Advance in Practical Subwavelength Imaging

Technion – Israel Institute of Technology present a new approach to imaging evanescent waves that allows, among other things, tackling this challenge with the help of “nonlinear wave-mixing,” a combination of two or more light beams that generate a new electromagnetic wave of a different color. At least one of the light beams must be very intense. It works 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.

This enables real-time measurement of light waves bound to surfaces.

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 indicated. “For many applications, all you really need is what you already have in your optics lab.”

In their manuscript, Bartal’s research team, comprised of Kobi Frischwasser, Kobi Cohen, Jacob Kher-Alden, Shimon Dolev, and Shai Tsesses, demonstrated the strength of their scheme in imaging various patterns of electromagnetic surface waves, called surface plasmons, while they change in real-time. “We have been working on simple methods to shape such waves for a while, so it was easy to design field patterns we could freely control,” said Jacob Kher-Alden.

“The interesting bit was the information we could extract,” added Bartal. “By changing the polarization of the high-intensity pulses, we could see different shapes. We then found out that we are not just measuring the evanescent waves, but we can choose what information to take out of them.” Particularly, the team could separate and visualize the information stored on the “spin” of the evanescent waves, i.e. the clockwise and anti-clockwise rotation of the electric field on the interface.

“When you process the optical information in free-space, everything is much easier,” said Kobi Cohen. “We could see the spatial frequency content of the surface waves, not just the real-space shape, and through a reconstruction algorithm, we managed to extract their phase as well. From here on out, the road to a full-field reconstruction was clear.”

They demonstrated the application of NNOM by monitoring the changes in digitally encoded surface waves via the use of a spatial light modulator (SLM). “We wanted to show that this new microscopy scheme can have practical applications,” explained Shai Tsesses. “Since there are times when you need to make sure of the exact evanescent pattern, such as in optical trapping and manipulation experiments or when trying to optically address quantum emitters in nanophotonic platforms.”

“We haven’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.”

SOURCES – Nature Photonics – Real-time sub-wavelength imaging of surface waves with nonlinear near-field optical microscopy
Written By Brian Wang,

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