Sep 20 2019
An incandescent lightbulb created in an engineering laboratory at Rice University may be viewed as the smallest in the world with the potential of advances in photonics, sensing, and maybe computing platforms beyond the boundaries of silicon.
A new study by Rice University electrical and computer engineer Gururaj Naik and graduate student Chloe Doiron demonstrates nanoscale selective thermal emitters, incandescent light sources with two or more elements. (Courtesy: The Naik Lab/Rice University)
Gururaj Naik from Rice’s Brown School of Engineering and graduate student Chloe Doiron have built unconventional “selective thermal emitters”—groups of near-nanoscale materials that absorb heat and discharge light.
The study, which was reported in Advanced Materials, stays ahead of a new method formulated by the lab that uses carbon nanotubes to direct heat from mid-infrared radiation to enhance the efficiency of solar energy systems.
The new approach brings together various known phenomena into a distinctive configuration that also converts heat into light—but in this case, the system is strongly configurable.
According to Naik, essentially, the incandescent light source was made by breaking down a single-element system—a bulb’s glowing filament—into two or more subunits. Blending and matching the subunits could offer the system a range of capabilities.
The previous paper was all about making solar cells more efficient. This time, the breakthrough is more in the science than the application. Basically, our goal was to build a nanoscale thermal light source with specific properties, like emitting at a certain wavelength, or emitting extremely bright or new thermal light states.
Gururaj Naik, Assistant Professor of Electrical and Computer Engineering, Brown School of Engineering, Rice University
“Previously, people thought of a light source as just one element and tried to get the best out of it,” added Naik. “But we break the source into many tiny elements. We put sub-elements together in such a fashion that they interact with each other. One element may give brightness; the next element could be tuned to provide wavelength specificity. We share the burden among many small parts.”
“The idea is to rely upon collective behavior, not just a single element,” Naik continued. “Breaking the filament into many pieces gives us more degrees of freedom to design the functionality.”
The system is dependent on non-Hermitian physics, a quantum mechanical means to define “open” systems that discharge energy—here, heat—instead of retaining it.
As part of their experiments, Naik and Doiron integrated two types of near-nanoscale passive oscillators that are electromagnetically coupled up on being heated to approximately 700 °C. The thermal light emitted by the metallic oscillator causes the coupled silicon disk to store the light and emit it in a preferred manner, stated Naik.
According to Doiron, the output of the light-emitting resonator can be regulated by damping the lossy resonator or by manipulating the level of coupling through a third element between the resonators.
Brightness and the selectivity trade off. Semiconductors give you a high selectivity but low brightness, while metals give you very bright emission but low selectivity. Just by coupling these elements, we can get the best of both worlds.
Chloe Doiron, Graduate Student, Brown School of Engineering, Rice University
“The potential scientific impact is that we can do this not just with two elements, but many more,” stated Naik. “The physics would not change.”
He observed that though commercial incandescent bulbs have been shoved aside in favor of LEDs for their energy efficiency, incandescent lamps are still the only concrete means to create infrared light.
“Infrared detection and sensing both rely on these sources,” Naik noted. “What we’ve created is a new way to build light sources that are bright, directional and emit light in specific states and wavelengths, including infrared.”
The prospects for sensing lie at the system’s “exceptional point,” he said.
There’s an optical phase transition because of how we’ve coupled these two resonators. Where this happens is called the exceptional point, because it’s exceptionally sensitive to any perturbation around it. That makes these devices suitable for sensors. There are sensors with microscale optics, but nothing has been shown in devices that employ nanophotonics.
Gururaj Naik, Assistant Professor of Electrical and Computer Engineering, Brown School of Engineering, Rice University
The prospects may also be significant for next-level classical computing. “The International Roadmap for Semiconductor Technology (ITRS) understands that semiconductor technology is reaching saturation and they’re thinking about what next-generation switches will replace silicon transistors,” stated Naik.
Naik added, “ITRS has predicted that will be an optical switch, and that it will use the concept of parity-time symmetry, as we do here, because the switch has to be unidirectional. It sends light in the direction we want, and none comes back, like a diode for light instead of electricity.”
The study was backed by the National Science Foundation.