Novel diffractive feedback structures for semiconducting polymer lasers

Abstract

Semiconducting conjugated polymers have recently attracted significant interest as amplifying media for solid-state lasers due to their functional photo-physical properties and simple fabrication. Distributed feedback (DFB) cavities are proving to be the most attractive for polymer lasers, since they can combine the properties of transverse optical pumping, low threshold and practical output beams. To date, in most polymer DFB lasers the feedback is provided by second order diffraction. This has the advantage of surface emission, though it also imposes extensive scattering losses. In this work, we present the use of alternative structures that attempt to reduce the threshold of polymer DFB lasers, and also achieve dual wavelength operation. The former was addressed with cavities formed by alternative symmetries of the Brillouin zone of a square lattice. Using the diagonal ΓM symmetry first order feedback was attained. The threshold energy was thus reduced by almost an order of magnitude as compared with the more commonly used ΓX symmetry of second order square gratings. Finally, we show that two lasing wavelengths may be set independently in a semiconducting polymer laser by using a doubly periodic (i.e. Moiré) DFB grating.