Influence of gain material concentration on an organic DFB laser

In this work, we investigate the properties of an organic distributed feedback laser as the concentration of the gain material in the waveguide core is varied across two orders of magnitude, from 5% down to 0.025%. The laser dye DCJTB (4-(Dicyanomethylene)-2-tert-butyl-6-(1,1,7,7-tetramethyljulolidin-9-enyl-vinyl)-4H-pyran) incorporated into a PVK (poly(9-vinylcarbazole)) host matrix provided the gain. The composite layer of PVK:DCJTB was spin-cast onto a silica grating with second order periodicity, and upon nanosecond optical excitation lasing was generated in the wavelength range of 600 nm. The threshold pulse energy for achieving lasing increased as the concentration of DCJTB was reduced, however the threshold excitation density quantified in terms of number of excited molecules per unit area remained nearly constant at 1.3×1013 molecules/cm2. In contrast, the relative slope efficiency for lasing decreased considerably as the gain concentration was reduced. We show that this effect can not be explained by a standard 4-level lasing model, but rather that it is due to optically induced charge separation for the DCJTB molecules situated in the PVK host matrix. Our findings suggest that fast charge separation and long back recombination times can be a significant factor in limiting further reduction of the gain concentration in organic DFB lasers.