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Wednesday, 19 June 2019

Plastic Laser finally becomes reality

This is the first time that laser emission is achieved in plastic or organic diodes. [Image: COPER / Kyushu University]

Researchers from Japan have demonstrated that an organic semiconductor-based laser diode is indeed possible, paving the way for the expansion of lasers in applications such as biosensors, screens, medical devices and optical communications.

Atula Sandanayaka and her colleagues at Kyushu University claim to have convincingly demonstrated for the first time that organic semiconductor laser diodes have finally come true - earlier allegations of electrically induced laser generation using organic materials have proved to be false on several occasions with other phenomena being confused with the laser emission.

A critical step in the laser is to inject 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 on the materials before they warm up and burn - the organic materials are polymers, or plastics.

In addition, losses inherent to most organic materials and operation under high currents reduce efficiency, further increasing the required current.

Laser emission


To overcome these obstacles, Sandanayaka used a highly efficient organic light emitting material with relatively low resistance to electricity and a low amount of losses - the material is known as BSBCz (4,4'-bis [(N-carbazole) ] biphenyl).

But finding the right stuff was not enough.

He also had to design a grid structure of insulation material on one of the electrodes to inject electricity into the thin organic films. These networks - called distributed feedback structures - were already known to be capable of producing the necessary optical effects for the laser.

"By optimizing such networks, we were able to not only obtain the desired optical properties but also control the flow of electricity in the devices and minimize the amount of electricity needed to observe the laser emission from the thin organic film," said Professor Chihaya Adachi.

Organic laser prototype scheme and photo. [Image: 10.7567 / 1882-0786 / ab1b90] 

Organic laser diode

For a long time considered a "holy grail" in the area of ​​light emitting components, organic laser diodes use carbon-based materials to emit light instead of inorganic semiconductors such as arsenide and gallium nitride used in commercial devices.

Organic 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 the best choice for cell phone screens because of their high efficiency and vibrant colors, which can be easily altered by synthesizing new organic molecules.

It turns out that organic laser diodes produce much purer light, allowing additional applications, but require currents with magnitudes higher than those used in OLEDs to achieve coherent light emission. These extreme conditions caused the prototypes built so far to sink well before the laser could be observed.

Researchers are so confident in their new components that they founded a company to make the missing developments to create a commercial product and launch the organic laser diodes in the market.


References:

Indication of current-injection lasing from an organic semiconductor
Atula SD Sandanayaka, Toshinori Matsushima, Fatima Bencheikh, Shinobu Terakawa, William J. Potscavage Jr., Chuanjiang Qin, Takashi Fujihara, Kenichi Goushi, Jean-Charles Ribierre, Chihaya Adachi
Applied Physics Express
DOI: 10.7567 / 1882-0786 / ab1b90

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