Web Date: October 1, 2014
Engineers Build Ultrasmall Organic Laser
Researchers have made the tiniest organic laser reported to date (ACS Nano 2014, DOI: 10.1021/nn504444g). The 8-µm-long device, which looks like a suspended bridge riddled with holes, is carved into a silicon chip coated with an organic dye. Integrated into microprocessor chips, such tiny lasers could one day speed up computers by shuttling data using light rather than electrons. They also could be valuable for sensors and lab-on-a-chip devices.
Like other organic lasers reported so far, the new laser is powered by pulses from another laser. But the device has a very low threshold—the energy required to start lasing—of 4 microjoules per square centimeter. Thresholds for organic lasers are typically in the tens of µJ/cm2, although some thresholds of a few µJ/cm2 have been reported. The low threshold brings the device closer to engineers’ ultimate goal of creating an organic laser that can run on electric current, says Max Shtein, a materials scientist at the University of Michigan, Ann Arbor, who was not involved in the new work. Electrically powered lasers would be key for on-chip use.
Compared with lasers made from inorganic materials, devices using organic semiconductors promise to emit a wide range of wavelengths, while being tiny, flexible, and inexpensive to make. “We wanted to make a small organic laser that could easily be integrated into a silicon chip,” says Parag Deotare of Massachusetts Institute of Technology.
In conventional lasers, a laser material emits light when excited by an energy source. The material is sandwiched between two reflectors in a cavity that bounces light back and forth to amplify it. Deotare and his colleagues replaced this traditional design with a novel optical structure that Deotare developed in 2009 as a graduate student at Harvard University (Appl. Phys. Lett., DOI: 10.1063/1.3107263). The structure is based on photonic crystals, which are periodic microscopic structures that control the flow of light and are composed of alternating layers of materials with different refractive indexes.
To construct these photonic crystals, Deotare and colleagues used electron-beam lithography and xenon gas etching to drill a series of holes into a 440-nm-wide sliver of silicon suspended across a cavity. Then they coated the strip with a 150-nm-thick film of a material commonly used in organic lasers: tris(8-hydroxyquinolinato)aluminum doped with a dicyanomethylene-based dye. The air in the holes and the organic laser material between them act as the periodic photonic crystal structure.
The holes are elliptical, with six smaller holes in the middle that are carefully designed with varying spacing and sandwiched by strings of periodically spaced larger ones. “The wide holes at the extremes act like mirrors, while the smaller center holes are where the light gets trapped,” Deotare explains. When the device is excited with 100-femtosecond pulses of 400-nm laser light, it gives off 610-nm laser light.
The bridge structure and the tapered holes trap light into a very small volume, allowing less light to leak out and thus lowering the laser’s threshold, says Thomas S. Mahony, one of Deotare’s MIT colleagues. By reducing the roughness of the deposited organic film, they could lower the threshold further, he says.
- Chemical & Engineering News
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