Reporting their findings in the journal Applied Physics Express under the title “Indication of current-injection lasing from an organic semiconductor”, the researchers circumvented the traditional issues of organic materials (which due to their inherent resistance, tend to break down under the high currents required for lasing).
A critical step in lasing is the injection of a large amount of electrical current into the organic layers to achieve a condition called population inversion. However, the high resistance to electricity of many organic materials makes it difficult to get enough electrical charges in the materials before they heat up and burn out. On top of that, a variety of loss processes inherent to most organic materials and devices operating under high currents lowers efficiency, pushing the necessary current up even higher.
To overcome these obstacles, the research group led by Prof. Chihaya Adachi used a highly efficient organic light-emitting material, 4,4′-bis[(N-carbazole)styryl]biphenyl (BSBCz) with a relatively low resistance to electricity and a low amount of losses—even when injected with large amounts of electricity.
They also designed a device structure with a mixed-order distributed feedback grating sitting on top of one of the electrodes used to inject electricity into the organic thin films. The grating alternated first- and second-order Bragg scattering regions in order to provide strong lateral optical feedback and efficient vertical outcoupling of the laser emission. It was further
optimized to also control the flow of electricity in the devices and minimize the amount of electricity required to observe lasing from the organic thin film.
Electrically driven under pulse operation, the organic semiconductor laser diode (OSLD) exhibited spectral line narrowing with increasing current density, with lasing occurring at the long-wavelength band edge (480.3nm) of the Bragg stopband.
Last March, the researchers have founded a startup called KOALA Tech Inc. (short for Kyushu Organic Laser Technology Inc.) and plan to overcome the final obstacles to the commercialization of organic laser diodes which they anticipate could be simple to manufacture, cheap, and tunable. Such organic laser diodes could then be integrated into other organic-based optoelectronic platforms for low cost spectroscopy, displays, medical devices and LIFI telecommunications.
Kyushu University – www.kyushu-u.ac.jp