Imaging method measures particle size and position with nanometer precision

Researchers have developed a new measurement and imaging approach that can resolve nanostructures smaller than the diffraction limit of light without requiring dyes or markers. This work represents an important step towards a new and powerful microscopy method that could one day be used to see the fine features of complex samples beyond what is possible with conventional microscopes and techniques.

The new method, described in Optical, is a modification of laser scanning microscopy, which uses a highly focused laser beam to illuminate a sample. The researchers developed the technique by not only measuring the brightness or intensity of light after it interacts with a specimen under study, but also by detecting other parameters encoded in the light field.

“Our approach could help extend the microscopy toolbox used to study nanostructures in a variety of samples,” said research team leader Peter Banzer from the University of Graz in Austria. “Compared to super-resolution techniques based on a similar scanning approach, our method is completely non-invasive, meaning it requires no injection of fluorescent molecules into a sample prior to imaging.”

The researchers show that they can measure the position and size of gold nanoparticles with an accuracy of several nanometers, even when several particles touch each other.

“Our new approach to laser scanning microscopy could bridge the gap between conventional resolution-limited microscopes and super-resolution techniques that require modification of the sample under study,” Banzer said.

Capture more light

In laser scanning microscopy, a beam of light is scanned through the sample and the transmitted, reflected or scattered light from the sample is measured. Although most microscopy methods measure the intensity or brightness of light coming from the sample, a lot of information is also stored in other characteristics of the light such as its phase, polarization and angle of reflection. diffusion. To capture this additional information, the researchers looked at the spatial resolution of the intensity and polarization information.

“The phase and polarization of light, as well as its intensity, vary in space in a way that incorporates fine detail about the sample it is interacting with – much like the shadow of an object tells us something about the shape of the object itself,” says Banzer. “However, much of this information is ignored if only the overall light output is measured after the interaction.”

They demonstrated the new approach by using it to study single samples containing metal nanoparticles of different sizes. They did this by scanning the area of ​​interest and then recording the polarization and angular resolution images of the transmitted light. The measured data was evaluated using an algorithm that creates a model of the particles that automatically adapts to resemble the measured data as accurately as possible.

“Although the particles and their distances were much smaller than the resolution limit of many microscopes, our method was able to resolve them,” Banzer said. “Additionally, and more importantly, the algorithm was able to provide other parameters about the sample, such as precise particle size and position.”

The researchers are now working to adapt the method so that it can be used with more complex samples. The functionality of the approach can also be extended by adapting the structure of the light that interacts with the sample and by incorporating artificial intelligence-based approaches into the image processing steps. On the detection side, the authors, in collaboration with other experts, are currently developing a special camera as part of a European project called SuperPixels. This next-generation detection device will be able to resolve polarization and phase information in addition to intensity.

“Our study is another demonstration of the central role that the structure of light can play in the field of optics and light-based technologies,” Banzer said. “Many intriguing applications and phenomena have already been demonstrated, but there is more to come.”

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Material provided by Optical. Note: Content may be edited for style and length.

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