet. An error analysis of the drop size and drop evaporation measurements was carried out and showed that accurate measurements of evaporation rates were feasible.

Single-particle scattering effects on the resonance modes of microdroplets

Scattering patterns captured at two different directions from ink-doped ethanol droplets containing submicrometer-sized particles, which had been pumped by nanosecond pulses, were obtained simultaneously. By use of ray optics we interpreted the various scattering patterns of the lasing in droplets. The scattering image of one side and the spectral image of the opposite side were also detected simultaneously. We observed that particles scatter with similar output couplings among modes of the same order. We also found that a single particle could react to modes of different orders with different output couplings.

Lasing emission from an evaporating layered microdroplet

We report the first observations, to our knowledge, of lasing emission from a single, evaporating, layered, micrometer-sized droplet. The droplet consists of an ~7.9-microm-radius glass core covered by a liquid shell of an initial 3 x 10(-4) M Rhodamine 6G dye-doped glycerin/water solution. After pumping by a doubled (532 nm) Nd:YAG pulsed laser, droplet lasing occurs over a range of shell thicknesses from ~14.5 to ~0.4 microm. At the smallest shell thickness, a dramatic increase in both the number and intensity of the lasing peaks occurs, in addition to a large (>40-nm) blue shift in the spectral bandwidth of lasing emission. A discussion of possible enhancement mechanisms responsible for these novel findings is given.

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.