Engineers bend light to enhance wavelength conversion

Date:
July 30, 2021
Source:
University of California - Los Angeles
Summary:
Engineers 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.



FULL STORY ========================================================================== Electrical engineers from the 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.


==========================================================================
Mona Jarrahi, professor of electrical and computer engineering at UCLA
Samueli, led the Nature Communications-published research.

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 UCLA-led team has devised a solution to enhance 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 bind to, 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 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, the study's lead author and a member of
Jarrahi's research laboratory who recently graduated with his doctorate
in electrical engineering from UCLA Samueli.

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.

Two addition members of Jarrahi's research group, Ping Keng Lu and
Nezih Yardimci, are co-authors of the study. Other co-authors are from Technical University Darmstadt in Germany and the Ames Laboratory, a
U.S. Department of Energy (DOE) lab affiliated with Iowa State University.

The Office of Naval Research supported the research, and the DOE provided
a grant for Turan.

========================================================================== Story Source: Materials provided by
University_of_California_-_Los_Angeles. Note: Content may be edited for
style and length.


========================================================================== Journal Reference:
1. Deniz Turan, Ping Keng Lu, Nezih T. Yardimci, Zhaoyu Liu, Liang Luo,
Joong-Mok Park, Uttam Nandi, Jigang Wang, Sascha Preu, Mona Jarrahi.

Wavelength conversion through plasmon-coupled surface states. Nature
Communications, 2021; 12 (1) DOI: 10.1038/s41467-021-24957-1 ==========================================================================

Link to news story: https://www.sciencedaily.com/releases/2021/07/210730104257.htm

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