Distributed feedback lasers up to the 400th Bragg order with an organic active layer

Abstract

The output characteristics and lasing threshold behaviour of higher order Bragg lasers is explored using an organic active layer spin-cast over substrate-defined fused-silica gratings. Gratings ranging from 1st to the 400th Bragg order of varying duty cycle are fabricated with standard e-beam lithography. Distinct diffraction orders are observed at lower Bragg orders but smears out towards higher orders due to overlapping diffracted orders. Significant variation in thresholds is observed with duty cycle for most Bragg orders. A dramatic reduction in threshold is observed with increasing cavity length. The lowest lasing thresholds obtained for 4th and 400th order distributed feedback (DFB) lasers are ~1.4 µJ cm-2 and 4 µJ cm-2, respectively, using F80.9BT0.1 as an active layer. 400th order Bragg lasers are fabricated with direct-write photolithography using a UV laser diode, with comparable thresholds to e-beam lithography fabricated devices.