Tang loves working in the realm of the invisible, using specialized tools to fabricate and test devices at the atomic scale.
A key challenge of the current project was to develop a tunneling junction from material sturdy enough to remain stable when voltage is applied but thin enough to allow quantum particles of electrons to pass through it.
Tang first developed algorithms to model simulations aimed at this goal, then drew on the results to fabricate the components.
A technique known as sputtering, which can be thought of like spray painting with high-energy ions, allowed him to coat a glass side with a thin layer of indium tin oxide, an electrically conductive material. He used the same technique to add an ultrathin layer of silicon dioxide, only a handful of atoms thick, to serve as the tunnel junction.
“I want to help create new nano-devices that benefit the world,” Tang says. (Photo by Carol Clark)
“I want to help create new nano-devices that benefit the world,” Tang says. (Photo by Carol Clark)
Tang created a mold for gold electrodes by using electron-beam lithography, a type of computer assisted design for the atomic scale. He then heated gold in a high-vacuum chamber until it vaporized and condensed, forming a thin, uniform coating within the polymer mold.
He applied a thin layer of chromium as an adhesive layer to affix these gold electrodes onto the nanolayer of silicon dioxide.
With the painstaking process of fabricating and integrating the components complete, Tang applied voltage to run an experiment and … Zap!
The silicon dioxide nanolayer essentially shorted out, frying the component.
“It wasn’t stable enough to hold the charge for more than a few minutes,” Tang explains.
He repeated the lengthy modeling, fabrication and integration process, this time using aluminum oxide for the tunnel junction.
It shorted out again.
“This was the most challenging time during this project,” Tang says. “We tried so many ideas and materials for more than a year and couldn’t make it work.”
Schematic of the integrated component shows the gold electrodes and the base topped with an ultra-thin layer of lutetium oxide. (Harutyunyan lab)
Schematic of the integrated component shows the gold electrodes and the base topped with an ultra-thin layer of lutetium oxide. (Harutyunyan lab)
The breakthrough came when the Harutyunyan lab connected with specialists in developing ultra-thin quantum materials — members of the Ariando group at the National University of Singapore.
“While we are experts in working with nonlinear light, they are the experts when it comes to fabricating thin oxide films,” Harutyunyan says. “For a project this complex, we realized that we needed to bring together unique areas of expertise.”
The Singapore team chose lutetium oxide, known for its high melting point and stability even in extreme conditions, as the material for the tunnel junction. They used pulsed laser deposition to coat zirconia with indium tin oxide, then topped this base with an ultra-thin, yet highly stable, layer of lutetium oxide.
Tang crafted the gold electrodes to integrate precisely with the components fabricated by the Singapore researchers.
This time, when Tang applied the voltage the tunnel junction held up. The device worked as predicted.
“It was an amazing feeling,” Tang says.
“To our knowledge, this is the first demonstration of electrically tunable, second-harmonic generation via a tunnel junction,” Harutyunyan says.
Experiments showed the large modulation range for the device, making it a versatile platform for both fundamental studies of light-matter interaction and a novel concept for integrated circuitry to improve photonic chips.
“If you’re replacing electric currents with photon flows, you need to be able to create photons on demand and control the rate of flow,” Harutyunyan explains.
Over the longer term, the new method to control optical processes at the nanoscale provides another tool in the quest to improve quantum computing processes. Using light particles to encode and process “qubits,” as the fundamental units of quantum information are known, offers the potential for high-speed transmission at room-temperature operations as researchers strive to create scalable, networked quantum computing systems.
Story by Carol Clark