A thin layer of glass makes “sub-wavelength” lasers more practical.
Scientists have created the smallest ever laser capable of operating at room temperature. The device is less than one cubic micron–less than the wavelength of the light it emits. It is the first sub-wavelength laser that doesn’t require cryogenic cooling.
Yeshaiahu Fainman, head of the Ultrafast and Nanoscale Optics Group at the University of California, San Diego, who led the work, says it should be possible to pack the microlasers close together without interference between devices. This paves the way for, among other things, faster optical communications devices that use sub-wavelength lasers in dense arrays.
The researchers modified what’s known as a microdisc laser. In this type of laser, a microscopic disc containing different materials is optically pumped by a larger laser. This stimulates its semiconductor core to emit light, which bounces around the disc’s edges before being released. Adding metal to this disc can prevent the laser from behaving in a way that would interfere with other devices in close proximity. But this reduces the laser’s efficiency, and until now the only way to counteract this loss in performance has been to cool it cryogenically to around 77 degrees Kelvin (-196 degrees Celsius) using liquid nitrogen, which is far from practical.
Fainman, together with postdoc Maziar Nezhad and other UCSD colleagues, found a simpler way to improve the efficiency of their laser, and to remove the need for cooling. They added a layer of silica, followed by a layer of aluminum around a laser cavity made of indium gallium arsenide phosphide. The outer metal layer acts like a shield, isolating the laser from other devices, and acting like a highly efficient heat sink. The silica layer prevents the metal from reducing the lasers’s overall efficiency.
Aluminum was chosen because its optical properties make it highly reflective. But the key to making it work lies in precisely controlling the thickness of the silica layer that separates the metal from the semiconductor core, says Fainman. If the layer is too thin, the metal shield will interact too strongly with the optical field, resulting in high losses.
“This is very exciting work, and introduces important advances in the new field of nanolasers,” says Naomi Halas, the Stanley C. Moore Professor of Electrical and Computer Engineering at Rice University, and director of the University’s Laboratory for Nanophotonics. “Making use of metallic layers and clever design geometries has allowed this group to begin to build refinements into these structures that will expand how these devices are used in communications systems.”
In a paper published in the journal Nature Photonics, the UCSD group shows that its laser can produce emissions with a wavelength of 1.43 microns at room temperature. The group has received funding from the National Science Foundation as well as DARPA’s Nanoscale Architectures for Coherent Hyper-Optic Sources program.
In theory, the efficiency of the laser could be improved further by using other metals that have even more favorable optical properties, such as silver or gold, says Fainman.
A bigger challenge is finding a way for the lasers to be fully integrated into optoelectronic devices, by replacing the complex optical pump with an electrical one. “Electrical pumping would be more desirable, because it is much more efficient,” says Richard De La Rue, an optoelectronics professor at the University of Glasgow, in the U.K.
Besides high-speed communications, sub-wavelength lasers could find applications in biomedical imaging and near-field optical microscopy, says Fainman. In the latter case, there are difficulties in mechanically scanning lasers across a surface, he says, “so the goal would be to make an array of light sources that would be scanned electrically rather than mechanically.”
Halas says the work is also a scientifically important. “They exploit a regime where the cavity design can alter the properties of the gain medium, which actually introduces a whole new way to think about lasers,” she says.