Researchers have created a powerful new imaging method that reveals far more detail about ultrafast events in the microscopic world than ever before. These processes unfold in incredibly short times, often within hundreds of femtoseconds, and have traditionally been difficult to study. The new approach allows scientists to observe and analyze 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 in Optica, Optica Publishing Group's journal for high-impact research. The technique is known as compressed spectral-temporal coherent modulation femtosecond imaging (CST-CMFI). Using this system, the researchers 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 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 Brightness in Ultrafast Imaging
This work is part of ongoing efforts at the Extreme Optical Imaging Laboratory at East China Normal University to advance ultrafast camera technologies. A key focus is single-shot ultrafast optical imaging, which captures events that cannot be repeated by recording everything in a single exposure, similar to snapping a single frame that contains an entire sequence.
In the past, these techniques mainly recorded changes in brightness, also known as light intensity. However, light also carries phase information, which reveals how it bends or changes speed as it passes through materials. The researchers set out to capture both intensity and phase at the same time, providing a more complete picture of ultrafast processes.
To achieve this, they combined time-spectrum mapping, compressive spectral imaging and coherent modulation imaging. Each method contributes a specific advantage, including the ability to follow extremely fast changes, gather more data in one measurement and preserve fine image details.
How the CST-CMFI Technique Works
The system uses a chirped laser pulse made up of multiple wavelengths that arrive at slightly different times. This setup effectively links time to wavelength. When the pulse interacts with a fast-changing event, the scattered light carries detailed spatial, spectral and phase information. This information is then compressed into a single image through dispersion-encoded coherent modulation imaging.
A physics-informed neural network processes this data by separating the wavelengths and reconstructing both intensity and phase over time. Since each wavelength represents a specific moment, the result is a sequence of frames that forms an ultrafast movie captured in a single shot.
Real-Time Views of Plasma and Electron Behavior
To test the technique, the researchers examined two types of ultrafast phenomena. One experiment focused on plasma created in water by a femtosecond laser. Understanding how this plasma forms and evolves could support applications such as laser-based medical procedures. The imaging results revealed both brightness and phase changes within the plasma channel, including the formation of a dense free-electron plasma that affects how light is absorbed and how it travels through water.
The team also studied carrier dynamics in ZnSe to better understand how electrical charges move after being excited by light. Insights like these are important for improving optical and electronic devices made from this material, potentially leading to faster and more efficient technologies.
"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."
Expanding Applications and Future Improvements
Looking ahead, the researchers plan to apply the method to study additional phenomena, including interface dynamics and ultrafast phase transitions. These areas require detecting extremely small changes in the phase of light, making the new technique especially valuable.
At present, CST-CMFI converts spectral information into temporal information, which limits its ability to study processes that are highly sensitive to spectral changes. To address this, the team aims to combine CST-CMFI with compressive ultrafast photography. This next step would allow spectral and temporal information to be captured separately, significantly expanding the range of applications and improving the overall versatility of the technology.
