New blue light laser microscopy has been shown to be effective in measuring nanoscale structures in semiconductors and other materials used in devices like mobile phones and laptops.
A nanometre is a billionth of a metre.
Optical instruments like microscopes which allow us to look at tiny objects, work by bouncing light off or through the object back into our eyes.
Advanced microscopes use a method called “scattering-type scanning near-field microscopy” (s-SNOM). This involves scattering light from a sharpened tip, itself only a few tens of nanometres across. The tip is positioned above the sample to be imaged. When the light scatters off the tip, it bathes the sample in the laser light, a portion of which contains information about the nanoscale structure.
But the sensitivity of optical instruments depends on the wavelength of the light used. The longer the wavelength, the less detailed the nanoimage. Most microscopes use red or infrared light which can be 600 to thousands of nanometres in wavelength.
This has limited our understanding of exactly what is happening in nanoscale structures which could help in the development of more energy-efficient semiconductors and electronics.
“There is a lot of interest these days in studying materials with nanoscale resolution using optics,” says Professor Daniel Mittleman from Brown University in the US. “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.”
It is harder for researchers to find the right spot near the metal tip used in s-SNOM to focus on the material when using shorter wavelength light.
But the Brown University researchers have now performed s-SNOM with blue light for the first time. They tested their procedure on a silicon sample that can’t be measured using red light.
“We were able to compare these new measurements to what one might expect to see from silicon, and the match was very good,” Mittleman says. “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.”
They achieved this by adding an extra step in the process.
The scientists used the blue light to both illuminate the sample as well as inducing the production of radiation from the sample itself, in the terahertz band of the electromagnetic spectrum (between infrared and microwave frequencies).
It adds more data for the scientists to analyse, but the terahertz burst contains information about the electronic structure of the sample.
The researchers say that their study may help in producing blue LED technology. Mittleman is already working on using blue light to analyse materials never investigated before.
The study is published in the journal Light Science & Applications.