First nanostructured material for broad mixing of light waves

A multicolor laser pointer you can use to change the color of the laser with a button click — similar to a multicolor ballpoint pen — is one step closer to reality thanks to a new tiny synthetic material made at Sandia.

A flashy laser pointer may be fun to envision, but changing the color of a laser has many other uses, from discovering hidden archeological sites in dense forests and detecting signs of extraterrestrial life in the air to the possibility of speeding up and increasing the capacity of long-distance communication via fiber-optic networks. 

Research on the new light-mixing metamaterial was published in Nature Communications last week. Sandia Senior Scientist Igal Brener led the work, along with collaborators at Friedrich Schiller University Jena, Germany. They describe how a metamaterial made up of an array of nanocylinders mixed two laser pulses of near infrared light to produce 11 light waves that ranged in color from the near infrared, through all the colors of the rainbow, to ultraviolet.

A metamaterial is a material made up of tiny, repeating structures that interact with electromagnetic waves in ways conventional materials cannot. The structures are much smaller than the wavelength of light they are designed to manipulate. They are somewhat similar to the natural structures that give blue morpho butterfly wings their spectacular iridescence. The wings have scales with tiny repeating structures, which reflect light to produce the blue color.

For this optical mixer, the team made the array of nanocylinders from gallium arsenide, a semiconductor used in many kinds of electronics. Gallium arsenide bends, or refracts, light strongly, which is essential for this kind of metamaterial, Igal said. Each nanocylinder is about 500 nanometers tall — or 100 times smaller than the width of a human hair — with a diameter of about 400 nanometers. They are laid out in a square pattern about 840 nanometers apart from one another.

Igal said the usual process of mixing light to make green laser pointers or other products currently requires phase matching, or perfectly aligning the light waves, with specially crafted crystals. Each crystal can only efficiently match the phases of one color of incoming light to produce one different color of light.

Sandia’s metamaterial is completely different.

The team selects two near infrared lasers with wavelengths tuned to the metamaterial’s resonant frequencies, or the wavelengths that bounce around inside the nanocylinders best, said Polina Vabishchevich, a Sandia postdoctoral appointee and first author on the paper. The light from these two lasers — call them frequencies A and B — mix to produce 11 colors from different mixing products including A+A, A+B, B+B, A+A+B, A+B+B, and other more complex mixing products.

“With this tiny device and two laser pulses we were able to generate 11 new colors at the same time, which is so cool,” Polina said. “We don’t need to change angles or match phases.”

Optical metamixer has potential for widespread research applications

The team used processes borrowed from semiconductor device fabrication to make the metamaterial. They made it at several Sandia facilities, principally the Microsystems Engineering, Sciences, and Applications complex and the Center for Integrated Nano-technologies, a DOE Office of Science user facility jointly operated with Los Alamos National Laboratory.

“If we didn’t have access to the instrumentation we have at Sandia, this research would have been impossible,” Igal said. “Without CINT’s specialized femtosecond laser system, it would have been very challenging to perform these measurements.” A femtosecond is one millionth of a billionth of a second; femtosecond lasers produce brighter light than traditional lasers.

Though the conversion efficiency for the optical metamixer is very low — for example, the resulting red-orange light is very weak compared to the incoming light — Igal believes they can greatly improve the efficiency, perhaps by stacking multiple layers of metamaterial.

Many different areas of chemical and biological research require light at specific wavelengths, such as using specialized microscopes to study how diseases evade the immune system or studying the chemistry of combustion to improve vehicle efficiency. The optical metamixer could convert light from lasers to a new wavelength where a laser might not be available or allow researchers to switch from one wavelength to another without having to buy a different laser, Igal said.

Switchable, tunable lasers also could be valuable in biological, chemical and atmospheric research; remote sensing; fiber-optic communication; even quantum optics. 

Igal is a leader for nanophotonics and optical nanomaterials research at CINT. 

The research team included collaborators from Friedrich Schiller University Jena, Germany; John Reno, a CINT materials scientist who grew the semiconductors; Sandia physicist Mike Sinclair, who was involved in the modeling and theory; and former Sandia researchers Sheng Liu and Gordon Keeler.

This work was funded by the DOE Office of Science.