An all-optical ultrafast imaging system developed by scientists at Shenzhen University has reached 15 trillion frames per second. The system is useful for visualizing ultrafast phenomena such as femtosecond laser ablation, ignition for nuclear fusion energy systems, shockwave interactions in living cells, and certain chemical reactions.

Sequential imaging of microscopic ultrafast dynamic processes requires high frame rates and high spatial and temporal resolutions, which in current imaging systems are in trade-off with one another. As the researchers’ technique is all-optical, it is free from bottlenecks arising from scanning with mechanical and electronic components.

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Four sequential images of a rotating optical field spinning at 10 trillion radians per second. Courtesy of Zeng et al.

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The design focuses on noncollinear optical parametric amplifiers (OPAs). An OPA is a crystal that amplifies the signal beam and produces another light beam known as an idler when it is simultaneously irradiated with a desired signal light beam and a higher frequency pump light beam. The crystal used in the study is noncollinear, meaning the idler is fired in a different direction from that of the signal beam.

The OPA maps the information of the target contained in the signal beam onto the idler while the pump beam is active. The idler moves in a different direction, allowing its capture by a conventional CCD camera while the signal beam moves toward the stage in the OPA cascade.

Similar to how water would descend in a waterfall, the signal beam reaches the subsequent OPA, and the pump beam generated from the same laser source activates it. A delay in the pump beam’s arrival causes the CCD adjacent the OPA in the second stage to snap a picture later in the process. Through a cascade of four OPAs with four associated CCD cameras and four different delay lines for the pump laser, the scientists created a system that can take four pictures in rapid succession.

The speed at which the camera is able to snap consecutive pictures is limited, however, by how small the size of the difference between two laser delay lines can be. In this regard, the imaging system achieved an effective frame rate of 15 trillion frames per second. Conversely, the temporal resolution depends on the duration of the laser pulses triggering the OPAs and generating the idler signals. In this case, the pulse width was 50 femtoseconds. That, combined with the frame rate, allows the technique to observe ultrafast physical phenomena, such as an air plasma grating and a rotating optical field spinning at 10 trillion radians per second.

The research was published in Advanced Photonics (www.doi.org/10.1117/1.AP.2.5.056002).

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