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Lasers & Sources

Low-threshold organic semiconductor lasers: moving out of the laboratory

Initial synthesis and investigation of a ‘star’ macromolecule show it to be a promising material, especially in terms of the purity and stability required for practical application of organic lasers.
29 November 2010, SPIE Newsroom. DOI: 10.1117/2.1201010.003214

The recent, rapid development of organic semiconductor lasers builds on that of organic LEDs, which are now commercially available in displays. It opens up the prospect of compact, low-cost (even disposable) visible lasers suitable for a range of uses from point-of-care diagnostics to sensing.1 However, practical application of organic semiconductor lasers faces two main challenges. The first concerns the ability to use ultracompact pump sources with sufficient output (spectral, power) above the lasing threshold of a particular gain material and resonator. Second, lasing tests are typically conducted in vacuum to inhibit photo-oxidative degradation by excluding oxygen and moisture. Consequently, we must emulate these conditions by having adequate encapsulation.

The choice of materials for lasing has mainly focused on available light emitters designed for organic LEDs (OLEDs). It is desirable to design organic semiconductors from the outset that will address the main challenges. The correct material will have excellent solution-processing properties, address the issue of photo-excitation by having a low lasing threshold and low optical losses, and spectrally match the output of the pump source. Furthermore, it can be expected that residual catalyst will accelerate degradation, so a very pure and stable material is required for the laser.

Star-shaped macromolecules are branched materials consisting of linear oligomeric arms joined together by a central core. Combining such an architecture with π-electron-conjugated arms would result in new electrical, optical, and morphological properties. In addition, because of the monodispersed nature of the materials, the product of the synthesis is pure and crystallizable. The purity provides a material that is stable to photo-oxidative processes.2,3 Here, we present the oligofluorene truxene T4. Figure 1 (top left) shows the material's structure, where R = C6H13. The molecule consists of a hexahexylated truxene core to which fluorene arms are attached, with each arm consisting of four 9,9-dihexylfluorene units. Synthesis of the star-shaped oligofluorene requires attaching quaterfluorene arms directly to the truxene core. We made the T4 material through a convergent route, involving first preparation of a tribromo truxene core and boronic acid-functionalized quaterfluorene arms in two parallel and independent syntheses. The final step involves Suzuki coupling of these two components to afford the T4 system. This route makes it possible to efficiently separate the target material from its side products.

Amplified spontaneous emission (ASE) allows us to determine key photophysical properties, such as optical gain and waveguide losses. T4 waveguide losses are very low for an organic-semiconductor gain medium, particularly for neat films. The results suggest that truxenes are promising for loss reduction, a key parameter in the operation of organic semiconductor lasers. Distributed-feedback lasers fabricated from solution by spin coating show a low lasing threshold of 270W/cm2 and broad tunability across 25nm in the blue part of the spectrum: see 1D lasing output, Figure 1 (top right).4 We previously described a set of stringent criteria that must be met for classification as an organic semiconductor laser, which these characteristics satisfy.5

Figure 1. (top left) Oligofluorene truxene T4, with R =C6H13. (top right) Laser emission from T4 spin-coated on a 2D distributed-feedback grating. (Photo courtesy of Georgios Tsiminis, University of St Andrews, UK.) (bottom) Flip-chip CMOS micro-LED with 368nm emission.

The synthesis of star-shaped materials by our group at the University of Strathclyde (UK) and the ASE and lasing measurements conducted by Ifor Samuel and his group at the University of St Andrews (UK) are the product the HYPIX project (hybrid organic semiconductor/gallium nitride/CMOS smart-pixel arrays). The overall aims of the project are to construct electrically driven organic semiconductor lasers using CMOS, gallium nitride LEDs, and organics. The CMOS design is led by the group of Robert Henderson at the University of Edinburgh (UK) and allows computer-controlled nanosecond pulsing, and individual addressing of the micropixelated LEDs:6 see Figure 1 (bottom).

The HYPIX grouping is unique in combining experts in solid-state physics, materials chemistry, and electronic engineering. By the end of the project we plan to have demonstrated a fully packaged hybrid organic/inorganic laser system that will have many applications in sensing, communication, and other areas.5 We are also examining the possibility of using composite lasers and encapsulation of the gain material. Already, we have shown that T4 in an inert photocurable matrix can last for hours under high laser fluencies, as opposed to minutes in ambient air.

Peter J. Skabara, Richard Pethrick, Allan R. MacKintosh
Department of Pure and Applied Chemistry
University of Strathclyde
Glasgow, UK

Peter Skabara is a professor of materials chemistry. From 1995 to 2000, he was a lecturer in organic chemistry at Sheffield Hallam University (UK), and from 2000 to 2005 lecturer then senior lecturer in inorganic materials chemistry at the University of Manchester (UK). He was a Leverhulme Trust Research Fellow for the year 2005–2006.

Martin Dawson
Institute of Photonics
University of Strathclyde
Glasgow, UK
Graham Turnbull, Ifor Samuel
Organic Semiconductor Centre School of Physics and Astronomy
University of St Andrews
St Andrews, UK