Heavy Atom Effect in Halogenated mCP and Its Influence on the Efficiency of the Thermally Activated Delayed Fluorescence of Dopant Molecules.

In this study, we explore the impact of halogen functionalization on the photophysical properties of the commonly used organic light-emitting diode (OLED) host material, 1,3-bis(N-carbazolyl)benzene (mCP). Derivatives with different numbers and types of halogen substituents on mCP were synthesized. By measuring steady-state and transient photoluminescence at 6 K, we study the impact of the type, number, and position of the halogens on the intersystem crossing and phosphorescence rates of the compounds. In particular, the functionalization of mCP with 5 bromine atoms results in a significant increase of the intersystem crossing rate by a factor of 300 to a value of (1.5 ± 0.1) × 1010 s-1, and the phosphorescence rate increases by 2 orders of magnitude. We find that the singlet radiative decay rate is not significantly modified in any of the studied compounds. In the second part of the paper, we describe the influence of these compounds on the reverse intersystem crossing of the 7,10-bis(4-(diphenylamino)phenyl)-2,3-dicyanopyrazino-phenanthrene (TPA-DCPP), a TADF guest, via the external heavy atom effect. Their use results in an increase of the reverse intersystem crossing (RISC) rate from (8.1 ± 0.8) × 103 s-1 for mCP to (2.7 ± 0.1) × 104 s-1 for mCP with 5 bromine atoms. The effect is even more pronounced for the mCP analogue containing a single iodine atom, which gives a RISC rate of (3.3 ± 0.1) × 104 s-1. Time-dependent DFT calculations reveal the importance of the use of long-range corrected functionals to predict the effect of halogenation on the optical properties of the mCP, and the relativistic approximation (ZORA) is used to provide insight into the strength of the spin-orbit coupling matrix element between the lowest-lying excited singlet and triplet states in the different mCP compounds.

Photonic enhancement in photoluminescent metal halide perovskite-photonic crystal bead hybrids.

We report two photonic crystal-perovskite nanocrystal microbead hybrids with photoluminescence matching that of the parent nanocrystals but with increased photoluminescence quantum yields. Time-resolved photoluminescence spectroscopy quantifies the radiative enhancement afforded by the photonic environment of the microbeads. The reported hybrids also demonstrate markedly better resistance to degradation in water over 30 days of immersion.

Room-temperature polariton quantum fluids in halide perovskites.

Quantum fluids exhibit quantum mechanical effects at the macroscopic level, which contrast strongly with classical fluids. Gain-dissipative solid-state exciton-polaritons systems are promising emulation platforms for complex quantum fluid studies at elevated temperatures. Recently, halide perovskite polariton systems have emerged as materials with distinctive advantages over other room-temperature systems for future studies of topological physics, non-Abelian gauge fields, and spin-orbit interactions. However, the demonstration of nonlinear quantum hydrodynamics, such as superfluidity and Cerenkov flow, which is a consequence of the renormalized elementary excitation spectrum, remains elusive in halide perovskites. Here, using homogenous halide perovskites single crystals, we report, in both one- and two-dimensional cases, the complete set of quantum fluid phase transitions from normal classical fluids to scatterless polariton superfluids and supersonic fluids-all at room temperature, clear consequences of the Landau criterion. Specifically, the supersonic Cerenkov wave pattern was observed at room temperature. The experimental results are also in quantitative agreement with theoretical predictions from the dissipative Gross-Pitaevskii equation. Our results set the stage for exploring the rich non-equilibrium quantum fluid many-body physics at room temperature and also pave the way for important polaritonic device applications.

Halide perovskites enable polaritonic XY spin Hamiltonian at room temperature.

Exciton polaritons, the part-light and part-matter quasiparticles in semiconductor optical cavities, are promising for exploring Bose-Einstein condensation, non-equilibrium many-body physics and analogue simulation at elevated temperatures. However, a room-temperature polaritonic platform on par with the GaAs quantum wells grown by molecular beam epitaxy at low temperatures remains elusive. The operation of such a platform calls for long-lifetime, strongly interacting excitons in a stringent material system with large yet nanoscale-thin geometry and homogeneous properties. Here, we address this challenge by adopting a method based on the solution synthesis of excitonic halide perovskites grown under nanoconfinement. Such nanoconfinement growth facilitates the synthesis of smooth and homogeneous single-crystalline large crystals enabling the demonstration of XY Hamiltonian lattices with sizes up to 10 × 10. With this demonstration, we further establish perovskites as a promising platform for room temperature polaritonic physics and pave the way for the realization of robust mode-disorder-free polaritonic devices at room temperature.

Enhanced nonlinear interaction of polaritons via excitonic Rydberg states in monolayer WSe2.

Strong optical nonlinearities play a central role in realizing quantum photonic technologies. Exciton-polaritons, which result from the hybridization of material excitations and cavity photons, are an attractive candidate to realize such nonlinearities. While the interaction between ground state excitons generates a notable optical nonlinearity, the strength of such interactions is generally not sufficient to reach the regime of quantum nonlinear optics. Excited states, however, feature enhanced interactions and therefore hold promise for accessing the quantum domain of single-photon nonlinearities. Here we demonstrate the formation of exciton-polaritons using excited excitonic states in monolayer tungsten diselenide (WSe2) embedded in a microcavity. The realized excited-state polaritons exhibit an enhanced nonlinear response ∼[Formula: see text] which is ∼4.6 times that for the ground-state exciton. The demonstration of enhanced nonlinear response from excited exciton-polaritons presents the potential of generating strong exciton-polariton interactions, a necessary building block for solid-state quantum photonic technologies.

Langmuir-Blodgett fabrication of large-area black phosphorus-C60 thin films and heterojunction photodetectors.

Black phosphorus (BP) has emerged as a promising two-dimensional (2D) semiconductor for applications in electronics, optoelectronics, and energy storage. As is the case for many 2D materials, the fabrication of large-area BP thin films remains a considerable challenge. Here, we report the assembly of BP nanosheets into compact thin films using the Langmuir-Blodgett (LB) technique. The overlapping stacking between BP nanosheets is suppressed when the nanosheets are surrounded by fullerene C60 molecules due to physisorption. This allows for the fabrication of large-area BP films (20 mm × 18 mm) with a homogenous nanosheet distribution and negligible oxidation. The fabricated films show measurable absorption up to 2.3 µm. We use these films as active layers to demonstrate mm-sized BP heterojunction photodetectors with mA W-1 scale responsivities from the visible to the near-infrared. Photodetector internal quantum efficiencies at 660 nm and 808 nm are 5% and 1%, respectively.

Mid-infrared Polarized Emission from Black Phosphorus Light-Emitting Diodes.

We demonstrate a mid-infrared light-emitting diode based on the 2D semiconductor black phosphorus (BP). The device is composed of a mechanically exfoliated BP/molybdenum disulfide heterojunction. Under forward bias, it emits polarized electroluminescence at λ = 3.68 µm, with room-temperature internal and external quantum efficiencies of ∼1% and ∼0.03%, respectively. In our structure, outcoupling losses are dominated by radiation toward the high refractive index substrate. The ability to tune the bandgap of BP and consequently its emission wavelength with layer number, strain, and electric field make these LEDs particularly attractive for heterointegration into mid-infrared photonic platforms.

Directional Light Emission from Layered Metal Halide Perovskite Crystals.

Metal halide perovskites are being increasingly explored for use in light-emitting diodes (LEDs), with achievements in efficiency and brightness charted across the spectrum. One path to further boosting the fraction of useful photons generated by injected electrical charges will be to tailor the emission patterns of devices. Here we investigate directional emission from layered metal halide perovskites. We quantify the proportion of in-plane versus out-of-plane transition dipole components for a suite of layered perovskites. We find that certain perovskite single crystals have highly anisotropic emissions and up to 90% of their transition dipole in-plane. For thin films, emission anisotropy increases as the nominal layer thickness decreases and is generally greater with butylammonium cations than with phenethylammonium cations. Numerical simulations reveal that anisotropic emission from layered perovskites in thin-film LEDs may lead to external quantum efficiencies of 45%, an absolute gain of 13% over equivalent films with isotropic emitters.

Bose-Einstein Condensation of Exciton-Polaritons in Organic Microcavities.

Bose-Einstein condensation describes the macroscopic occupation of a single-particle mode: the condensate. This state can in principle be realized for any particles obeying Bose-Einstein statistics; this includes hybrid light-matter excitations known as polaritons. Some of the unique optoelectronic properties of organic molecules make them especially well suited for the realization of polariton condensates. Exciton-polaritons form in optical cavities when electronic excitations couple collectively to the optical mode supported by the cavity. These polaritons obey bosonic statistics at moderate densities, are stable at room temperature, and have been observed to form a condensed or lasing state. Understanding the optimal conditions for polariton condensation requires careful modeling of the complex photophysics of organic molecules. In this article, we introduce the basic physics of exciton-polaritons and condensation and review experiments demonstrating polariton condensation in molecular materials.

Inverting singlet and triplet excited states using strong light-matter coupling.

In organic microcavities, hybrid light-matter states can form with energies that differ from the bare molecular excitation energies by nearly 1 eV. A timely question, given the recent advances in the development of thermally activated delayed fluorescence materials, is whether strong light-matter coupling can be used to invert the ordering of singlet and triplet states and, in addition, enhance reverse intersystem crossing (RISC) rates. Here, we demonstrate a complete inversion of the singlet lower polariton and triplet excited states. We also unambiguously measure the RISC rate in strongly coupled organic microcavities and find that, regardless of the large energy level shifts, it is unchanged compared to films of the bare molecules. This observation is a consequence of slow RISC to the lower polariton due to the delocalized nature of the state across many molecules and an inability to compete with RISC to the dark exciton reservoir.

Interacting polariton fluids in a monolayer of tungsten disulfide.

Atomically thin transition metal dichalcogenides (TMDs) possess a number of properties that make them attractive for realizing room-temperature polariton devices1. An ideal platform for manipulating polariton fluids within monolayer TMDs is that of Bloch surface waves, which confine the electric field to a small volume near the surface of a dielectric mirror2-4. Here we demonstrate that monolayer tungsten disulfide can sustain Bloch surface wave polaritons (BSWPs) with a Rabi splitting of 43 meV and propagation lengths reaching 33 µm. In addition, we show strong polariton-polariton nonlinearities within BSWPs, which manifest themselves as a reversible blueshift of the lower polariton resonance. Such nonlinearities are at the heart of polariton devices5-11 and have not yet been demonstrated in TMD polaritons. As a proof of concept, we use the nonlinearity to implement a nonlinear polariton source. Our results demonstrate that BSWPs using TMDs can support long-range propagation combined with strong nonlinearities, enabling potential applications in integrated optical processing and polaritonic circuits.

Polariton-Assisted Singlet Fission in Acene Aggregates.

Singlet fission is an important candidate to increase energy conversion efficiency in organic photovoltaics by providing a pathway to increase the quantum yield of excitons per photon absorbed in select materials. We investigate the dependence of exciton quantum yield for acenes in the strong light-matter interaction (polariton) regime, where the materials are embedded in optical microcavities. Starting from an open-quantum-systems approach, we build a kinetic model for time-evolution of species of interest in the presence of singlet quenchers and show that polaritons can decrease or increase exciton quantum yields compared to the cavity-free case. In particular, we find that hexacene, under the conditions of our model, can feature a higher yield than cavity-free pentacene when assisted by polaritonic effects. Similarly, we show that pentacene yield can be increased when assisted by polariton states. Finally, we address how various relaxation processes between bright and dark states in lossy microcavities affect polariton photochemistry. Our results also provide insights on how to choose microcavities to enhance similarly related chemical processes.

Alkali Metal Halide Salts as Interface Additives to Fabricate Hysteresis-Free Hybrid Perovskite-Based Photovoltaic Devices.

A new method was developed for doping and fabricating hysteresis-free hybrid perovskite-based photovoltaic devices by using alkali metal halide salts as interface layer additives. Such salt layers introduced at the perovskite interface can provide excessive halide ions to fill vacancies formed during the deposition and annealing process. A range of solution-processed halide salts were investigated. The highest performance of methylammonium lead mixed-halide perovskite device was achieved with a NaI interlayer and showed a power conversion efficiency of 12.6% and a hysteresis of less than 2%. This represents a 90% improvement compared to control devices without this salt layer. Through depth-resolved mass spectrometry, optical modeling, and photoluminescence spectroscopy, this enhancement is attributed to the reduction of iodide vacancies, passivation of grain boundaries, and improved hole extraction. Our approach ultimately provides an alternative and facile route to high-performance and hysteresis-free perovskite solar cells.

The road towards polaritonic devices.

Polaritons are quasiparticles that form in semiconductors when an elementary excitation such as an exciton or a phonon interacts sufficiently strongly with light. In particular, exciton-polaritons have attracted tremendous attention for their unique properties, spanning from an ability to undergo ultra-efficient four-wave mixing to superfluidity in the condensed state. These quasiparticles possess strong intrinsic nonlinearities, while keeping most characteristics of the underlying photons. Here we review the most important features of exciton-polaritons in microcavities, with a particular emphasis on the emerging technological applications, the use of new materials for room-temperature operation, and the possibility of exploiting polaritons for quantum computation and simulation.

Confined surface plasmon-polariton amplifiers.

We demonstrate the realization of confined surface plasmon polariton amplifiers using a thin layer of the organic gain medium 4-dicyanomethylene-2-methyl-6-(p-dimethylaminostyryl)-4H-pyran dispersed in a tris(8-hydroxy-quinolinato)aluminum matrix. Complete loss compensation, which occurs at a pump fluence of approximately 200 µJ/cm(2), is directly observed in the time domain and studied for a range of waveguide lengths. The power dependence is also reported, and a significant net gain of 93 dB/mm is observed at the highest fluence.

Nanoparticle-assisted stimulated-emission-depletion nanoscopy.

We show that metal nanoparticles can be used to improve the performance of super-resolution fluorescence nanoscopes based on stimulated-emission-depletion (STED). Compared with a standard STED nanoscope, we show theoretically a resolution improvement by more than an order of magnitude, or equivalently, depletion intensity reductions by more than 2 orders of magnitude and an even stronger photostabilization. Our scheme may allow improvement of existing STED nanoscopes and assist in the development of low-power, low-cost nanoscopes. This has the potential to increase the availability of STED nanoscopes and lead to a significant expansion of our understanding of biological and biochemical phenomena occurring on the nanoscale.

Quantum statistics of surface plasmon polaritons in metallic stripe waveguides.

Heralded single surface plasmon polaritons are excited using photons generated via spontaneous parametric down conversion. The mean excitation rates, intensity correlations, and Fock state populations are studied. The observed dependence of the second-order coherence in our experiment is consistent with a linear uncorrelated Markovian environment in the quantum regime. Our results provide important information about the effect of loss for assessing the potential of plasmonic waveguides for future nanophotonic circuitry in the quantum regime.

Plasmonic sinks for the selective removal of long-lived states.

The use of plasmonic nanostructures for the removal of unwanted long-lived states is investigated. We show that the total decay rate of such a state can be increased by up to 4 orders of magnitude, as compared to its intrinsic radiative decay rate, while leaving other neighboring optical transitions unaffected. For the specific case of molecular triplet excited states, we show that the use of a "plasmonic sink" has the potential to reduce photobleaching and ground-state depletion by at least 2 orders of magnitude. We consider, in addition, the impact of such structures on the performance of organic semiconductor lasers and show that, under realistic device conditions, plasmonic sinks have the capacity to increase the achievable laser repetition rate by a factor equal to the triplet decay rate enhancement. We conclude by studying the effect of exciton diffusion on the triplet density in the presence of metallic nanoparticles.