Schematic of an indium arsenide lattice in contact with a nanoantenna array that bends incoming light so it is tightly confined around the shallow surface of the semiconductor. Image: Deniz Turan/UCLA.Researchers from the Samueli School of Engineering at the University of California, Los Angeles (UCLA) have developed a more efficient way of converting light from one wavelength to another. This opens the door for improving the performance of imaging, sensing and communication systems. Led by Mona Jarrahi, professor of electrical and computer engineering at UCLA Samueli, the researchers report their advance in a paper in Nature Communications.
Finding an efficient way to convert wavelengths of light is crucial for improving many imaging and sensing technologies. For example, converting incoming light into terahertz wavelengths allows imaging and sensing in optically opaque environments. However, previous conversion systems were inefficient and required bulky and complex optical setups.
The UCLA-led team has now devised a way to enhance the wavelength-conversion efficiency by exploring a generally undesirable but natural phenomenon called semiconductor surface states.
Surface states occur when surface atoms have an insufficient number of other atoms to bond with, causing a breakdown in atomic structure. These incomplete chemical bonds, also known as '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 Jarrahi, who leads the UCLA Terahertz Electronics Laboratory.
In fact, since these incomplete bonds create a shallow but huge electric field across the semiconductor surface, the researchers decided to take advantage of the surface states to improve wavelength conversion.
When incoming light hits electrons in a semiconductor lattice, it moves 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 away 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 for practical use. 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,” explained Deniz Turan, lead author of the paper and a member of Jarrahi’s research laboratory, who recently graduated with his doctorate in electrical engineering from UCLA Samueli.
Using this approach with the semiconductor indium arsenide, the researchers successfully and efficiently converted a 1550nm-wavelength light beam into the terahertz part of the spectrum, which encompasses wavelengths from 100µm to 1mm. To demonstrate the conversion efficiency, they incorporated 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. This advance could also be used for optical wavelength conversion in other parts of the electromagnetic spectrum, ranging from microwave to far-infrared wavelengths.
This story is adapted from material from UCLA Samueli, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.