YSM Issue 95.2

Wave Physics
FOCUS
random array of holes in the wafer. When
the laser beam was sent in, some of the
scattered light would come out of the holes
and into the path of a reference beam. A
camera then recorded these interference
patterns. “Basically, by shaping the incident
wavefront using a spatial light modulator,
we can control how we’re going to send
the light in,” Cao said. “By finding the
correct wavefront, we can deposit light
into different target areas deep inside.”
This experimental platform allowed
the researchers to directly map the
diffusive system at any depth. “Once
we have this system that allows us to
see what light is doing inside a random
disordered system, we can essentially
say, ‘Okay, instead of just describing
the relationship from the input to the
output, we can describe the relationship
of the light from the input to anywhere
inside,’” Bender said.
This experimental setup allowed the
researchers to create a system where the
disorder could be controlled, precisely
tuned, and analyzed. By optimizing the
laser input wavefront, they were then
able to maximize energy delivery.
This is a unique and novel set up—
one that has fundamentally changed
the ways light can interact with opaque
systems. “We’re the only people who can
actually do this study,” Bender said. “We
can control the input with very, very
good precision using the spatial light
modulators we have available. We can
make the waveguides any way we want
just because of the technical capabilities
we have in the facilities at Yale.”
The Theory
The research team also built a theoretical
model to predict the maximum amount
of energy that could be delivered to a
certain depth in the system. Theoretical
modeling was important in showing
the researchers the limits of their new
technology. “What’s the fundamental
limit? How well can we reach it, and what
determines this limit?” Cao said.
Through mathematical calculations,
the team found that energy enhancement
depended on the sample thickness and
depth of the region. Energy enhancement
was also affected by the transport
mean free path of the system, which
www.yalescientific.org
The unique experimental platform designed for this study.
is the distance light can travel in a
random system before it loses all of the
information about the initial direction of
propagation. They then experimentally
measured the internal field distribution
at different depths to find the deposition
matrix for regions in a diffusive system.
They found that the highest possible
energy enhancement occurs at threefourths
of the system’s thickness.
Moment of Discovery
This research differs from
previous studies in the field
because it focuses on deposition
instead of energy transmission.
“Everybody doing wavefront
shaping was looking at the
transmission matrix or some version
of the transmission matrix,” Bender
said. Compared to transmission,
having disorder after the region of
light deposition can result in light
traveling backward, interfering with
the light going forwards, leading to a
greater energy enhancement than
in the case of transmission.
ABOUT THE AUTHOR
Controlling Randomness
IMAGE COURTESY OF HUI CAO
The ability to control random wave
scattering allows for energy deposition
into specific regions of opaque systems.
“This is interesting for medical applications
or anything dealing with a real system
because most systems are disordered
to some extent,” Bender said. Research
into this type of targeted energy delivery
could be used in applications ranging
from the optogenetic control of
neurons to tissue imaging. “There
are people in [the] community
trying to do imaging through
the skull. They try to send the
laser beams through the skull for
both a diagnosis and also to try to
simulate neurons,” Cao said.
This study pushes the boundaries
of what was previously thought to
be possible. “Traditionally, people
[think that] if something looks white,
then you just cannot see through it,”
Cao said. “I wish more people knew
that actually, that is not true. Random
scattering is not something just
impossible to control.” ■
EUNSOO HYUN
EUNSOO HYUN is a junior in Berkeley College majoring in Biomedical Engineering. Outside of writing
for YSM, Eunsoo enjoys painting, learning new languages, and dancing with the Yale Jashan Bhangra
team.
THE AUTHOR WOULD LIKE TO THANK Professor Hui Cao and Dr. Nicholas Bender for their time
and enthusiasm about their research.
FURTHER READING
Bender, N., Yamilov, A., Goetschy, A., Yılmaz, H., Hsu, C.W., & Cao, H. (2022). Depth-targeted energy
delivery deep inside scattering media. Nature Physics. 18: 309–315. https://doi.org/10.1038/s41567-
021-01475-x.
May 2022 Yale Scientific Magazine 21