Newswise — PROVIDENCE, R.I. [Brown University] —

Using a novel microscopy method that employs blue light to gauge electrons in semiconductors and other nanoscale substances, a group of researchers from Brown University is introducing a fresh domain of potential in the research of these crucial constituents, which have the potential to energize gadgets such as cellphones and laptops.

The discoveries represent a breakthrough in nanoscale visualization and offer a solution to a persistent issue that has significantly restricted the examination of crucial phenomena in numerous substances that may eventually result in semiconductors and electronics that are more energy-efficient. The research was published in Light: Science & Applications.

"Presently, there is a substantial interest in examining materials with nanoscale precision through optics," stated Daniel Mittleman, an author of the research paper and a professor at Brown's School of Engineering. "As the wavelength decreases, this becomes significantly more challenging to execute. Therefore, up until now, no one had accomplished this using blue light."

Typically, researchers employ optics such as lasers that emit longer wavelengths such as red or infrared light to examine nanoscale materials. The approach investigated by the researchers in this study is known as scattering-type scanning near-field microscopy (s-SNOM). It involves scattering light from a sharpened tip that measures only a few tens of nanometers in width. The tip floats slightly above the material being imaged. When the sample is illuminated with optical light, the light scatters and some of the scattered light contains details about the nano-sized region of the sample precisely beneath the tip. The researchers scrutinize that scattered radiation to obtain information about this minute volume of material.

The s-SNOM method has underpinned numerous technological breakthroughs, but it encounters a roadblock when attempting to employ light with a significantly shorter wavelength, such as blue light. This indicates that utilizing blue light, which is more appropriate for examining specific materials that red light is inadequate for, to obtain fresh insights from semiconductors that have already been well-investigated has remained unattainable since the technique's inception in the 1990s.

In the recent investigation, the scientists from Brown University describe how they overcame this hurdle to conduct what is thought to be the initial empirical verification of s-SNOM employing blue light in lieu of red.

To carry out the experiment, the researchers employed blue light to obtain measurements from a silicon sample that could not be attained using red light. These measurements offered a valuable proof-of-concept regarding the use of shorter wavelengths to study materials at the nanoscale.

Mittleman stated, "We were able to compare these fresh measurements to what one would anticipate seeing from silicon, and the correlation was exceptional. It validates that our measurement is functional and that we comprehend how to decode the outcomes. At this point, we can commence scrutinizing all these substances in a manner that was previously unfeasible."

To conduct the experiment, the scientists had to get inventive. Essentially, they resolved to make things simpler by making them more complex. For instance, with the conventional technique, blue light is difficult to use due to its short wavelength, which makes it more difficult to focus on the correct area near the metal tip. If the alignment is not precise, the measurement will not work. With red light, this focusing requirement is less stringent, making it easier to align the optics to gather the scattered light effectively.

Taking into account these obstacles, the investigators employed the blue light not only to illuminate the sample and cause light scattering but also to generate a burst of terahertz radiation from the sample, which carries crucial information about the sample's electrical features. Although the solution adds an additional step and increases the quantity of data the researchers must scrutinize, it negates the necessity for a precise alignment of the tip over the sample. The crucial aspect here is that the terahertz radiation has a considerably longer wavelength, which makes it easier to align.

Mittleman remarked, "It still has to be quite close, but not as close. When the sample is illuminated with light, valuable terahertz information can still be obtained."

The researchers are enthusiastic to explore what new information and discoveries this method may bring, such as providing better insights into semiconductors used to create blue LED technology. Mittleman is currently formulating strategies to use blue light to investigate materials that have not been studied before.

The work was led by Angela Pizzuto, a Brown physics Ph.D. student who will graduate in May. Pingchuan Ma, a Ph.D. student in Brown’s School of Engineering, also contributed.

The work was supported by the National Science Foundation Division of Electrical Communications and Cyber Systems, the Kansas City National Security Campus and the Department of Energy.

Journal Link: Light: Science & Applications