Metamaterial Enhancement of Metal-Halide Perovskite Luminescence

The Nanoplasmonic Purcell Effect in Ultrafast and High-Light-Yield Perovskite Scintillators

The development of X-ray scintillators with ultrahigh light yields and ultrafast response times is a long sought-after goal. In this work, a fundamental mechanism that pushes the frontiers of ultrafast X-ray scintillator performance is theoretically predicted and experimentally demonstrated: the use of nanoscale-confined surface plasmon polariton modes to tailor the scintillator response time via the Purcell effect. By incorporating nanoplasmonic materials in scintillator devices, this work predicts over tenfold enhancement in decay rate and 38% reduction in time resolution even with only a simple planar design. The nanoplasmonic Purcell effect is experimentally demonstrated using perovskite scintillators, enhancing the light yield by over 120% to 88 ± 11 ph/keV, and the decay rate by over 60% to 2.0 ± 0.2 ns for the average decay time, and 0.7 ± 0.1 ns for the ultrafast decay component, in good agreement with the predictions of our theoretical framework. Proof-of-concept X-ray imaging experiments are performed using nanoplasmonic scintillators, demonstrating 182% enhancement in the modulation transfer function at four line pairs per millimeter spatial frequency. This work highlights the enormous potential of nanoplasmonics in optimizing ultrafast scintillator devices for applications including time-of-flight X-ray imaging and photon-counting computed tomography.

Bio-inspired optical structures for enhancing luminescence

Luminescence is an essential signal for many plants, insects, and marine organisms to attract the opposite sex, avoid predators, and so on. Most luminescent living organisms have ingenious optical structures which can help them get high luminescent performances. These remarkable and efficient structures have been formed by natural selection from long-time evolution. Researchers keenly observed the enhanced luminescence phenomena and studied how these phenomena happen in order to learn the characteristics of bio-photonics. In this review, we summarize the optical structures for enhancing luminescence and their applications. The structures are classified according to their different functions. We focus on how researchers use these biological inspirations to enhance different luminescence processes, such as chemiluminescence (CL), photoluminescence (PL), and electroluminescence (EL). It lays a foundation for further research on the applications of luminescence enhancement. Furthermore, we give examples of luminescence enhancement by bio-inspired structures in information encryption, biochemical detection, and light sources. These examples show that it is possible to use bio-inspired optical structures to solve complex problems in optical applications. Our work will provide guidance for research on biomimetic optics, micro- and nano-optical structures, and enhanced luminescence.

Interplay of Purcell effect and extraction efficiency in CsPbBr3 quantum dots coupled to Mie resonators

Inorganic halide perovskite quantum dots have risen in recent years as efficient active materials in numerous optoelectronic applications ranging from solar cells to light-emitting diodes and lasers, and have lately been tested as quantum emitters. Perovskite quantum dots are often coupled to photonic structures either to enhance their emission properties, by accelerating their emission rate thanks to the Purcell effect, or to increase light extraction. From a theoretical point of view, the first effect is often considered at the single-dipole level while the latter is often treated at the mesoscopic level, except possibly for quantum emitters. In this work we employ a layer of perovskite quantum dots coupled to dielectric Mie resonators to exploit both effects simultaneously and achieve an 18-fold increase in luminescence. Our numerical simulations, combined with spatially- and time-resolved photoluminescence measurements, reveal how the macroscopic response of the perovskite-on-Mie resonator structure results from the interplay of the two effects averaged over the whole spatial distribution of emitters. Our work provides thus guiding principles for maximizing the output intensity of quantum emitters embedded into photonic resonators as well as classical emitters integrated in perovskite-based optoelectronic devices.

Polarization-Tunable Perovskite Light-Emitting Metatransistor

Emerging immersive visual communication technologies require light sources with complex functionality for dynamic control of polarization, directivity, wavefront, spectrum, and intensity of light. Currently, this is mostly achieved by free space bulk optic elements, limiting the adoption of these technologies. Flat optics based on artificially structured metasurfaces that operate at the sub-wavelength scale are a viable solution, however, their integration into electrically driven devices remains challenging. Here, a radically new approach to monolithic integration of a dielectric metasurface into a perovskite light-emitting transistor is demonstrated. It is shown that nanogratings directly structured on top of the transistor channel yield an 8-fold increase of electroluminescence intensity and dynamic tunability of polarization. This new light-emitting metatransistor device concept opens unlimited opportunities for light management strategies based on metasurface design and integration.

Phase-Change Perovskite Microlaser with Tunable Polarization Vortex

Metasurfaces supporting optical bound states in the continuum (BICs) are emerging as simple and compact optical cavities to realize polarization-vortex lasers. The winding of the polarization around the singularity defines topological charges which are generally set by the cavity design and cannot be altered without changing geometrical parameters. Here, a subwavelength-thin phase-change halide perovskite BIC metasurface functioning as a tunable polarization vortex microlaser is demonstrated. Upon the perovskite structural phase transitions, both its refractive index and gain vary substantially, inducing reversible and bistable switching between distinct polarization vortexes underpinned by opposite topological charges. Dynamic tuning and switching of the resulting vector beams may find use in microscopy imaging, particle trapping and manipulation, and optical data storage.

Menezes L, Tittl A, Ren H, Maier SA. Optical Metasurfaces for Energy Conversion

Nanostructured surfaces with designed optical functionalities, such as metasurfaces, allow efficient harvesting of light at the nanoscale, enhancing light-matter interactions for a wide variety of material combinations. Exploiting light-driven matter excitations in these artificial materials opens up a new dimension in the conversion and management of energy at the nanoscale. In this review, we outline the impact, opportunities, applications, and challenges of optical metasurfaces in converting the energy of incoming photons into frequency-shifted photons, phonons, and energetic charge carriers. A myriad of opportunities await for the utilization of the converted energy. Here we cover the most pertinent aspects from a fundamental nanoscopic viewpoint all the way to applications.

Ultrafast resonant exciton-plasmon coupling for enhanced emission in lead halide perovskite with metallic Ag nanostructures

Integrating metal halide perovskites onto plasmonic nanostructures has recently become a trending method of enabling superior emissive performance of perovskite nanophotonic devices. In this work, we present an in-depth study on the spontaneous emission properties of hybrid systems comprising CsPbBr₃ nanocrystals and silver nanostructures. Specifically, a 5.7-fold increment of the photoluminescence (PL) intensity and a 1.65-fold enhancement of the PL relaxation rate is attained when the transition energy of CsPbBr₃ is spectrally resonant with the oscillational frequency of Ag nanodisks (NDs), which is attributed to the intense exciton-plasmon coupling-induced Purcell effect. Furthermore, a 540-fs ultrafast energy transfer from the CsPbBr₃ excitons to Ag plasmons is revealed by femtosecond pump-probe experiments, suggesting the key mechanism responsible for the Purcell-enhanced radiative emission. Our finding offers a unique understanding of the enhanced emissive behavior in the plasmon-coupled perovskite system and paves the way for further applications.

Perovskite metasurfaces with large superstructural chirality

Recent attempts to synthesize hybrid perovskites with large chirality have been hampered by large size mismatch and weak interaction between their structure and the wavelength of light. Here we adopt a planar nanostructure design to overcome these limitations and realize all-dielectric perovskite metasurfaces with giant superstructural chirality. We identify a direct spectral correspondence between the near- and the far- field chirality, and tune the electric and magnetic multipole moments of the resonant chiral metamolecules to obtain large anisotropy factor of 0.49 and circular dichroism of 6350 mdeg. Simulations show that larger area metasurfaces could yield even higher optical activity, approaching the theoretical limits. Our results clearly demonstrate the advantages of nanostructrure engineering for the implementation of perovskite chiral photonic, optoelectronic, and spintronic devices.

Though chiral hybrid organic-inorganic perovskites are attractive for next-generation optoelectronics, imparting strong chirality through chemical synthesis has proved challenging. Here, the authors report all-dielectric perovskite metasurfaces with giant superstructural chirality via planar nanostructuring.

Optical Rashba Effect in a Light-Emitting Perovskite Metasurface

The Rashba effect, i.e., the splitting of electronic spin-polarized bands in the momentum space of a crystal with broken inversion symmetry, has enabled the realization of spin-orbitronic devices, in which spins are manipulated by spin-orbit coupling. In optics, where the helicity of light polarization represents the spin degree of freedom for spin-momentum coupling, the optical Rashba effect is manifested by the splitting of optical states with opposite chirality in the momentum space. Previous realizations of the optical Rashba effect relied on passive devices determining the surface plasmon or light propagation inside nanostructures, or the directional emission of chiral luminescence when hybridized with light-emitting media. An active device underpinned by the optical Rashba effect is demonstrated here, in which a monolithic halide perovskite metasurface emits highly directional chiral photoluminescence. An all-dielectric metasurface design with broken in-plane inversion symmetry is directly embossed into the high-refractive-index, light-emitting perovskite film, yielding a degree of circular polarization of photoluminescence of 60% at room temperature.

Plasmonic-perovskite solar cells, light emitters, and sensors

The field of plasmonics explores the interaction between light and metallic micro/nanostructures and films. The collective oscillation of free electrons on metallic surfaces enables subwavelength optical confinement and enhanced light-matter interactions. In optoelectronics, perovskite materials are particularly attractive due to their excellent absorption, emission, and carrier transport properties, which lead to the improved performance of solar cells, light-emitting diodes (LEDs), lasers, photodetectors, and sensors. When perovskite materials are coupled with plasmonic structures, the device performance significantly improves owing to strong near-field and far-field optical enhancements, as well as the plasmoelectric effect. Here, we review recent theoretical and experimental works on plasmonic perovskite solar cells, light emitters, and sensors. The underlying physical mechanisms, design routes, device performances, and optimization strategies are summarized. This review also lays out challenges and future directions for the plasmonic perovskite research field toward next-generation optoelectronic technologies.

Enhanced Multiphoton Processes in Perovskite Metasurfaces

Multiphoton absorption and luminescence are fundamentally important nonlinear processes for utilizing efficient light-matter interaction. Resonant enhancement of nonlinear processes has been demonstrated for many nanostructures; however, it is believed that all higher-order processes are always much weaker than their corresponding linear processes. Here, we study multiphoton luminescence from structured surfaces and, combining multiple advantages of perovskites with the concept of metasurfaces, we demonstrate that the efficiency of nonlinear multiphoton processes can become comparable to the efficiency of the linear process. We reveal that the perovskite metasurface can enhance substantially two-photon stimulated emission with the threshold being comparable with that of the one-photon process. Our modeling of free-carrier dynamics and exciton recombination upon nonlinear photoexcitation uncovers that this effect can be attributed to the local field enhancement in structured media, a substantial increase of the mode overlap, and the selection rules of two-photon absorption in perovskites.

Tuning Hybrid exciton-Photon Fano Resonances in Two-Dimensional Organic-Inorganic Perovskite Thin Films

As easy-to-grow quantum wells with narrow excitonic features at room temperature, two-dimensional (2D) Ruddleson-Popper perovskites are promising for realizing novel nanophotonic devices based on exciton-photon interactions. Here, we demonstrate a distinct hybrid exciton-photon Fano resonance in (C₄H₉NH₃)₂PbI₄ thin films prepared via spin coating. Using a classical coupled-oscillator model and finite-difference time-domain simulations, we link the Fano interference to the coupling of the exciton with the Rayleigh-like scattering of the film microstructure. Combining colloidal plasmonic cavities with the 2D perovskite films, we demonstrate tuning of the Fano resonance. In combination with silver nanoparticles, the exciton-photon Fano interference couples to the in-plane plasmonic modes with indications of Rabi splitting. By creating a nanoparticle on mirror geometry, we address the out-of-plane excitonic component, reaching an intermediate coupling regime. These structures suggest possible photonic targets for biomolecular self-assembly applications.