A new microfluidic dye laser switches its emission wavelength at frequencies of up to 3.6 kilohertz, faster than any previously reported dye laser, according to a new report (Lab Chip, DOI: 10.1039/b914066b). Such a device could be used for on-chip spectroscopy and flow cytometry. George M. Whitesides and Sindy K. Y. Tang of Harvard University and coworkers create the dye laser in a microfluidic channel from a train of alternating droplets containing rhodamine 560 or rhodamine 640. The droplets flow through a region in which they are optically excited by a single 532-nm laser and lase at a wavelength determined by the dye in the droplet. The rhodamine 560 lases over the 570- to 595-nm range, and the rhodamine 640 lases over the 665- to 695-nm range, with a switching time of 30 milliseconds between the two colors. Because each droplet is excited only once, photobleaching of the dyes is not an issue. The current system requires an external excitation laser, but the researchers predict that improvements will allow the use of an on-chip excitation laser.