We demonstrate broadband, tunable wavelength lasing emission using spherical CdS/CdSe/CdS quantum shells (QS) incorporated into distributed feedback (DFB) nanopillar Si cavities. Such QSs have recently attracted considerable attention as they exhibit strongly suppressed Auger recombination, ultralong biexciton (BX) lifetimes and broad gain bandwidth. Using only one QSs size (i.e., confinement), we demonstrate emission coupling and low threshold, narrowband lasing across wide spectral range, from single exciton (X~640 nm) to biexciton (BX~625 nm) to multiple exciton (MX~615-565 nm) transitions. The ensemble-averaged gain threshold of < N>~1.4 electron-hole pairs per QS particle and lowest pump fluence of ~ 4 uJ/cm2 result from almost completely impeded Auger recombination and low optical losses in the nanopillar cavity. These results represent a significant advance towards the development of future electrically pumped, colloidal nanocrystal lasers
Using recently developed laser-assisted synthesis technique (LAST), we demonstrate, for the first time, transient absorption (TA) exciton dynamics of W- and Mo-families of TMDs that have strong excitation fluence dependence and reach extremely long, several nanosecond lifetimes at the highest fluences. To put this in the context, all previous observations have shown tens of picosecond lifetimes at high powers (due to Auger recombination). Using a variety of experimental approaches (excitation wavelength and temperature dependent measurements) and comparing the results with TMD samples in a freestanding form, we trace our observations to the amount of strain produced in the LAST samples by the specifics of high temperature growth and substrate cooling. Using kinetic modeling, we relate these observations to the strain-induced modifications of electronic bands and associated population of intervalley dark excitons that can now interplay with intravalley (bright) excitations.
Cs-based perovskite nanocrystals (PNCs) possess alluring optoelectronic properties through compositional and structural versatility, tunable bandgap, high photoluminescence (PL) quantum yield (QY) and facile chemical synthesis. However, PL properties of solid-state samples suffer from environmental and operational degradation. Here we report alumina (AlOx) encapsulation of 0D Cs4PbBr6 nanocrystal thin films and individual nanoparticles using a modified atomic layer deposition (ALD) method with concurrent exposure of both Al and water precursors in the gas flow. We observed stronger PL intensity, increased PL lifetimes and much improved long-term stability at both film and single PNC level. These findings provide roadmap for ALD utilization to create solid-state perovskite devices.
Cesium lead halide perovskite materials have recently attracted attention in view of their optical and electronic properties which make them excellent candidates for potential applications in lasers, light emitting diodes and photodetectors. In this work, we provide the experimental and theoretical evidence for sequential photon absorption/re-emission in CsPbBr3 perovskite microwires. Using two-photon excitation, we recorded PL lifetimes and emission spectra as a function of the lateral distance between PL excitation and collection positions along the microwire, with separations exceeding 100 µm. As the propagation length increases, the PL spectrum develops a new emission peak that is red-shifted by 20 nm from the main emission and is accompanied by the appearance of the well-resolved rise times in the PL kinetics. We undertake quantitative modeling that accounts for bimolecular recombination and photon recycling within the microwire waveguide, and find that it is sufficient to account for the observed decay modifications. The model relies on a high radiative efficiency in CsPbBr3 perovskite microwires to explain the photon recycling observed. Such findings provide crucial information about the potential impact of photon recycling and waveguide trapping on optoelectronic properties of cesium lead halide perovskite materials
Perovskite nanocrystals (PNCs) have attracted a lot of scientific interest in the recent years due to the extraordinary optical and electronic properties such as compositional and structural versatility, tunable bandgap, high photoluminescence (PL) quantum yield (QY) and facile chemical synthesis. Among them, all inorganic CsnPbXm perovskites have attracted particular attention due to enhanced light emission and photo/thermal stability. Lower dimensionality polymorphs can be formed by manipulation of chemical- synthesis conditions where Cs+ can stabilize 3D [PbX6] framework, resulting in 2D (nanosheet), 1D (nanowire) and 0D (nanodot) internal octahedra arrays within the bulk of the perovskite. The gamut of available experimental approaches are further expanded in colloidal PNCs where both external size quantization and internal 0D structure may combine to achieve “multidimensional” electronic properties that are engineered both on atomic scale and nanoscale. In this work, we explore the photon emission statistics from individual 3D (CsPbBr3) and 0D (Cs4PbBr6) PNCs in order to address the origin of their PL emission. Using time-correlated, time-stamped single photon counting (TCSPC) we obtain PL intensity trajectories and extract PL lifetimes and second–order correlation functions at different excitation levels. Blinking traces show “burst-like” intensity behavior, with large bin-to-bin fluctuations, akin to molecular fluorophores. Recorded single photon emission statistics indicate that some of the measured PNCs are single photon emitters, others contain several emissive centers with very similar lifetimes. Few of the PNCs exhibited effects of photobrightening – superlinear increase of PL emission intensity due to the activation of an additional number of emissive centers within the PNC. Such emission behavior, independent of the confinement effects afforded by quantization in the medium/large sized Cs-based PNCs, supports theoretical framework that points towards Br-vacancy states localized within isolated octahedra.
KEYWORDS: Energy transfer, Semiconductors, Nanocrystals, Solar energy, Energy coupling, Dielectrics, Light-matter interactions, Optoelectronics, Quantization, Excitons
Monolayers of transition-metal dichalcogenides (TMDCs) are recently isolated materials combining strong light-matter interactions and high charge mobilities. Many TMDCs possess direct bandgap - a necessary property for optoelectronic and photonic applications. On the other hand, colloidal semiconductor nanocrystals (NQDs) exhibit high emission efficiency, chemical lability and excellent bandgap tunability via size quantization. Joining the two classes of materials in hybrid structures aims to utilize their respective strengths.
Among those, hybrids where two constituents are coupled via non-radiative energy transfer (NRET) present a particular interest. In the NRET process, exciton energy is transferred from NQD donor to TMDC acceptor via near-field, dipole-dipole energy coupling. This process plays an important role in photosynthetic plants and has been recently considered for energy harvesting in NQD/semiconductor architectures. Its efficiency depends on the distance, spectral overlap and dielectric screening properties of the acceptor material and its dimensionality. With the emergence of 2D materials, there is strong motivation, both for fundamental reasons and for the new applications, to study NRET in these novel systems.
We have studied NRET coupling between several types of NQDs and MoS2 monolayers using photoluminescence (PL) and femtosecond transient absorption (TA) spectroscopies. Both methods indicate very efficient NRET into the MoS2 acceptor, with donor PL intensity quenching concurrent with energy influx into acceptor as observed by TA. These effects are facilitated by reduced dielectric screening inherent to strongly polarizable TMDC materials as described by classical electromagnetic model. We envision energy coupling in 0D-2D hybrids enabling applications in photosensing, photovoltaics and light emission.
We present our recent advances toward the development of high-performance solid-state optical limiting devices using reverse saturable absorption (RSA) dyes doped into optical host materials. Femtosecond transient absorption spectroscopy was employed to determine both the spectral regions of strong RSA, and the singlet-triplet excited-state dynamics. The optical limiting in the visible spectrum in both metallo-phthalocyanines and metallo-porphyrins is due to a combination of singlet and triplet RSA. Optical limiting performance was studied for RSA dyes in dual tandem limiters (both in solution and solid-state). Our best results in the solid-state yielded an attenuation of 400X, and a damage threshold of up to several mJ at f/5 focusing. The optical limiting at f/5 is further enhanced, particularly in the solid-state, by self-defocusing thermal nonlinearities.
Parameters of refractive-index profile, the birefringence value, and mode interference pattern of buried planar waveguides fabricated by proton irradiation with doses from 1014 to 1017 cm-2 have been investigated experimentally. It has been shown that the Epsteyn model is a good approximation of waveguide refractive-index profile. The mechanical stress distribution in such waveguide layers has been studied by optical methods. A method for reconstructing the radiation induced defect distribution in the collision region has been proposed. The value of the proportionality factor in the refractive-index increment dependence on dose was determined more exactly on the basis of experimental data. Changes of the waveguide parameters dependent on the temperature of isochronous annealing also have been investigated.
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