Tech Briefs Magazine - November 2024 - PIT-13

New Ultrafast Imaging Technique Provides Insight into
Combustion Processes
A team of scientists has developed an ultrafast imaging technique, called femtosecond laser sheetcompressed
ultrafast photography that can compile videos of incredibly transient details.
California Institute of Technology (Caltech), Pasadena, CA
D
etails of combustion - the chemical
reactions that take place when, for
example, a flame is lit - are fleeting and
therefore, difficult to study. But scientists
would like to better understand the
complex processes that occur in those
billionths of seconds, not only to make engines
more efficient but also to shed light
on how candle flames, cars, and airplanes
produce gases and particles that are harmful
to humans and the environment.
A team of scientists from Caltech's
campus and Jet Propulsion Laboratory
(JPL - which Caltech manages for
NASA), and the University of Erlangen-Nuremberg,
in Germany, have developed
an ultrafast imaging technique,
called
femtosecond laser
sheet-compressed
ultrafast photography (fsLSCUP),
that can compile videos of those
incredibly transient details.
Capturing 250 billion frames per second,
the new technique is 20,000 times
faster than conventional high-speed imaging
cameras, and about 100 times faster
than state-of-the-art imaging systems.
It has already revealed some of the underlying
dynamics involved in the formation
of soot particles during combustion.
A paper highlighting the new technique
appeared online on August 29 in
the journal Light: Science & Applications.
Soot is a major contributor to global
warming, thought to be number two
behind carbon dioxide in terms of its
impact on climate. Yet how exactly soot
particles form is still an open question.
While scientists have previously suggested
that the particles might be built
from polycyclic aromatic hydrocarbons
(PAHs), a class of chemicals that are
formed during the incomplete burning
of coal, oil, gasoline, and even charbroiled
meat, the precise mechanism by
which this happens has not been fully
determined, leaving room for debate.
" With the movies we have been able to
capture with our new technique, we now
have a very clear indication that soot parPhotonics
& Imaging Technology, November 2024
ticles do indeed grow from PAHs, " said
Yogeshwar Nath Mishra, a co-lead author
of the new paper and a Swedish Research
Council-funded researcher in the Caltech
Optical Imaging Laboratory. " We can see
that the PAHs attach together and form
nascent soot particles that further grow
in the flame to form very large particles. "
The new technique involves shining
a femtosecond laser - one that emits
a single pulse of light for just a quadrillionth
of a second - on a sample flame
and pairing that with a modified version
of a method called compressed ultrafast
photography, which was developed in
the lab of Lihong Wang, the Bren Professor
of Medical Engineering and Electrical
Engineering at Caltech, who is also
an author of the new paper.
" We are thrilled to see a high-impact
application of the world's fastest camera
that captures events in a single shot without
repetition, " said Wang, who is also the
Andrew and Peggy Cherng Medical Engineering
Leadership Chair and executive
officer for medical engineering at Caltech.
" This technology opens new doors across
both biological and physical sciences, with
applications ranging from microscopy to
telescopy, and holds the potential for significant
societal impact in understanding
underlying biology and physics. "
Without disturbing the flow of the
sample flame, the single femtosecond
laser in fsLS-CUP accomplishes a couple
of things. First, its wavelength is selected
such that it excites the PAHs present, allowing
their fluorescence to be detected.
And second, it causes the soot nanoparticles
present to rapidly heat up. As they
relax back to normal temperatures, the
particles emit a signal of incandescence.
The fluorescence and incandescence
can then be used to study the lifespan
of the PAHs and soot particles created
through combustion.
Still, both the fluorescence of PAHs and
the laser-induced incandescence of soot
particles have extremely short lifespans,
lasting only nanoseconds to thousands of
nanoseconds. To measure such fleeting
signals in real time, the new technique
uses something called a light sheet to define
a two-dimensional plane within the
Yogeshwar Nath Mishra, a co-lead author of a paper describing a new ultrafast imaging technique capable
of capturing the fleeting details of combustion at 250 billion frames per second. (Image:Caltech)
13
Tech Briefs Magazine - November 2024

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