Strong Exciton-Photon Coupling

• Strong exciton-photon coupling is a regime where photons and excitons merge into hybrid quasiparticles, manifesting clear energy level splitting and anticrossing features.
• The phenomenon is modeled by coupled oscillator and quantum Rabi frameworks, emphasizing coherent interactions that surpass dissipation rates.
• Applications range from room-temperature polariton devices to scalable quantum photonics, leveraging engineered photonic modes and collective material effects.

Strong exciton-photon coupling refers to the regime in which electromagnetic modes of a photonic structure and the optical transitions of an electronic excitation (exciton) coherently hybridize, leading to the formation of mixed light-matter quasiparticles—exciton-polaritons—characterized by energy level splitting and quantum interference phenomena. This regime is defined by the coupling strength exceeding dissipation rates, manifesting universal signatures across diverse material systems, including inorganic and organic semiconductors, quantum wells, two-dimensional materials, perovskites, and hybrid photonic structures.

1. Physical Principles and Theoretical Frameworks

Strong exciton-photon coupling arises when the coherent interaction rate $g$ between a photonic mode (e.g., cavity photon, optical mode of a photonic crystal, magnetic dipole resonance in a nanosphere) and the transition dipole of an exciton outpaces their respective dissipative rates ($\gamma$, $\kappa$). The coupled system is described by the quantum Rabi/Jaynes–Cummings or, more generally, coupled oscillator/Fano-Hamiltonian models: $$\Omega = 2\hbar g,\quad g \propto \sqrt{\frac{n f}{V}}$$ where $\Omega$ is the vacuum Rabi splitting, $n$ the number of oscillators, $f$ the oscillator strength, and $V$ the photonic mode volume. Hybrid eigenstates (upper and lower polariton branches) split by $\Omega$ emerge at resonance, observed as anticrossing behavior in optical spectra.

Extensions incorporate multi-level systems, vibrational sidebands, and frequency-dependent self-energies: $$E_{UP,LP} = \frac{E_X + E_{Ph}}{2} \mp \frac{1}{2}\sqrt{\Omega^2 + (E_X-E_{Ph})^2}$$ or, for three-level/complex systems, matrix Hamiltonians and Dyson/Green's function approaches. In nanostructured or spatially dispersive platforms, collective and coherent effects (spatial mode profiles, planar momentum, symmetry, and phase) critically modulate radiative and nonradiative properties of polaritons.

2. Experimental Platforms and Key Observables

Microcavities and Photonic Crystals

Strong coupling has been demonstrated in a variety of solid-state microcavity systems:

• Planar and open-access microcavities: Employing distributed Bragg reflectors (DBRs), metallic or dielectric mirrors, or hemispherical/concave geometries to realize tunable, high-finesse optical modes (Dufferwiel et al., 2014, Dovzhenko et al., 2021, Mikhin et al., 10 Sep 2024).
• Photonic crystals: Patterned two-dimensional lattices in waveguides providing engineered band structure, zone folding, and robust field localization (0907.2813, Jia et al., 5 Mar 2024).
• Nanowires and nanospheres: Subwavelength perovskite nanowires (NW) as Fabry–Pérot microcavities, or all-dielectric (Si–WS$_2$) hybrid nanostructures coupling localized Mie resonances with monolayer excitons (Coles et al., 2017, Wang et al., 2017).

Observables:

• Anticrossing (Rabi splitting): PL, reflectance, scattering, or transmission spectra resolve an avoided crossing, with observed splittings spanning $\sim7$ meV (GaAs QWs in PhCs) (0907.2813), $>70$ meV (WS$_2$ in Bragg microcavities) (Mikhin et al., 10 Sep 2024), up to $390$–$560$ meV (MAPbBr$_3$ perovskite NWs, especially with plasmonic enhancement) (Zhang et al., 2017, Shang et al., 2017).
• Polariton linewidth and lifetime: Strong coupling can dramatically narrow polariton linewidths even in low-Q (broadband) cavities, with radiative decay dominated by collective interference rather than sum of bare losses (Cerda-Méndez et al., 25 Oct 2025, Song et al., 28 Aug 2025).

Material Systems

• Quantum wells and III–V semiconductors: QW excitons strongly coupled to microcavity photons or photonic crystal modes (Zhang et al., 2012, Dufferwiel et al., 2014).
• Colloidal nanocrystals/nanoplatelets: CdSe nanoplatelets and colloidal QDs with substantial oscillator strengths and quantum yields enable room-temperature strong coupling, with Rabi splittings $>50$ meV (Flatten et al., 2016, Dovzhenko et al., 2021).
• 2D transition metal dichalcogenides (TMDCs): Strong coupling of CVD-grown or exfoliated WS$_2$, MoSe$_2$, and WSe$_2$ to diverse optical cavities demonstrates scalability, with Rabi splittings of 17–77 meV and tunable polaritonic phenomena (Gillard et al., 2020, Wang et al., 2017, Mikhin et al., 10 Sep 2024, Khatoniar et al., 2022).

Ultrastrong coupling (USC: $g/\omega_{ex} > 0.1$) is achieved, for example, in CrSBr: $g = 169$ meV persists up to room temperature and can be tuned magnetically (Wang et al., 2023).

3. Engineering and Modulation of Coupling Regimes

Photonic Mode Engineering

• Mode volume reduction: Decreasing $V$ (nanoscale confinement, use of nanowires, plasmonic enhancement, dielectric antennas in topological environments) increases $g$ (Zhang et al., 2017, Shang et al., 2017, Jia et al., 5 Mar 2024).
• Dispersion engineering: Photonic crystal lattice parameter or cavity geometry controls polariton dispersion, enabling phase matching for nonlinear processes such as entangled photon pair generation (0907.2813).
• Topological vacuum backgrounds: Harnessing edge states in topological photonic crystals enables ultra-narrow linewidths (<3.5 meV) and robust, high-collection polariton emission via the “topological vacuum effect” (Jia et al., 5 Mar 2024).

Multi-exciton and Loss Engineering

• Multiple excitonic levels: Coupling to several excitonic transitions (e.g., heavy/light hole, vibrational sidebands) yields multiplet polariton branches, with hybridization between transitions (Flatten et al., 2016, Coles et al., 2017, Cerda-Méndez et al., 25 Oct 2025).
• Linewidth narrowing in lossy (low-Q) cavities: Strong coupling can dramatically “purify” photon modes—contrary to the high-Q paradigm—by transfer of coherence to multiple narrow excitons, not predictable by standard two-level sum rules (Cerda-Méndez et al., 25 Oct 2025, Song et al., 28 Aug 2025).
• Dark excitons and hot electrons: In metal-organic microcavities, dark (non-radiative) excitons and free-carrier (hot electron) populations modulate strong-coupling strength dynamically, imparting Fano-like features and two-temperature decay in transient spectra (Kolesnichenko et al., 26 Jan 2024).

4. Beyond Conventional Models: Coherence, Collective Effects, and Topology

Conventional coupled-oscillator or Jaynes–Cummings models neglect spatial, phase, and collective correlations. Recent theory (Song et al., 28 Aug 2025) shows:

• Polaritonic bound states in the continuum (BICs): Destructive interference, both within the excitonic ensemble (collective mode structure) and between excitonic and photonic decay pathways, can fully suppress radiative decay, producing BICs with infinite radiative lifetime, limited only by non-radiative processes.
• Mathematical condition for BIC: Given Hopfield coefficients $c_p$, $c_x$ and radiative couplings $\kappa$, $\beta$,

$\tau = c_p \kappa \pm c_x \beta = 0$

• Topological and non-reciprocal states: Coupling photonic topology (edge/surface states, magnetic order) with polaritons allows manipulation of selection rules, robustness against disorder, and reconfigurable transport properties (Wang et al., 2023, Jia et al., 5 Mar 2024).

5. Applications and Implications

Room-temperature and Scalable Platforms

• Polaritonic devices enable operation at room temperature with Rabi splittings exceeding exciton linewidths, applicable to polariton lasers, nonlinear optics, quantum light sources, and slow-light devices—realized in perovskites (Zhang et al., 2017, Shang et al., 2017), colloidal QDs (Dovzhenko et al., 2021), and 2D materials (Gillard et al., 2020, Mikhin et al., 10 Sep 2024).
• Large-area CVD growth and heterostructure encapsulation permit wafer-scale integration and high optical quality, eliminating the scalability barrier posed by exfoliation (Gillard et al., 2020).

Quantum and Topological Technologies

• Entangled photon-pair generation: Engineering polariton dispersion and phase matching in patterned photonic crystals facilitates parametric scattering suitable for on-chip solid-state quantum light sources (0907.2813).
• Valley-layer degree of freedom manipulation: TE-TM splitting in microcavities acts as a pseudomagnetic field, allowing optical control of valley coherence in bilayer WS$_2$ polaritons, opening routes towards valleytronics without applied fields (Khatoniar et al., 2022).
• Hybrid quantum platforms: Magnetically dressed polaritons in vdW magnetic semiconductors (CrSBr) provide avenues for combined photonic, electronic, and magnetic quantum information processing (Wang et al., 2023).

Design and Methodological Insights

System/material	Rabi splitting (meV)	Scalability/Techniques	Special features
GaAs QW/DBR MC	7–20	Epitaxy, Bragg cavities, air gap MC	“Very strong” $g > E_B$ (Zhang et al., 2012)
CdSe/CdS QDs/nanoplatelets	50–154	Hot injection, solution-processed, open MC	High oscillator strength, tunable
MAPbBr$_3$ NWs	268–560	Vapour growth, plasmonic enhancement	Room-temp, surface plasmon boost
Monolayer WS$_2$/DBR	17–77	CVD, dry transfer, chip-integrated Bragg MC	On-chip compact strong coupling
CrSBr/TPMC	169	vdW exfoliation, Tamm plasmon MC	Magnetic tuning, ultrastrong room-T
TMDC/hBN (CVD)	17–34	Wafer-scale CVD, DBR microcavities	High optical quality, scalablility
Nanographene/PMMA	104 (electronic), 40 (vib)	Spin-coating, open-plinth MC	Polariton-vibrational hybridization
Si nanosphere–WS$_2$	77	Colloid, dry transfer, coupled MDR/WS$_2$	All-semiconductor, robust coupling
Carbon nanotube/fiber MC	23–40 (μeV)	Fiber MC, Purcell regime, phonon wings	Efficient tunable photon source

6. Limitations and Controversies

• “High-Q dogma” challenged: Experiments demonstrate that low-Q, high-loss photonic structures can still exhibit strong coupling and even linewidth narrowing due to hybridization with multiple excitonic states (Cerda-Méndez et al., 25 Oct 2025).
• Role of dark excitons and non-radiative channels: Non-emissive excitons, hot electrons, and correlated loss channels can obscure or modulate the polaritonic response, producing Fano-like features and altered relaxation dynamics (Kolesnichenko et al., 26 Jan 2024).
• Conventional models insufficient: In subwavelength and multi-excitonic platforms, coupled oscillator descriptions must be replaced by full Hamiltonians capturing collective, coherent, and symmetry-induced interference phenomena (Song et al., 28 Aug 2025).

7. Future Prospects

Ongoing progress in material synthesis, cavity and photonic device nanofabrication, hybrid system engineering (e.g., topological reservoirs, magnetic tuning), and theoretical modeling (open quantum systems, non-Hermitian quantum optics) is expected to further expand the range and utility of strong exciton-photon coupling. Application domains encompass scalable polaritonics, quantum light sources, on-chip logic, valleytronics, topological photonics, and strongly correlated quantum simulators. The universal design principles—maximizing oscillator strength, minimizing mode volume, engineering the electromagnetic vacuum, and exploiting coherence and interference—are applicable across a spectrum of next-generation hybrid quantum photonic platforms.