Fabrication of refractive index and relief gratings in polymer films for DFB lasers
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Fabrication of refractive index and relief gratings in polymer films for DFB lasers *
T. Kavc1, G. Langer1, W. Kern1 , G. Kranzelbinder2, E. Toussaere2, G.A. Turnbull3, I.D.W. Samuel3, K.F. Iskra4, T. Neger4 1 Institute for Chemistry and Technology of Organic Materials, Graz University of Technology, 8010 Graz, Austria 2 LPQM, École Normale Supérieure de Cachan, 94230 Cachan, France 3 Ultrafast Photonics Collaboration, School of Physics and Astronomy, University of St. Andrews, Fife KY16 9SS, UK 4 Institute of Experimental Physics, Graz University of Technology, 8010 Graz, Austria * To whom all correspondence should be adressed ABSTRACT A styrene copolymer of 4-vinylbenzyl thiocyanate (PST-co-VBT) was employed as recording material for optical interference patterns with periods Λ < 1 µm. Using lower intensity laser irradiation (4 mJ cm-2, λ = 266 nm), refractive index gratings were produced in PST-co-VBT by an UV induced photoisomerization SCN – NCS. Subsequent modification of the patterns with gaseous amines yielded surface relief gratings via the formation of derivatives of thiourea. Laser irradiation with higher pulse energies (7 mJ cm-2, λ = 266 nm) directly produced surface relief gratings (modulation depth 30 nm). These gratings were also reactive towards amine reagents and allowed a selective functionalization of the grooves of the relief (“reactive gratings”). Optically inscribed gratings in PST-co-VBT were employed as optical resonators for distributed feedback (DFB) lasing. With a laser dye (DCM) dissolved in PST-co-VBT, optically pumped DFB laser action was observed after inscribing index and relief gratings. The pumping threshold for lasing Ith was 250 nJ cm-2 at λ = 532 nm. INTRODUCTION Distributed feedback (DFB) lasers based upon inorganic laser materials are well developed. Since the pioneering work of Kogelnik and Shank [1] DFB lasers based upon organic compounds as gain materials have been realized as a further step to applications of organic materials in the field of optoelectronics. Low molecular weight laser dyes in polymer matrices and conjugated polymers (e.g., derivatives of poly-p-phenylene-vinylene (PPV) [2,3] and ladder-type poly-p-phenylene (m-LPPP) [4,5]) have been used as active materials. In most cases the active layer is spin-cast as a thin film on a substrate (e.g., SiO2 or PET). For DFB lasing, a periodic change of the effective refractive index neff and / or the gain in the waveguide is required. The formation of relief and index patterns in polymer films is common practice in the field of holography. For DFB laser structures based upon index gratings, it is important that the range of irradiation wavelengths used in the patterning step are different from the wavelengths of laser action. Otherwise the inscribed index patterns would be wiped out during operation of the device.
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For laser operation in the visible, the recording material should thus only change its refractive index under UV irradiation. With the aim of writing short grating periods, deep UV irradiation with
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