Plasmon Etching Technique Produces Ultrathin Flat Optics

A simplified approach for producing flat, ultrathin optics foregoes acids and other hazardous chemical etching agents. The method could enable do-it-yourself (DIY) optics, particularly those related to low-cost microcontroller boards.

The process incorporates a nanostructured template that can be used to create many different types of optical components, said professor Kimani Toussaint of the University of Illinois at Urbana-Champaign. Current methods require cleanroom access to create new template for each new component.

“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,” Toussaint said. “We present plasmon-assisted etching (PAE) as an approach to extend the DIY theme to optics with only a modest tradeoff 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,” said Hao Chen, a former postdoctoral researcher in Toussaint’s lab.

Experimentally obtained image of a Fresnel zone plate (left) for focusing light that is fabricated with plasmon-assisted etching. A 2D array of pillar-supported bowtie nanoantennas [zoomed in image (right)] comprises this flat lens.

The PAE method used laser light to scan a template — a 2D array of gold pillar-supported bowtie nanoantennas with an area of 80 × 80 μm² — which was submerged in water, in a desired pattern in a microscope. The light-matter interaction, enhanced by the nanoantennas, produced a strong heating effect. As a result, the gold layer of the nanoantennas underwent thermal expansion that worked against its adhesion with a glass substrate. With a certain amount of optical power, the force provided by thermal expansion allowed the gold layer to break away from the substrate, etching the metal.

“Overall, the workload in cleanroom is greatly reduced,” Chen said. “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 ultrathin, 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 about 150 μm, a diffraction grating and a holographic converter that imparts angular momentum to a standard optical beam.

The PAE method and specialized template could also be used to enable preferential trapping and sorting of particles, to create optofluidic channels “without walls,” the researchers said.

The research was published in Nature Communications (doi: 10.1038/ncomms10468).