Ultrafast laser camera films combustion in real time

Scientists from the California Institute of Technology (Caltech), the University of Gothenburg and the University of Erlangen–Nuremberg have developed an ultrafast laser camera that can create videos with a record-fast speed of 12.5 billion images per second. This has enabled the researchers to study the combustion of hydrocarbon fuel — which produces nano-sized soot particles, various light phenomena and polycyclic aromatic hydrocarbons (PAHs) — with a time resolution that has never been recorded before.

Soot particles from hydrocarbons constitute 70% of the material in interstellar space and are also interesting nanomaterials with applications in electronics and energy, making their study important — but soot particles and aromatic hydrocarbons are extremely short-lived, with a lifespan measured in nanoseconds when they burn up, so ultrafast imaging approaches are required to resolve combustion in space and time. Current state-of-the-art planar imaging systems are limited to just a few million (10⁶) frames per second and require multiple consecutive laser pulses in order to extract 2D maps of flame species, causing undesired thermal issues. Furthermore, traditional pump-probe ultrafast imaging methods can only capture processes which are ‘repeatable’ because several images of the same process are taken at different time instances to extract a complete picture of spatiotemporal dynamics, and combustion is characterised by reactions that are not repeated.

The researchers developed a planar imaging camera utilising single-shot laser-sheet compressed ultrafast photography (LS-CUP), through which they have captured entire movies of laser-flame dynamics at 12.5 billion (10⁹) frames per second — at least three orders of magnitude higher (or 1000 times faster) than the current state-of-the-art systems. Using a single laser pulse, LS-CUP has enabled wide-field real-time imaging of laser-induced fluorescence from PAHs, elastic light scattering and laser-induced incandescence from soot particles. The results are published in the journal Light: Science & Applications.

“Laser sheet imaging is one of the most popular techniques for characterising flows and combustion in two dimensions because it preferably resolves a plane in both time and space,” said Caltech’s Dr Yogeshwar Nath Mishra, a leading author on the paper.

“Using LS-CUP, we can perform many exciting studies and ‘film’ fast chemical reactions and non-repeatable flame-laser interactions using a single laser pulse in real time beyond the MHz imaging range. We can combine it with pre-existing planar imaging methods for combustion research. Further, we can apply LS-CUP for real-time observation of hydrogen combustion, plasma-assisted combustion and metal powder combustion — some of the recent hot topics in the field. Temperature is a crucial property in many thermodynamic systems and, to the best of our knowledge, we have reported its fastest wide-field measurements.”

Dr Peng Wang, also from Caltech, added, “LS-CUP is perfect: it is single-shot, only needs a single laser pulse, has a wide field of view, and can be easily adapted to observe all kinds of laser-induced signals over the entire lifetime of soot particles. We extracted critical parameters from the fast dynamics, such as fluorescence lifetimes of PAH molecules, soot nanoparticle sizes and cluster sizes, particle temperature, etc. LS-CUP, in general, allows us to study extremely fast phenomena from a completely new and unique perspective.

“Reaching far beyond combustion research, the applications of our technique are extremely broad in physics, chemistry, biology and medicine, energy, and environmental research. The capability of capturing ultrafast phenomena represents an important metric of our human’s technology development.”

Image credit: iStock.com/dzika_mrowka