Polariton plurality

Materials discovery and advances in synthesis are driving the fields of exciton and exciton–polariton physics, moving towards on-demand engineering of many-body quasiparticle interactions in solid-state systems.

Condensed-matter physicists have long been fascinated by the generation and manipulation of quasiparticles, originating from the interactions of quantum particles with their surroundings, in solid-state platforms. Among various quasiparticle platforms, exciton–polaritons — quasiparticles arising from the strong coupling of cavity-confined photons and bound electron–hole pairs or excitons — are becoming a prime platform for the study of quantum many-body physics and quantum phases of matter, such as Bose–Einstein condensation¹ or superfluidity². While fundamental understanding remains the overarching motivation, advances in experimental demonstrations, as enabled in part by new material systems, and their realization at room temperature provide early glimpses of the technological potential of exciton–polaritons, ranging from room-temperature polariton lasing to quantum polaritonics and quantum simulations.

Angle-resolved transmission spectrum of Rydberg exciton–polaritons in a Cu₂O microcavity. Credit: Sai Kiran Rajendran and Hamid Ohadi, University of St Andrews

In recent years, the field of polariton physics has received a boost from emerging materials such as halide perovskites and atomically thin van der Waals semiconductors. In a Q&A in this issue of Nature Materials, Hui Deng discusses how these materials not only bring plurality in terms of the active material properties, but also provide flexibility in the design of the optical cavity. Deng further discusses the intriguing possibilities opened up by the realization of strong coupling in moiré polaritons³.

Perovskites are another emerging class of materials in the quest for practical room-temperature polariton condensate systems. Indeed, polariton condensation at room temperature has been reported in microcavity-confined perovskite lattices⁴. However, the realization of large-scale perovskite polariton condensate lattices has been hindered by stringent growth requirements where high crystalline quality, homogeneity and uniformity in crystal thickness are essential. In an Article in this issue, Renjie Tao and colleagues report polariton condensate lattices that have a large lateral size (up to 50 μm) with good control of crystal thickness and uniformity. The film uniformity was achieved by adopting a solution synthesis approach based on nanoconfinement growth, where the perovskite crystal is grown directly inside a Fabry–Pérot nanocavity formed by two dielectric mirrors separated by gold pillar spacers.

Unlike perovskites or van der Waals materials, Cu₂O is not a newcomer when it comes to the study of quasiparticles. In fact, it was in a Cu₂O crystal that excitons were first observed nearly 70 years ago⁵. More recently, interest in this material has been rekindled by the demonstration of giant Rydberg excitons where, by exploiting the large Rydberg energy of Cu₂O, highly excited excitons with the largest principal quantum numbers were reported⁶. To put the dimensions of Rydberg excitons into context, a bound electron orbits its electron–hole pair at distances that reach the micrometre scale. Now, in an Article in this issue, Konstantinos Orfanakis and colleagues report the strong coupling of highly excited Rydberg excitons and cavity photons by embedding a single bulk Cu₂O crystal of micrometre thickness into a Fabry–Pérot microcavity, resulting in the formation of Rydberg exciton–polaritons with principal quantum numbers up to n = 6. A related News and Views article by HeeBong Yang and Na Young Kim discusses the importance of preserving the crystal’s optical properties by minimizing strain-induced degradation and highlights the value of the Cu₂O Rydberg exciton–polariton platform in the study of strongly correlated states of matter, as is the case, for example, for polaritonic systems with reduced dimensionality.

Hybrid dimensionality is explored in an Article by Ling Zhou and colleagues reporting the emergence of quasi-one-dimensional (1D) bright excitonic states in exfoliated SiP₂ bulk flakes and their strong coupling to optical phonons. The anisotropic crystallization of SiP₂ leads to the formation of structures with hybrid dimensionality where quasi-1D phosphorous chains are embedded in quasi-2D sheets. From such an anisotropic structure, linearly dichroic quasi-1D excitonic states arise, where electrons are tightly confined to the 1D phosphorus–phosphorus chains while the correlated holes extend over the 2D SiP₂ atomic plane. SiP₂ is an indirect-bandgap semiconductor, which hinders its potential as an optical material, although symmetry engineering by forming heterostructures with van der Waals semiconductors could enable optoelectronic functionalities. As Matthieu Fortin-Deschênes and Fengnian Xia mention in a related News and Views article, the unusually strong phonon coupling of these hybrid dimensionality excitons makes them an interesting platform for the study of exciton–phonon coupling in low-dimensional materials. It further discusses potential approaches, such as strain engineering or alloying, that may be explored to turn SiP₂ into a direct-bandgap material.

These examples show how materials science is driving the advance of exciton and polariton physics. It is stimulating to see how researchers are continuing to realize new polariton phenomena by capitalizing on the progress made in parallel fields, such as materials synthesis for perovskite solar cells or moiré heterostructures. Indeed, polaritonics is becoming an increasingly multifaceted field, where the plurality of active materials and diverse dimensionalities is promoting new opportunities and presenting diverse challenges.