Ultra-fast imaging or capturing images at the speed of light

Short abstract

Many physical, chemical or biological phenomena take place on so-called “ultra-short” time scales, from nanoseconds to hundreds of femtoseconds, and in a non-reproducible manner. However, their capture is generally impossible using conventional technologies, which has led in the last ten years to the emergence of ultra-fast imaging techniques known as “single-shot”. The remarkable progress made recently in this field, notably through the control and shaping of ultra-short light pulses and the development of original capture techniques, based or not on the use of a posteriori numerical methods, has made it possible to follow short or unpredictable events with unprecedented precision. Previously inaccessible phenomena have been captured with an adapted temporal resolution: shock wave propagation, laser ablation, plasma generation, fluorescence, or even the propagation of light itself, to name but a few. In this talk we will give an overview of the advances in ultrafast imaging and detail the key technologies at the heart of them.

BIO

Thomas Godin is a lecturer at the University of Rouen Normandy since 2014 and a researcher at the CORIA laboratory (CNRS – University and INSA of Rouen) in the Optics and Lasers Department. He did his thesis at the University of Caen in collaboration with the University of Sao Carlos (Brazil) and the National Laser Center of Pretoria (South Africa) on laser beam shaping and then a post-doc at the FEMTO-ST Institute (Besançon) in an international consortium for the study of velocity waves and extreme events in optics. He co-manages with Ammar Hideur (Pr. Univ. Rouen) the CORIA activities on ultrafast metrology and laser sources and coordinates several research projects on the application of ultrafast imaging methods to topics beyond the field of fundamental optics.

Extended abstract

Fast optical imaging is an essential technology for observing moving scenes without blurring, and this in all scientific fields. Since the first photograph by Nicéphore Nièpce in 1828, which required several days (!) of exposure time, man has constantly sought to reduce this exposure time. Until the end of the 19th century, photography was only a means of sharing with others what the eye could already see… Everything changed with the first “chronophotography” by Muybridge in 1878 and his iconic sequence (see Figure 1) breaking down the movement of a galloping horse. For the first time, a phenomenon inaccessible to human vision was captured, and Man could answer an existential question: does a galloping horse lift all four legs off the ground as it runs?!

Fig. 1 : «The Horse in Motion». E. Muybrigde, 1878, 10 images in 40 ms

Since then, technologies have evolved, but fast imaging has remained a field in the spotlight until today, with the aim of pushing back the limits of instrumentation and capturing ever more fleeting events… Acquisition rates of a few thousand images per second (fps – frame per second) were achieved in the 1980s, but a real “revolution” was initiated by the advent of CCD and CMOS sensors at the end of the 1990s, which today make it possible to find so-called ultra-fast commercial cameras that are relatively democratised and that allow several hundred thousand fps (= ? ? second between each image). They are used in a variety of ways, from youtubers making ‘consumer’ videos in ultra-slow-motion to scientists in different fields exploiting these new technological capabilities to analyse a myriad of ultra-short dynamic phenomena. It would be difficult to make an exhaustive list of these phenomena, as the number of fields impacted is so great, but ultra-fast imaging has made a significant contribution to subjects as varied as biomechanics, fluid dynamics, the physics of materials or molecular dynamics.

However, for time scales of less than a nanosecond, these ultra-fast cameras prove to be blind and new metrology techniques have had to be developed. Knowing that in order to capture a short phenomenon and thus “decompose” its movement, an optical signal at least as short must be used (remember the stroboscope in a nightclub…), the recent advent of ultra-short and ultra-intense pulsed lasers (cf. Nobel Prize 2018) has enabled significant progress in the monitoring of fast dynamics. The best-known methods are known as “pump-probe” methods, in which an ultrashort light pulse is used to trigger the phenomenon of interest and another to capture it (see femto-chemistry, Nobel Prize 1999). However, this large family of techniques is limited to events that are perfectly reproducible (= repeatable) and thus does not allow the capture of non-repetitive, transient or irreversible phenomena…

This constraint has been overcome recently (since the 2010s) with the development of so-called “single-shot” imaging techniques [1,2], which are now able to record short sequences at very high frame rates of more than Tera-fps (>1012 frames/s). This remarkable progress, achieved in particular through the control and shaping of ultra-short light pulses and the development of original capture techniques, based or not on the use of digital methods a posteriori, has made it possible to follow short or unpredictable events with unprecedented precision. Previously inaccessible phenomena have been captured with an adapted temporal resolution [3-5]: shock wave propagation, laser ablation, plasma generation, fluorescence, or even the propagation of light itself – the fastest phenomenon ever! Have we reached a technological limit in terms of capturing short events? Are these new techniques compatible with ‘real world’ use?

Fig. 2 : Ultrafast imaging of a Mach cone [6]

References

[1] Liang, J. Y. & Wang, L. V. “Single-shot ultrafast optical imaging”, Optica 5 (2018). https://doi.org/10.1364/OPTICA.5.001113

[2] Goda, K., Tsia, K. K. & Jalali, B. “Serial time-encoded amplified imaging for realtime observation of fast dynamic phenomena”, Nature 458 (2009) https://doi.org/10.1038/nature07980

[3] Gao, L. et al. “Single-shot compressed ultrafast photography at one hundred billion frames per second”, Nature 516 (2014) https://doi.org/10.1038/nature14005

[4] Nakagawa, K. et al. “Sequentially timed all-optical mapping photography (STAMP)”, Nature Photonics 8 (2014) https://doi.org/10.1038/nphoton.2014.163

[5] Touil M, Idlahcen S, Becheker R, et al. “Acousto-optically driven lensless single-shot ultrafast optical imaging”, Light: Science & Applications 11 (2022) https://doi.org/10.1038/s41377-022-00759-y

[6] Liang, J. Y. et al. “Single-shot real-time video recording of a photonic mach cone induced by a scattered light pulse”, Science Advances 3 (2017) https://doi.org/10.1126/sciadv.1601814