(12h) Continuously Tunable Dye Laser Using Dissolving Drops in Microchannels

We describe a new type of tunable dye laser using a stream of dye-containing droplets possessing decreasing diameters in a microchannel. The size of the drops decreases as they dissolve into the carrier fluid when they travel downstream of a microchannel; the lasing wavelengths blue-shift accordingly. This laser operates in whispering gallery modes, and has the attractive feature that the droplet itself forms the optical cavity; extra fabrication steps to integrate structures to provide optical amplification (e.g. mirrors) are not necessary. We have previously demonstrated a droplet-based dye laser that is capable of switching between discrete sets of lasing wavelengths at rates > 1 kHz.[1] Here we demonstrate the ability to sweep and fine-tune the lasing wavelengths continuously. We generated drops containing solution of rhodamine 640 in benzyl alcohol using water as the carrier fluid with a microfluidic flow-focusing device.[2] Benzyl alcohol dissolves in water, up to 4 wt% solution.[3] As a result of these dissolutions, the drops of benzyl alcohol decreased in size over time, as they travelled through the microchannel (Figure a,b). We excited the drops optically to cause them to lase in whispering gallery modes. The lasing spectrum depended on the size of the drops, and was thus continuously tunable by positioning the excitation beam at different parts of the channel. The contrast in refractive index between benzyl alcohol (nD = 1.54) and water (nD = 1.33) was sufficiently high to allow confinement of light for lasing in whispering gallery modes. Minimization of surface energy between the drop and the carrier fluid caused the surface of the drop to be optically smooth; optical loss due to scattering by the surface was, therefore, minimal. The droplets were excited optically by a frequency-doubled Nd:YAG directed perpendicular to the plane of the microchannel. The optical output from the drops was collected through an objective in the same direction as the pump beam and coupled to a spectrometer. Figure c,d show that the size of the drops decreased as they travelled downstream. The rate at which the drops dissolved was dependent on the composition of the carrier fluid. When the carrier fluid was pre-saturated with benzyl alcohol, the decrease in the size of the drops was much slower than when the carrier fluid was water. We obtained lasing spectrum of drops at different positions in the channel (Figure e). The lasing wavelengths decreased from about 680 nm to 620 nm as the diameter of the drops decreased from 42 um to 18 um. The free-spectral range (FSR)?the spacing between two modes (or lasing peaks) in nanometers?also increased as drops became smaller.[4] Numerical simulation of lasing behavior from drops possessing different sizes is under progress. To summarize, we demonstrated a dye laser with continuously tunable wavelength. Many pairs of fluids that have limited mutual solubility can be used for this strategy. These types of droplet-based optofluidic lasers can be useful for on-chip spectroscopy and flow cytometry, where multiple lasing frequencies are necessary for the excitation of different fluorescent labels. References: 1. ?A multi-color fast-switching microfluidic droplet dye laser?, S.K.Y. Tang, Z. Li, A.R. Abate, J.J. Agresti, D.A. Weitz, D. Psaltis, and G.M. Whitesides , Lab Chip, 2009, 9, 2767?2771. 2. ?Formation of Monodisperse Bubbles in a Microfluidic Flow-Focusing Device", P. Garstecki, I. Gitlin, W. DiLuzio, G. M. Whitesides, Applied Physics Letters, 2004, 85, 2649-2651. 3. Benzyl alcohol Material Safety Data Sheet: http://www.sciencelab.com/xMSDS-Benzyl_alcohol-9927099 4. ?Optical Processes in Microcavities?, R. K. Chang and A. J. Campillo, World Scientific Pub. Co. Inc., 1996.