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.
Reference:
Turan, D., Lu, P.K., Yardimci, N.T. et al. Wavelength conversion through
plasmon-coupled surface states. Nat Commun 12, 4641 (2021).
https://doi.org/10.1038/s41467-021-24957-1