Meta-optics shows physical processes in the attosecond range

April 07, 2023

(Nanowerk News) Developed at Harvard, and successfully tested at Graz University of Technology (TU Graz), the revolutionary new meta-optics for microscopy with very high spatial and temporal resolution has proven its functional capability in laboratory tests at the Institute of Experimental Physics at TU Graz. Microscopes using this type of lens hold promise for completely new research and development approaches, particularly in semiconductor and solar cell technology.

A team of researchers from Graz and Boston now report on successful construction and laboratory experiments with this new meta-optic in the journal. Science (“Vacuum Guiding Extreme Ultraviolet Metalens”).

The lens of the microscope made it possible to use extreme ultraviolet radiation for the first time. Its very short wavelength allows it to follow very fast physical processes in the attosecond range. For example, real-time images of the interior of modern transistors or the interaction of molecules and atoms with light. Marcus Ossiander came up with the idea for the new lens during his research work in Federico Capasso’s group at Harvard University, and since January 2023, winner of the ERC Starting Grant and FWF START Award has been conducting research at the Institute of Experimental Physics at TU Graz . This team of experimental physicists from the Graz University of Technology successfully demonstrated the functionality of meta-optics. From left to right: Martin Schultze, Maryna Meretska (Harvard), Marcus Ossiander and Hana Hampel. (Image: TU Graz)

Success together for Boston and Graz

Atthodetic physics uses extreme ultraviolet light. Because this light oscillates rapidly and all the materials in the optical development construction kit are opaque to this light, no imaging system has yet been usable until now. Marcus Ossiander: “I asked myself whether the classical principles of optics cannot be reversed. Can you use the absence of matter in a small area as a basis for optical elements?”

Lenses developed at Harvard based on this idea and successfully tested at TU Graz implement this design principle. An array of precisely counted pinholes in a very thin silicon foil conducts and focuses the incident light in attoseconds. The research team’s remarkable observation was that the vacuum tunnels were sending out much more light energy than they would have due to the surface covered in holes. This means that the innovative meta-optics actually draws ultraviolet light to the point of focus.

The holes are several nanometers in diameter

Very small, precisely controlled structures were required for this breakthrough. Their production is approaching technically feasible limits today. The technical implementation was achieved by Federico Capasso’s team at Harvard, who are world leaders in this field, after a pilot phase of about two years.

Proof of functionality was achieved in collaboration with TU Graz, where Martin Schultze’s group at the Institute of Experimental Physics is dedicated to the creation and application of ultra-short ultraviolet flashes. “This is a great success for the cooperation between Boston and Graz. Now we want to quickly use them to study microelectronics, nanoparticles and similar things,” explains Marcus Ossiander.

Meta-optics consists of a film as thin as approximately 200 nanometers in which pinhole structures have been etched. The entire lens is made up of hundreds of millions of holes; there are about ten of these structures per micrometer on the membrane. One hole is between 20 and 80 nanometers in diameter. For comparison: a human hair is about 60 to 100 micrometers thick, a tiny virus is 15 nanometers in diameter. The hole diameter varies and tapers from the center of the membrane outwards. Depending on the size of the hole, the incident light radiation is delayed and thus collapses into a small focal point.

The laser meets the gas cloud

To measure the new type of lens, Martin Schultze and Hana Hampel of the Institute for Experimental Physics at TU Graz had unique expertise in generating the extreme ultraviolet radiation required.

“Reliably generating short, high-energy pulses of light requires precise control of light-controlled atomic processes and very precise optical adjustments. For this project, we have developed a light source that is highly efficient at generating radiation at the wavelengths designed for this meta-optics,” says Martin Schultze.

In an experimental setting at Graz, where a laser was focused onto a jet of inert gas, extreme ultraviolet radiation could be generated and concentrated in very short pulses. The effectiveness of meta-optics is proven by this light source optimized for atthodetic physics.

The next step: a microscope with meta-optics

Development of a microscope that works with this lens is now the next step. Marcus Ossiander received an ERC Initiative Grant for this advanced research work and investigation of light-absorbing nanoparticles. Following the Birgitta Schultze-Bernhardt award in 2020, this is the second of a high-end research prize taken at the Institute of Experimental Physics at TU Graz in a short period of time.

Possible applications for the new research field of attosecond microscopy are manifold. Semiconductor and solar cell technologies in particular will benefit from the possibility of being able to track the ultrafast movement of charge carriers in space and time for the first time. In modern transistor and optoelectronic circuits, the relevant processes occur in a spatial expansion of a few nanometers and in a time frame of a few attoseconds. The new meta-optics will make it possible to see these key components of information technology in action and optimize them even further.

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