New blue light techniques could enable advances in understanding nanotechnology

(Nanowerk News) With a new microscopy technique that uses blue light to measure electrons in semiconductors and other nanoscale materials, the Brown University research team opens up new possibilities in studying these important components, which can help power devices such as cell phones and laptops.

The findings are a first in nanoscale imaging and provide a solution to a long-standing problem that has severely limited the study of key phenomena in materials that could one day produce more energy-efficient semiconductors and electronics.

Works published in Light: Science & Applications (“Near-range terahertz nonlinear optics with blue light”).

“There is a lot of interest today in studying materials at nanoscale resolution using optics,” said Daniel Mittleman, a professor in the Brown School of Engineering and author of a paper describing the work. “As the wavelengths get shorter, it becomes much more difficult to implement. As a result, no one has ever done it with blue light until now.”

Typically, when researchers use optics such as lasers to study nanoscale materials, they use light that emits long wavelengths such as red or infrared light. The method the researchers looked at in this study is called scattering-type scanning near-field microscopy (s-SNOM). This involves scattering light from a pointed tip that is only a few tens of nanometers. The tip hovers just above the sample material to be imaged. When the sample is exposed to optical light, the light is scattered and some of the scattered light leaves information about the nano-sized region of the sample just below the tip. The researchers analyzed the scattered radiation to extract information about this tiny volume of material.

This technique has been the basis of many technological advances, but has hit a dead end when it comes to using much shorter wavelength light, such as blue light. This means that using blue light, which is better suited for studying certain materials for which red light is ineffective, to gain new insights from well-studied semiconductors has been out of reach since the 1990s when this technique was discovered.

In the new study, researchers from Brown present how they overcame this obstacle to carry out what is believed to be the first experimental demonstration of s-SNOM using blue light instead of red.

For their experiment, the researchers used blue light to obtain measurements from silicon samples that couldn’t be obtained using red light. The measurements provide a valuable proof-of-concept of using shorter wavelengths to study materials at the nanoscale.

“We were able to compare these new measurements to what one would expect from silicon, and the fit was excellent,” said Mittleman. “This confirms that our measurements were successful and we understand how to interpret the results. Now we can start studying all of this material in a way we couldn’t before.”

To conduct an experiment, researchers have to get creative. Basically, they decided to make things easier by making things more complicated. With typical techniques, for example, blue light is difficult to use because its wavelength is so short, meaning it’s more difficult to focus on a precise point near the tip of the metal. If it is not aligned properly, the measurement will not work. With red light, these focusing conditions are more relaxed, making it easier to align the optics to efficiently extract scattered light.

With these challenges in mind, the researchers used blue light to not only illuminate the sample so that the light scatters, but also generate terahertz bursts of radiation from the sample. Radiation carries important information about the electrical properties of the sample. While the solution adds an extra step and increases the amount of data scientists must analyze, it eliminates the need to precisely align sample tips. The key here is that because terahertz radiation has a much longer wavelength, it is easier to align.

“It still has to be really close, but it doesn’t have to be that close,” Mittleman said. “When you hit it with light, you can still get information in terahertz.”

The researchers are excited to see what comes next in terms of the new information and discoveries this method brings, better insight into the semiconductors used to produce blue LED technology. Mittleman is currently developing plans to use blue light to analyze matter in ways researchers have never been able to do before.