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Absorption and amplification singularities in metasurface etalons with gain
Full text linkPassive reflective metasurfaces can possess perfect absorption conditions: Singular scattering anomalies at which all impinging light is absorbed. Perfect absorption is a common yet powerful metasurface design option with applications in energy harvesting, sensing, and more. Less common is the inclusion of optical gain to the system, which can give rise to a singular condition for perfect amplification. We analyze absorption and amplification singularities in plasmon antenna metasurface etalons with gain with a simple transfer matrix model. Our etalon follows the Salisbury screen design: A metal ground plate spaced by dielectric medium from an array of resonant plasmonic scatterers. We include frequency dispersive models for gain media and discuss the limitations of time reversal symmetry arguments for relating gain singularity conditions (reflectivity poles) to the well-known perfect absorption conditions (reflectivity zeros) of metasurface etalons. We show that for metasurface etalons with both gain and loss, gain can induce both perfect absorption and gain singularities, and we describe topological constraints on their creation and annihilation. Our findings have implications for the fields of non-Hermitian photonics, parity-time symmetric scattering systems, and dynamically controllable active metasurface pixels
Broadband localization of light at the termination of a topological photonic waveguide
Full text linkLocalized optical field enhancement enables strong light-matter interactions necessary for efficient manipulation and sensing of light. Specifically, tunable broadband energy localization in nanoscale hotspots offers many applications in nanophotonics and quantum optics. We experimentally demonstrate a mechanism for the local enhancement of electromagnetic fields based on strong suppression of backscattering. This is achieved at a designed termination of a topologically nontrivial waveguide that nearly preserves the valley degree of freedom. The symmetry origin of the valley degree of freedom prevents edge states to undergo intervalley scattering at waveguide discontinuities that obey the symmetry of the crystal. Using near-field microscopy, we reveal that this leads to strong confinement of light at the termination of a topological photonic waveguide, even without breaking reciprocity. We emphasize the importance of symmetry conservation by comparing different waveguide termination geometries, confirming that the origin of suppressed backscattering lies with the near conservation of the valley degree of freedom, and show the broad bandwidth of the effect
Exotic mechanical properties enabled by countersnapping instabilities
Full text linkFrom an umbrella flipping inside out during a gust of wind to a slender stick bowing when compressed, mechanical instabilities are often seen as undesirable. However, they can also be leveraged, as illustrated by the snapping-based prey capture strategies of the Venus flytrap and mantis shrimp. Inspired by these observations, researchers have started to harness such nonlinear effects to design materials with exotic and programmable functions. Here, we expand this repertoire by experimentally demonstrating countersnapping, where a combination of geometrically nonlinear building blocks cooperate to suddenly contract when increasingly tensioned. We demonstrate that this behavior unlocks exotic mechanical and dynamical behavior, potentially useful for metamaterials, sensors, and smart structures
Optical Reabsorption Effects in Photoluminescence of Perovskites Conformally Coated on Textured Silicon
Full text linkTwo-terminal fully textured perovskite silicon tandem solar cells have recently advanced significantly and are quickly moving toward scalable production. While µm-sized texturing of the silicon solar cell enables minimizing reflection losses, and tuning of the perovskite layer thickness allows optimizing the photo-generated current distribution between subcells, both approaches introduce challenges at the development stage. One of these challenges is the accurate optoelectronic assessment of perovskite films with photoluminescence (PL) spectroscopy. In this work, we study effects of optical self-absorption on the PL of perovskite films conformally coated on industry-compatible textured silicon with pyramid heights ranging from 6 µm. Our findings indicate that with increasing pyramid height, the PL peak energy shows a redshift of 20–30 meV. Similarly, increasing the perovskite thickness on a fixed texture pattern induces a redshift. Three-dimensional confocal laser scanning PL microscopy, combined with statistical ray optical simulations, reveals that photon reabsorption in the perovskite film plays an important role in the texture-dependent and thickness-dependent PL responses. This optical effect, besides previously reported changes in perovskite mechanical properties due to silicon texture, is crucial to consider for accurate assessment of PL, and for efficient optimization of perovskite silicon tandems with advanced optical designs
Interview with 2025 ACS Energy Lectureship Outstanding Mid-Career Award Winner Dr. Bruno Ehrler
Full text linkDr. Bruno Ehrler, the recipient of the 2025 ACS Energy Lectureship Outstanding Mid-Career Award, has established himself as a leading researcher in the field of solar energy materials. Dr. Ehrler has made significant contributions to the understanding of ion migration effects in perovskite solar cells, which is crucial in advancing the efficiency and stability of PSC devices. In this interview, Dr. Ehrler shares insights into his academic path, current research, and perspectives on the future of energy materials scienc
Transition graphs of interacting hysterons: structure, design, organization and statistics
Full text linkTransition graphs capture the memory and sequential response of multistable media, by specifying their evolution under external driving. Microscopically, collections of bistable elements, or hysterons, provide a powerful model for these materials, with recent work highlighting the crucial role of hysteron interactions. Here, we introduce a general framework that links transition graphs and the microscopic parameters of interacting hysterons. We first introduce a systematic framework, based on so-called scaffolds, which structures the space of transition graphs and provides tools to deal with their combinatorial explosion. We then connect the topology of transition graphs to partial orders of the microscopic parameters. This allows us to understand the statistical properties of transition graphs, as well as determine whether a given graph is realizable, i.e. compatible with the hysteron framework. Our approach paves the way for a deeper theoretical understanding of memory effects in complex media and opens a route to rationally design pathways and memory effects in materials
Intestinal tuft cell subtypes represent successive stages of maturation driven by crypt-villus signaling gradients
Full text linkIntestinal tuft cells are epithelial sentinels that trigger host defense upon detection of parasite-derived compounds. While they represent potent targets for immunomodulatory therapies in inflammation-driven intestinal diseases, their functioning and differentiation are poorly understood. Here, we reveal common intermediary transcriptomes among the previously described tuft-1 and tuft-2 subtypes in mouse and human. Tuft cell subtype-specific reporter knock-ins in organoids show that the two subtypes reflect successive post-mitotic maturation stages within the tuft cell lineage. In vitro stimulation with interleukin-4 and 13 is sufficient to fuel the generation of new Nrep+ tuft-1 cells, arising from tuft precursors (tuft-p). Subsequently, changes in crypt-villus signaling gradients, such as BMP, and cholinergic signaling, are required to advance maturation towards Chat+ tuft-2 phenotypes. Functionally, we find chemosensory capacity to increase during maturation. Our tuft subtype-specific reporters and optimized differentiation strategy in organoids provide a platform to study immune-related tuft cell subtypes and their unique chemosensory properties
Contact Transfer Epitaxy of Halide Perovskites
Full text linkHalide perovskite materials are very exciting because of their excellent optoelectronic properties and simple deposition with both solution and vapor-phase methods. Until now, solution deposition has received more attention, but there are growing indications that residual solvent may limit performance and in particular long-term stability. Evaporation is a promising alternative, but is more complicated to implement; it requires vacuum, multiple sources at different temperatures, is difficult to switch between different materials due to cross-contamination and typically leads to films with very small grain size. Here a novel contact transfer method is presented for fabricating single crystalline perovskites that maintains the simplicity and flexibility of solution deposition while avoiding the use of solvent. This contact transfer epitaxy method uses an acceptor substrate consisting of self-assembled perovskite nanocubes to control crystal orientation and a donor substrate of the desired perovskite film to determine the ultimate composition. By heating the two substrates under close contact in atmospheric conditions, the perovskite film is transferred from the donor to the acceptor substrate, showing cubic phase (100) orientation even with hexagonal donor films. It is shown that contact transfer epitaxy is compatible with a variety of compositions and does not require specialized evaporators or vacuum conditions
Excitation Intervals Enhance Performance in Perovskite Solar Cells
Full text linkHalide perovskites face intrinsic stability challenges primarily due to light- and bias-induced ion migration. To mitigate ion-mediated degradation on operationally relevant time scales, this work investigates how introducing brief periodic intervals of light and darkness (LD cycling) can stabilize the average efficiency of perovskite films and devices. Systematic photoluminescence (PL) studies reveal that dark intervals on the order of seconds significantly suppress nonradiative recombination and slow degradation. The extent of PL enhancement depends on the duration of the dark time, the material composition, and critically, the sample’s age. Remarkably, LD cycling increases PL by more than 7-fold even in aged samples that would otherwise undergo photodarkening under continuous illumination. Moreover, the PL kinetics under LD cycling mirror the corresponding open-circuit voltage dynamics in full solar cells, showing that local emission changes provide a direct measure of device-level behavior. Device measurements similarly show that LD cycling enhances the power conversion efficiency compared to continuous illumination and mitigates deterioration over extended operation. This strategy highlights a potential pathway to dynamically preserve or even improve perovskite performance in future optoelectronic applications
Photonics in Flatland: challenges and opportunities for nanophotonics with 2D semiconductors
Full text linkTwo-dimensional (2D) semiconductors are emerging as a versatile platform for nanophotonics, offering unprecedented tunability in optical properties through exciton resonance engineering, van der Waals heterostructuring, and external field control. These materials enable active optical modulation, single-photon emission, quantum photonics, and valleytronic functionalities, paving the way for next-generation optoelectronic and quantum photonic devices. However, key challenges remain in achieving large-area integration, maintaining excitonic coherence, and optimizing amplitude-phase modulation for efficient light manipulation. Advances in fabrication, strain engineering, and computational modeling will be crucial to overcoming these limitations. This Perspective highlights recent progress in 2D semiconductor-based nanophotonics, emphasizing opportunities for scalable integration into photonics