Two-photon pumped lasing from methanol micro-droplets doped by a weakly fluorescent dye M. Anand a , A.K. Dharmadhikari a , J.A. Dharmadhikari a , A. Mishra b , D. Mathur a, * , M. Krishnamurthy a a Atomic and Molecular Sciences, Tata Institute of Fundamental Research, 1 Homi Bhabha Road, Mumbai 400 005, India b Department of Chemistry and Chemical Engineering, Polytechnic University, Brooklyn, NY 11201, USA Received 21 October 2002; in final form 24 February 2003 Abstract We report lasing induced by two-photon absorption (TPA) from micro-droplets of a weakly fluorescent dye (DASPI) doped in methanol. Dye-doped methanol droplets, of uniform diameter of 20 lm, are generated using a piezo- driven capillary. The droplet size is determined by a direct imaging technique as well as from peak separations of lasing (morphology dependent resonances – MDRs) spectra. The threshold for TPA-induced lasing of droplets with 0.01 M concentration of the dye is determined to be about 0.4 GW cm 2 . The lasing spectral emission from the micro-droplets exhibits well-resolved structure that is related to the MDR. Ó 2003 Elsevier Science B.V. All rights reserved. 1. Introduction Micrometer-sized spherical liquid droplets constitute optical cavities that can possess ex- tremely high values of quality factor Q. Repetitive total internal reflections of light at the spherical boundary of such micro-cavities can give rise to Q-values which are as high as 10 6 –10 8 at optical wavelengths. The emission spectra from the micro- droplets show structure due to the constructive interference of the light field inside the micro- cavity after multiple total internal reflections from the boundary of the droplet sphere. The structure in the emission spectra is correlated to the mor- phology of the drops and hence the usage of the term morphology dependent resonance (MDR). There has been longstanding interest in studies of lasing processes in micro-droplets from the view- point of applications, most notably those con- cerned with development of high efficiency optical devices [1,2]. Recent resurgence of interest has focused on issues relating to the uniformity of the shapes of spherical micro-cavities and their de- formations as these lead to possibilities of tailoring Q-values in such fashion that stronger and more directional light emitting optoelectronic devices can be designed [3–5]. From a fundamental perspective, lasing in micro-droplets has wide ramifications. The high Q-values that can be achieved become responsible Chemical Physics Letters 372 (2003) 263–268 www.elsevier.com/locate/cplett * Corresponding author. Fax: +91-22-2215-2181. E-mail address: atmol1@tifr.res.in (D. Mathur). 0009-2614/03/$ - see front matter Ó 2003 Elsevier Science B.V. All rights reserved. doi:10.1016/S0009-2614(03)00426-3