Polymer lasers: recent advances

Abstract

The development of organic thin film lasers has seen tremendous progress over the past few years. Only a few materials are necessary to allow for continuous wavelength tunability in the spectral region from the UV to the near IR. At the same time, the lasing thresholds of organic thin film lasers have been reduced considerably both due to improved low-loss distributed feedback (DFB) resonator structures and highly efficient gain materials based on guest-host energy transfer. Aside from the as yet open issue of electrical operation of organic lasers, which we will address briefly in this paper, there are numerous applications (e.g. in biotechnology, spectroscopy) where optically driven organic lasers may be the more cost effective and versatile solution. In this context, tunable polymer lasers pumped by compact and inexpensive InGaN laser diodes will be shown. These lasers are based on a modified poly(9,9'-dioctylfluorene) derivative (BN-PFO) containing 12% of -6,6'-(2,2'-octyloxy-1,1'-binaphthyl) spacer groups doped with a few wt% of the stilbene dye 1,4-Bis(2-(4-(N,N-di(p-tolyl)amino)phenyl)vinyl-benzene (DPAVB). With the same host polymer (BN-PFO) quasi continuous wave operation (up to 5 MHz) can be demonstrated. Highly repetitive lasers are especially desirable for many spectroscopic applications. This regime of operstion is found to be impeded by the photo-physics in doped organic systems where the accumulation of absorptive species in the gain medium leads to piled-up absorption losses and consequently to termination of the lasing process. The presence of the dopand molecules seems to strongly promote the formation and stabilization of the species which we relate to triplet excitons. Therefore, the concentration of the dopand affects the feasibility of quasi-cw operation of thin-film organic lasers. Strategies and results to achieve highly repetitive operation in low-threshold guest-host systems BN-PFO:DPAVB or BN-PFO:poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene] (MEH-PPV) will be presented.