Polymer Lasers Produced by Soft Lithography
Scientists at the University of St. Andrews in the UK have fabricated semiconducting polymer distributed feedback lasers in less than two minutes using solvent-assisted microcontact molding. The researchers' work, which was published in the June 9 issue of Applied Physics Letters, suggests that it should be possible to produce active photonic devices cheaply and easily using soft lithography techniques.
Soft lithography reduces process complexity and manufacturing costs because it enables the user to replicate many copies of a polymer device from a high-quality master whose negative structure is imprinted into a reusable stamp or mold. The solvent-assisted approach further eases fabrication by employing a solvent on the stamp that dissolves the polymer to be processed, enabling it to flow into the printed pattern. Other techniques may involve the application of heat or pressure, which can alter the optical properties of the materials involved.
In the St. Andrews experiments, the researchers used poly[2-methoxy-5-(3,7-dimethyloctyloxy) parphenylenevinylene] as the laser material and chlorobenzene as the solvent. They applied a microstructured elastomeric mold to a 120-nm-thick film of the conjugated polymer to produce a crossed grating structure with a 400-nm period -- as illustrated in the atomic force microscope image shown -- which offered distributed laser feedback and output coupling by Bragg scattering. The imprinting process took 100 seconds.
Excited by the 532-nm output of a frequency-doubled Q-switched Nd:YVO4 microchip laser, the patterned films emitted 638-nm light, with a linewidth of 0.6 nm at pump powers 200 percent of the 225-nJ threshold. The researchers expect that increasing the depth of the grating structures will reduce the lasing threshold of the optically pumped devices.
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