Researchers have unveiled a new imaging method that can capture ultrafast events in the microscopic world with unprecedented detail. These processes, which unfold in timescales of hundreds of femtoseconds (a trillionth of a second), have traditionally been fiendishly difficult to study. The new technique, however, allows scientists to observe these rapid changes with exceptional clarity and speed.
"In the fields of physics, chemistry, biology and materials science, many important phenomena happen incredibly fast," said research team leader Yunhua Yao from East China Normal University. "Our new technique can capture the complete evolution of both the brightness and internal structure of an object in a single measurement. This is a big step forward for understanding the fundamental nature of matter, designing new materials and even uncovering the mysteries of biological processes."
The team described their method, known as compressed spectral-temporal coherent modulation femtosecond imaging (CST-CMFI), in the journal Optica. Using this system, they were able to track ultrafast activity such as plasma forming in water after a femtosecond laser pulse and the behavior of excited charge carriers in a material called ZnSe.
"Beyond helping scientists study materials that change instantly in response to laser light, chemical reactions that rearrange atoms at lightning speed and the dynamic behavior of biomolecules over incredibly short timescales, CST-CMFI could help improve high-power laser technologies used for clean energy research, advanced manufacturing and scientific instrumentation," said Yao. "It might also lead to the development of more efficient electronics, improved solar cells and faster devices by enabling a better understanding of how materials behave at extremely fast timescales."
Capturing More Than Just a Flash in the Pan
This work is part of ongoing efforts at the Extreme Optical Imaging Laboratory at East China Normal University. A key focus is single-shot ultrafast optical imaging, which captures non-repeatable events in a single exposure. Past techniques mainly recorded changes in brightness, or light intensity. But light also carries phase information, which reveals how it bends or changes speed. The researchers set out to capture both intensity and phase simultaneously, providing a more complete picture.
To achieve this, they combined time-spectrum mapping, compressive spectral imaging and coherent modulation imaging. The system uses a chirped laser pulse made up of multiple wavelengths that arrive at slightly different times, effectively linking time to wavelength. When the pulse interacts with a fast-changing event, the scattered light carries detailed spatial, spectral and phase information, which is compressed into a single image. A physics-informed neural network then processes this data, separating wavelengths and reconstructing both intensity and phase over time to create an ultrafast movie from a single shot.
Real-Time Views of Plasma and Electron Shenanigans
In testing, the team examined plasma created in water by a femtosecond laser, which could support applications like laser-based medical procedures. The imaging revealed both brightness and phase changes within the plasma channel, including the formation of a dense free-electron plasma. They also studied carrier dynamics in ZnSe to understand how electrical charges move after being excited by light, which is crucial for improving optical and electronic devices.
"Using CST-CMFI, we were able to see phase variations associated with the carrier dynamics, even when there were no significant changes in intensity," said Yao. "This highlights a key advantage of our method: Phase measurements can be much more sensitive than intensity measurements in detecting subtle ultrafast processes."
Future Plans: Because Even a Trillionth of a Second Isn't Fast Enough
Looking ahead, the researchers plan to apply the method to study additional phenomena like interface dynamics and ultrafast phase transitions. Currently, CST-CMFI converts spectral information into temporal information, which limits its ability to study processes highly sensitive to spectral changes. To address this, the team aims to combine CST-CMFI with compressive ultrafast photography, which would allow spectral and temporal information to be captured separately, significantly expanding the technology's versatility.