Electrical engineers from the University of California, Los Angeles, UCLA Samueli School of Engi­neering, have developed a more efficient way of converting light from one wavelength to another, opening the door for improve­ments in the performance of imaging, sensing and communi­cation systems. 

Finding an efficient way to convert wavelengths of light is crucial to the improvement of many imaging and sensing techno­logies. For example, converting incoming light into terahertz wave­lengths enables imaging and sensing in optically opaque environ­ments. However, previous conversion frameworks were ineffi­cient and required bulky and complex optical setups. The team has devised a solution to enhance wavelength-conver­sion efficiency by exploring a generally undesirable but natural phenomenon of semi­conductor surface states. Surface states occur when surface atoms have an insufficient number of other atoms to bind to, causing a breakdown in atomic structure. These incomplete dangling bonds cause roadblocks for electric charges flowing through semi­conductor devices and affect their performance.

“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,” said Mona Jarrahi, who leads the UCLA Terahertz Electronics Laboratory. In fact, since these incomplete bonds create a shallow but giant built-in electric field across the semi­conductor surface, the researchers decided to take advantage of surface states for improved wavelength conversion.

Incoming light can hit the electrons in the semiconductor lattice and move them to a higher energy state, at which point they 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 gained by radiating it at different optical wavelengths, thus converting the wavelengths. However, this energy exchange can only happen at the surface of a semi­conductor and needs to be more efficient. In order to solve this problem, the team incorporated a nanoantenna array that bends incoming light so it is 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,” said Deniz Turan, a member of Jarrahi’s research labora­tory. The researchers successfully and efficiently converted a 1,550-nanometer wavelength light beam into the terahertz part of the spectrum, ranging from wave­lengths of 100 micrometers up to 1 millimeter. 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.

Without this breakthrough in wavelength conversion, it would have required 100 times the optical power level to achieve the same terahertz waves, which the thin optical fibers used in the endo­scopy probe cannot support. The advance can apply to optical wavelength conversion in other parts of the electro­magnetic spectrum, ranging from microwave to far-infrared wavelengths. (Source: UCLA)

Reference: D. Turan et al.: Wavelength conversion through plasmon-coupled surface states, Nat. Commun. 12, 4641 (2021), DOI: 10.1038/s41467-021-24957-1

Link: Terahertz Electronics Laboratory, Electrical and Computer Engineering Dept., University of California, Los Angeles, USA