Longer Duration Time Achieved for Continuous-Wave Organic Lasing

An optically pumped organic thin-film laser has shown the ability to continuously emit light for 30 ms — more than 100 times longer than previous devices. Organic thin-film lasers use a thin layer of organic molecules as the laser medium. By designing and synthesizing molecules with novel structures, they can achieve a wider range of colors than inorganic lasers, making them well-suited for photonics applications.

Photograph of a blue organic thin-film laser under operation along with microscope images and a schematic of the lasers. By optimizing the device structure and using appropriate materials, the duration of lasing was extended up to 30 ms, which is more than 100 times longer than previously possible. The schematic is a generalized representation of the structure and is not to scale. Courtesy of Atula S. D. Sandanayaka and Willam J. Potscavage, Jr.

However, organic thin-film lasing remains a challenging objective. Thermal degradation of the organic gain medium impedes long-term laser operation, as do losses caused by the generation of long-lived triplet excitons.

“People have been studying organic thin-film lasers for a long time, but degradation and loss processes have greatly limited the duration of emission,” said researcher Atula S. D. Sandanayaka.

A research team from Kyushu University’s Center for Organic Photonics and Electronics Research (OPERA) was able to reduce losses by using a laser with triplet excitons that absorbed a different color of light than that emitted by the laser.

Thermal degradation caused by overheating of the lasers during operation was reduced by building the devices on a crystalline silicon wafer and gluing a piece of sapphire glass on top of the organic laser medium with a special polymer. The silicon and sapphire helped to quickly conduct heat away from the device while at the same time encapsulating it. The encapsulation technique reduced the laser-induced thermal degradation and suppressed the ablation of the gain medium.

The team reported low-threshold surface-emitting organic distributed feedback lasers operating in the quasi-CW regime at 80 MHz as well as under long-pulse photoexcitation of 30 ms. This performance was achieved using an organic semiconductor thin film with high optical gain, high photoluminescence quantum yield, and no triplet absorption losses at the lasing wavelength, combined with a mixed-order distributed feedback grating to achieve a low lasing threshold.

“These devices really operate at the extreme, so we have to keep finding new tricks to eliminate any inefficiencies and prevent the devices from burning themselves out,” said professor Chihaya Adachi.

The study indicates that the development of a CW organic semiconductor laser technology is possible by engineering the gain medium and the device architecture. Use of organic lasing devices with inorganic lasers could provide a way to achieve colors that are difficult to produce using conventional lasers, for applications in spectroscopy, communications, displays and sensing.

The research team plans to work on sustaining the emission of its organic thin-film laser for even longer durations.

“Our ultimate goal is realizing organic thin-film lasers that directly use electricity as the energy source, and this is an important step in that direction,” said Adachi.

The research was published in Science Advances (doi: 10.1126/sciadv.1602570).