Enhanced spontaneous emission of CsPbI3 perovskite nanocrystals using a hyperbolic metamaterial modified by dielectric nanoantenna

In this work, we demonstrate, theoretically and experimentally, a hybrid dielectric-plasmonic multifunctional structure able to provide full control of the emission properties of CsPbI3 perovskite nanocrystals (PNCs). The device consists of a hyperbolic metamaterial (HMM) composed of alternating thin metal (Ag) and dielectric (LiF) layers, covered by TiO2 spherical MIE nanoresonators (i.e., the nanoantenna). An optimum HMM leads to a certain Purcell effect, i.e., an increase in the exciton radiative rate, but the emission intensity is reduced due to the presence of metal in the HMM. The incorporation of TiO2 nanoresonators deposited on the top of the HMM is able to counteract such an undesirable intensity reduction by the coupling between the exciton and the MIE modes of the dielectric nanoantenna. More importantly, MIE nanoresonators result in a preferential light emission towards the normal direction to the HMM plane, increasing the collected signal by more than one order of magnitude together with a further increase in the Purcell factor. These results will be useful in quantum information applications involving single emitters based on PNCs together with a high exciton emission rate and intensity

Superradiance Emission and Its Thermal Decoherence in Lead Halide Perovskites Superlattices

Self-assembled nanocrystals (NCs) into superlattices (SLs) are alternative materials to polycrystalline films and single crystals, which can behave very differently from their constituents, especially when they interact coherently with each other. This work concentrates on the Superradiance (SR) emission observed in SLs formed by CsPbBr3 and CsPbBrI2 NCs. Micro-Photoluminescence spectra and transients in the temperature range 4–100 K are measured in SLs to extract information about the SR states and uncoupled domains of NCs. For CsPbBr3 SLs with mostly homogeneous SR lines (linewidth 1–5 meV), this work measures lifetimes as short as 160 ps, 10 times lower than the value measured in a thin film made with the same NCs, which is due to domains of near identical NCs formed by 1000 to 40 000 NCs coupled by dipole–dipole interaction. The thermal decoherence of the SR exciton state is evident above 25 K due to its coupling with an effective phonon energy of ≈8 meV. These findings are an important step toward understanding the SR emission enhancement factor and the thermal dephasing process in perovskite SLs.Financial support from the Spanish Ministry of Science (MICINN) through project no. PID2020- 120484RB-I00 is gratefully acknowledged. G.M.M. also thanks the support from the Spanish MICINN & AEI (project RTI2018-099015-J-I00). I.M.S. thanks the funding of MCIN/AEI/10.13039/501100011033 with the project STABLE PID2019-107314RB-I00. S.G. acknowledges her “Grisolia” grant from Generalitat Valenciana, and G.M.M. thanks the Ramon y Cajal programme (contract RYC2020-030099-I). Thanks are also due to Dr. Raúl Iván Sánchez Alarcón for his help with X-ray diffraction characterization of NC films and SLs

Homogeneous and inhomogeneous broadening in single perovskite nanocrystals investigated by micro-photoluminescence

Metal halides with perovskite crystalline structure have given rise to efficient optoelectronic and photonic devices. In the present work, we have studied the light emission properties of single CsPbBr3 and CsPbI3 semiconductor perovskite nanocrystals (PNCs), as the basis for a statistical analysis of micro-photoluminescence (micro-PL) spectra measured on tens of them. At room temperature, the linewidth extracted from PL spectra acquired in dense films of these nanocrystals is not very different from that of micro-PL measured in single nanocrystals. This means that the homogeneous linewidth due to exciton-phonon interaction is comparable or larger than the inhomogeneous effect associated to the micro-PL peak energy dispersion due to the nanocrystal size distribution defined by the chemical synthesis of the PNCs. Contrarily, we observe very narrow micro-PL lines in CsPbBr3 and CsPbI3 PNCs at 4 K, in the range of 1–5 meV and 0.1–0.5 meV, respectively, because they are limited by spectral diffusion. Aging of PNCs under ambient conditions has been also studied by micro-PL and a clear reduction of their nanocube edge size in the order of the nm/day is deduced

Revealing giant exciton fine-structure splitting in 2D perovskites using van der Waals passivation

The study of two-dimensional (2D) van der Waals materials has been an active field of research in the development of new optoelectronics and photonic applications over the last decade. Organic-inorganic layered perovskites are currently some of the most promising 2D van der Waals materials, due to their exceptional optical brightness and enhanced excitonic effects. However, low crystal quality and spectral diffusion usually broaden the exciton linewidth, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach for reducing the effect of spectral diffusion by means of hBN-capping on layered perovskites with different thicknesses, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral linewidth with the population of active charge fluctuation centres present in the organic spacer taking part in the dynamical Stark shift. Active fluctuation centres are reduced by a factor of 3.7 to 7.1 when we include hBN-capping according to our direct spectral measurements. This rate is in good agreement with the analysis of the overlap between the squared perovskite lattice and the hexagonal hBN lattice. Van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence, the amplitude of the dynamical Stark shift characteristic of the spectral diffusion effect is reduced. Our work provides an easy and low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission

Purcell Enhancement and Wavelength Shift of Emitted Light by CsPbI3 Perovskite Nanocrystals Coupled to Hyperbolic Metamaterials

Manipulation of the exciton emission rate in nanocrystals of lead halide perovskites (LHPs) was demonstrated by means of coupling of excitons with a hyperbolic metamaterial (HMM) consisting of alternating thin metal (Ag) and dielectric (LiF) layers. Such a coupling is found to induce an increase of the exciton radiative recombination rate by more than a factor of three due to the Purcell effect when the distance between the quantum emitter and HMM is nominally as small as 10 nm, which coincides well with the results of our theoretical analysis. Besides, an effect of the coupling-induced long wavelength shift of the exciton emission spectrum is detected and modeled. These results can be of interest for quantum information applications of single emitters on the basis of perovskite nanocrystals with high photon emission rates

Optical emission at single level and cavity coupling of low-dimensional perovskite semiconductors

Materials with perovskite lattices have caught the attention of a large community of researchers worldwide. Despite the relative simplicity of its crystal structure, ABX3, the perovskite lattice. Currently, metal halide perovskites with the same crystal structure as that of ABX3, have become limelight in the field of optoelectronics and photonics. In this context, all inorganic CsPbX3 (with X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently emerged as an outstanding material with remarkable optical properties. Additionally, two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and optoelectronic applications in the nanoscale. The crystal lattice in 2D perovskites is composed of an inorganic octahedral layer sandwiched by long organic cations. Lead and other hazardous heavy metals, on the other hand, are common in perovskites. As a result, lead-free perovskites were developed as a low-cost, non-toxic, earth-abundant material for the next generation of optoelectronic applications. The goal of this Ph.D. thesis is to fully reveal the significance of perovskite materials as an active material for quantum photonics, from both a fundamental and an application standpoint. For the first research objective within that goal, all physical mechanisms responsible for spontaneous emission in PNCs must be investigated. Determining the decay time of these single NCs are all important steps toward this objective. Once the optimal conditions for PNCs are established, they can be used as quantum light sources, with the quality of the light being evaluated using a second order photon correlation function. The next landmark will be to investigate 2D perovskite, a type of 2D van der Waals nanomaterial, for its potential use in photonic and optoelectronic applications at the nanoscale. Optimizing the conditions for mechanical exfoliation of bulk crystals of two different phases of 2D perovskites with the lowest quantum well thickness of n=1,2 to achieve sufficiently thin 2D is an important step toward achieving this goal. The final objective is focused on the incorporation of perovskite materials in an open access fiber-based microcavity. The cavity specific geometry provides strong optical confinement of the modes. The open style of the fiber cavity allows for independent movement of the fiber mirror, allowing for in-situ tuning of the cavity resonance, and typically over a wider spectral range, which this tuning is not possible in monolithic microcavity. As a result, controlling the various parts of this cavity setup, as well as properly understanding the possible light-matter interactions and Light mode coupling based on numerical models, is a significant step toward achieving this objective. Considering the above-written results, this Ph.D. thesis suggests that perovskite materials are promising candidates for opening the way for a new generation of quantum light sources and their applications.Materials with perovskite lattices have caught the attention of a large community of researchers worldwide. Despite the relative simplicity of its crystal structure, ABX3, the perovskite lattice. Currently, metal halide perovskites with the same crystal structure as that of ABX3, have become limelight in the field of optoelectronics and photonics. In this context, all inorganic CsPbX3 (with X = Cl, Br, I) perovskite nanocrystals (PNCs) have recently emerged as an outstanding material with remarkable optical properties. Additionally, two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and optoelectronic applications in the nanoscale. The crystal lattice in 2D perovskites is composed of an inorganic octahedral layer sandwiched by long organic cations. Lead and other hazardous heavy metals, on the other hand, are common in perovskites. As a result, lead-free perovskites were developed as a low-cost, non-toxic, earth-abundant material for the next generation of optoelectronic applications. The goal of this Ph.D. thesis is to fully reveal the significance of perovskite materials as an active material for quantum photonics, from both a fundamental and an application standpoint. For the first research objective within that goal, all physical mechanisms responsible for spontaneous emission in PNCs must be investigated. Determining the decay time of these single NCs are all important steps toward this objective. Once the optimal conditions for PNCs are established, they can be used as quantum light sources, with the quality of the light being evaluated using a second order photon correlation function. The next landmark will be to investigate 2D perovskite, a type of 2D van der Waals nanomaterial, for its potential use in photonic and optoelectronic applications at the nanoscale. Optimizing the conditions for mechanical exfoliation of bulk crystals of two different phases of 2D perovskites with the lowest quantum well thickness of n=1,2 to achieve sufficiently thin 2D is an important step toward achieving this goal. The final objective is focused on the incorporation of perovskite materials in an open access fiber-based microcavity. The cavity specific geometry provides strong optical confinement of the modes. The open style of the fiber cavity allows for independent movement of the fiber mirror, allowing for in-situ tuning of the cavity resonance, and typically over a wider spectral range, which this tuning is not possible in monolithic microcavity. As a result, controlling the various parts of this cavity setup, as well as properly understanding the possible light-matter interactions and Light mode coupling based on numerical models, is a significant step toward achieving this objective. Considering the above-written results, this Ph.D. thesis suggests that perovskite materials are promising candidates for opening the way for a new generation of quantum light sources and their applications

Extrinsic Effects on the Optical Properties of Surface Colour Defects generated in Hexagonal Boron Nitride Nanosheets publised in ACS Appl Mater & Interf 13, 46105 2021

Preprint of the ACS Appl Mater & Interf 13, 46105 2021Hexagonal boron nitride (hBN) is a wide-bandgap van der Walls material able to host light emitting centres behaving as single photon sources. Here, we report the generation of colour defects in hBN nanosheets dispersed on different kind of substrates by thermal treatment processes. The optical properties of these defects have been studied by micro-spectroscopy techniques and far-field simulations of their light emission. By these techniques, we have found that subsequent ozone treatments of the deposited hBN nanosheets improve the optical emission properties of created defects, as revealed by their zero-phonon linewidth narrowing and reduction of background emission. Micro-localized colour defects deposited on dielectric substrates show bright (≈1 MHz) and stable room temperature light emission with zero-phonon line peak energy varying from 1.56 eV to 2.27 eV, being the most probable value 2.16 eV. In addition to this, we have observed a substrate dependence of the optical performance of the generated colour defects. The energy range of the emitters prepared on gold substrates is strongly reduced, as compared to that observed in dielectric substrates or even alumina. We attribute this effect to the quenching of low-energy colour defects (these of energies lower than 1.9 eV) when gold substrates are used, which reveals the surface nature of the defects created in hBN nanosheets. Results described here are important for future quantum light experiments and their integration in photonic chips

Origin of discrete donor-acceptor pair transitions in 2D Ruddlesden-Popper perovskites

Two-dimensional (2D) van der Waals nanomaterials have attracted considerable attention for potential use in photonic and light-matter applications at the nanoscale. Thanks to their excitonic properties, 2D perovskites are also promising active materials to be included in devices working at room temperature. In this work, we study the presence of very narrow and spatially localized optical transitions in 2D lead halide perovskites by μ-photoluminescence and time-decay measurements. These discrete optical transitions are characterized by sub-millielectronvolt linewidths (⁠ ⁠) and long decay times (5-8 ns). X-ray photoemission and density-functional theory calculations have been employed to investigate the chemical origin of electronic states responsible of these transitions. The association of phenethylammonium with methylammonium cations into 2D Ruddlesden-Popper perovskites, ⁠, particularly in phases with ⁠, has been identified as a mechanism of donor-acceptor pair (DAP) formation, corresponding to the displacement of lead atoms and their replacement by methylammonium. Ionized DAP recombination is identified as the most likely physical source of the observed discrete optical emission lines. The analysis of the experimental data with a simple model, which evaluates the Coulombic interaction between ionized acceptors and donors, returns a donor in Bohr radius of the order of 10 nm. The analysis of the spectral and electronic characteristics of these single donor-acceptor states in 2D perovskites is of particular importance both from the point of view of fundamental research, as well as to be able to link the emission of these states with new optoelectronic applications that require long-range optically controllable interactions

Revealing giant exciton fine-structure splitting in two-dimensional perovskites using van der Waals passivation

Organic–inorganic layered perovskites are currently some of the most promising 2D van der Waals materials. Low crystal quality usually broadens the exciton line width, obscuring the fine structure of the exciton in conventional photoluminescence experiments. Here, we propose a mechanical approach to reducing the effect of spectral diffusion by means of hBN capping on layered perovskites, revealing the exciton fine structure. We used a stochastic model to link the reduction of the spectral line width with the population of charge fluctuation centers present in the organic spacer. van der Waals forces between both lattices cause the partial clamping of the perovskite organic spacer molecules, and hence the amplitude of the overall spectral diffusion effect is reduced. Our work provides a low-cost solution to the problem of accessing important fine-structure excitonic state information, along with an explanation of the important carrier dynamics present in the organic spacer that affect the quality of the optical emission