Framework model for determining atomic structure Light-matter interactions in nonlinear optical microscopy

Materials scientists can learn a lot about a sample’s material by shooting lasers at it. with nonlinear optical microscopy—a specialized imaging technique that looks for changes in the color of intense laser light.

Now, researchers at Pennsylvania State University have developed a computational framework that can interpret nonlinear optical microscopy images to characterize materials in microscopic detail.

The team has published their approach in the journal Optica.

“Nonlinear optical microscopy is an important tool that can reveal structural information about a variety of materials,” said lead author Albert Soswa, a doctoral student in materials science and engineering at Penn State.

“This method, which looks for exotic interactions between matter and light, can be used to see things in material samples that are normally invisible to us otherwise.

The way our eyes see the world is through linear optical interactions such as reflection, refraction and absorption, Soswa explained.

“Whereas in nonlinear optical microscopy, we use a focused laser beam to get light that’s much more intense than what you can get from everyday light sources like sunlight,” Sosova said. “And this intense light can produce new types of optical signals that are detectable. We can understand something about the structure of the material by looking at how these new signals change in the sample, or how they change with something like the polarization of a laser source.

“From there, we used our understanding of classical optical microscopes to develop a computational tool for interpreting these images, enabling the determination of material properties at the microscopic scale.”

The researchers said, when they observed unexpected phenomena in the microscopy images and questioned whether it was due to the sample or the microscope.

“This whole project started when we were doing nonlinear microscopy on a sample that we thought we understood very well but we were seeing things in our images that we couldn’t explain, almost like an optical illusion,” Sosova said.

“Therefore, we took a very long time to make sure that the observations were not just an optical illusion but valid data. We had to make sure that when we were able to focus the microscope light and our probes perfectly tightly.

Light travels in the form of electromagnetic waves with unique frequencies, and the interaction of atoms and molecules with light – also known as electromagnetic radiation – provides information about their structure.

“Light is really central to how we see the world. In fact, it dominates our sense of physical reality,” said Venkatraman Gopalan, a professor of materials science and engineering at Penn State and a co-author on the paper. “Imaging with light is very fundamental and we’re constantly looking at new ways of imaging things. All of this light interacts with atoms and scatters.”

There are many types of light waves in the electromagnetic spectrum, ranging from radio waves to gamma rays. Each type of light has different wavelengths and frequencies, and scientists can use information about how objects and materials emit, absorb, transmit, or reflect light to investigate their properties.

“Atoms vibrate differently and make music. They dance to different beats, and light is like music,” Gopalan said. “From electrons to nuclei to clusters of atoms to their spins, they dance at all kinds of different frequencies. It’s almost like an opera. And when, for example, you want to know how atoms are moving, you can send a color of light, and the atoms vibrate and absorb some of that light.

“The light that reflects back is slightly less and different in color. It has a slightly longer wavelength and a lower frequency, because it reflects less energy.

There are many techniques that use light to study the properties of materials, from X-rays to thermal imaging. For this research, the team used a technique called second-harmonic generation microscopy.

“Second harmonic generation is where a material changes the color of light by doubling its frequency,” Gopalan said. “It can detect signals that indicate a large dance of electrons, which can reveal the specificity of the material. This doubling of frequency can turn an infrared into a blue color, which comes from this lopsided dance of electrons within the atoms in these solids.”

Scientists say they can make an image out of signals, but it takes more than just making an image to really characterize a substance.

“We need to know what’s going on, what the nukes are doing, what’s happening with local properties, but the picture that’s telling us has been a challenge because there’s more information than show and tell,” Gopalan said.

The goal was to develop a framework that accurately models the interaction of tightly focused light with a sample in nonlinear optical microscopy, providing reliable quantitative information, according to the researchers.

The team tested their framework on several reference materials, comparing the results to known properties. By doing this, they were also able to extract quantitative information from the sample, Soswa noted. Understanding specific properties, along with quantitative information, is critical to developing new materials and understanding their properties, Soseva said.

“Our framework tries to go beyond ‘see and see,’ to actually say that,” Sosova said. We envision this framework to help standardize approaches to data analysis in the nonlinear optics community to improve material consistency and reproducibility.

“We think we’ve found a way to look at this problem that’s simpler than how other people have done it and still agrees very well with known patterns. By mapping material properties instead of just snapping an image, we can help build a library of material properties that can be used in a variety of applications.”