Tuning Light and Vibration: New Control Over Matter’s Interactions

Author: Denis Avetisyan

Researchers have achieved precise control over the strong interaction between light and atomic vibrations in perovskite materials, opening new avenues for manipulating material properties.

The hybrid structure of MAPbI$_3$ nanoslots exhibits ultrastrong coupling between cavity modes and phonon modes TO1 and TO2, giving rise to three distinct polaritonic branches-lower (LP), middle (MP), and upper (UP)-while a further branch emerges near 0.83 THz below a critical temperature, indicative of coupling with a newly activated third phonon mode.

Symmetry-controlled ultrastrong coupling of terahertz photons and optical phonons in perovskites demonstrates a stable phase transition and enhanced light-matter hybridization.

Engineering strong light-matter interactions requires precise control over the coupling between photons and material excitations, yet achieving tunable, ultrastrong coupling remains a significant challenge. This work, ‘Symmetry-Controlled Ultrastrong Phonon-Photon Coupling in a Terahertz Cavity’, demonstrates reversible modulation of this coupling by leveraging a structural phase transition in lead halide perovskites embedded within nanoslot cavities. Specifically, the researchers observe the emergence of new polariton branches correlated with symmetry changes, confirming persistent ultrastrong coupling across the transition. Could this symmetry-based approach unlock new avenues for dynamically controlling and exploiting light-matter hybridization in advanced photonic devices?

Unlocking the Quantum Realm: Engineering Light-Matter Interaction

The pursuit of strong and ultrastrong coupling (USC) between light and matter represents a pivotal advancement in quantum technologies. This phenomenon, where the rate of interaction between light and a material system surpasses the energy of the material excitations, allows for the creation of hybrid light-matter states known as polaritons. These polaritons exhibit unique properties, differing significantly from either light or matter alone, and open pathways to realize novel devices with enhanced functionalities. Specifically, ultrastrong coupling-a regime exceeding even strong coupling-promises access to non-perturbative quantum effects and allows exploration of previously inaccessible physical regimes, potentially revolutionizing fields like quantum computation, low-power electronics, and highly sensitive sensors. The ability to engineer and control this interaction is therefore paramount to unlocking the full potential of these emerging technologies, pushing the boundaries of what is possible with light and matter.

The pursuit of robust polariton-based devices – which leverage the hybrid light-matter quasiparticles known as polaritons – is frequently constrained by limitations in current material science and optical cavity engineering. Traditional approaches often struggle to achieve the deep coupling necessary for the ultrastrong coupling (USC) regime, where the interaction energy between light and matter surpasses all other energy scales in the system. This shortfall arises from challenges in maximizing light-matter interaction strength, typically hindered by weak oscillator strengths in materials or limited light confinement within cavities. Without reaching USC, the resulting polaritons exhibit only modest quantum properties, impeding the realization of advanced functionalities like Bose-Einstein condensation of polaritons or the creation of highly efficient nonlinear optical devices. Consequently, innovative materials and cavity designs – including those employing metamaterials, 2D materials, and tailored photonic structures – are vital to overcome these barriers and unlock the full potential of strong light-matter interactions.

Temperature-dependent THz spectroscopy reveals a consistent tetragonal-to-orthorhombic phase transition near 162.5 K in hybrid systems with varying cavity frequencies, despite differences in light-matter hybridization.

The Perovskite Code: Structural Phase Transitions as Control Knobs

Lead halide perovskites, specifically methylammonium lead iodide ($MAPbI_3$), exhibit a notable sensitivity to alterations in their crystalline structure. This characteristic arises from the relatively weak interactions between the organic cation and the inorganic lattice, allowing for facile structural distortions in response to external stimuli such as temperature or pressure. Unlike many other semiconductor materials with rigidly defined lattices, $MAPbI_3$ can undergo phase transitions that directly modify its electronic and optical properties. These structural changes manifest as shifts in the arrangement of atoms within the perovskite framework, affecting parameters like lattice constants and symmetry, and ultimately influencing the material’s behavior in optoelectronic devices.

The light-matter coupling strength in MAPbI$_3$ can be actively tuned by exploiting the structural phase transition between its tetragonal and orthorhombic phases. This transition, occurring at 162.5 K, results in a change in the crystal lattice parameters and symmetry, directly modulating the electronic band structure and dielectric properties of the material. Alterations to these properties influence the interaction between incident photons and the perovskite’s excitons, thereby controlling the strength of the resulting light-matter coupling and the characteristics of any formed polaritons. This provides a pathway for dynamically controlling optoelectronic properties through external stimuli such as temperature adjustments.

The structural phase transition in lead halide perovskites, specifically MAPbI3, induces a measurable change in the material’s transverse optical (TO) phonon modes, designated TO1 and TO2. These alterations directly affect the strong light-matter coupling regime, influencing both the formation and properties of polaritons. Experimental observation confirms the stability of this phase transition at a consistent temperature of 162.5 K; deviations from this temperature result in a return to the original structural configuration. The modified phonon modes impact the exciton-phonon interactions, thereby tuning the polariton dispersion and lifetime.

Terahertz spectroscopy reveals a phase transition in MAPbI3, evidenced by a shift from two phonon modes at 165 K to three below 162.5 K, and demonstrates resonance within a 60μm nanoslot cavity at 1.52 THz.

Probing the Hybrid Realm: Experimental Evidence from THz Spectroscopy

Time-domain terahertz spectroscopy (THz-TDS) was utilized to characterize the dispersion of polaritons within the material system. This technique provides a means to map the energy and momentum of these quasiparticles. To enhance light-matter interaction and improve signal detection, the THz-TDS measurements were performed using nanoslot cavities. These cavities are specifically engineered to confine the terahertz radiation, increasing the probability of interaction with the sample and enabling the observation of subtle features in the polariton spectrum. The resulting data allows for direct observation of the relationship between energy and momentum of the polaritons, and informs analysis of coupling strengths.

Nanoslot cavities are utilized to significantly enhance light-matter interactions, a critical requirement for observing polariton signatures within the terahertz (THz) spectrum. These cavities confine the electromagnetic field, increasing the probability of interaction between incident THz radiation and the active material. This enhancement is achieved through the cavity’s geometry, which supports strong field localization and resonance. Consequently, the normally weak coupling effects that define polariton formation become readily observable in the THz-TDS spectra, allowing for precise characterization of their energy and linewidth, and enabling the study of their dependence on material properties.

Correlation of MAPbI₃ structural phase with polariton properties was achieved through experimental analysis. Observed coupling strengths varied depending on temperature relative to the critical temperature (T_c) and the specific phonon mode being examined. Above T_c, coupling strengths of 0.36, 0.35, and 0.25 were recorded for different phonon modes, while below T_c, values of 0.28, 0.36, and 0.25 were observed for the same phonon modes, indicating a direct relationship between material phase and the resulting polariton dispersion characteristics.

Decoding the Interaction: A Theoretical Framework with the Hopfield Model

The multimode Hopfield model is employed to describe light-matter interactions in systems exhibiting strong coupling between cavity photon modes and material excitations, specifically phonons. This model treats both the photonic and phononic degrees of freedom as harmonic oscillators, allowing for the calculation of hybrid light-matter states known as polaritons. By considering multiple cavity modes and their interaction with multiple phonon modes – including transverse optical (TO) modes – the model provides a means to analyze the resulting polariton dispersion and understand the energy exchange between light and matter. The framework facilitates the investigation of how variations in system parameters, such as cavity resonance or structural phase, impact the properties of these polariton states, offering a predictive capability for experimental observations.

The multimode Hopfield model accounts for polariton dispersion by explicitly incorporating the relative contributions of photonic and phononic modes. The model represents the system’s Hamiltonian with terms proportional to the energy of each mode – the cavity photon and the optical phonon – as well as interaction terms representing the coupling between them. The photonic contribution to the polariton dispersion is determined by the cavity resonance frequency, while the phononic component is defined by the phonon frequency and associated momentum. By weighting these contributions according to the mode’s spatial overlap and the strength of the light-matter interaction, the model accurately predicts the energy and momentum of the resulting polariton states. The relative weights are determined by the system’s parameters, allowing for a quantitative analysis of the interplay between photonic and phononic character in the polariton dispersion.

The multimode Hopfield model accurately reproduces experimental observations regarding the impact of structural phase transitions on polariton properties. Specifically, simulations demonstrate a 30% reduction in the coupling strength of the TO1 phonon mode when the material undergoes a cooling-induced transition from the tetragonal to the orthorhombic phase. This quantitative agreement between modeled and experimental data validates the model’s capacity to describe the interplay between cavity photon modes, phonon modes, and the resulting polariton states under varying structural conditions. The model achieves this by allowing the cavity resonance frequency to be adjusted, effectively simulating the changes in the material’s dielectric environment as it transitions phases.

Beyond the Experiment: Towards a Future of Tunable Polaritonic Devices

Recent investigations highlight lead halide perovskites as a remarkably adaptable material for the creation of tunable polaritonic devices. These materials, already known for their excellent optoelectronic properties, exhibit a unique ability to strongly couple with light, forming polaritons – quasi-particles that combine light and matter. Crucially, the properties of these polaritons can be dynamically adjusted by manipulating the perovskite’s structural phase, offering a pathway to ‘tune’ the devices for specific functionalities. This versatility stems from the material’s inherent structural flexibility and responsiveness to external stimuli, presenting a significant advantage over traditional materials used in polaritonics. The potential impact extends to a wide range of applications, including the development of advanced optical sensors, high-speed modulators, and even the building blocks for future quantum technologies, making lead halide perovskites a promising platform for realizing the full potential of polariton-based devices.

The ability to dynamically manipulate the structural phase transition within lead halide perovskites presents a pathway to precisely tailor their polariton properties. This control is achieved by externally influencing the material’s crystal structure, effectively tuning the strong light-matter coupling that defines polariton formation. Such dynamic control isn’t merely a theoretical possibility; it enables the creation of polaritonic devices optimized for specific quantum optics applications, like single-photon sources and entanglement generation. Furthermore, the tunability extends to information processing paradigms, offering potential routes towards all-optical switching and the development of novel quantum bits – or qubits – where information is encoded in the properties of these hybrid light-matter quasiparticles. The resulting devices promise enhanced functionality and adaptability compared to static polaritonic systems, paving the way for advanced photonic technologies.

The unique properties of polaritons-quasiparticles arising from the strong coupling of light and matter-pave the way for innovative device concepts. Researchers envision sensors leveraging the extreme sensitivity of polariton dispersions to environmental changes, enabling the detection of minute quantities of substances or subtle shifts in temperature. Furthermore, the ability to rapidly modulate polariton properties suggests the development of high-speed optical modulators for data transmission. Perhaps most ambitiously, the coherent nature and quantized energy levels of polaritons offer a potential platform for realizing robust quantum bits, or qubits, for quantum information processing. These qubits, potentially offering advantages in coherence and scalability, could contribute to the advancement of quantum computing and communication technologies, representing a significant leap beyond conventional electronics.

The research meticulously details a controlled manipulation of light-matter interactions, specifically ultrastrong coupling between terahertz photons and optical phonons. This pursuit of control resonates with a fundamental principle: to truly understand a system, one must push its boundaries. As John Bell observed, “No phenomenon is a mystery, given sufficient knowledge.” The study’s use of perovskite materials within nanoslot cavities exemplifies this reverse-engineering approach. By inducing a structural phase transition, the researchers don’t merely observe the interaction; they actively modulate it, probing the limits of light-matter hybridization and revealing, crucially, that transition temperatures remain largely unaffected by the coupling strength. This isn’t passive observation, but a deliberate interrogation of the system’s rules.

Beyond the Resonance

The demonstration of modulated ultrastrong coupling, while elegant, predictably opens more questions than it closes. The observed invariance of the perovskite’s phase transition temperature despite significant light-matter hybridization begs further scrutiny. Is this a fundamental limitation of the system, or merely a consequence of the specific material and cavity design? Perhaps the transition itself is being subtly altered, manifested in properties beyond simple temperature measurements – a distortion of the order parameter, for instance, detectable through more sensitive probes. The current work successfully exploits a structural transition; however, the true potential may lie in inducing such transitions via tailored photonic interactions-effectively using light as a sculpting tool for material properties.

The reliance on naturally occurring phase transitions feels…constrained. A more ambitious direction would involve engineering artificial transitions-designer materials with precisely tuned responses to terahertz stimuli. This moves beyond passively observing a system and towards actively controlling it, blurring the lines between material science and optical circuit design. The current nano-slot cavities, while functional, represent a local solution. Scaling these interactions-creating extended, interconnected networks of strongly coupled polaritons-remains a significant hurdle, demanding innovations in nanofabrication and cavity design.

Ultimately, the pursuit of ultrastrong coupling isn’t about achieving ever-larger coupling strengths; it’s about fundamentally altering the rules of light-matter interaction. It’s about probing the limits of the very concept of a ‘photon’ or a ‘phonon’ when they become inextricably linked. The present work provides a foothold, a controlled perturbation. The interesting part, naturally, is what happens when the system inevitably breaks.

Original article: https://arxiv.org/pdf/2511.16285.pdf

Contact the author: https://www.linkedin.com/in/avetisyan/