Longitudinal pumping of polymer microring lasers T. Ben-Messaoud, S.X. Dou, E. Toussaere * , A. Potter, D. Josse, G. Kranzelbinder, J. Zyss Laboratoire de Photonique Quantique et Mole ´culaire (UMR 8537), ENS-Cachan, 61 Av. du Pre ´sident Wilson, 94235 Cachan, France Abstract Pulsed, longitudinal photopumped multi-mode laser emission is demonstrated in the visible spectral range from cylindrical microcavities formed by luminescent polymer thin films coated around optical fibers. Thresholds for laser oscillation for picosecond excitation are 80 pJ/ mm for longitudinal pumping as compared to 4 nJ/mm (5 mJ/cm 2 or 0.05 MW/cm 2 ) for transversal pumping. An important advantage of the longitudinal pumping configuration lies in a significantly lower excitation density at threshold in comparison with the transverse pumping configuration. This improvement is ascribed to a larger overlap factor between the optical pump and the gain medium in the longitudinal geometry. The emission spectra of plastic microrings under these different excitation configurations are reported and analyzed. We note that lowering the lasing threshold is an important prerequisite criterion towards the development of electrically pumped plastic lasers. # 2002 Elsevier Science B.V. All rights reserved. Keywords: Microcavities; Polymer lasers; Longitudinal pumping; Whispering gallery modes 1. Introduction Organic solid state lasers have been at the origin of the development of laser sources in waveguiding integrated optics format [1] and they experience a revival of interest due to the recent emergence of a wide range of conjugated polymers in the solid state. These polymers have already displayed lasing in various optically pumped configurations including planar microcavities [2,3], distributed feedback (DFB) lasers on plastic substrate [4] or tunable DFB lasers [5,6] as well as in microdisk or microring configurations [7,8]. However, in this last configuration, optical pumping has been reported only in a transverse geometry with respect to the ring. We present here evidence of the interest of long- itudinal pumping whereby the optical pump is guided by the optical fiber supporting the microring laser and evanescently coupled to the active medium. 2. Experiment Microring samples are being coated around glass multi- mode optical fibers that were etched in hydrofluoric acid from their original diameter, 125 mm, to the desired size. Low concentration acid was used to maintain adequate surface quality. The etched optical fiber was dipped into a solution of DCM–PMMA in anisole, which after quick evaporation, forms a coating around the fiber shaping a complete cylinder of 1.5 mm in thickness, 150 mm in length, and a diameter predetermined by the size of the fiber. The concentration of DCM in PMMA was 2  10 4 mol/ cm 3 . The samples can then be used without any further processing. The polymer microrings were photopumped by 100 ps- long pulses at 532 nm from a Nd:YAG laser. The two different, longitudinal and transversal, pumping configura- tions are shown in Fig. 1. In the transverse pumping configuration, the laser beam was first focused through a cylindrical lens, creating a stripe perpendicular to the microring axis, in order to excite modes in a thin cross-section of a single microring sample. The beam was then focused through a spherical lens to further reduce the size of the pumped region. In the longitudinal pumping configuration, the laser beam was coupled directly into the optical fiber. The fiber with the microrings was contained in a cell under nitrogen gas flushing so as to prevent possible degradation of the samples as a result of photo-oxidation. The microring was probed by two micro- scopic objectives set in the plane of the ring, and the output was carried by a multi-mode optical fiber to a spectrometer and cooled charge coupled device (CCD) camera. The overall spectral resolution of the system is 0.4 nm. Synthetic Metals 127 (2002) 159–163 * Corresponding author. Tel.: þ33-1-4740-5557; fax: þ33-1-4740-5567. E-mail address: eric.toussaere@lpqm.ens-cachan.fr (E. Toussaere). 0379-6779/02/$ – see front matter # 2002 Elsevier Science B.V. All rights reserved. PII:S0379-6779(01)00614-2