🔬 mesoscale physics

Spin injection of exciton-polaritons with halide perovskites at room temperature

This study demonstrates room-temperature spin injection and preservation in exciton-polaritons within a monolithic Tamm-plasmon microcavity embedding 2D halide perovskites, where rapid polariton decay outcompetes spin relaxation mechanisms to enable potential applications in chiral lasers and switches.

Original authors: Elena Sendarrubias Arias-Camisón, Maksim Lednev, Jorge Cuadra, Raúl Gago, Luis Viña, Francisco José García Vidal, Johannes Feist, Ferry Prins, Carlos Antón Solanas

Published 2026-07-22

📖 4 min read☕ Coffee break read

CC BY 4.0

Original authors: Elena Sendarrubias Arias-Camisón, Maksim Lednev, Jorge Cuadra, Raúl Gago, Luis Viña, Francisco José García Vidal, Johannes Feist, Ferry Prins, Carlos Antón Solanas

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

Imagine a world where light doesn't just shine; it dances with matter so closely that they become a single, hybrid creature. This is the playground of quantum physics, specifically a field called "polaritonics." To understand the story, you first need to meet the main characters. Think of an exciton as a tiny, energetic couple: an electron and a "hole" (a missing electron) holding hands, zipping around inside a material. Now, imagine a photon, a particle of light, zooming by. Usually, they just pass each other. But if you trap them in a super-tiny, mirrored box, they can get so excited they start swapping energy back and forth faster than they can escape. They merge into a new hybrid particle called an exciton-polariton. It's like a ghost that is half-light and half-matter, capable of flowing like a liquid and even forming lasers.

Why do we care? Because these particles carry a secret code called "spin," which is basically their direction of rotation or, in the world of light, their polarization (whether they spin left or right). If we can control this spin, we could build super-fast, ultra-efficient computers and lasers that process information using light instead of electricity. The big question scientists have been asking is: Can we keep this spin code intact when these particles are created and moving at room temperature, where things are usually too chaotic and hot for delicate quantum tricks?

This paper takes a bold step toward answering "yes." The researchers built a special sandwich-like device using a type of crystal called a halide perovskite, which is famous for being a great host for these excitons. They trapped a thin film of this material between a mirror made of stacked layers and a thin silver cap, creating a "Tamm-plasmon" cavity. They then shined a laser on it, but with a twist: they used circularly polarized light, which acts like a spinning key to inject excitons with a specific spin direction.

The team found that when they pumped the system with this spinning light, the resulting polaritons actually kept a memory of that spin. It wasn't a perfect copy, but it was a measurable one. The key to this success was speed. The polaritons in this setup are incredibly short-lived, decaying in just 30 to 100 femtoseconds (that's 0.00000000000003 seconds!). Because they vanish so quickly, they don't have enough time to get "confused" by the heat and jostling of the room, which would normally scramble their spin. It's like trying to spin a top on a bumpy table; if you knock it over instantly before the bumps can shake it, it stays upright.

However, the paper also draws a clear line in the sand. The researchers showed that if you look at the excitons before they become polaritons (the "bare" excitons), they lose their spin completely. This proves that the magic isn't just in the material itself, but in the specific, ultra-fast journey the particles take inside the cavity. The study suggests that the more "light-like" the polariton is, the better it holds onto its spin. As the particles become more "matter-like" (more exciton-heavy), they lose their polarization faster. The authors used a mathematical model to explain this, suggesting that the cavity's ability to speed up the emission (a concept called the Purcell factor) is crucial for preserving the spin. While the numbers from their simple model didn't perfectly match the experiment, the trend was clear: the cavity helps.

Ultimately, this work doesn't claim to have built a perfect spin-based computer today. Instead, it suggests that halide perovskites in these specific micro-cavities are a promising playground for keeping spin information alive at room temperature. It opens the door for future devices, like chiral lasers or optical switches, that could one day use the spin of light to process information faster and more efficiently than ever before.

Technical Summary: Spin Injection of Exciton-Polaritons with Halide Perovskites at Room Temperature

Problem and Context
Exciton-polaritons, hybrid quasiparticles formed by the strong coupling of light and matter, offer a platform for studying light-matter interactions and developing nonlinear photonic applications. Lead-halide perovskites are particularly promising for room-temperature (RT) polaritonics due to their large exciton binding energies, high oscillator strengths, and tunable bandgaps. While RT lasing and long-range propagation have been demonstrated, a significant challenge remains: achieving spin control of polaritons via quasi-resonant laser driving with the exciton reservoir at room temperature. Previous demonstrations of spin injection in similar systems have largely been limited to cryogenic temperatures (e.g., in II-VI and III-V semiconductors) or strictly non-resonant driving regimes. Furthermore, while electrical generation of spin-polarized polaritons in perovskite metasurfaces has been shown at liquid-nitrogen temperatures, such architectures involve complex nanofabrication and are limited to continuous driving.

Methodology
The authors engineered a monolithic Tamm-plasmon microcavity operating in the strong coupling regime at room temperature. The device structure consists of:

A dielectric distributed Bragg reflector (DBR) made of 8 pairs of SiO₂/TiO₂.
A thin film of two-dimensional (2D) phenethylammonium lead iodide ((PEA)₂PbI₄) with a thickness of 50 nm (approx. 30 layers).
A tunable polymer spacer layer of poly(methyl methacrylate) (PMMA) with thicknesses varying between 45 nm and 105 nm to control the exciton-photon detuning.
A 35 nm silver capping layer acting as the top mirror.

The study utilized angle-resolved optical spectroscopy, including reflection and photoluminescence (PL) measurements. Experiments were conducted in the linear regime, well below any lasing threshold. To investigate spin injection, the authors employed quasi-resonant laser excitation at 2.38 eV (matching the exciton energy) using circularly polarized light (σ±) and linearly polarized light (H). Full polarization tomography was performed to measure the Stokes parameters ( $S_0, S_1, S_2, S_3$ ) of the lower polariton branch (LPB) emission. The circular degree of polarization was defined as $S_3 = (I_{\sigma+} - I_{\sigma-}) / (I_{\sigma+} + I_{\sigma-})$ .

Key Results

Polariton Formation: The microcavities exhibited a vacuum Rabi splitting of $322 \pm 13$ meV, confirming strong coupling. The quality factor was measured at $Q \sim 100$ , corresponding to a cavity photon lifetime of $\tau_C \sim 30$ fs. The lower polariton branch (LPB) lifetime was estimated to be between 30 and 100 fs.
Spin Injection and Preservation: Under circularly polarized quasi-resonant excitation, the LPB emission retained a measurable degree of circular polarization ( $S_3$ ). The spin injection was observed across different exciton-photon detunings (ranging from -56 meV to +111 meV).
Detuning Dependence: The degree of circular polarization ( $|S_3|$ ) was found to be dependent on the detuning. For negative detunings (photon-like polaritons), $|S_3|$ was approximately 20%. As the detuning became more positive (exciton-like polaritons), $|S_3|$ decreased to below 10%. This trend correlates with the polariton lifetime; photon-like polaritons have shorter lifetimes, allowing them to decay radiatively before spin relaxation mechanisms (which occur on a picosecond scale at RT) can fully depolarize the population.
Comparison with Bare Excitons: Crucially, no spin conservation was observed in the bare perovskite exciton emission under the same excitation conditions. This indicates that the ultrafast decay channel provided by the polariton state is essential for retaining spin information at room temperature.
Theoretical Modeling: A semiclassical rate-equation model was developed to describe the population dynamics. The model suggests that the observed spin retention requires a Purcell factor ( $P$ ) on the order of 80 (specifically the $k_{\parallel}$ -resolved factor) to reproduce the experimental $|S_3|$ values. Without the cavity enhancement ( $P=1$ ), the theoretical $|S_3|$ drops significantly.

Significance and Claims
The authors claim that this work demonstrates the first successful spin injection of exciton-polaritons in a halide perovskite microcavity at room temperature using quasi-resonant excitation. The primary significance lies in the demonstration that the ultrafast lifetime of polaritons (30–100 fs) is shorter than the spin relaxation time of the excitons at room temperature (picosecond scale). This temporal mismatch allows a fraction of the injected spin polarization to be preserved and emitted as circularly polarized light before thermalization occurs.

The paper concludes that halide perovskite Tamm-plasmon microcavities serve as a promising platform for developing RT spin-based polaritonic devices, such as chiral lasers and switches, controlled under quasi-resonant excitation. The authors note that while the current system shows partial spin retention, future improvements could involve perovskite compositions with longer spin relaxation times (e.g., specific doped perovskites) or advanced photonic architectures to further enhance spin control.

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