Two very low power lasers that produce the smallest continuous-wave, telecommunications frequency demonstrated at room temperature to date may be a step toward the development of the “ultimate” nanolaser. The new lasers — the smallest room-temperature nanolaser to date, and a highly efficient “thresholdless” laser that funnels all of its protons into lasing without any waste — were developed by a team of electrical engineers at the University of California, San Diego. Their extremely low operating power is an important breakthrough since lasers usually require greater and greater pump power to begin lasing as they shrink to nano sizes. This quality, along with the nanolasers’ small size, could make them useful components for future optical circuits packed into tiny computer chips. Postdoctoral researcher Mercedeh Khajavikhan at work in the optics laboratory. (Image: Josh Knoff, UC San Diego Jacobs School of Engineering) Mercedeh Khajavikhan and her colleagues at the UC San Diego Jacobs School of Engineering suggest that the thresholdless laser may also help researchers as they develop new metamaterials – artificially structured materials for applications ranging from superlenses for seeing individual DNA molecules or viruses, to “cloaking” devices that can bend light around an object and make it appear invisible. All lasers require a certain amount of pump power from an outside source to begin emitting a coherent beam of light, said Yeshaiahu (Shaya) Fainman, a professor at UC San Diego. The smaller a laser is, the greater the pump power needed to reach the point of lasing. Left to right, electrical engineers Michael Kats, Aleksandar Simic and Mercedeh Khajavikhan in the lab. (Image: Josh Knoff, UC San Diego Jacobs School of Engineering) To overcome this problem, the researchers developed a design that uses quantum electrodynamic effects in coaxial nanocavities to alleviate the threshold constraint. Like a coaxial cable hooked up to a television, only at a much smaller scale, the laser cavity consists of a metal rod enclosed by a ring of metal-coated quantum wells of semiconductor material. The team built the thresholdless laser by modifying the geometry of this cavity. The new design allowed them to build the smallest room-temperature, continuous-wave laser to date. The nanoscale coaxial laser is more than an order of magnitude smaller than the group’s previous record-smallest nanolaser, described in Nature Photonics less than two years ago. Upper left: A schematic of the coaxial laser cavity. Lower left: A scanning electron micrograph image of the constituent ring of the coaxial structure containing the gain section and the cover silica layer. Right: the TEM-like mode of the coaxial structure that is symmetric in the transverse plane, has no degeneracy and is extremely confined. (Image: Mercedeh Khajavikhan and Aleksandar Simic) Fainman said that these highly efficient lasers would be useful in augmenting future computing chips with optical communications, where the lasers are used to establish communication links between distant points on the chip. Only a small amount of pump power would be required to reach lasing, reducing the number of photons needed to transmit information. The nanolaser designs appear to be scalable, meaning that the devices could be shrunk to even smaller sizes, an extremely important feature that would make it possible to harvest laser light from even smaller nanoscale structures, the researchers said. This feature eventually could make them useful for creating and analyzing metamaterials with structures smaller than the wavelength of light currently emitted by the lasers. For more information, visit: www.jacobsschool.ucsd.edu