Electrical engineers from the UCLA Samueli School of Engineering have introduced a solution to enhance wavelength-conversion efficiency by exploring the phenomenon of semiconductor surface states. The work, which establishes a more efficient way of converting light from one wavelength to another, opens doors to improvements in the performance of imaging, sensing, and communication systems. Surface states occur when surface atoms have an insufficient number of other atoms to which they can bind, causing a breakdown in atomic structure. These incomplete chemical bonds, also known as “dangling bonds,” cause roadblocks for electric charges flowing through semiconductor devices. Specifically, the incomplete bonds create a shallow-but-giant built-in electric field across the semiconductor surface. “There have been many efforts to suppress the effect of surface states in semiconductor devices without realizing they have unique electrochemical properties that could enable unprecedented device functionalities,” Mona Jarrahi, a professor of electrical and computer engineering who leads the UCLA Terahertz Electronics Laboratory, said. Schematic of InAs lattice in contact with a nanoantenna array that bends incoming light so it is tightly confined around the shallow surface of the semiconductor. Courtesy of Deniz Turan. Incoming light can hit the electrons in the semiconductor lattice and move them to a higher energy state, at which point the electrons are free to jump around within the lattice. The electric field created across the surface of the semiconductor further accelerates these photo-excited, high-energy electrons, which then unload the extra energy they gain by radiating it at different optical wavelengths — thus converting the wavelengths. However, this energy exchange can only happen at the surface of a semiconductor. The UCLA team overcame this problem with a nanoantenna array that it incorporated to bend incoming light so that it was tightly confined around the shallow surface of the semiconductor. “Through this new framework, wavelength conversion happens easily and without any extra added source of energy as the incoming light crosses the field,” Deniz Turan, lead author of the study, said. The researchers converted a 1550-nm wavelength light beam into the terahertz part of the spectrum, ranging from wavelengths of 100 µm up to 1 mm. The team demonstrated the wavelength-conversion efficiency by incorporating the new technology into an endoscopy probe that could be used for detailed in vivo imaging and spectroscopy using terahertz waves. Photograph, microscopy, and scanning electron microscopy images of a fabricated nanoantenna array placed at the tip of a fiber for optical-to-terahertz wavelength conversion. Courtesy of Deniz Turan. Without the wavelength conversion approach, the researchers said it would have required 100× the optical power level to achieve the same terahertz waves, which the thin optical fibers used in the endoscopy probe cannot support. The advancement can apply to optical wavelength conversion in other parts of the electromagnetic spectrum, ranging from microwave to far-infrared wavelengths, as well. Additional co-authors are from Technical University Darmstadt (Germany) and Ames Laboratory, a U.S. Department of Energy lab. The Office of Naval Research supported the research, and DOE provided a grant for Turan. The research was published in Nature Communications (www.doi.org/10.1038/s41467-021-24957-1).