Scientists Tune Light-Matter Coupling with Crystal Symmetry

A new study demonstrates how the symmetry of a crystal lattice can be used as a switch to control the strength of interactions between light and matter in a quantum regime. This breakthrough, achieved with lead halide perovskites in terahertz cavities, allows researchers to reversibly tune ultrastrong coupling—a state where light and matter hybridize so intensely that traditional models break down. By leveraging a temperature-driven structural phase transition, the team has provided a clean way to vary coupling strength in situ, addressing a long-standing in polaritonics where such control is typically fixed by cavity design. This work not only advances fundamental understanding of light-matter hybrids but also establishes perovskites as a versatile platform for engineering quantum materials with tailored optical properties.

The key finding is that the coupling strength between terahertz photons and optical phonons in MAPbI3 perovskites can be modulated by changing the crystal symmetry through temperature. Above a critical temperature of about 162.5 Kelvin, the material is in a tetragonal phase with two infrared-active phonon modes, while below this temperature, it transitions to an orthorhombic phase, activating an additional phonon mode. In experiments, this symmetry change led to the emergence of a new polariton branch—a hybrid light-matter state—below the transition, with the system remaining in the ultrastrong coupling regime throughout. Specifically, normalized coupling strengths ranged from 0.25 to 0.36 relative to phonon frequencies, all exceeding the threshold for ultrastrong coupling. The researchers confirmed that the transition temperature itself was unchanged within experimental uncertainty, indicating that light-matter hybridization did not measurably affect phase stability under these conditions.

Ology combined terahertz time-domain spectroscopy with nanoslot cavities to achieve and probe ultrastrong coupling. The team fabricated arrays of nanoslots on quartz substrates, with lengths varying from 30 to 160 micrometers to tune cavity resonance frequencies, as shown in Figure 2(b). They then deposited 200-nanometer-thick MAPbI3 films onto these cavities, creating hybrid structures where the confined electric fields in the slots enhanced light-matter interactions. Temperature-dependent measurements across the phase transition allowed them to track changes in transmission spectra, revealing polariton branches. To interpret the data, they employed a multimode Hopfield model, a theoretical framework that accounts for the diamagnetic contribution and anti-resonant terms essential in the ultrastrong coupling regime. This model was numerically diagonalized to fit experimental dispersions and extract coupling parameters, using phonon frequencies determined from bare film measurements.

Analysis, based on Figures 1 through 5, shows clear evidence of symmetry-controlled tuning. In the tetragonal phase above 162.5 K, three polariton branches were observed from the hybridization of two phonon modes with the cavity mode, as depicted in Figure 2(c). Below the transition, an additional branch appeared near 0.83 terahertz, corresponding to the new phonon mode in the orthorhombic phase. The multimode Hopfield model accurately reproduced these dispersions, with coupling strengths for the TO1 mode decreasing by approximately 30% upon cooling, while the TO2 mode remained stable. Figure 3 and Figure 4 illustrate the phononic and photonic fractions of the polariton branches, highlighting how the emergent mode redistributes weight. Temperature-dependent maps in Figure 5 further confirmed the transition, with polariton features evolving consistently across different cavity frequencies without shifting the critical temperature beyond a 2 Kelvin resolution.

Of this research extend to the development of quantum optoelectronic materials where precise control over light-matter interactions is crucial. By using a structural phase transition as a built-in tuning parameter, the study offers a pathway to engineer material properties—such as vibrational, electronic, or magnetic responses—through ultrastrong coupling without introducing ambiguities from external modifications. This could lead to applications in advanced sensors, quantum information processing, or energy-harvesting devices that leverage hybrid states. Moreover, the ability to reversibly modulate coupling strength in perovskites, materials already prominent in photovoltaics, suggests potential for integrating quantum optical functionalities into existing technologies. The work sets a foundation for exploring cavity-modified critical behavior, especially if coupled to soft phonon modes near phase boundaries.

Limitations of the study, as noted in the paper, include the experimental resolution of 2 Kelvin, which may obscure subtle cavity-induced effects on the phase transition. The researchers acknowledge that more pronounced influences, such as modified lattice fluctuations, could emerge under stronger vacuum-field confinement or different coupling geometries. Additionally, the fourth phonon mode expected in the orthorhombic phase was not resolved due to its weak oscillator strength, leaving aspects of the full phonon spectrum incompletely characterized. Future investigations might address these gaps by employing cavities with even smaller mode volumes or exploring other perovskite compositions to enhance control and observe potential quantum vacuum effects. Despite these constraints, provide robust insight into multimode polariton formation and establish a clear experimental framework for symmetry-driven tuning in light-matter hybrids.