Physicists developed a microscopic, nonlinear light source that can be switched on, off or tuned to a particular intensity by an electrical "knob."
The journal Optica published the work led by Emory University, which may aid in the design of smaller, more flexible technologies for communications, sensing and quantum computing.
The new method focuses on a type of nonlinear optics known as second harmonic generation, or SHG, where two photons of the same frequency interact with a material and combine into a single photon with twice the frequency.
"Nobody had previously shown that you can tune second harmonic generation with an electric knob in such a small device," says Hayk Harutyunyan, senior author of the paper and Emory professor of physics.
The entire integrated component is a little more than 200 nanometers wide, or more than 100 times smaller than the width of a human hair. The active area, where light is generated, is just two-to-six nanometers wide — tens of times smaller than most preexisting devices for second harmonic generation and far more controllable.
"We can switch on our device, completely shut it off, and raise or lower its intensity within a range of 500 percent," Harutyunyan says.
Yuankai Tang, a PhD student in Emory's Laney Graduate School, is first author of the paper.
Co-authors include Amit Agrawal from the University of Cambridge in Cambridge, England; Ariando Ariando and Saurav Prakash from the National University of Singapore; and Monica Allen and Jeffrey Allen from the Air Force Research Laboratory at Eglin Air Force Base in Florida.
Blazing new frontiers
Second harmonic generation is already widely used in applications such as doubling laser frequencies and in high-resolution optical microscopy for biological, medical and materials science.
"It's a very important process," Harutyunyan explains, "because it gives you the means to tune optical interactions and generate new light."
A key focus of Harutyunyan's research is to enhance, control and characterize optical properties and energy flow at the nanoscale to work on grand challenges in photonic technology.
A major revolution in telecommunications occurred in the 1950s, driven by the development of silicon semiconductors as miniature transistors to control electrical current. These transistors led to smaller, faster computers and paved the way for everything from flatscreen TVs to cell phones.
Transistors continued to get tinier and more efficient, but around 15 years ago, this progress stopped as transistors could not become any smaller and still function well.
Meanwhile, light is blazing new technological frontiers. The gradual replacement of copper wiring with fiber optics is speeding up transmission between computers and other electronic devices.
Size matters
A few examples of semiconductor chips that process light rather than electricity also exist. Further improvements are needed, however, to make this technology broadly practical and boost processing speeds while also lowering energy use.
"If we're going to improve photonic chips, we're going to need to develop better nanoscale light sources that we can control," Harutyunyan explains.
One approach to ongoing challenges of modulating light at the nanoscale are components known as plasmonic electric-field-induced second harmonic (plasmonic-EFISH) devices. They provide a bridge between electronics and high-speed photonics, allowing for smaller, faster and more efficient optical switching than traditional devices.
Most plasmonic-EFISH devices, however, are not highly tunable and are relatively large.
"Small is critical," Harutyunyan says. "That's the name of the game in both electronics and photonics. You need to squeeze as many features as you can into a small space."
Harutyunyan and Tang set out to see if they could develop a better, smaller plasmonic-EFISH device. They took a new approach by focusing on a component known as the tunneling junction — a semi-permeable barrier that acts as an insulating layer in an optoelectronic component.
A lengthy quest
Tang loves working in the realm of the invisible, using specialized tools to fabricate and test devices at the atomic scale.
"I want to help create new nano-devices that benefit the world," he says.
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.
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."
A key collaboration
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.
