Artist’s rendition of optically-defined quantum circuits in a topological insulator. Credit: Peter Allen
In contrast to using advanced nanofabrication facilities based on chemical processing of materials, this flexible technique allows for rewritable ‘optical fabrication’ of devices. The electrons in topological insulators have unique quantum properties useful for developing low-power spin-based electronics and quantum computers. However, making even the simplest experimental circuits with these materials has proved difficult because traditional semiconductor engineering techniques tend to destroy their fragile quantum properties. Even a brief exposure to air can reduce their quality.
The researchers report the discovery of an optical effect that allows them to “tune” the energy of electrons in these materials using light, and without ever having to touch the material itself. They have used it to draw and erase p-n junctions—one of the central components of a transistor—in a topological insulator for the first time.
The researchers found that the surface of strontium titanate, the substrate material on which they had grown their samples, becomes electrically polarized when exposed to UV light, and their room lights happened to emit at just the right wavelength. The electric field from the polarized strontium titanate was leaking into the topological insulator layer, changing its electronic properties. By intentionally focusing beams of light on their samples, they could draw electronic structures that persisted long after the light was removed.
Graduate students Andrew Yeats (left) and Peter Mintun at Jones Laboratory. UChicago researchers found that the lab’s room lights emitted at a wavelength that changed the electronic properties of the materials they were studying. Photo by Joel Wintermantle
“It’s like having a sort of quantum etch-a-sketch in our lab,” he said. They also found that bright red light counteracted the effect of the ultraviolet light, allowing them to both write and erase. “Instead of spending weeks in the cleanroom and potentially contaminating our materials,” said Awschalom, “now we can sketch and measure devices for our experiments in real time. When we’re done, we just erase it and make something else. We can do this in less than a second.”
To test whether the new technique might interfere with the unique properties of topological insulators, the team measured their samples in high magnetic fields. They found promising signatures of an effect called weak anti-localization, which arises from quantum interference between the different simultaneous paths electrons can take through a material when they behave as waves.
“One exciting aspect of this work is that it’s noninvasive” said Prof Nitin Samarth. “Since the electrical polarization occurs in an adjacent material, and the effect persists in the dark, the topological insulator remains relatively undisturbed.”
The researchers conducted a number of control measurements which showed that the optical effect is not unique to topological insulators, but that it can act on other materials grown on strontium titanate as well. “…it should be applicable to a wide range of nanoscale materials such as complex oxides, graphene, and transition metal dichalcogenides,” said Awschalom. “It’s not just that it’s faster and easier. This effect could allow electrical tuning of materials in a wide range of optical, magnetic and spectroscopic experiments where electrical contacts are extremely difficult or simply impossible.”
http://news.uchicago.edu/article/2015/10/09/scientists-paint-quantum-electronics-beams-light
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