DNA-Templated Nanowire Fabrication Technique Developed
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Dye-DNA-Lipid Thin Films Exhibit Durable Amplified Spontaneous Emission A considerable obstacle in the current development of solid-state lasers is durability. Researchers at the Chitose Institute of Science and Technology, Japan, have demonstrated that thin films composed of DNA, lipid, and a hemicyanine dye have the potential for a practical, durable, solid-state laser. As reported in the August 19 issue of Applied Physics Letters, Y. Kawabe and coworkers fabricated thin films by casting on Teflon plates a dichloromethane-methanol solution containing DNA-hexadecyltrimethylammonium (HTMA) and 4-[4(dimethyamino) stylyl]-1-dococylpyridinium bromide (DMASDPB), a derivatized hemicyanine dye known for its nonlinear optical properties. Self-standing films were formed by evaporating the solvent in vacuum. Film thickness was controlled by varying the concentration. Fluorescence enhancement of DMASDPB in DNA was previously confirmed. Threshold-energy levels were determined by measuring the light intensity emitted from the samples as a function of pump-pulse energy. The output of a frequency-doubled Nd3+:YAG laser (λ = 532 nm, τp = 7 ns), used as the pump source, was focused with a normal incident angle onto a 1 mm × 5 mm stripe on the sample. The emitted light was observed perpendicular to the incident beam along the direction of the stripe. Emission peaked at 631 nm under lowintensity pumping. A shift in the emission peak to 624 nm with concomitant spectral narrowing were observed under highintensity pumping. While output energy is proportional to pump energy in the low-intensity region, superlinear dependence is evident for energies greater than 0.5 mJ/cm2. The researchers attribute the spectral narrowing to amplification because the narrowing and superlinear enhancement occur at the same intensity. Amplification occurs at relatively low selfabsorption. In addition, the emitted light is linearly polarized parallel to the film surface, indicating enhanced amplification. The researchers conclude from these results that amplified spontaneous emission occurs in the DMASDPB-HTMA films. The role played by DNA was investigated in two control experiments. First, thin films were fabricated with poly(methyl methacrylate) and DMASDPB. The resulting films showed strong fluorescence under the same pumping conditions, but neither intensity enhancement nor spectral narrowing was observed. In the second control experiment, a simple laser cavity
containing a chloroform solution of DMASDPB was prepared. No indication of lasing was observed. The same apparatus confirmed lasing for some conventional laser dyes as well as for a chloroform solution of DMASDPB and HTMA. The researchers believe that this clearly demonstrates that DMASDPB acquires the ability to lase through its interaction with DNA, which likely alters the dye’s electronic state. Further structural and spectroscopic studies to investigate the details of the dye–DNA interaction are planned by the researchers. In this preliminary study, which was performed in air at room tempera
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