A new metacanvas for metamaterials
Metamaterials are famous for photonic tricks like bending light backwards, making “perfect” lenses that resolve details smaller than a wavelength, or acting as invisibility cloaks — not to mention a host of mundane roles to improve communications.
What makes a material “meta” is its microstructure, not (as with normal materials) its chemical composition. Typically, photolithography is used to join a metal with a dielectric (an insulator) in a two- or three-dimensional pattern. Different patterns yield different effects. The catch is that, once made, they can't be altered without destroying their purpose.
Junqiao Wu and Jie Yao, professors of materials science and members of Berkeley Lab's Materials Sciences Division, asked themselves how it might be possible to create metamaterial devices that could be erased and written over again — like an Etch A Sketch drawing toy — to quickly yield a quite different device.
Wu, who describes himself as “the materials guy” of the team, proposed that an Etch A Sketch-like screen could be made from a film of vanadium dioxide just 200 nanometers thick, on which two-dimensional patterns could be drawn, not by twisting knobs but by programming a low-powered laser.
Yao, the “photonics person” of the team, named the rewritable surface a “metacanvas” and proposed a number of light-manipulating devices that could be made to prove the principle. Wu and Yao proceeded to demonstrate the metacanvas with the aid of their colleagues, and are lead authors of a paper describing the work in the journal Advanced Materials.
A malleable material
Vanadium dioxide's odd properties are what make this first sketchable, erasable and resketchable metacanvas possible. Changing its temperature can radically change its color, structure, and other features, including phase changes — switching from an insulator to a metal and back. “I've probably published 50 papers about vanadium dioxide in the last 10 years,” says Wu, “and the discoveries just keep coming.”
Phase changes allow the existence of a metamaterial made from a single component. Most such materials change phase at very high temperatures, but vanadium dioxide changes from an insulator to a metal at 67 degrees Celsius (152 degrees Fahrenheit) — not exactly room temperature, but cooler than boiling water.
To create patterns, a fixed laser beam turns on and off as the metacanvas moves underneath it. Near the critical temperature, the beam heats the thin film enough to change it from insulator to metal where it touches.
By keeping the metacanvas near the phase-transition temperature, the insulating surroundings remain unchanged while the laser-drawn metallic pattern persists (a property known as hysteresis). If the overall temperature is lowered, the pattern fades and vanishes.
“We came up with three demonstrations of dynamic manipulation of light waves” to demonstrate the potential of the metacanvas, Wu says. First was a beam steerer, a grid pattern that deflected an infrared beam in a specific direction.
Next, circular and linear polarization patterns were written on the metacanvas. By changing the temperature at a controlled rate, the opacity of the imprinted metallic patterns was manipulated, “dissolving” one into the other to make a smooth transition among different fields and focuses.
The third demonstration was the simulation of a two-cube hologram, created with a design program. “In practice, the hologram may deviate from what we expected it to be, like the one here; it was distorted,” says Yao. (The distortion was deliberate.) “We analyzed the distortion, corrected it and wrote a modified pattern that solved the problem.”
Wu says that fixing a design problem like this with existing metamaterials “could take a week. With the metacanvas, it took an hour.” Easily programmed, a metacanvas creates numerous structures rapidly and repeatedly, almost 100 so far on a single vanadium dioxide thin film. Thousands of these nanoscale patterns could be stacked to yield intricate systems.
Using the metacanvas, Yao says, “Experimental optical elements can be arranged as if they were physical logic blocks to create a complex, programmable system.” The results could be analogous to the electronics world's field-programmable gate arrays (FPGAs), integrated circuits designed to be readily modified to the user’s needs.
Besides vanadium oxide, other metacanvas materials may be within practical reach. The metacanvas promises an economical, fast and convenient technology — versatile and possibly even disruptive.