ORGANIC LASERS
New resonator geometry advances polymer laser
A flexible, conjugated-polymer laser can be made by incorporating into the design a periodically modulated substrate that acts as a resonator to create a distributed-feedback effect. This optically pumped design is a ste¥toward electrically pumped all-polymer lasers.
Uli Lemmer and Jochen Feldmann, both at the Ludwig-Maximilians-Universität München (Munich), note that conjugated polymers can exhibit high optical gain comparable to the best inorganic semiconductor laser material. To achieve lasing, however, a suitable optical resonator geometry is still needed. Lemmer and Feldmann are part of a team investigating such structures at the Ludwig-Maximilians-Universität München, the Max-Planck-Institut für Polymerforschung Main¥(Mainz), and the Fraunhofer Institut für Solare Energiesysteme (Freiburg).
The researchers designed a structured substrate that provides distributed feedback in a lateral direction by the periodic modulation of the refractive index. To achieve this modulation, a thin film of a ladder-type poly(p-phenylene) (LPPP) was spin-coated on to¥of a flexible, 125-µm poly(ethylene terephtalate) (PET) substrate that contained an embossed periodic height modulation of 300 nm. The stamper for embossing the periodic modulations was produced by holographic exposure of a positive photoresist and then transfer of the surface-relief structure into a nickel stamper by electroforming.
A frequency-doubled, regeneratively amplified Ti:sapphire laser provided the optical excitation, with a pulse duration of less than 150 fs at 400 nm. Pulse energies varied from 0.1 to 100 nJ. The pum¥beam was focused to a spot of approximately 200 µm in diameter.
The researchers believe that the subsequent blue-green lasing results from a complex interaction of stimulated emission, photoinduced absorption, and optical feedback. The interaction occurs when the modulated heights diffract the light as it travels in waveguide mode along the LPP¥layer. The first-order diffracted light is coupled out of the waveguide and propagates perpendicular to the film. The second-order diffraction is fed into the counterpropagating wave and induces the optical feedback.
This device has proved to be remarkably stable and should provide the basis for further experiments toward an electrically pumped polymer laser. The researchers hope that this approach will open the possibility of producing low-cost, large-area laser devices of virtually any shape.
W. Conard Holton