2 July 1999 Ž . Chemical Physics Letters 307 1999 163–166 www.elsevier.nlrlocatercplett Effective stimulated emission and excited state absorption measurements in the phenylene–vinylene oligomer ž ž / // 1,4-bis- alpha-cyanostyryl -2,5-dimethoxybenzene Fryad Z. Henari a, ) , Hacene Manaa a , Kevin P. Kretsch b , Werner J. Blau b , H. Rost c , S. Pfeiffer c , A. Teuschel c , H. Tillmann c , H.H. Horhold c ¨ a Physics Department, UniÕersity of Bahrain, Isa Town, Bahrain b Physics Department, Trinity College, Dublin 2, Ireland c Friedrich-Schiller UniÕersity, Institute for Organic Chemistry and Macromolecular Chemistry, Humboldtstr. 10, Jena, Germany Received 10 December 1998; in final form 6 May 1999 Abstract The pump probe technique was used to measure the effective stimulated emission and excited state absorption Ž Ž . . cross-sections of the phenylene–vinylene oligomer 1,4-bis- alpha-cyanostyryl -2,5-dimethoxybenzene dissolved in toluene for different probe wavelengths. The effective stimulated emission cross-section was calculated from s ss ys SE-eff SE ESA and found to be positive. The possible laser tuning ranges have been determined. The spectral and temporal behavior of the optical transitions were investigated. The dynamics of these optical transitions permitted direct measurements of the lifetime of the lowest triplet state, and was found to be of the order of 11 ms. q 1999 Elsevier Science B.V. All rights reserved. Conjugated polymers have been used in a wide range of optoelectronic applications, such as photolu- w minescent, electroluminescent and laser devices 1– x 3 . Conjugated phenylene–vinylene polymers have proved to be good candidates for a lasing medium due to their wide range of tunability, and high photo- luminescence quantum yield in both solution and w x solid state 4–9 . The effective stimulated emission cross-section and excited absorption cross-section of para-phenylene–ethynylene was determined in the w x fluorescence region 10 using a recently developed w x fluorescence amplification technique 11 . ) Corresponding author. E-mail: fryad@sci.uob.bh; fax: q973- 682-582 Excited state absorption plays an important role in determining the efficiency of a laser medium. In the pump region, the excited state absorption reduces the pumping efficiency and in the stimulated emission wavelength region, it reduces the laser gain and the w x laser tuning range 12,13 . If the excited state absorp- tion cross-section is larger than the stimulated emis- sion cross section, then the lasing effect cannot be obtained in such a medium. In wavelength regions where the excited-state absorption is larger than ground-state absorption, reverse saturation absorp- tion can occur. Due to the vast range of available vibrational and rotational degrees of freedom in con- jugated polymers, the optical transitions are charac- terized by broad ground-state absorption and emis- sion bands. Consequently, the overlapping of these 0009-2614r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. Ž . PII: S0009-2614 99 00536-9