Microstructured polymer lasers: diode-pumped lasing and extending operation lifetimes

摘要

The field of organic semiconductors lasers is developing rapidly, building on the major advances in organic light-emitting diodes. Semiconducting conjugated polymers have many attractive features for lasers, such as the very strong absorption (and hence gain), broad spectra, scope to tune emission across the visible spectrum, and simple processing [1,2]. Indeed solution processing and soft lithography can be used to rapidly form resonant structures with features on the wavelength scale. At present, all organic semiconductor lasers are pumped optically, and as a result the nature of the pump source is a key factor determining the size, cost and complexity of the overall system. Early polymer lasers were pumped by a regenerative amplifier, but steady progress has been made to improve the optical design to allow pumping by much more compact sources, such as a microchip laser [2]. We now report the demonstration of a very compact, all-solid-state polymer laser system comprising of a GaN semiconductor diode laser as the pump source with a novel surface-emitting resonator structure and an innovative energy-transfer gain medium. We also report improved operating lifetimes in micromoulded polymer DFB lasers through simple device encapsulation. For diode pumping, the polymer laser was configured as a surface-emitting, distributed Bragg reflector (DBR) laser. The resonator comprised an organic film waveguide deposited onto a silica substrate with two 2nd order ～400 nm period gratings of typically 40μm length, separated by lengths of between 10 and 100 μm (Fig. 2). The gain medium was a novel energy transfer blend of Coumarin 102 and the conjugated polymer poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV). This blend was chosen to significantly improve harvesting of diode laser pump photons (at 409 nm) to the MEH-PPV, which has only moderate absorption around 400 nm. We studied the energy transfer from dye host and its viability for lasing in the polymer guest. We found that the energy transfer efficiency was >80% and that blending actually reduced the level of non-radiative decay in MEH-PPV. Amplified spontaneous emission measurements confirmed the successful light harvesting even at high excitation densities, and showed the optimum blend ratio for high gain to be 50:50. Fig. 1 shows a typical surface emission spectrum above and below threshold for a diode-pumped polymer DBR laser (mirror separation of 20 μm). Below threshold, spontaneous emission couples to four distinct optical modes. Above a threshold energy of 420 pJ, we observe lasing with a more rapid growth of one DBR mode and a saturation of the surrounding spontaneous emission. Similar behaviour was observed for a range of cavity lengths. When pumping at higher excitation densities, the lasing mode rapidly grows to dominate the surface emission. We will discuss the origin of the frequency selection in these resonators, and their advantages for low threshold, high efficiency operation compared with 2nd order DFB lasers.