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Novel technique enables table-top production of flat optics

28 January 2016

Researchers have developed a simplified approach to fabricating flat, ultra-thin optics. The new approach enables etching without the use of acids or hazardous chemicals.

Experimentally obtained image of a Fresnel zone plate (left) for focusing light that is fabricated with plasmon-assisted etching. A two-dimensional array of pillar-supported bowtie nanoantennas (inset right) comprises this flat lens

“Our method brings us closer to making do-it-yourself optics a reality by greatly simplifying the design iteration steps,” saya Kimani Toussaint, an associate professor of mechanical science and engineering the University of Illinois at Urbana-Champaign who led the research (published in Nature Communications). “The process incorporates a nanostructured template that can be used to create many different types of optical components without the need to go into a cleanroom to make a new template each time a new optical component is needed.

“In recent years, the push to foster increased technological innovation and basic scientific and engineering interest from the broadest sectors of society has helped to accelerate the development of do-it-yourself (DIY) components, particularly those related to low-cost microcontroller boards,” Toussaint adds. “Simplifying and reducing the steps between a basic design and fabrication is the primary attraction of DIY kits, but typically at the expense of quality. We present plasmon-assisted etching as an approach to extend the DIY theme to optics with only a modest trade-off in quality, specifically, the table-top fabrication of planar optical components.”

“Our method uses the intuitive design aspects of diffractive optics by way of simple surface modification, and the electric-field enhancement properties of metal nanoantennas, which are typically the building blocks of metasurfaces towards do-it-yourself planar optical components using plasmon-assisted etching,” says Hao Chen, a former postdoctoral researcher in Toussaint’s lab and first author of the Nature Communications paper.

According to Chen, laser light scans the template—a 2D array of gold pillar-supported bowtie nanoantennas (with an area of 80 x 80 square micrometres), which is submerged in water, in a desired pattern in a microscope. The light-matter interaction, enhanced by the nanoantennas, produces a strong heating effect. As a result, the gold layer of the nanoantennas undergoes thermal expansion that works against its adhesion with their glass substrate. With a certain amount of optical power, the force provided by thermal expansion allows the gold layer to break away from the substrate, etching the metal.

“Overall, the workload in cleanroom is greatly reduced,” Chen adds. “Once the template is ready, it is like a paper sheet. You can ‘draw’ all the optical elements you need on a ‘canvas’ using a conventional laser-scanning optical microscope.”

The study demonstrated fabrication of various ultra-thin (characteristic dimension less than the optical wavelength), flat optical components using the same template. The specific optical components fabricated by the researchers included a flat focusing lens (also known as a Fresnel zone plate) with focal length of around 150 micrometres, a diffraction grating, and a holographic converter that imparts angular momentum to a standard optical beam.

According to the researchers, the plasma assisted etching method and specialised template could also be used to enable preferential trapping and sorting of particles, to create so-called optofluidic channels 'without walls'.


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