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Changchun Team Investigates Threshold and Light Amplification in Electrically Pumped OSLDs

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CHANGCHUN, China, Dec. 22, 2017 — Organic semiconductor laser diodes (OSLDs) can be prepared using simple processing technologies and can be integrated easily with other optoelectronic devices. This makes them attractive for use in low-cost, compact, flexible and tunable lasers with spectral regions from the UV to NIR. Although lasing has been widely demonstrated under optical pumping, electrically pumped OSLDs have posed a challenge, because the expected high threshold current is hard to reach in low electrical conductivity organic semiconductors, and electroluminescence (EL) efficiency decreases under high current.

Electrically pumped organic semiconductor laser, Changchun Institute of Optics, Fine Mechanics and Physics.
Threshold and light amplification behaviors in organic semiconductor lasers. Courtesy of Science China Press.

Microcavities with high quality factor (Q) can result in a low threshold and reduce the difficulty of achieving electrically pumped lasing in OSLDs. A planar microcavity of wavelength size can effectively control spontaneous emission and stimulated emission characteristics of organic semiconductors.

Based on a high-Q microcavity OSLD, researchers at Changchun Institute of Optics, Fine Mechanics and Physics have demonstrated an electrically pumped, vertical-cavity organic semiconductor optical amplifier and its light gain amplification phenomenon. The research team has further demonstrated that electrically pumped quasi-continuous-wave (QCW) lasing can be obtained with a much lower Jth (i.e., threshold current density).

The Changchun research team was able to overcome the challenge of high loss of electrodes within the cavity, and design and demonstrate a high quality factor microcavity organic laser device. The team observed obvious gain amplification phenomenon at a lower threshold current under quasi continuous operating, indicating that the realization and practical application of continuous-wave (CW) OSLDs could be achieved.

The optically pumped lasing characteristics of the OSLD sample were examined under QCW excitation from a 405 nm laser diode (LD). The lasing characteristics were further examined under QCW electrical pumping.

The organic laser used a small molecule doping system as the gain medium. The laser peak wavelength was located at 621.7 nm, and this remained unchanged with the increase of current. The consistent emission peak could indicate that the optical constant and thickness of each organic layer remain unchanged with the increase of current density. The threshold current density was about 1.8 mA/cm2, above which the spectral linewidth was narrowed to 0.835 nm. The developed vertical cavity organic semiconductor optical amplifier showed a maximum optical gain of 5.25 dB under 16 mA/cm2 electrical pumping.

The researchers showed that optical loss in the high-Q organic semiconductor microcavity could be significantly reduced. Obtaining low laser threshold could be a significant step toward the practical application of organic lasers, and could pave the way for room temperature continuous-wave lasing. The research team, led by professor X. Liu, believes that the underlying physics deserve further investigation.

The research was published in Science Bulletin (doi: 10.1016/j.scib.2017.12.010).
Dec 2017
Research & TechnologyeducationAsia-PacificlasersTunable Laserscontinuous wave laserssemiconductorslaser diodesorganic semiconductor laser diodesOSLDOLEDs

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