LS-CUP for time-resolved 2D imaging of laser-induced flame signals. (a) Photograph of the studied flame. The regions of investigation at heights h1 and h2 are labeled. (b) Illustrations of four optical signals from the flame: LIF laser-induced fluorescence (cyan dotted line), elastic scattering (green solid line), LII laser-induced incandescence (yellow dashed line), and luminosity (orange dash-dotted line). The first three are excited by laser light. (c) Schematic of the LS-CUP system. BS: beam splitter, CyL cylindrical lens, DMD digital micro-mirror device, HWP half-wave plate, IIP intermediate image plane, IL imaging lens assembly, M mirror, Pol polarizer, RL relay lens, SF spectral filter, SL stereoscopic lens. (d) Top-view of the laser sheet probing the central vertical plane of the flame. Credit: Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01095-5
The soot produced by unburnt hydrocarbon flames is the second largest contributor to global warming, while also harming human health. Researchers have developed state-of-the-art, high-speed imaging techniques to study turbulent flames, yet they are limited to an imaging rate of million-frames-per-second. Physicists are therefore keen to obtain a complete picture of flame-laser interactions via single-pulse imaging.
In a new report published in Light: Science & Applications, Yogeshwar
Nath Mishra and a research team at the Caltech Optical Imaging Laboratory, the
NASA Jet propulsion lab, department of physics, and the Institute of
Engineering Thermodynamics in the U.S., and Germany, used single-shot
laser-sheet comprised ultrafast photography per billion frames per second, for
the first time, to observe the dynamics of laser-flames.
The team noted laser-induced incandescence, elastic light scattering and the fluorescence of soot precursors such as polycyclic aromatic hydrocarbons in real-time, with a single nanosecond laser pulse. The research outcomes provide strong experimental evidence to support soot inception and growth mechanisms in flames. Mishra and the team combined a variety of techniques to probe the short-lived species in turbulent environments to unravel the mysteries of hot plasma, nuclear fusion and sonoluminescence.
Real-time
dynamics of laser-induced fluorescence (LIF) of PAH molecules at height ℎ1 of
the kerosene flame, captured by laser-sheet compressed ultra-fast photography
(LS-CUP) at 1.25 Gfps using a 450 nm shortpass spectral filter. The interval
between neighboring frames is 0.8 ns and there are 150 frames. LIF is excited
by a single 532-nm pulse with a fluence of 0.01 J cm-2. Credit: Light: Science
& Applications (2023). DOI: 10.1038/s41377-023-01095-5
The environmental and health impacts of
polycyclic aromatic hydrocarbons (PAH)
Modern day hydrocarbons including kerosene, gasoline
and diesel produce harmful emissions such as polycyclic aromatic hydrocarbons (PAHs) that contribute to poor health effects.
Soot emissions have a large impact on the quality of human life due to its nanoscale size that can easily penetrate
the lungs or bloodstream to cause health defects. While PAHs form precursors of
soot particles that contribute to toxic carcinogenicity, their role is also
significant in materials science due to their use as carbon nanomaterials.
The high energy efficiency, low cost and rapid
production of the plasmonic nanoparticles have led to applications with excellent optical properties. Incidentally, about 70% of interstellar space is composed of carbonaceous particles, and soot
formation from gaseous PAHs have remained a mystery in both combustion science
and astronomy. In 2014, physicists first reported a method of compressed ultrafast photography (CUP) for single-shot 2D imaging, with an
imaging speed of 70 trillion frames per second.
Real-time
dynamics of elastic light scattering at height ℎ1 of the kerosene flame,
captured by LS-CUP at 12.5 Gfps using an ultra-narrow bandpass spectral filter
centered at 532 nm. The interval between neighboring frames is 0.08 ns and
there are 200 frames. This is induced by a single 532 nm pulse with a fluence
of 0.24 J cm-2. Credit: Light:
Science & Applications (2023). DOI: 10.1038/s41377-023-01095-5
In
this work, the researchers used laser-sheet compressed ultrafast photography
(LS-CUP) technology as an experimental framework to observe laser-induced
fluorescence of polycyclic aromatic hydrocarbons to determine primary particle
sizes, for soot temperature mapping and light scattering applications. The
engineers and physicists synergized planar imaging and compressed ultrafast
photography (CUP) to view flame-laser interactions in real-time. They combined
laser-sheet time planar imaging to explore a comprehensive experimental
outcome, where the multi-channel capabilities of the network allowed the team
to investigate qualities of the flame in real-time and explore high dimensional
imaging.
Laser-sheet compressed ultrafast photography (LS-CUP) at 1.25-Gfps, imaging of laser-induced fluorescence (LIF) of PAH molecules. Credit: Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01095-5
Laser-sheet compressed ultrafast
photography (LS-CUP) technology
During the experiments, the team examined a laminar,
symmetric and relatively stable kerosene flame. They selected kerosene as a
fuel due to its broad industrial and
domestic applications and
characterized the flame by using four optical signals and included time-gated
cameras to collect a specific signal of interest.
Based on laser-sheet imaging, the scientists optically
sectioned a 2D plane of a volumetric flame, where they extracted a 2D map of
the species of interest and collected it on a camera thereafter, which was
covered by screens to reduce turbulence. The imaging lens assembly projected
the flame dynamics to two intermediate image planes separated by a
non-polarizing beam splitter. The scientists selected different flame signals
via dual operation of the LS-CUP technology to simultaneously image the two
species.
1.25-Gfps LS-CUP imaging of one-color laser-induced incandescence (LII) of soot particles and primary soot particle size distributions determined by time-resolved LII. Credit: Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01095-5
Observing the polycyclic aromatic
hydrocarbons (PAHs) laser-induced fluorescence decay in real-time
The research team sought new experimental insights to
better understand the growth chemistry of polycyclic aromatic hydrocarbons; the
molecular precursors of soot. Physicists had previously studied the spatially
resolved, averaged 2D light-induced fluorescence of PAHs in flames and
obtained time-resolved
measurements.
Nevertheless, reports on single-shot, high-speed
spatiotemporal imaging of polycyclic aromatic hydrocarbons were yet to be explored,
which Mishra and colleagues conducted by exciting laser-induced fluorescence
with a single 532-nm pulse. The researchers studied a combined view of the
flame luminosity with polycyclic aromatic
hydrocarbons, light
induced fluorescence, soot-laser induced incandescence and elastic light
scattering extracted from time-integrated images via three sequential
measurements.
The team studied the time-resolved, one-color
laser-induced incandescence (LII) after examining the evolution of soot and
deduced the size of the soot particles from the LII signals via energy and mass balancing. They further obtained time-resolved two-color
laser-induced incandescence as well as the soot temperature dynamics with two
optical bandpass filters, which they simultaneously recorded with two channels
via laser-shield compressed ultrafast photography. The accompanying temperature
maps showed varied temperatures throughout the flame that was highest at the
flame edges and lowest at the center and at the bottom of the flame. The
researchers additionally observed elastic light scattering from soot particles
in real-time.
1.25-Gfps LS-CUP imaging of two-color LII of soot particles and soot temperature dynamics. (a–d) Reconstructed spatiotemporal dynamics of two-color LII signals at height h1: a short wavelength channel centered at 460 nm and (b) long wavelength channel centered at 666 nm. There are totally 200 frames. Spatiotemporal dynamics of soot temperature after laser heating: (c) 3D representation; (d) selected snapshots. Scale bar in (d): 2 mm. (e) Spatially averaged temperature of heated soot particles against time and its exponential fit. (f) Soot particle size distribution in the x-y plane at height h1. It is calculated using the reconstructed temperature decay of soot particles shown in (c). (g) Soot particle size distributions along the x direction, at vertical locations y1 to y8, indicated by the short lines in (f), using both one-color LII dynamics (red solid lines) and two-color LII dynamics (black dashed line). Credit: Light: Science & Applications (2023). DOI: 10.1038/s41377-023-01095-5
Outlook for the new invention
Yogeshwar Nath Mishra and colleagues achieved the
world's fastest single-pulse, real-time, 2D imaging of combustion with an
unprecedented imaging speed of 12.5 Gfps, with a sequence depth of up to 200
frames via laser-sheet compressed ultrafast photography (LS-CUP). The imaging
speed exceeded the existing high-speed rate in the Mfps regime, and the team
used the new invention to explore active and passive imaging options. The work
opens new paths for real-time ultrafast imaging of polycyclic aromatic
hydrocarbon molecules via femtosecond pulses to gain new details to their
origins.
These insights will shed light on the development of
carbon-based nanomaterials and allow materials scientists and physicists to
understand the fundamentals of combustion relative to jet propulsion. Broader
research applications include using LS-CUP to examine sonoluminescence; a mysterious phenomenon in condensed matter physics,
where the gradual build-up of sound in fluid can produce bubbles with a plasma
temperature greater than 10,000 K that emit a light flash in tense of
picoseconds.
By using the LS-CUP method, the team envision
exploring ultrafast temperature sensing of sonoluminescence bubbles that have a
broad scope of applications across condensed
matter physics and
as therapeutic strategies in the life sciences.
by
Thamarasee Jeewandara , Phys.org
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