Room-Temperature Nanoimprinting Enables Fabrication of 1D Photonic-Crystal Oligomer-Based Lasers
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concentration was increased into the range of 0.05–0.1 M, instead of the range of 0.002–0.1 M, polyaniline rod-like structures formed together with the nanofibers. Typical nanorod diameters were 250–500 nm and lengths were 5–10 µm. Although the mechanism of polyaniline nanofiber formation is unknown, the researchers found that nanoparticles resembling hollow spheres formed during irradiation, and their aggregation seemed to play an important role in the fiber formation. According to the researchers, further studies on the characteristics and precise formation conditions of these hollow nanoparticles are currently being conducted. LUCY YUE HU
Room-Temperature Nanoimprinting Enables Fabrication of 1D Photonic-Crystal Oligomer-Based Lasers The photoluminescence and stimulatedemission properties of some conjugated compounds make them well suited for use as distributed feedback resonators (DFBs) in solid-state organic lasers. Organic lasers are attractive alternatives to conventional
semiconductor lasers, as they are likely to be more easily and cheaply manufactured. However, many fabrication techniques for forming grooved DFB structures can cause degradation of the optical properties by incorporating oxygen at high temperatures or by poor transfer of the topography. To avoid these issues, D. Pisignano (Università di Lecce, Italy), G. Barbarella (Istituto per la Sintesi Organica e la Fotoreattività, Bologna, Italy), and colleagues used a room-temperature nanoimprinting process to print one-dimensional photonic crystals made of a thiophene-based pentamer (T5oCx). By using these photonic crystals as resonators in DFB lasers, the research team printed single-mode lasers with low lasing thresholds. Resonator fabrication began by spincasting the T5oCx oligomer onto quartz substrates. A silicon grating formed by electron-beam lithography and reactive ion etching was pressed into the organic film, leaving behind a replica of the grating structure in the oligomer. The gratings had a period of 400 nm and areas as large as 3 mm × 7 mm. Pisignano, Barbarella, and their colleagues then studied the emission proper-
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MRS BULLETIN • VOLUME 30 • MARCH 2005
ties of these resonators, as reported in the February 1 issue of Optics Letters (p. 260). After illuminating a small area with a HeCd laser (λ = 325 nm), the researchers collected the angle-resolved photoluminescence (PL) spectra of the oligomer gratings. They found that there is a strong normalincidence PL peak at 611 nm that is not present in as-cast films. As the collection angle increased, the peak split into two, with the peak separation eventually spanning the amplified stimulated emission band of the T5oCx. The researchers also demonstrated the performance of the oligomer gratings used as a DFB laser by pumping a large area (7 mm × 0.5 mm) with a frequency-tripled Nd:YAG laser (λ = 355 nm, 3 ns pulses) under vacuum. These organic lasers operated at a wavelength of 637 nm in the second order of the diffraction grating and
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