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More than 60 years have passed since the development of the theory of strong light-matter interactions by Hopfield [1], a milestone that ignited a series of fundamental breakthroughs across the fields of photonics and condensed matter. Over the last three decades, the field has witnessed exponential growth, driven by the relentless progress in modern nanofabrication techniques, material growth, imaging, spectroscopic and time-resolved technologies that have allowed the realization of increasing advanced polariton resonances up to room temperature with applications in a wide range of scientific disciplines [2]. Polaritons are quasiparticles that arise from the strong coupling between quanta of light (photons) and matter oscillators, including atoms, molecules, excitons, phonons, magnons, and plasmons [3]. The diverse array of polaritonic systems spanning various materials and optical platforms is likely the linchpin of polaritons’ success, propelling them to the forefront in applications requiring sophisticated properties such as coherence, nonlinearity, nontrivial topology, and straightforward electron-light conversion [4].

This special issue serves as a platform to showcase state-of-the-art research in polaritonics, encompassing both fundamental discoveries and transformative technological advancements. The contributions within this collection explore a multitude of topics, including the generation and manipulation of polaritons, novel materials and devices, quantum simulators, and emerging applications in photonic and opto-mechanical technologies. By bringing together experts in the field, we have curated an assemblage of contributions aiming to shed light on the latest breakthroughs and pave the way for the future of polaritonics.

This feature issue opens with an editorial article by P. V. Santos and A. Fainstein that discusses the exciting opportunities arising from the convergence of cavity exciton-polariton physics and cavity optomechanics [5]. In the following article, T. Liew shares his insights on the future of quantum phenomena in polariton systems, shedding light on crucial aspects of the term “quantum” in this rapidly evolving field of exciton-polaritons [6]. S. L. Harrison et al. present their recent efforts in realizing a Chern insulator based on cavity polariton vortices [7]. Their theoretical results suggest a pathway toward the development of optical-based nonlinear topological signal processing schemes.

C. Bennenhei et al. report on the condensation of exciton-polaritons emerging from fluorescent protein-filled microcavities, laying the foundation for achieving full linear polarization control in room-temperature polariton lasers [8]. M. Król et al. demonstrate a general scheme for constructing open and tunable microcavities, enabling the observation of the strong coupling regime in WS₂ monolayers, spin-coated PEPI perovskites, and CdTe quantum wells [9]. V. Janonis et al. undertake a numerical and experimental investigation into the coherent thermal emission of hybrid surface plasmon-phonon polaritons in n-GaN gratings [10].

A. Opala and M. Matuszewski provide a review of recent studies and significant findings regarding the potential use of exciton-polaritons in digital computing and optical neural networks [11]. D. Li et al. realize discretized exciton-polariton modes in a micro-sized spatial trap inside a microcavity with an embedded 2D van-der-Waals heterostructure, offering new insights for dynamically controlling polariton relaxation and phonon generation through spatial confinement in future applications [12]. S. Rajabali et al. demonstrate how plasmonic reflectors can effectively prevent energy leakage due to propagating plasmons, a factor known to limit the formation of polaritonic resonances [13]. Finally, C. Lüders et al. present a review that explores the use of continuous-variable spectroscopy techniques for investigating quantum coherence and light-matter interactions in semiconductor systems with ultrafast dynamics [14].