Exciton–polaritons are among the strangest creatures in the solid-state menagerie. They arise when excitons — bound pairs of an electron and a hole — couple so strongly to photons trapped in an optical cavity that the distinction between light and matter blurs entirely. The resulting quasiparticles inherit the best of both worlds: the tiny effective mass and coherence of photons, and the strong mutual interactions of excitons. When enough of them crowd into a single quantum state, they undergo a form of Bose–Einstein condensation, spontaneously locking their phases together into a single macroscopic wavefunction that emits coherent light. These condensates have long been touted as candidates for quantum communication, cryptography and novel lasers, but a persistent problem has been control: in conventional semiconductor microcavities, the interactions are weak, and tuning the condensate typically means physically rebuilding the device.

The German-led collaboration, spearheaded by Heng Zhang, Niloufar Nilforoushan and Christian Weidgans at the University of Regensburg, together with colleagues in Munich, Prague and elsewhere, saw a way around this limitation by turning to a material in which the excitons themselves are intimately tied to magnetic order. CrSBr is an air-stable, A-type antiferromagnet in which the spins within each layer are aligned ferromagnetically, while adjacent layers are magnetized in opposite directions. Crucially, its excitons are quasi-one-dimensional, tightly bound, and strongly polarized along the crystallographic b axis, with an enormous oscillator strength. Because the electronic orbitals that form these excitons are fully spin-polarized, any change in the magnetic order of the crystal directly reshapes the exciton wavefunction — and with it, the polaritons built from that exciton.

To trap light and matter together, the researchers fabricated microscopic cavities of unusual elegance. Few-nanometer-thick flakes of CrSBr, exfoliated from bulk crystals, were sandwiched between two gold mirrors separated by thin spacers of polymethyl methacrylate. The flakes naturally cleave into elongated microwires, tens of micrometers long along the a axis but only about a micrometer wide along the b axis. This geometry acts as a slab waveguide with additional lateral confinement: light is squeezed tightly in the vertical direction, quantized into discrete modes across the narrow width, and left essentially free to propagate along the length of the wire. Modeling the polaritons as particles in an elongated two-dimensional rectangular potential well, the team predicted a ladder of discrete quantized states along the short axis, with energy splittings of tens of millielectronvolts — exactly what the experiments revealed.

When the researchers pumped one such microwire with femtosecond laser pulses, the angle-resolved photoluminescence showed a strikingly anisotropic picture. Along the narrow b axis, the emission resolved into discrete quantized states — a ground state and two excited states at 1.248, 1.265 and 1.286 electronvolts — while along the long a axis the modes dispersed parabolically, as expected for particles confined in only one lateral direction. A global fit with a coupled oscillator model confirmed that the system had entered the strong-coupling regime, with a vacuum Rabi splitting of 200 millielectronvolts, matching earlier observations of polaritonic effects in bulk CrSBr. Two additional, dispersionless resonances were identified as surface excitons, sitting slightly below the bulk exciton energy — a detail that would prove decisive for driving condensation.

The hallmarks of condensation appeared when the pump laser was tuned into resonance with those surface excitons. Above a threshold fluence of roughly 30 microjoules per square centimeter, the emission from the first excited quantized state surged by more than two orders of magnitude as the pump fluence rose by just a factor of three, while the emission linewidth collapsed from 19 to 8.7 millielectronvolts and the peak energy shifted upward by 5 millielectronvolts. Off-resonant pumping at higher photon energy, by contrast, produced only linear, lackluster emission. Interferometric measurements drove the point home: below threshold, the photoluminescence showed no interference fringes; above it, crisp fringes appeared, and the measured coherence time of 427 femtoseconds exceeded the polariton lifetime by an order of magnitude. Together, the superlinear intensity scaling, linewidth narrowing, energy shift and long-range spatial and temporal coherence constitute an unambiguous fingerprint of exciton–polariton condensation.

Two peculiarities of the condensation process hint at the deep magnetic physics at play. First, condensates formed only under near-resonant excitation of the surface excitons, never under high-energy pumping, suggesting that relaxation into the condensate requires a carefully matched sequence of scattering events. Second, the condensates formed not in the ground state of the microwire but in excited quantized states — and in several samples, the energy gap between the pump photons and the condensate emission closely matched the energy of an optical magnon mode of CrSBr of about 45 millielectronvolts. Density functional theory calculations supported a complementary pathway through lattice vibrations: excitons in CrSBr localize electrons on chromium atoms and holes on sulfur atoms, inducing a structural distortion that increases the interlayer separation and couples efficiently to the interlayer breathing phonon. The picture that emerges is one of resonant phonon- and magnon-assisted scattering shuttling excitation energy from surface states into the condensate — a direct handshake between light, lattice and spin.

The centerpiece of the study, however, is the magnetic control. Placing a microwire in a magneto-optical cryostat with an out-of-plane magnetic field, the researchers tracked the polariton modes as the field swept from zero to 2 tesla. Both the polariton branches and the surface exciton resonance redshifted by more than 10 millielectronvolts, while defect emission stayed put — evidence that the shifts stem from a field-driven reconfiguration of the collective spin order rather than ordinary Zeeman splitting. The spectral shift saturated above 2 tesla, exactly where CrSBr is known to flip from its antiferromagnetic to a ferromagnetic configuration under out-of-plane bias. Most strikingly, the condensed state itself followed suit: the macroscopically coherent emission shifted by up to 10.5 millielectronvolts between the antiferromagnetic and ferromagnetic phases, with condensation achievable in both regimes.

What makes this tuning so powerful is its mechanism. Previous electric-field-based strategies perturbatively modify the potential experienced by polaritons; in CrSBr, the magnetic field instead changes the magnetic order and symmetry of the material itself, transforming the exciton from a quasi-one-dimensional species into a spatially extended three-dimensional wavefunction as the spins reorganize. Because the polaritons emerge from a manifold of spin-polarized electronic orbitals, spin order becomes a native control knob for the condensate — one that operates at the level of the quantum wavefunction rather than merely its environment. The authors even suggest that the fluence-dependent energy shifts they observed may involve not only repulsive exciton–exciton interactions but also magnon-induced magnetic disorder, which reduces the exciton oscillator strength, shrinks the Rabi gap, and pushes the lower polariton energy upward.

The implications stretch well beyond a single laboratory demonstration. Because the condensate is coupled to the magnetic order, the researchers envision modulating its coherent emission on ultrafast timescales by linking it to coherent, high-frequency magnons — spin waves that can be launched and manipulated with picosecond precision. Such a platform could enable magnetic-memory integration and efficient microwave-to-optical transduction for quantum information processing, with microwave photons coupling to condensate emission through the mediating magnons. For a field that has spent two decades searching for robust ways to control macroscopic quantum states of light, the message of this work is tantalizing: sometimes the most elegant control knob is simply a magnet, and the material that responds to it may be sitting in a vial of layered crystals, waiting to be exfoliated.

Subject of Research: Magnetically tunable exciton–polariton condensation in the van der Waals antiferromagnet CrSBr

Article Title: Magnetic control of an exciton–polariton condensate in a van der Waals magnet

Article References: Zhang, H., Nilforoushan, N., Weidgans, C., Inzenhofer, T., Liebich, M., Riepl, J., Hirschmann, J., Gronwald, I., Mosina, K., Sofer, Z., Tyagi, R., Wilhelm, J., Mooshammer, F., Dirnberger, F., & Huber, R. (2026). Magnetic control of an exciton–polariton condensate in a van der Waals magnet. Nature Materials. https://doi.org/10.1038/s41563-026-02751-y

Image Credits: AI Generated

DOI: 10.1038/s41563-026-02751-y

Keywords: exciton–polaritons, condensate, CrSBr, van der Waals magnet, antiferromagnetism, magnons, microcavity, strong coupling, quantum light sources, spintronics, photoluminescence, coherence

Top-emitting OLEDs are the workhorses of high-resolution active-matrix displays because their light exits away from the substrate, decoupling the emitting aperture from the transistor backplane beneath. They also lend themselves naturally to a Fabry–Perot microcavity, formed between a reflective bottom electrode and a semitransparent top cathode. The cavity narrows the electroluminescence spectrum and concentrates emission toward the forward direction, boosting frontal efficiency and color purity. But the same physics that sharpens the spectrum also makes extraction design unforgiving: much of the generated light ends up confined in waveguide modes traveling laterally through the organic stack and in surface plasmon polaritons hugging the metal electrodes. Fixing the air-side interface alone cannot recover those internal losses.

Conventional internal outcoupling structures, such as wrinkles, corrugations, and strongly curved microlens textures, scatter trapped modes back toward useful directions, but they carry a hidden cost. When the underlying texture is steep, the organic layers deposited on top fail to follow it conformally, so the local optical thickness of the cavity varies across the device and the resonance condition drifts from point to point. Stronger scattering then does not translate into a better device, because the cavity-defined forward emission benefit is simultaneously eroded. The key insight of the new work is that the geometry of an internal extraction layer must be judged not just by how much light it scatters but by how gently it does so.

The researchers call their solution a gentle internal corrugation layer, or GICL. Atomic force microscopy over a 50 by 50 micrometer area revealed a peak-to-valley height of about 1.12 micrometers with root-mean-square roughness of 144.5 nanometers, but the defining feature is the slope: representative surface angles of only 2.1 to 6.9 degrees along one axis and 1.8 to 3.5 degrees along the other, spread over lateral spans of nearly 9 micrometers. Cross-sectional electron microscopy showed the entire multilayer stack following this soft undulation without discontinuities. The contrast with reference structures is instructive. A steep imprinted random microlens array reached surface angles of 25.3 degrees with a peak-to-valley height of 7.87 micrometers, while one wrinkle reference achieved low roughness yet still exhibited angles above 53 degrees, demonstrating that roughness amplitude alone says little about geometric severity.

Finite-difference time-domain simulations confirmed that gentler is genuinely better. Among the compared structures, GICL yielded the highest relative total simulated power, outperforming both wrinkle references and the steep microlens array, even though the latter produced far stronger local field perturbation. Ray tracing sharpened the picture: GICL directed 77.3 percent of input power into the forward collection region, versus 43.5 percent for the microlens array, which instead diverted light into wide-angle, internally reflected, and backward trajectories. Under identical conditions, the calculated light extraction efficiency rose from 37.6 percent for the planar microcavity device to 74.1 percent with GICL, an increase of nearly a factor of two. An intermediate-index clear resin with a refractive index of 1.59 was applied at the outer surface to soften the air interface and let the redistributed light actually escape.

The emissive system matters as much as the geometry. In a microcavity, only the spectral portion of the emitter that overlaps the resonance band contributes efficiently to the enhanced external output, so a broad emitter wastes much of its spectrum on off-resonant wavelengths. The team used a phosphor-assisted thermally activated delayed fluorescence sensitization scheme, in which a TADF host and a phosphorescent assistant funnel excitation energy to a terminal multi-resonance TADF emitter called tCzphB-Fl, producing green emission with a full width at half maximum of only about 19 to 22 nanometers. When the microcavity was formed, the narrowband system’s peak shifted just 2 nanometers and its FWHM narrowed from 22 to 19 nanometers, whereas a conventional phosphorescent comparator shifted 13 nanometers and narrowed from 65 to 25 nanometers, evidence that most of the narrowband emission already sat inside the resonance band.

Device results bore out the combined strategy. All GICL devices shared the same 2.25-volt turn-on voltage and essentially unchanged current density, ruling out electrical effects, and the forward spectra showed only minor changes, ruling out spectral reshaping. Under matched resin conditions, adding GICL raised the external quantum efficiency from 34.8 to 69.2 percent, current efficiency from 302.7 to 378.4 candela per ampere, and power efficiency from 199.3 to 396.7 lumens per watt, with maximum luminance reaching nearly 120,000 candela per square meter. Across five samples the efficiencies averaged 68.2 plus or minus 0.7 percent, indicating good reproducibility. The angular color shift at 60 degrees off-axis also fell from 0.0197 to 0.0086 in CIE 1976 coordinates, and the luminance distribution became more Lambertian, both valuable for display uniformity.

Polarization-resolved measurements identified where the recovered light had been hiding. Because waveguide modes are predominantly transverse-electric like and surface plasmon polaritons are transverse-magnetic like, separating the s- and p-polarized emission provides a mode-sensitive diagnostic. Both polarized components, and the non-polarized total, increased across the viewing-angle range in the corrugated device, confirming genuine extraction rather than mere redistribution between channels. Quantitative modal analysis showed the outcoupled fraction rising from 29.7 to 38.6 percent, with the waveguide fraction dropping from 40.0 to 32.0 percent and the plasmon-related fraction changing only modestly. The roughly 30 percent simulated gain in outcoupled power matched the approximately 25 percent uncorrected device-level increase closely, lending quantitative coherence to the extraction interpretation.

Notably, the same corrugation helped the broad phosphorescent system far less, lifting its current efficiency by only about 15.7 percent compared with 25.0 percent for the narrowband system, a difference the authors attribute to spectral overlap with the cavity resonance rather than to bandwidth alone, since the two emissive chemistries differ in other respects as well. The gains did come with a small price: efficiency roll-off at high brightness was moderately larger in the corrugated devices, possibly reflecting local current crowding on the textured surface, and operational lifetime at 1,000 candela per square meter was slightly shorter, though still comparable at about 101 hours to 95 percent of initial luminance. The broader lesson stands: in microcavity OLEDs, internal texture geometry, outer-interface index matching, and emitter-resonance alignment must be engineered together, and gentleness, it turns out, extracts more light than force.

Subject of Research: Cavity-compatible internal light extraction in top-emitting microcavity OLEDs using a gentle corrugation layer and resonance-matched narrowband emission

Article Title: Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission

Article References: Kim, Y. R., Nimbalkar, A., & Suh, M. C. (2026). Cavity‐Compatible Light Extraction in Top‐Emitting Microcavity OLEDs via a Gentle Internal Corrugation Layer and Resonance‐Matched Narrowband Emission. Advanced Science, Article e78013. https://doi.org/10.1002/advs.78013

Image Credits: AI Generated

DOI: 10.1002/advs.78013

Keywords: OLED, microcavity, light extraction, external quantum efficiency, corrugation, TADF, narrowband emission, waveguide modes, surface plasmon polariton, display technology, FDTD simulation, phosphorescence

The central idea behind the new work is the polariton, a hybrid quasiparticle formed when the excited state of an organic molecule couples strongly enough to a confined optical mode that the two lose their separate identities. In this regime, the energy levels split into upper and lower polariton branches, and emission can be funneled toward the lowest energy state of the system. Because the optical mode imposes its own dispersion and density of states on the hybrid excitation, the emitted spectrum can be dramatically narrowed compared with the broad fluorescence band of the bare molecule. For display applications, where each subpixel must produce a saturated primary color, this spectral compression is enormously valuable, since narrow emitters translate directly into wider color gamuts as defined by standards such as Rec. 2020.

Historically, however, polariton light emitting devices have struggled to convert this spectral elegance into practical performance. Many early demonstrations relied on phosphorescent emitters containing iridium or platinum, materials that are expensive, supply constrained, and increasingly undesirable from a sustainability standpoint. Moreover, the strong coupling cavity structures used to form polaritons often introduce their own problems. The angular dispersion of the cavity mode means that the emission wavelength can shift substantially as the viewer moves off axis, producing the familiar color drift that plagues conventional OLEDs at large viewing angles. And the process of relaxing from high energy states down to the emissive ground state of the polariton landscape can be inefficient, throttling the maximum achievable external quantum efficiency.

The research team behind the new device addressed these challenges by building their polariton OLED around thermally activated delayed fluorescence, or TADF, a mechanism that has emerged over the past decade as a metal free route to harvesting both singlet and triplet excitons in organic emitters. In a TADF molecule, the energy gap between the lowest singlet and triplet states is made small through careful molecular design, so that triplet excitons can be thermally upconverted back to emissive singlet states at room temperature. In principle this allows every electrically generated exciton to contribute to light emission, matching the internal efficiency of phosphorescent systems without any rare metal content. Combining this exciton harvesting machinery with a strong coupling microcavity, the researchers created a device in which delayed fluorescence feeds the polariton modes rather than simply radiating through the usual broad molecular transition.

The resulting emission characteristics are striking. The devices produce spectrally narrow output with linewidths far below those of standard fluorescent and TADF OLEDs, which typically emit over tens of nanometers. The polariton mediated emission concentrates the radiated light into a tight spectral band whose position is set primarily by the cavity design rather than by the full molecular vibronic envelope. Equally important, the emission remains stable as a function of viewing angle. In conventional microcavity OLEDs, the resonance wavelength blue shifts as the observation angle increases because the optical path length through the cavity effectively shortens. The polariton architecture reported in the new study suppresses this angular dependence, so the color a viewer sees remains essentially unchanged across a wide range of angles, a property that is critical for large panel displays viewed by multiple people at once.

Efficiency is the third pillar of the demonstration. The researchers report highly efficient device operation, with external quantum efficiency figures that place the polariton OLED among the best performing narrowband organic emitters, while retaining the metal free character of the TADF emitter. Achieving this required careful balancing of the optical and electrical design. The cavity must be strong enough to reach the regime of genuine strong coupling, where the energy exchange between exciton and photon outpaces all loss processes, yet the device must still inject charge carriers efficiently and allow excitons to form and relax into the polariton states without excessive nonradiative loss. The team optimized the layer stack, the emitter concentration, and the mirror structures to satisfy these competing requirements simultaneously.

The physics underlying the angular stability deserves a closer look. In a planar microcavity, photon modes obey a parabolic dispersion, with energy increasing as the in plane wavevector grows. Excitons, by contrast, are essentially dispersionless because molecules are fixed in place. When the two hybridize, the resulting polariton branches inherit a mixture of both characters. Near the crossing point of the bare exciton and photon energies, the lower polariton branch flattens relative to the pure photon dispersion, reducing the rate at which its energy changes with angle. By engineering the detuning, that is, the energy offset between the exciton resonance and the cavity resonance, the researchers positioned their emission on a portion of the polariton dispersion where this flattening is pronounced, locking the output color in place for off axis viewers.

The narrowband character arises from a complementary mechanism. The density of optical states in the cavity is strongly frequency dependent, and in the strong coupling regime the lowest polariton state acts as an efficient sink into which excitations relax before radiating. Instead of every molecule emitting independently across its inhomogeneously broadened spectrum, the ensemble funnels its energy into a single well defined hybrid mode. This relaxation funnel effect compresses the emission linewidth and can also shorten the effective radiative lifetime, since the polariton carries photonic character that couples efficiently to the outside world. Faster emission is not merely a curiosity; it reduces the time excitons spend in states vulnerable to annihilation processes that degrade efficiency at high brightness, one of the persistent bottlenecks in OLED development.

The choice of TADF as the gain medium is what makes the whole scheme electrically practical. Fluorescent emitters can, in principle, address only the twenty five percent of excitons formed as singlets under electrical excitation, capping their internal efficiency at a low level. Phosphorescent emitters harvest everything but require precious metals. TADF molecules harvest everything using only abundant organic elements, and the delayed fluorescence channel provides a steady supply of singlet excitons that can couple to the cavity mode. The new work demonstrates that this supply can be routed into polariton states efficiently enough to sustain bright, narrowband output, resolving a long standing question about whether metal free emitters could power high performance polariton devices.

The implications extend beyond displays. Narrowband, angle stable organic sources are attractive for optical communications, sensing, and spectroscopy, wherever a compact, tunable, low cost light source with well defined color is needed. The demonstration also energizes the broader field of polaritonic engineering, in which researchers seek to use strong light matter coupling to modify chemical reaction rates, energy transport, and material properties. Showing that a technologically mature emitter class like TADF can be integrated into a strongly coupled device with high efficiency suggests that polariton concepts are moving from laboratory physics toward manufacturable technology. As fabrication techniques for high quality optical cavities mature and molecular design of TADF emitters continues to advance, the combination of saturated color, viewing angle robustness, and metal free efficiency reported here may well define the next generation of organic optoelectronics.

Subject of Research: Polariton organic light emitting diodes employing thermally activated delayed fluorescence for narrowband, angle-stable, high-efficiency emission

Article Title: Narrowband, angle-stable, and highly efficient polariton organic light emitting diodes employing thermally activated delayed fluorescence

Article References: Mischok, A., Lennartz, S., Gruber, V., Tenopala-Carmona, F., Witt, J., Hillebrandt, S., & Gather, M. C. (2026). Narrowband, angle-stable, and highly efficient polariton organic light emitting diodes employing thermally activated delayed fluorescence. Light: Science & Applications, 15(1), Article 378. https://doi.org/10.1038/s41377-026-02415-1

Image Credits: AI Generated

DOI: 10.1038/s41377-026-02415-1

Keywords: organic light emitting diodes, polaritons, thermally activated delayed fluorescence, strong light-matter coupling, narrowband emission, angular color stability, microcavity, external quantum efficiency, display technology, metal-free emitters, exciton harvesting, optoelectronics