“There is a lot of interest these days in studying materials with nanoscale resolution using optics. As the wavelength gets shorter, this becomes a lot harder to implement. As a result, nobody had ever done it with blue light until now.”Daniel Mittleman, Brown University
With a new microscopy technique that uses blue light to analyze semiconductors and other nanoscale materials, a team of researchers at Brown University is opening up new possibilities for the study of these critical components.
The new technique, reported in a paper in Light: Science & Applications, is a first in nanoscale imaging. It provides a workaround to a longstanding problem that has greatly limited the study of key phenomena in a wide variety of materials and could one day lead to more energy-efficient semiconductors and electronics.
“There is a lot of interest these days in studying materials with nanoscale resolution using optics,” said Daniel Mittleman, a professor in Brown’s School of Engineering and author of the paper. “As the wavelength gets shorter, this becomes a lot harder to implement. As a result, nobody had ever done it with blue light until now.”
Typically, when researchers employ optics like lasers to study nanoscale materials, they use long wavelengths of light such as red light or infrared. This is the case with a method called scattering-type scanning near-field microscopy (s-SNOM), which is based on scattering light from a sharpened tip that is only a few tens of nanometers across.
The tip hovers just above the sample material to be imaged. When the sample is illuminated with light from a laser, the light scatters and a portion of this scattered light contains information about the nano-sized region of the sample directly beneath the tip. Researchers can then analyze the scattered light to extract information about this small volume of material.
This method has been the foundation of many technological advances, but it hits a wall when it comes to light with shorter wavelengths. As a result, the use of blue light, which is better suited for studying certain materials than red light, to gain new insights from semiconductors has been out of reach since the 1990s when the method was invented.
In this new study, the Brown researchers were able to overcome this problem to perform what’s believed to be the first-ever experimental demonstration of s-SNOM using blue light instead of red. They used the blue light to derive measurements from a silicon sample that cannot be obtained using red light. These measurements provided a valuable proof-of-concept about the use of shorter wavelengths for studying materials at the nanoscale.
“We were able to compare these new measurements to what one might expect to see from silicon, and the match was very good,” Mittleman said. “It confirms that our measurement works and that we understand how to interpret the results. Now we can start studying all these materials in a way that we couldn't before.”
To conduct the experiment, the researchers had to get creative. Essentially, they decided to make things easier by making them more complicated. In the typical version of s-SNOM, blue light is hard to use because its wavelength is so short, making it difficult to focus blue light over the right spot near the metal tip. If not aligned just right, the measurement won’t work. With red light, this focusing condition is less stringent, making it easier to align the optics to extract the scattered light efficiently.
With those challenges in mind, the researchers used the blue light not only to illuminate the sample so that the light scatters, but also to produce a burst of terahertz radiation from the sample. This radiation carries important information about the sample’s electrical properties. While this approach adds an extra step and increases the amount of data the scientists have to analyze, it eliminates the need to be as precise in how they align the tip over the sample. Because the terahertz radiation has a much longer wavelength, it can be aligned more easily.
“It still has to be really close, but it doesn’t have to be as close,” Mittleman said. “When you hit it with the light, you’ll still be able to get information in the terahertz.”
The researchers are excited to see what new information and discoveries come out of their new technique, such as better insights into semiconductors used to produce blue LED technology. Mittleman is currently developing plans to use blue light to analyze materials that researchers haven’t been able to study before.
This story is adapted from material from Brown University, with editorial changes made by Materials Today. The views expressed in this article do not necessarily represent those of Elsevier. Link to original source.