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| Dasom Kim Photo Credit: Jorge Vidal/Rice University |
Atoms in crystalline solids sometimes vibrate in unison, giving rise to emergent phenomena known as phonons. Because these collective vibrations set the pace for how heat and energy move through materials, they play a central role in devices that capture or emit light, like solar cells and LEDs.
A team of researchers from Rice University and collaborators have found a way to make two different phonons in thin films of lead halide perovskite interact with light so strongly that they merge into entirely new hybrid states of matter. The finding, reported in a study published in Nature Communications, could provide a powerful new lever for controlling how perovskite materials harvest and transport energy.
To get a specific light frequency in the terahertz range to interact with phonons in the halide perovskite crystals, the researchers fabricated nanoscale slots ⎯ each about a thousand times thinner than a sheet of cling wrap ⎯ into a thin layer of gold. The slots acted like tiny metallic traps for light, tuning its frequency to that of the phonons and thus giving rise to a strong form of interaction known as “ultrastrong coupling.”
In order to tune the effect, the researchers made nanoslots of seven different sizes: Longer slots trapped lower-frequency light, while shorter ones trapped higher frequencies. The goal was to precisely match the confined light frequency to the vibration frequencies of the perovskite material.
“We fabricated arrays of nanoscale slots with seven slightly different lengths to tune a single terahertz resonance and deposited perovskite thin films on top,” Kim said. “Designing the slot geometry allowed us to shape the interaction between light and the perovskite phonons without using high-power laser pulses or bulky crystals.”
The payoff was the appearance of three distinct hybrid quantum states known as phonon-polaritons, each a new blend of vibration and light.
“The coupling ratio reached roughly 30% of the phonon frequency at room temperature,” Kim said.
The ability to stage such strong, exotic interactions between multiple quantum modes without resorting to external driving factors opens the door to new ways of steering energy flow in optoelectronics.
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| Dasom Kim Photo Credit: Jorge Vidal/Rice University |
The experimental results were validated by numerical simulations and a theoretical quantum model, which allowed the researchers to calculate the actual coupling strengths and confirm that the two phonon modes were indeed operating in the ultrastrong coupling regime.
“Advances in nanofabrication and perovskite film quality made it possible to reach this regime reliably,” Kim said.
“This offers a gentle, device-compatible way to influence processes that matter for light harvesting and light emission, potentially improving performance and reducing energy losses,” said Junichiro Kono, the Karl F. Hasselmann Professor in Engineering, professor of electrical and computer engineering and materials science and nanoengineering and the corresponding author on the study.
“What makes this result stand out is that we were able to uncover entirely new phonon behavior without extreme conditions just by carefully designing the nanoscale environment,” added Kono, who also serves as director of the Smalley-Curl Institute at Rice.
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Published in journal: Nature Communications
Title: Multimode phonon-polaritons in lead-halide perovskites in the ultrastrong coupling regime
Authors: Dasom Kim, Jin Hou, Geon Lee, Ayush Agrawal, Sunghwan Kim, Hao Zhang, Di Bao, Andrey Baydin, Wenjing Wu, Fuyang Tay, Shengxi Huang, Elbert E. M. Chia, Dai-Sik Kim, Minah Seo, Aditya D. Mohite, David Hagenmüller, and Junichiro Kono
Source/Credit: Rice University | Silvia Cernea Clark
Reference Number: phy093025_01
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