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Lloyd-mirror setup used for modulation of the film (a) and picture of a typical surface ... (a) Setup for DFB laser experiment: The pump beam of a. Nd3:YAG is ...
JOURNAL OF APPLIED PHYSICS

VOLUME 89, NUMBER 5

1 MARCH 2001

Laser emission in periodically modulated polymer films Licinio Rocha,a) Vincent Dumarcher, Christine Denis, Paul Raimond, Ce´line Fiorini, and Jean-Michel Nunzi LETI (CEA-Technologies Avance´es), DEIN/SPE, Laboratoire Composants Organiques, CEA Saclay, F-91191 Gif-sur-Yvette Cedex, France

共Received 7 June 2000; accepted for publication 2 November 2000兲 We report on the realization of a compact distributed feedback laser using luminescent polymer films where the optical feedback is provided by Bragg diffraction on an index grating. Permanent modulation of the polymer refractive index is achieved using an original technique for photoinduced patterning of surface relief grating, using laser-controlled mass-transport in azoaromatic polymers. We describe the fabrication of such surface gratings and show the laser emission properties resulting from a transversal one-photon pumping of the sinusoidally modulated polymer films upcovered with a luminescent-dye-doped film. Control of the laser wavelength by the grating pitch is evidenced. © 2001 American Institute of Physics. 关DOI: 10.1063/1.1335636兴

The principle of distributed feedback 共DFB兲 lasers was first described by Kogelnik and Shank.1 In these mirrorless lasers the optical feedback is provided by Bragg scattering from periodic perturbations of the gain and/or refractive index of the medium. Optical feedback and gain are distributed throughout the medium and the emitted laser wavelength depends on the modulation period. The interest for organic lasers2–6 fabricated by means of a low cost technique has matured in recent years with the development of polymers with high laser damage resistance. Threshold and laser emission features have been studied in various luminescent polymer films where the modulation was obtained by a spatially periodic absorption in the medium.7–10 As a next step, studies are now performed towards designing and realizing devices with permanent modulations resulting from modulation of the polymer refractive index. A recent achievement in this respect is the realization of surface gratings by means of lithography11,12 or embossing 13 techniques. Optical feedback allowing laser oscillation is then provided by diffraction on a permanent index grating due to the modulation of the film surface 共more precisely at the interface between two media with different indices兲. Here, we propose a more flexible method of film patterning which consists in a single step optical process and permits control of the grating pitch in a simple way. Laser emission properties resulting from one-photon pumping of such modulated films are presented. Azodye aromatic polymers present remarkable properties for patterning and structuring using light matter interactions.14–16 More particularly, it was demonstrated that irradiation by an interference pattern between coherent light beams can induce a migration of the polymer from high to low intensity regions in films bearing azodye molecules.17 Photoinduced mass transportation occurs at temperatures much below the glass transition temperatures. It is controlled by the polarization direction and by the shape of the interference pattern of the pump beams on the film. For a given

polarization of the interfering beams the exposure dose controls the surface modulation amplitude, which can be equal to twice the film thickness, corresponding to the case where all the matter has been removed from the high intensity regions of the interference pattern. Practical realization of the device consists of the modulation of an azopolymer substrate on top of which a luminescent film is deposited. The azopolymer film 共index⫽1.6兲 is spin-coated on a glass slide 共index⫽1.5兲. It is made of a yellow azodye aromatic molecule, 4-phenylazophenol, offering similar properties than its more extensively studied analog DR1 关4-共N-共2-hydroxyethyl兲-N-ethyl兲-amino-4⬘-nitroazobenzene兴 as concerns photoinduced microengineering.18 The photoactive molecules are covalently bound onto 35% of the monomers of a poly共methylmethacrylate兲 skeleton. In order to record a sinusoidal modulation on the initially planar surface of the azopolymer film we use the Lloyd mirror setup represented in Fig. 1共a兲. The incident beam of an Argon laser is divided into two parts: the first half is directly incident onto the sample and the second half reflects first onto a metallic mirror rigidly held at 90° to the sample. The wavelength of the Argon laser 共␭⫽514 nm兲 falls into the absorption band of the chromophores and the intensity used is chosen around 200 mW cm⫺2 . The polarization is set in the plane of incidence in order to favor a mass transportation following a direction perpendicular to the fringes of the interference pattern. A typical surface relief grating imaged with an atomic force microscope 共AFM兲 is shown in Fig. 1共b兲. The grating pitch ⌳ corresponds to the interfringe spacing of the interference pattern given by ⌳⫽

共1兲

where ␭ p is the wavelength of the Argon laser and ␪ the incidence angle of the pump beams onto the sample. Simply by changing the inclination of the pump beam onto the film surface, we can modify the grating pitch which is an important characteristic in order to control DFB laser emission.

a兲

Electronic mail: [email protected]

0021-8979/2001/89(5)/3067/3/$18.00

␭p , 2 sin ␪

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© 2001 American Institute of Physics

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Rocha et al.

J. Appl. Phys., Vol. 89, No. 5, 1 March 2001

FIG. 1. Lloyd-mirror setup used for modulation of the film 共a兲 and picture of a typical surface relief grating obtained with an AFM 共b兲.

After realization of the photoinduced grating, a luminescent film is spin-coated onto the modulated azopolymer surface. The luminescent polymer is a 共20 g/l兲 polyacrylic acid 共PAA兲 polymer doped with 4⫻10⫺3 M of Rhodamine 6G 共Rh6G兲. The film thickness is near 250 nm and the index is 1.52. The samples are pumped transversally, close to the Rh6G maximum absorption, with the ␭ exc⫽532 nm second harmonic, of a Q-switched mode locked Nd3⫹ :YAG laser delivering 33 ps pulses at a 10 Hz repetition rate 关Fig. 2共a兲兴. In order to improve lateral confinement we use a 30 cm focal length cylindrical lens which concentrates the pump beam surface along a narrow line of 80 ␮m width. Pump beam surface onto the film is 2⫻10⫺3 cm2 . The signal emitted by the film is collected with the help of a 5 cm focal length lens into an optical fiber coupled with a spectrometer. The light is then dispersed by a 12 cm focal length monochromator, using a 1200 lines/mm grating, and detected with a charged coupled device. Dispersion of the spectrometer is 0.15 nm per pixel. As demonstrated by Kogelnik and Shank1 the DFB laser modes being able to propagate must verify the following equation: ␭ L⫽

2n⌳ , m

共2兲

where ␭ L is the laser wavelength emitted by the film, ⌳ the grating pitch, n the effective index of the mode propagating inside the medium, and m the diffraction order onto the grating. The device laser emission wavelength is controlled with the grating pitch. The choice of the DFB laser wavelength is

FIG. 2. 共a兲 Setup for DFB laser experiment: The pump beam of a Nd3⫹ :YAG is focused onto the luminescent film surface 共D兲 by a cylindrical lens 共A兲. The film emission is collected with the help of a lens 共B兲 into an optical fiber 共C兲 coupled to a spectrometer by a computer 共E兲. 共b兲 Laser emission from PAA films doped with Rhodamine 6G for different grating pitches.

limited by the spectral bandwidth of the stimulated emission spectrum of the dye used. It is close to 40 nm in the case of the PAA/Rh6G film.10 For pitches near 410 nm we obtain a very narrow laser emission centered near 610 nm corresponding to the second order of diffraction 共 m⫽2兲 with a full width at half maximum equal to 0.4 nm, showing a high spectral selectivity due to the Bragg grating nature of the device. In Fig. 2共b兲 we show that laser emission can be tuned over 40 nm using different grating pitches around 420 nm. The laser threshold measured at the maximum of the stimulated spectrum is 0.2 mJ/cm2 . The threshold can be lowered by optimizing the confinement of the laser mode into the gain layer. Indeed, in our case, the azodye polymer film has a higher index than the active layer which means that guiding occurs mainly in the modulated substrate. This wave guiding aspect must be studied in more detail in order to optimize laser properties of the device and compare them with transient grating experiments.9,10 The main advantage of the present technique with respect to transient gratings is the possibility to achieve larger modulation amplitudes of the refractive index. The same experiment was also realized in an azopolymer film doped with the laser dye and modulated in the same conditions for larger pitches. The refractive index modulation was then provided by modulation of the film/air interface. Laser emission at 600 nm was obtained for a pitch around 610 nm, corresponding to the third order of diffraction 共m⫽3兲. This configuration involves a one step preparation process but is limited to short exposure times following

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J. Appl. Phys., Vol. 89, No. 5, 1 March 2001

laser degradation of the luminescent dye 共a bleaching effect兲, which prevents the realization of larger modulation amplitudes. In conclusion, we have implemented a new fabrication technique for polymer DFB lasers. The originality of this method is that patterning involves no development process: the gratings are obtained by photoinduced mass transportation. Moreover, patterning is a one stage process in which the grating parameters can simply be chosen by setting the incidence angle of the pump beams onto the film and the exposure time. The resulting mirrorless laser is compact and can be deposited on many different solid or plastic substrates. The authors would like to thank Dr. Kevin P. Kretsch for his critical reading of the article and Dr. Pierre-Allain Chollet for his help in the indices measurements. H. Kogelnik and C. V. Shank, J. Appl. Phys. 43, 2327 共1972兲. M. Kuwata-Gonokami, R. H. Jordan, A. Dodabalapur, H. E. Katz, M. L. Schilling, R. E. Slusher, and S. Ozawa, Opt. Lett. 20, 2093 共1995兲. 3 N. Tessler, G. J. Denton, and R. H. Friend, Nature 共London兲 382, 695 共1996兲. 4 F. Hide, M. A. Diaz-Garcia, B. J. Schwartz, M. R. Anderson, Q. Pei, and A. J. Heeger, Science 273, 1833 共1996兲. 5 A. Schu¨lzgen, Ch. Spiegelberg, M. M. Morell, S. B. Mendes, B. Kippelen, 1 2

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N. Peyghambarian, M. F. Nabor, E. A. Mash, and P. M. Allemand, Appl. Phys. Lett. 72, 269 共1998兲. 6 V. G. Kozlov, G. Parthasarathy, P. E. Burrows, S. R. Forrest, Y. You, and M. E. Thompson, Appl. Phys. Lett. 72, 144 共1998兲. 7 H. Kogelnik and C. V. Shank, Appl. Phys. Lett. 18, 152 共1971兲. 8 M. Maeda, Y. Oki, and K. Imamura, IEEE Trans. Quantum Electron. 33, 2146 共1997兲. 9 KP. Kretsch, W. J. Blau, V. Dumarcher, L. Rocha, C. Fiorini, J-M. Nunzi, S. Pfeiffer, H. Tillmann, and H.-H. Ho¨rhold, Appl. Phys. Lett. 76, 2149 共2000兲. 10 V. Dumarcher, L. Rocha, C. Denis, C. Fiorini, J. M. Nunzi, F. Sobel, B. Sahraoui, and D. Gindre, Pure Appl. Opt., 4 共2000兲. 11 N. Mukherjee, B. J. Eapen, D. M. Keicher, S. Q. Luong, and A. Mukherjee, Appl. Phys. Lett. 67, 3715 共1995兲. 12 G. Gigli, R. Rinaldi, C. Turco, P. Visconti, R. Cingolani, and F. Cacialli, Appl. Phys. Lett. 73, 3926 共1998兲. 13 M. Berggren, A. Dodabalapur, R. E. Slusher, A. Timko, and O. Nalamasu, Appl. Phys. Lett. 72, 410 共1998兲. 14 P. Rochon, E. Batalla, and A. Natansohn, Appl. Phys. Lett. 66, 136 共1995兲. 15 D. Y. Kim, L. Li, J. Kumar, and S. K. Tripathy, Appl. Phys. Lett. 66, 1166 共1995兲. 16 P. Lefin, C. Fiorini, and J.-M. Nunzi, Pure Appl. Opt. 7, 71 共1997兲; Opt. Mater. 9, 323 共1998兲. 17 C. Fiorini, N. Prudhomme, A.-C. Etile´, P. Lefin, P. Raimond, J.-M. Nunzi, Macromol. Symp. 137, 105 共1999兲; C. Fiorini, N. Prudhomme, G. de Veyrac, I. Maurin, P. Raimond, and J.-M. Nunzi, Synth. Meth. 共to be published兲. 18 N. K. Viswanathan, D. Y. Kim, S. Bian, J. Williams, W. Liu, L. Li, L. Samuelson, J. Kumar, and S. K. Tripathy, J. Mater. Chem. 9, 1941 共1999兲.

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