Metal halide perovskites have emerged as a promising class of materials for energy applications including photovoltaics, light-emitting devices and energy-storage systems. However, their carrier mobilities are lower than those of gallium arsenide and other traditional inorganic semiconductors, a limitation that is mostly attributable to higher electron–phonon coupling.

Now, an international team of researchers has developed a unique phonon control strategy that has the potential to modify fundamental material properties, such as charge carrier mobilities, in perovskite thin films (Nat. Commun., doi: 10.1038/s41467-025-63810-7). “This work opens a pathway to engineer phonons—and thereby tune charge transport and recombination—in perovskite solar cells and light-emitting devices,” said study author Junichiro Kono, Rice University, USA. “It also establishes a foundation for phonon-based quantum technologies that harness vacuum-field effects to control material properties.”

A quantum vacuum

Phonon engineering can be achieved with the use of strong external laser fields, such as intense terahertz radiation. Recent work has also investigated the coupling of phonons to the vacuum field of a cavity resonator as a way to control phonon properties. For example, a study on the impact of phonon–photon coupling on electron–phonon interactions in perovskites did not show a change in carrier mobility.

However, the experiment was conducted in the strong coupling regime, where the ground state is a standard vacuum. Kono and his colleagues set out to explore this behavior in the ultrastrong coupling regime to determine how a quantum vacuum can mediate interactions between vibrational modes in solids.

The researchers achieved, for the first time, multimode ultrastrong coupling between two optical phonon modes and a terahertz nanoslot cavity at room temperature.

“In perovskites, phonons play a key role in charge transport and light emission,” he said. “By coupling multiple phonons to a terahertz cavity in the ultrastrong coupling regime, we aimed to find a way to control these vibrations passively—without the need for external driving fields.”

Correlated phonon emission

The researchers achieved, for the first time, multimode ultrastrong coupling between two optical phonon modes and a terahertz nanoslot cavity at room temperature. They successfully tuned the nanoslot resonator frequency, producing three distinct phonon–polariton branches. A theoretical analysis revealed that the cavity mediates effective interactions between different phonon modes, resulting in correlated phonon emission even at thermal equilibrium.

The approach represents a possible pathway for engineering phononic properties for light-harvesting and light-emitting technologies. “Moving forward, we plan to investigate how such cavity-induced phonon correlations affect carrier mobility and heat transport in optoelectronic materials,” said Kono.