Working characteristics of external distributed feedback polymer lasers with varying waveguiding structures

We report the fabrication and characterization of second-order external distributed feedback (DFB) lasers based on blue-emitting polymer poly (9, 9-dioctyl-fluorene) (PFO). The relief grating prepared in the photo-resist layer on top of the gain medium was directly employed as the resonator. The effect of various structural parameters including the thickness of the active film, the thickness of the residue layer, and the depth of the relief grating on performance of these polymer lasers was investigated. An analytical approach based on the slab waveguiding theory and the Bragg condition was developed to accurately determine the lasing wavelength. We found that laser threshold increases monotonously as the thickness of the residue layer increases. The presence of the residue layer between the relief grating and the organic semiconductor layer weakens the strength of light coupling between the two layers, leading to a decreased gain for the specific lasing mode. The lowest laser threshold level of 80 μJ cm−2 was obtained when the thickness of the gain medium was around 250 nm, indicating an optimum balance between light feedback provided by the grating and optical amplification in the organic semiconductor. Our results not only provide insights into working mechanisms of external DFB polymer lasers, but also design rules for fabricating them.