Whispering-gallery modes are the resonances that occur inside round objects (spheres, cylinders, disks) when light bounces around the object’s circumference, restrained by total internal reflection to stay inside the object. The modes can be useful analytic tools because their resonant frequencies — that is, the frequencies corresponding to an integral number of wavelengths in one trip around the circumference — depend strongly on the refractive index. Thus, the properties of an unknown analyte dissolved in a droplet of fluid can be inferred by analyzing the resonant frequencies of the droplet’s whispering-gallery modes. The technique is especially attractive for biosensing in small samples because the volume of droplets typically is measured in the tens or hundreds of picoliters.Figure 1. Water from the 30-μm horizontal channel was injected into the 50-μm oil channel, where it initially formed small plugs in the downward-flowing oil (a). When the plugs reached the point where the vertical channel suddenly widened to 200 μm, they formed spherical droplets (b). Images reprinted from Optics Letters.Scientists at the University of California, Davis, recently reported what they believe is the first observation of lasing based on the whispering-gallery modes in microdroplets that were formed in a microfabricated channel. Because the characteristics of a laser depend not only on the resonant frequencies of its cavity but also on the intracavity gain-and-loss dynamics, the technique may lead to even more powerful analytic tools.The scientists created the droplets in a micromachined channel that injected water into a fast-flowing stream of low-refractive-index oil (Figure 1). The droplets then moved past two optical fibers, one of which illuminated them with light from a HeNe laser, while the other collected the HeNe light scattered from them. This scattered light provided a timing signal that ensured synchronization of the experiment. Not shown in Figure 1 are a camera and a spectrometer that collected experimental data through a window viewing the droplets as they moved past the fibers.Figure 2. The blue photographs show a single 50-μm droplet moving down the channel and being excited as it moves past the fibers (f, g and h). In k and l, a close-up of the droplet is shown as it is excited. In k, some scattered green light from the exciting laser is visible. The image in l was taken through a notch filter that blocked the green light.The scientists dissolved either Rhodamine 6G or fluorescein in the ∼55-μm-diameter water droplets, whose volume was roughly 100 pl. As the droplets flowed past the two fibers, they used the same fiber that illuminated the droplets with HeNe light to excite them with the light from either a frequency-doubled Nd:YAG laser or an argon-ion laser. In their initial experiments, the investigators saw no signs of lasing from the droplets. The refractive index ratio in the experiment (ndroplets:noil) was only 1.03, and they suspected that that was too low to provide sufficient containment by total internal reflection. When they boosted the ratio to 1.1 by adding glycerol to the water, they suddenly saw the sought-after result (Figure 2). Besides the normal yellowish fluorescence from the dye dissolved in the droplet, they saw a reddish emission from around the rim that was indicative of lasing action within the droplet.Figure 3. The fluorescence spectrum of the dye used in this experiment, fluorescein, is indicated by the black trace. When the whispering-gallery laser reached threshold, its individual modes were clearly visible in the red trace.Geometry dictates that light scattered from such modes would be most visible at the rim of the sphere. But much stronger evidence of lasing was present in the spectrum of the light collected from the droplets, where the peaks of the individual modes were clearly visible (Figure 3). The maximum of the laser spectrum (reddish) was shifted to longer wavelengths than the maximum of the fluorescence spectrum (yellowish) because fluorescein absorption, with a peak at 490 nm and a tail extending all the way to 535 nm, prevented the shorter wavelengths from reaching the lasing threshold.Optics Letters, Sept. 1, 2007, pp. 2529-2531.