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Organic Semiconductor Lasers with Two-Dimensional Distributed Feedback

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Organic Semiconductor Lasers with Two-Dimensional Distributed Feedback Organic semiconductor lasers with two-dimensional distributed feedback Dissertation der Fakultät für Physik der Ludwig-Maximilians-Universität München vorgelegt von Stefan Riechel aus München München, den 11. Februar 2002 1. Gutachter : Prof. Dr. Jochen Feldmann 2. Gutachter : Prof. Dr. Khaled Karrai Tag der mündlichen Prüfung: 15. Mai 2002 Front page: Photograph of a mechanically flexible 2D-DFB laser made from the conjugated polymer MeLPPP. The laser is optically pumped well above the threshold for amplified spontaneous emission. The complex far-field emission pattern (blue-green) results from a large detuning between the resonance wavelength and the gain maximum. The picture was taken in the course of the documentation for the Philip-Morris Forschungspreis 1999 awarded to Prof. Dr. J. Feldmann and Dr. U. Lemmer (Courtesy by the Philip Morris Stiftung, Fallstraße 40, D-81369 München). Scientific publications of results presented in this work • A nearly diffraction limited surface emitting conjugated polymer laser utilizing a 2D photonic bandstructure S. Riechel, C. Kallinger, U. Lemmer, J. Feldmann, A. Gombert, V. Wittwer, and U. Scherf, Applied Physics Letters 77, 2310 (2000). • Lasing modes in organic solid state distributed feedback lasers S. Riechel, U. Lemmer, J. Feldmann, T. Benstem, W. Kowalsky, U. Scherf, A. Gombert, and V. Wittwer, Applied Physics B 71, 897 (2000). • Very compact tunable solid-state laser utilizing a thin-film organic semiconductor S. Riechel, U. Lemmer, J. Feldmann, S. Berleb, A. G. Mückl, W. Brütting, A. Gombert, and V. Wittwer, Optics Letters 26, 593 (2001). • Conjugated Polymers: Lasing and Stimulated Emission U. Scherf, S. Riechel, U. Lemmer, and R. F. Mahrt, Current opinion in Solid State and Materials Science 5, 143 (2001) • Picosecond amplified spontaneous emission bursts from a molecularly doped organic semiconductor C. Kallinger, S. Riechel, O. Holderer, U. Lemmer, J. Feldmann, S. Berleb, A. G. Mückl, W. Brütting, Journal of Applied Physics 91, 6367 (2002) Further publications related to this work • Laser aus Plastik C. Kallinger, A. Haugeneder, S. Riechel, U. Lemmer und J. Feldmann Einsichten: Forschung an der Ludwig-Maximilians-Universität München 1/99, p. 22 • Polymer laser threshold plummets Opto&Laser Europe, May 1999, p. 19 • Laserlicht aus Polymeren Physikalische Blätter 56, January 2000 , p. 25 • Polymer laser is nearly diffraction limited Laser Focus World, December 2000, p. 49 • Thin-film organic laser tunes very compact DFB Laser Focus World, September 2001, p. 15 Contents Zusammenfassung 1 Summary 3 1 Introduction 4 2 Fundamentals of organic semiconductor lasers 6 2.1 Organic semiconductors ................................................................................................. 6 2.1.1 Electronic and optical properties of organic semiconductors ............................... 7 2.1.2 Transport phenomena in disordered organic semiconductors............................. 12 2.2 Lasing in organic semiconductors ................................................................................ 17 2.2.1 Stimulated emission............................................................................................ 17 2.2.2 Amplified spontaneous emission ........................................................................ 18 2.2.3 Resonant laser structures..................................................................................... 19 2.3 Distributed feedback lasers........................................................................................... 20 2.3.1 Slab waveguides.................................................................................................. 20 2.3.2 One-dimensional distributed feedback lasers ..................................................... 22 2.3.3 Two-dimensional distributed feedback lasers..................................................... 27 2.3.4 Randomly distributed feedback .......................................................................... 29 3 Experimental techniques 32 3.1 Sample preparation and characterization...................................................................... 32 3.2 Optical experiments at high excitation density............................................................. 33 3.2.1 Determination of the excitation density.............................................................. 34 3.2.2 Regeneratively amplified fs-laser system ........................................................... 34 3.2.3 Time-integrated emission.................................................................................... 35 3.2.4 Temporally resolved emission with sub-ps resolution........................................ 36 3.2.5 Differential transmission spectroscopy............................................................... 38 4 Materials for organic solid-state lasers 40 4.1 The conjugated polymer MeLPPP................................................................................ 40 4.1.1 Electronic, optical and transport properties of MeLPPP..................................... 40 4.1.2 Stimulated emission in MeLPPP......................................................................... 42 4.2 The composite molecular system Alq3:DCM............................................................... 44 4.2.1 Electronic, optical and transport properties of Alq3:DCM ................................. 44 4.2.2 Stimulated emission in Alq3:DCM ..................................................................... 46 5 Lasers with one-dimensional distributed feedback 49 5.1 Fabrication of mechanically flexible organic DFB lasers ............................................ 49 5.2 Characterization of the laser operation......................................................................... 51 5.3 Lateral laser modes....................................................................................................... 53 5.4 Wavelength tuning by variation of the film thickness.................................................. 55 5.5 Coupling mechanism in organic solid-state DFB lasers............................................... 57 5.5.1 Calculation of the coupling coefficient............................................................... 57 5.5.2 Coupling in 2nd order DFB structures ................................................................. 58 5.5.3 Prediction of coupling mechanisms in 1st order DFB structures ........................ 62 5.5.4 Implications for the design of optimized DFB lasers ......................................... 63 6 Lasers with two-dimensional distributed feedback 65 6.1 Fabrication.................................................................................................................... 65 6.2 Characterization............................................................................................................ 66 6.3 Laser modes of 2D-DFB lasers .................................................................................... 68 6.3.1 Far-field identification of lasers modes .............................................................. 71 6.3.2 Factors favoring monomode operation ............................................................... 72 6.4 Photonic band structure analysis .................................................................................. 74 7 Lasers with randomly distributed feedback 77 7.1 Sample preparation and characterization...................................................................... 78 7.2 Random laser operation................................................................................................ 80 7.2.1 Variation of excitation density............................................................................ 81 7.2.2 Variation of the excitation area........................................................................... 84 8 Emission dynamics 86 8.1 Polymer lasers .............................................................................................................. 86 8.1.1 ASE in slab waveguides...................................................................................... 86 8.1.2 Lasing dynamics ................................................................................................. 87 8.2 Emission dynamics in Alq3:DCM ................................................................................ 92 8.2.1 Gain switching oscillations in slab waveguides.................................................. 92 8.2.2 DFB-lasers .......................................................................................................... 96 9 Devices and applications of organic solid-state lasers 98 9.1 Very compact tunable organic solid-state laser............................................................ 98 9.2 Laser operation in the presence of electric contacts -- towards an organic injection laser........................................................................... 100 9.2.1 Problems associated with electrical pumping of organic lasers........................ 100 9.2.2 Design of suitable structures for organic diode lasers ...................................... 102 9.2.3 Optically pumped laser diodes.......................................................................... 105 9.2.4 Feasibility of organic diode lasers .................................................................... 110 10 Outlook 112 List of abbreviations and definitions 114 Bibliography 115 Acknowledgments 130 Curriculum Vitae 132 Zusammenfassung Die vorliegende Arbeit beinhaltet eine
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