Improved Temporal Stability of Polymethine Laser Dyes in Aqueous Solutions

Polymethine laser dyes which fluoresce in the near infrared in organic solvents generally exhibit poor temporal stability at room temperature. Two typical laser-pumped polymethine dyes in aqueous solutions containing nonionic surfactants are investigated. Temporal stability was improved 20-fold, as compared to organic solutions, with no significant changes in laser-pumping threshold or tuning range. Polymethine dye solutions were the first organic materials to operate successfully as lasing media [l], [2]. Since 1966 over 30 polymethine chromophoric dyes have exhibited lasing action in organic solvents; a bibliography of these dyes may be found in [3]. In using these dyes, we have shared with others the frustration of observing the lasing efficiency of prepared organic solutidns deteriorate erratically in times ranging from a few hours to a few days. In an attempt to increase the shelf life of solubilized polymethine laser dyes, and simultaneously decrease the temperature dependence of the solvent refractive index, we have investigated two typical dyes in aqueous solutions containing the nonionic surfactant Triton” X-100 [4].’ The dyes were DTDC iodide (3,3‘diethyl-2,2’-thiadicarbocyanine iodide) [2] and DMOTC iodide (3,3’-dimethyl-2,2’-oxatricarbocyanine iodide) [5], which we shall refer to as D-715 and D-735, respectively [3]. The absorption ( A ) and fluorescent (0 emission spectra for the two dyes in ethanolic and in aqueous solution are given in Fig. 1. These spectra were obtained using conventional spectrofluorometric techniques. The flporescent emission data were obtained with excitation at 632.8 nm; fluorescence was also observed using an excitation wavelength of 675 nm to more closely simulate the emission spectra produced by ruby-laser excitation, and the data exhibited no substantial differences from those shown in Fig. 1. Curves labeled ( F A ) represent fluorescence spectra which have been noticeably altered by self-absorption, and this selfabsorption is a general feature of polymethine dyes we have used to produce laser action in the near infrared (at least at the concentrations required for efficient laser pumping). Such a. selfabsorption is thought to be responsible for the increase in laseroutput wavelength with increasing dye concentration which was observed by Miyazoe and Maeda [5]. In Fig. l(c), the curves labeled (F) and (F-A) have been normalized to illustrate the general distortion of the fluorescence spectrum caused by selfabsorption. The absorption cross section per dye molecule was calculated from these experiments. Table I gives values of the absorption cross section at the ruby-laser emission wavelength (694.3 nm) for both dyes in ethanolic and aqueous solutions. ported by the Advanced Research Projects Agency and monitored by the Army Manuscript received March 21, 1973: revised July 5, 1973. This research was supResearch Office, Durham, N.C. The authors are with the Department of Mechanical Engineering, Massachusetts Institute of Technology, Cambridge, Mass. 02139. E! Registered trademark of Rohm & Haas Co. ’ Eastman Kodak Co., Rochester, N.Y. 14650, Cat. No. 13075. WAVELENGTH (NANOMETERS) 600 700 WAVELENGTH (NANOMETERS) 800 I I _, ‘. 0 0 I , , ’ 500 600 700 800 600 700 800 WAVELENGTH (NANOMETERS) WAVELENGTH (NANOMETERS)