Organic Lasers – Recent Developments on Materials, Device Geometries, and Fabrication Techniques Alexander J.C. Kuehne1* and Malte C. Gather2* 1DWI – Leibniz Institute for Interactive Materials, RWTH Aachen University, Germany 2Organic Semiconductor Centre, SUPA, School of Physics and Astronomy, University of St Andrews, United Kingdom *Email: [email protected]; [email protected] Abstract Organic dyes have been used as gain medium for lasers since the 1960s—long before the advent of today’s organic electronic devices. Organic gain materials are highly attractive for lasing due to their chemical tunability and large stimulated emission cross-section. While the traditional dye laser has been largely replaced by solid-state lasers, a number of new and miniaturized organic lasers have emerged that hold great potential for lab-on-chip applications, bio-integration, low-cost sensing and related areas, which benefit from the unique properties of organic gain materials. On the fundamental level, these include high exciton binding energy, low refractive index (compared to inorganic semiconductors) and ease of spectral and chemical tuning. On a technological level, mechanical flexibility and compatibility with simple processing techniques such as printing, roll-to-roll, self-assembly and soft-lithography are most relevant. Here, the authors provide a comprehensive review of the developments in the field over the past decade, discussing recent advances in organic gain materials — today often based on solid-state organic semiconductors — optical feedback structures, and device fabrication. Recent efforts towards continuous wave operation and electrical pumping of solid-state organic lasers are reviewed and new device concepts and emerging applications are summarized. Contents 1. Introduction ........................................................................................................................................................... 3 1.1. General conditions for lasing from organic materials .................................................................................. 4 1.2. Gain, loss and Q-factor ................................................................................................................................. 5 1.3. Triplet absorption and other parasitic losses ............................................................................................... 7 1.4. Amplified spontaneous emission and unambiguous detection of lasing ..................................................... 9 2. Materials ............................................................................................................................................................... 9 2.1. Small Molecules .......................................................................................................................................... 12 2.1.1. New matrices for well-known dyes.................................................................................................... 12 2.1.2. Rylene dyes ........................................................................................................................................ 14 2.1.3. BODIPY based laser dyes .................................................................................................................... 15 2.1.4. Thiophene based dyes ....................................................................................................................... 17 2.1.5. Triphenyldiamines – TPDs .................................................................................................................. 18 2.1.6. Phenylene vinylene small molecules ................................................................................................. 19 2.1.7. Fluorene based small molecules ........................................................................................................ 21 2.1.8. Towards orbital design via precise functionalization with donor and acceptor groups .................... 22 2.2. Defined star-shaped macromolecules ........................................................................................................ 25 2.3. Linear Macromolecules .............................................................................................................................. 27 2.4. Biological Dyes and Fluorescent Proteins ................................................................................................... 33 2.5. Perovskites.................................................................................................................................................. 35 2.6. Conclusion .................................................................................................................................................. 37 3. Optical Feedback Structures................................................................................................................................ 40 3.1. Planar cavities ............................................................................................................................................. 42 3.2. Distributed feedback and photonic crystal structures ............................................................................... 47 3.2.1. One dimensional DFB structures ....................................................................................................... 47 3.2.2. Two and three dimensional photonic crystal structures ................................................................... 53 3.2.3. Development of designs for future electrical operation ................................................................... 54 3.3. Fibers and wires .......................................................................................................................................... 58 3.4. Whispering gallery mode resonators ......................................................................................................... 59 3.4.1. Spherical WGM Resonators ............................................................................................................... 59 3.4.2. Microgoblet and microdisk WGM resonators ................................................................................... 63 3.4.3. Microring-type WGM resonators....................................................................................................... 64 3.4.4. Further considerations on WGM lasers ............................................................................................. 65 3.5. Organic crystal based cavities ..................................................................................................................... 66 3.6. Optofluidic resonator structures ................................................................................................................ 67 3.7. Random lasers ............................................................................................................................................ 70 3.8. Plasmonic structures .................................................................................................................................. 71 3.9. Conclusion .................................................................................................................................................. 73 4. Fabrication ........................................................................................................................................................... 74 4.1. Topical direct writing techniques ............................................................................................................... 74 4.2. Interference lithography and holographic patterning ................................................................................ 77 4.3. Nanoimprinting and micro-contact printing ............................................................................................... 78 4.4. Structure formation via colloidal self-assembly ......................................................................................... 81 4.5. Encapsulation ............................................................................................................................................. 82 4.6. Conclusion .................................................................................................................................................. 84 5. Routes towards continuous wave operation ...................................................................................................... 85 6. Strategies to overcome the challenges associated with electrical pumping of organic lasers ........................... 88 6.1. Absorption by conducting electrodes ......................................................................................................... 88 6.2. Increasing charge carrier mobility and reducing excitonic and polaronic losses ....................................... 91 6.3. Reducing triplet generation ........................................................................................................................ 93 6.4. Conclusion .................................................................................................................................................. 95 7. Outlook ................................................................................................................................................................ 95 7.1. Organic polariton lasers .............................................................................................................................