Nanolaser Operates Below 3-D Diffraction Limit

Growing a smooth layer of silver onto a silicon substrate has improved the performance of nanolasers, a discovery that could lead to the emergence of nanophotonic devices with applications ranging from computing to medicine.

Miniaturizing semiconductor lasers is crucial for the development of faster, smaller and lower-energy photon-based technologies such as ultrafast computer chips; highly sensitive biosensors for detecting, treating and studying diseases; and next-generation communication technologies.

Such photonic devices could use nanolasers to generate optical signals and transmit information; they also have the potential to replace electronic circuits. However, the size and performance of these devices have been restricted by what’s known as the 3-D optical diffraction limit. Currently, the diffraction barrier limits the ability of optical instruments to distinguish between two objects separated by a distance of less than about half the wavelength of light used to image the specimen.

An illustration of the nanoscale semiconductor structure used for demonstrating the ultralow-threshold nanolaser. A single nanorod is placed on a thin silver film (28 nm thick). The resonant electromagnetic field is concentrated at the 5-nm-thick silicon dioxide gap layer sandwiched by the semiconductor nanorod and the atomically smooth silver film. (Image: Science)

Now, physicists at the University of Texas at Austin, in collaboration with colleagues in Taiwan, have created a plasmon-based laser, or SPASER, that emits continuous green wavelengths at low energy levels and cold temperatures while operating below the 3-D diffraction limit. The laser is too small to be visible to the naked eye.

"We have developed a nanolaser device that operates well below the 3-D diffraction limit," said Chih-Kang "Ken" Shih, professor of physics at The University of Texas at Austin. "We believe our research could have a large impact on nanoscale technologies."

The SPASER is constructed of a gallium nitride nanorod that is partially filled with indium gallium nitride. Both alloys are semiconductors commonly used in LEDs. The nanorod is placed on top of a thin layer of silicon that covers a layer of silver film that is smooth at the atomic level. This material, created in Shih’s lab, took more than 15 years to perfect.

The material’s "atomic smoothness" is the key to building photonic devices that do not scatter and lose plasmons, which move large amounts of data.

Research by physics graduate student Charlotte Sanders and professor Ken Shih helped develop the world's smallest nanolaser. Sanders stands here with a molecular beam epitaxy machine that she designed and built in collaboration with the department of physics machine shop and with assistance from co-author Dr. Jisun Kim. The machine is used to create a smooth silver thin film critical to the function of the laser. (Courtesy of Alex Wang)

"Atomically smooth plasmonic structures are highly desirable building blocks for applications with low loss of data," Shih said.

The new nanolaser device could provide for the development of on-chip communication systems, which would prevent heat gains and information loss typically associated with electronic devices that transmit data between multiple chips.

"Size mismatches between electronics and photonics have been a huge barrier to realize on-chip optical communications and computing systems," said Shanjr Gwo, professor at National Tsing Hua University and formerly Shih’s doctoral student.

The findings will appear in the July 27 issue of Science.

For more information, visit: www.utexas.edu