Almost all modern communications rely on the guided waves of optical fibers to conduct enormous amount of information at roughly the speed of light. Large data centers, which are the central hubs for this ocean of information, rely on photonic integrated circuitry. This is another form of guided light waves. but within a silicon chip, quite like the chips of electrical circuitry.
Guided waves have attracted great attention in recent decades, stimulating the development of various generation and detection methods. These guided waves do not radiate outside their host structure but still leave a signature in air – a fast-decaying wave called an evanescent wave.
These waves can’t be picked up with standard microscopy methods, since their energy remains bound to the surface and cannot be seen by the microscope detector. Because of this, designated technologies were developed to detect these waves, using either a small needle approaching the surface, scattering out the electromagnetic power in its vicinity or by firing electrons on the surface and characterizing their spectrum afterwards. Although these two schemes provide an excellent spatial resolution, they require complex and designated infrastructure, as well as long acquisition times, which currently prevent them from imaging the guided waves in real time.
In an article published in the prestigious science journal Nature Photonics under the title “Real-time sub-wavelength imaging of surface waves with nonlinear near-field optical microscopy,” researchers from Technion-Israel Institute of Technology in Haifa present a new approach to imaging evanescent waves that allows, among other things, tackling this challenge with the help of “nonlinear wave-mixing.”
This refers to a combination of two or more light beams that generate a new electromagnetic wave of a different color. The phenomenon, which requires at least one of the light beams to be very intense, occurs in most semiconductors, dielectrics and metals. The Technion researchers mixed a wide and intense pulsed beam of light with evanescent waves traversing the surface, generating a new light wave that could be subsequently detected by regular means. By doing so, they were able to fully reconstruct the electromagnetic field of the evanescent waves and demonstrated real-time monitoring of changes in the wave pattern.
“The idea to overcome this challenge came to me when I was working on a different project” said Kobi Frischwasser, the leading author of the paper. “I was exploring ways to nonlinearly couple light into confined optical modes, when I realized that it could also work the other way around – the information in such modes can be coupled nonlinearly out. I never imagined that this new microscopy scheme could open up new and, so far, unattainable opportunities for near-field science.”
“Aside from bulk materials, nonlinear wave-mixing naturally takes place at any interface between two materials, making it an ideal platform for nanophotonics – which often deals with light at interfaces,” added Prof. Guy Bartal of the Viterbi Faculty of Electrical Engineering who headed the project. “Below some spatial limit, information remains bound to the surface and cannot be seen by any camera. Our technique “releases” this information into radiation that can be detected – even with a commercial camera.”
The new scheme, termed Nonlinear Near-field Optical Microscopy (NNOM), does not require anything other than a powerful commercial laser source and standard optical components and detectors. According to the researchers, this makes it not only affordable – but also approachable. “You don’t need expensive and complicated tools anymore,” Bartal declared. “For many applications, all you really need is what you already have in your optics lab.”
“We have’’t even begun to explore the limits of this scheme and its applications,” Frischwasser concluded, “It may very well help us to develop better methods of verification for photonic circuitry. We are very excited about the future, and hope that many groups around the world will join us on our quest.”