Able to produce white light, a chip-scale semiconductor laser brings the technology one step closer to becoming a mainstream light source and potential alternative to LEDs. Brighter and more energy efficient than LEDs, the laser design could be used in displays and visible light communications, according to researchers at Arizona State University. "The concept of white lasers first seems counterintuitive because the light from a typical laser contains exactly one color, a specific wavelength of the electromagnetic spectrum, rather than a broad range of different wavelengths," said professor Cun-Zheng Ning. "White light is typically viewed as a complete mixture of all of the wavelengths of the visible spectrum." Three parallel segments of a semiconductor nanosheet support lasing in three elementary colors. When the total field is collected, a white color emerges. Courtesy of Arizona State University/Nature Nanotechnology. Supercontinuum lasers used in bioimaging already produce broadband white light but through a different process and with a much bigger footprint. The Arizona device is based on interband transitions in semiconductors. At about 20 μm, it can be mounted on a chip for miniaturized applications. Supercontinuum lasers, on the other hand, require large-scale pump lasers and several meters of optical fiber. The two types of laser also generate white light differently. Supercontinuum lasers emit at every wavelength over a given range, whereas the Arizona device emits distinct colors that are blended. It is based on a novel zinc cadmium sulfur selenide nanosheet with three parallel segments, each supporting lasing in one of three elementary colors. The device is capable of lasing in any visible color and is tunable from red to green to blue or any color in between. When the total field is collected, white light emerges. Sandia National Laboratories in 2011 produced white light from four large lasers. The researchers showed that the human eye is as comfortable with white light generated by diode lasers as with that produced by LEDs, inspiring others to advance the technology. "While this pioneering proof-of-concept demonstration is impressive, those independent lasers cannot be used for room lighting or in displays," Ning said. "A single, tiny piece of semiconductor material emitting laser light in all colors or in white is desired." This photo collage shows the mixed-emission color from a multisegment nanosheet in the colors of red, green, blue, yellow, cyan, magenta and white. The top dots in each photograph are the direct image of laser emission, while the tails under the dots are the reflection from the substrate. Courtesy of Arizona State University/Nature Nanotechnology. Typically a semiconductor emits light of a single color determined by its atomic structure and energy bandgap. A semiconductor's lattice constant represents the distance between its atoms. Producing all possible wavelengths in the visible spectrum requires several semiconductors of very different lattice constants and energy bandgaps. "The key obstacle is an issue called lattice mismatch, or the lattice constant being too different for the various materials required," said Zhicheng Liu, who completed his doctorate after working on the project. "We have not been able to grow different semiconductor crystals together in high enough quality, using traditional techniques, if their lattice constants are too different." Over several years, Ning's team developed methods for reconciling lattice mismatches at the nanoscale, first merging red and green emitters. To produce blue, the last piece of the puzzle, the group later came up with a strategy to create the required shape first and then convert the materials into the right alloy contents. "To the best of our knowledge, our unique growth strategy is the first demonstration of an interesting growth process called dual ion exchange process that enabled the needed structure," said former doctoral student Sunay Turkdogan, now assistant professor at the University of Yalova in Turkey. Used in displays, the new laser design could provide 70 percent more perceptible colors than the current display industry standard, the researchers said. Another important application could be in visible light communications, in which light fixtures are used both for illumination and data transmission. The technology is called Li-Fi, for light-based wireless communication, as opposed to Wi-Fi, or wireless fidelity, which uses radio frequencies. Li-Fi could be more than 10 times faster than Wi-Fi, and Li-Fi using white lasers could be 10 to 100 times faster than LED-based Li-Fi. Multicolor fluorescence sensing could also benefit from the new laser design. While this first proof of concept is important, significant obstacles remain to make white semiconductor lasers applicable for real-life lighting or display applications. One of crucial next steps is to find a way to pump the laser electrically, rather than with another laser as was done in this demonstration. The research was published in Nature Nanotechnology (doi: 10.1038/nnano.2015.149). For more information, visit www.asu.edu.