Organic Laser Diodes Move from Dream to Reality

Schematic representation of an organic semiconductor laser diode producing blue laser emission under electrical excitation. (Source: OPERA, Kyushu U.)

Researchers from Japan have demonstrated that a long-elusive kind of laser diode based on organic semi­conductors is indeed possible, paving the way for the further expansion of lasers in applications such as biosensing, displays, healthcare, and optical communi­cations. Long considered a holy grail in the area of light-emitting devices, organic laser diodes use carbon-based organic materials to emit light instead of the inorganic semi­conductors, such as gallium arsenide and gallium nitride, used in traditional devices.

The lasers are in many ways similar to organic light-emitting diodes (OLEDs), in which a thin layer of organic molecules emits light when elec­tricity is applied. OLEDs have become a popular choice for smartphone displays because of their high efficiency and vibrant colors, which can easily be changed by designing new organic molecules. Organic laser diodes produce a much purer light enabling additional appli­cations, but they require currents that are magni­tudes higher than those used in OLEDs to achieve the lasing process. These extreme conditions caused previously studied devices to break down well before lasing could be observed.

Further compli­cating progress, previous claims of elec­trically generated lasing from organic materials turned out to be false on several occasions, with other phenomena being mistaken for lasing because of insufficient characterization. But now, scientists from the Center for Organic Photonics and Elec­tronics Research (OPERA) at Kyushu University have enough data to con­vincingly show that organic semi­conductor laser diodes have finally been realized.

“I think that many people in the community were doubting whether we would actually one day see the reali­zation of an organic laser diode,” says Atula S. D. Sanda­nayaka, “but by slowing chipping away at the various performance limi­tations with improved materials and new device structures, we finally did it.” A critical step in lasing is the injection of a large amount of electrical current into the organic layers to achieve a 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 Chihaya Adachi used a highly efficient organic light-emitting material (BSBCz) with a relatively low resistance to electricity and a low amount of losses – even when injected with large amounts of elec­tricity. But having the right material alone was not enough.

They also designed a device structure with a grid of insulating material on top of one of the electrodes used to inject elec­tricity into the organic thin films. Such distributed feedback structures are known to produce the optical effects required for lasing, but the researchers took it one step further. “By optimizing these grids, we could not only obtain the desired optical properties but also control the flow of electricity in the devices and minimize the amount of elec­tricity required to observe lasing from the organic thin film,” says Adachi.

The researchers are so confident in the promise of these new devices that they founded the startup company KOALA Tech Inc. – short for Kyushu Organic Laser Tech­nology Inc. – to accelerate research and overcome the final obstacles remaining for using the organic laser diodes in commercial appli­cations. The founding members are now hard at work improving the per­formance of their organic laser diodes to bring this most advanced organic light-emitting technology to the world. (Source: Kyushu U.)

Reference: A. S. D. Sandanayaka et al.: Indication of current-injection lasing from an organic semiconductor, Appl. Phys. Exp., online 31 March 2019; DOI: 10.7567/1882-0786/ab1b90

Link: Center for Organic Photonics and Electronics Research (OPERA), Kyushu University, Fukuoka, Japan

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