Showing papers on "Spontaneous emission published in 2022"
TL;DR: In this article , the emission of light from a radiation source placed inside a PTC was investigated and it was shown that radiation corresponding to the momentum bandgap is exponentially amplified, whether initiated by a macroscopic source, an atom, or vacuum fluctuations, drawing the amplification energy from the modulation.
Abstract: Photonic time crystals (PTCs), materials with a dielectric permittivity that is modulated periodically in time, offer new concepts in light manipulation. We study theoretically the emission of light from a radiation source placed inside a PTC and find that radiation corresponding to the momentum bandgap is exponentially amplified, whether initiated by a macroscopic source, an atom, or vacuum fluctuations, drawing the amplification energy from the modulation. The radiation linewidth becomes narrower with time, eventually becoming monochromatic in the middle of the bandgap, which enables us to propose the concept of nonresonant tunable PTC laser. Finally, we find that the spontaneous decay rate of an atom embedded in a PTC vanishes at the band edge because of the low density of photonic states. Description Amplification in photonic time crystals Regular photonic crystals are structures in which the refractive index is spatially periodic and can suppress the spontaneous emission of light from an emitter embedded in the structure. In photonic time crystals, the refractive index is periodically modulated in time on ultrafast time scales. Lyubarov et al. explored theoretically what happens when an emitter is placed in such a time crystal (see the Perspective by Faccio and Wright). In contrast to the regular photonic crystals, the authors found that time crystals should amplify emission, leading to lasing. —ISO Photonic time crystals can amplify emission from an embedded emitter.
47 citations
TL;DR: In this article , the influence of domain distribution on the laser emission of CsPbCl1.5Br 1.5-based quasi-2D perovskites was investigated.
Abstract: Quasi-2D perovskites, composed of self-organized quantum well structures, are emerging as gain materials for laser applications. Here we investigate the influence of domain distribution on the laser emission of CsPbCl1.5Br1.5-based quasi-2D perovskites. The use of 2,2-diphenylethylammonium bromide (DPEABr) as a ligand enables the formation of quasi-2D film with a large-n-dominated narrow domain distribution. Due to the reduced content of small-n domains, the incomplete energy transfer from small-n to large-n domains can be greatly addressed. Moreover, the photoinduced carriers can be concentrated on most of the large-n domains to reduce the local carrier density, thereby suppressing the Auger recombination. By controlling the domain distribution, we achieve blue amplified spontaneous emission and single-mode vertical-cavity surface-emitting lasing with low thresholds of 6.5 and 9.2 μJ cm-2, respectively. This work provides a guideline to design the domain distribution to realize low-threshold multicolor perovskite lasers.
23 citations
TL;DR: In this article , a free electron moving in a PTC spontaneously emits radiation, and when associated with momentum-gap modes, the electron emission process is exponentially amplified by the modulation of the refractive index.
Abstract: Photonic time-crystals (PTCs) are spatially homogeneous media whose electromagnetic susceptibility varies periodically in time, causing temporal reflections and refractions for any wave propagating within the medium. The time-reflected and time-refracted waves interfere, giving rise to Floquet modes with momentum bands separated by momentum gaps (rather than energy bands and energy gaps, as in photonic crystals). Here, we present a study on the emission of radiation by free electrons in PTCs. We show that a free electron moving in a PTC spontaneously emits radiation, and when associated with momentum-gap modes, the electron emission process is exponentially amplified by the modulation of the refractive index. Moreover, under strong electron-photon coupling, the quantum formulation reveals that the spontaneous emission into the PTC bandgap experiences destructive quantum interference with the emission of the electron into the PTC band modes, leading to suppression of the interdependent emission. Free-electron physics in PTCs offers a platform for studying a plethora of exciting phenomena, such as radiating dipoles moving at relativistic speeds and highly efficient quantum interactions with free electrons.
19 citations
TL;DR: In this paper , a three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, compensating its intrinsic nature of weak radiation.
Abstract: Spin-forbidden excitons in monolayer transition metal dichalcogenides are optically inactive at room temperature. Probing and manipulating these dark excitons are essential for understanding exciton spin relaxation and valley coherence of these 2D materials. Here, we show that the coupling of dark excitons to a metal nanoparticle-on-mirror cavity leads to plasmon-induced resonant emission with the intensity comparable to that of the spin-allowed bright excitons. A three-state quantum model combined with full-wave electrodynamic calculations reveals that the radiative decay rate of the dark excitons can be enhanced by nearly 6 orders of magnitude through the Purcell effect, therefore compensating its intrinsic nature of weak radiation. Our nanocavity approach provides a useful paradigm for understanding the room-temperature dynamics of dark excitons, potentially paving the road for employing dark exciton in quantum computing and nanoscale optoelectronics.
18 citations
TL;DR: In this paper , the collective radiative effects in an ensemble of cold atoms coupled to a single-mode optical nanofiber were investigated, showing that collective interactions between the atoms and a single guided photon gradually build-up along the atomic array in the direction of propagation.
Abstract: We experimentally and theoretically investigate collective radiative effects in an ensemble of cold atoms coupled to a single-mode optical nanofiber. Our analysis unveils the microscopic dynamics of the system, showing that collective interactions between the atoms and a single guided photon gradually build-up along the atomic array in the direction of propagation of light. These results are supported by time-resolved measurements of the light transmitted and reflected by the ensemble after excitation via nanofiber-guided laser pulses, whose rise and fall times are shorter than the atomic lifetime. Superradiant decays more than one order of magnitude faster than the single-atom free-space decay rate are observed for emission in the forward-propagating guided mode, while at the same time no speed-up of the decay rate are measured in the backward direction. In addition, position-resolved measurements of the light that is transmitted past the atoms are performed by inserting the nanofiber-coupled atomic array in a 45-m long fiber ring-resonator, which allow us to experimentally reveal the progressive growth of the collective response of the atomic ensemble. Our results highlight the unique opportunities offered by nanophotonic cold atom systems for the experimental investigation of collective light-matter interaction.
17 citations
TL;DR: In this article , the authors show that light emission can be observed at record low voltages of 36-60% of their bandgaps, exhibiting a large apparent energy gain of 0.6-1.4 eV per photon.
Abstract: For a light-emitting diode (LED) to generate light, the minimum voltage required is widely considered to be the emitter's bandgap divided by the elementary charge. Here we show for many classes of LEDs, including those based on perovskite, organic, quantum-dot and III-V semiconductors, light emission can be observed at record-low voltages of 36-60% of their bandgaps, exhibiting a large apparent energy gain of 0.6-1.4 eV per photon. For 17 types of LEDs with different modes of charge injection and recombination (dark saturation currents of ~10-39-10-15 mA cm-2), their emission intensity-voltage curves under low voltages show similar behaviours. These observations and their consistency with the diode simulations suggest the ultralow-voltage electroluminescence arises from a universal origin-the radiative recombination of non-thermal-equilibrium band-edge carriers whose populations are determined by the Fermi-Dirac function perturbed by a small external bias. These results indicate the potential of low-voltage LEDs for communications, computational and energy applications.
16 citations
TL;DR: In this article , the authors present the interplay between the mechanisms and their views/perspectives in determining the likely processes, which may dictate the recombination dynamics in the material.
Abstract: In methylammonium lead iodide (MAPbI3), a slow recombination process of photogenerated carriers has often been considered to be the most intriguing property of the material resulting in high-efficiency perovskite solar cells. In spite of intense research over a decade or so, a complete understanding of carrier recombination dynamics in MAPbI3 has remained inconclusive. In this regard, several microscopic processes have been proposed so far in order to explain the slow recombination pathways (both radiative and non-radiative), such as the existence of shallow defects, a weak electron–phonon coupling, presence of ferroelectric domains, screening of band-edge charges through the formation of polarons, occurrence of the Rashba splitting in the band(s), and photon-recycling in the material. Based on the up-to-date findings, we have critically assessed each of these proposals/models to shed light on the origin of a slow recombination process in MAPbI3. In this review, we have presented the interplay between the mechanisms and our views/perspectives in determining the likely processes, which may dictate the recombination dynamics in the material. We have also deliberated on their interdependences in decoupling contributions of different recombination processes.
14 citations
TL;DR: The mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr3) quantum dots suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination.
Abstract: Mechanically modulating optical properties of semiconductor nanocrystals and organic molecules are valuable for mechano-optical and optomechanical devices. Halide perovskites with excellent optical and electronic properties are promising for such applications. We report the mechanically changing excitons and photoluminescence of self-assembled formamidinium lead bromide (FAPbBr3) quantum dots. The as-synthesized quantum dots (3.6 nm diameter), showing blue emission and a short photoluminescence lifetime (2.6 ns), form 20-300 nm 2D and 3D self-assemblies with intense green emission in a solution or a film. The blue emission and short photoluminescence lifetime of the quantum dots are different from the delayed (ca. 550 ns) green emission from the assemblies. Thus, we consider the structure and excitonic properties of individual quantum dots differently from the self-assemblies. The blue emission and short lifetime of individual quantum dots are consistent with a weak dielectric screening of excitons or strong quantum confinement. The red-shifted emission and a long photoluminescence lifetime of the assemblies suggest a strong dielectric screening that weakens the quantum confinement, allowing excitons to split into free carriers, diffuse, and trap. The delayed emission suggests nongeminate recombination of diffusing and detrapped carriers. Interestingly, the green emission of the self-assembly blueshifts by applying a lateral mechanical force (ca. 4.65 N). Correspondingly, the photoluminescence lifetime decreases by 1 order of magnitude. These photoluminescence changes suggest the mechanical dissociation of the quantum dot self-assemblies and mechanically controlled exciton splitting and recombination. The mechanically changing emission color and lifetime of halide perovskite are promising for mechano-optical and optomechanical switches and sensors.
13 citations
TL;DR: In this paper , carbon nanodots (CDs) are used to localize the excited states and the symmetry break of the π-electron conjugation, leading to high emission efficiency.
Abstract: Carbon nanodots (CDs) have emerged as an alternative option for traditional nanocrystals due to their excellent optical properties and low toxicity. Nevertheless, high emission efficiency is a long‐lasting pursuit for CDs. Herein, CDs with near‐unity emission efficiency are prepared via atomic condensation of doped pyrrolic nitrogen, which can highly localize the excited states thus lead to the formation of bound excitons and the symmetry break of the π–electron conjugation. The short radiative lifetimes (<8 ns) and diffusion lengths (<50 nm) of the CDs imply that excitons can be efficiently localized by radiative recombination centers for a defect‐insensitive emission of CDs. By incorporating the CDs into polystyrene, flexible light‐converting films with a high solid‐state quantum efficiency of 84% and good resistance to water, heating, and UV light are obtained. With the CD–polymer films as light conversion layers, CD‐based white light‐emitting diodes (WLEDs) with a luminous efficiency of 140 lm W−1 and a flat‐panel illumination system with lighting sizes of more than 100 cm2 are achieved, matching state‐of‐the‐art nanocrystal‐based LEDs. These results pave the way toward carbon‐based luminescent materials for solid‐state lighting technology.
13 citations
TL;DR: In this article , the authors investigated the structural properties of two-dimensional layered perovskites (2DLPs) with different organic moieties and emission ranging from self-trapped exciton (STE)-dominated white light to blue band-edge photoluminescence.
Abstract: The soft hybrid organic-inorganic structure of two-dimensional layered perovskites (2DLPs) enables broadband emission at room temperature from a single material, which makes 2DLPs promising sources for solid-state white lighting, yet with low efficiency. The underlying photophysics involves self-trapping of excitons favored by distortions of the inorganic lattice and coupling to phonons, where the mechanism is still under debate. 2DLPs with different organic moieties and emission ranging from self-trapped exciton (STE)-dominated white light to blue band-edge photoluminescence are investigated. Detailed insights into the directional symmetries of phonon modes are gained using angle-resolved polarized Raman spectroscopy and are correlated to the temperature-dependence of the STE emission. It is demonstrated that weak STE bands at low-temperature are linked to in-plane phonons, and efficient room-temperature STE emission to more complex coupling to several phonon modes with out-of-plane components. Thereby, a unique view is provided into the lattice deformations and recombination dynamics that are key to designing more efficient materials.
10 citations
TL;DR: In this paper , a new combining photon strategy was proposed to achieve highly efficient broadband white-light emission in a new family of zero-dimensional (0D) indium halides based on an [InCl6]3- octahedron.
Abstract: Two-dimensional hybrid lead perovskites have attracted a great deal of attention in white-light-emitting diodes, but the serious toxicity of Pb2+ and the limited photoluminescence quantum yield (PLQY) still restrict further optoelectronic application. To address these issues, a new combining photon strategy was proposed to achieve highly efficient broadband white-light emission in a new family of zero-dimensional (0D) indium halides based on an [InCl6]3- octahedron. Remarkably, these 0D halides display dual-band white-light emission derived from the synergistic work of blue- and yellow-light-emitting bands, which can be ascribed to the radiative recombination of bound excitons in organic cations and self-trapped excitons in inorganic anions, respectively, based on spectroscopy and theoretical studies. In-depth first-principles calculation demonstrates that the increased structural deformability effectively improves the PLQY from 7.01% to 18.56%. As a proof of concept, this work provides a profound understanding for advancing the rational design of novel single-component 0D lead-free halides as high-performance white-light emitters.
TL;DR: In this article , the authors achieved effective modulation of spontaneous emission (SE) from a single-layer flat plate by changing the crystal directions of α-MoO3, achieving a modulation factor of more than three orders of magnitude.
Abstract: As a natural van der Waals crystal, α-MoO3 has excellent in-plane hyperbolic properties and essential nanophotonics applications. However, its tunable properties are generally neglected. Here, we achieve effective modulation of spontaneous emission (SE) from a single-layer flat plate by changing the crystal directions. Numerical results and theoretical analysis show that α-MoO3 exhibits good tunability when the crystal directions of α-MoO3 are different in y– z or x– y planes. A modulation factor of more than three orders of magnitude is obtained at 634 cm−1. This phenomenon is caused by the excitation of hyperbolic phonon polaritons in α-MoO3 at specific bands. However, when the crystal directions of α-MoO3 are different in the x– z plane, the SE of the material exhibits strong angle independence. Additionally, for the semi-infinite α-MoO3 flat structure, we determine the distribution of the modulation factor of SE using the wavenumber and rotation angle. Finally, we extend the calculation results from semi-infinite media to finite thickness films. We obtain the general evolution law of the peak angle of the modulation factor with thickness, increasing the modulation factor to approximately 2000, which exceeds the maximum modulation factor observed in previous works by 48 times. We believe this work could guide the SE modulation of anisotropic materials and benefit the field of micro-/nano-lasers and quantum computing.
TL;DR: In this article , the authors provide a comprehensive view of the competing radiative and non-radiative recombination processes in tin-based perovskite thin films to establish the interplay between doping and trapping by combining photoluminescence measurements with trapped-carrier dynamic simulations and first-principles calculations.
Abstract: Tin halide perovskites have recently emerged as promising materials for low band gap solar cells. Much effort has been invested on controlling the limiting factors responsible for poor device efficiencies, namely self-p-doping and tin oxidation. Both phenomena are related to the presence of defects; however, full understanding of their implications in the optoelectronic properties of the material is still missing. We provide a comprehensive picture of the competing radiative and non-radiative recombination processes in tin-based perovskite thin films to establish the interplay between doping and trapping by combining photoluminescence measurements with trapped-carrier dynamic simulations and first-principles calculations. We show that pristine Sn perovskites, i.e. sample processed with commercially available SnI2 used as received, exhibit extremely high radiative efficiency due to electronic doping which boosts the radiative band-to-band recombination. Contrarily, thin films where Sn4+ species are intentionally introduced show drastically reduced radiative lifetime and efficiency due to a dominance of Auger recombination at all excitation densities when the material is highly doped. The introduction of SnF2 reduces the doping and passivates Sn4+ trap states but conversely introduces additional non-radiative decay channels in the bulk that fundamentally limit the radiative efficiency. Overall, we provide a qualitative model that takes into account different types of traps present in tin-perovskite thin films and show how doping and defects can affect the optoelectronic properties.
TL;DR: In this paper , an interfacial chemistry strategy was proposed to increase the radiative recombination rate of perovskites by coating aluminum oxide on the lead halide perovsite.
Abstract: Abstract Efficient radiative recombination is essential for perovskite luminescence, but the intrinsic radiative recombination rate as a basic material property is challenging to tailor. Here we report an interfacial chemistry strategy to dramatically increase the radiative recombination rate of perovskites. By coating aluminum oxide on the lead halide perovskite, lead–oxygen bonds are formed at the perovskite‐oxide interface, producing the perovskite surface states with a large exciton binding energy and a high localized density of electronic state. The oxide‐bonded perovskite exhibits a ≈500 fold enhanced photoluminescence with a ≈10 fold reduced lifetime, indicating an unprecedented ≈5000 fold increase in the radiative recombination rate. The enormously enhanced radiative recombination promises to significantly promote the perovskite optoelectronic performance.
TL;DR: In this article , the authors improved the optical gain properties of 2D perovskites and achieved optically pumped lasing, and obtained PEA2SnI4 films with high crystallinity and favorable optical properties, resulting in amplified spontaneous emission (ASE) with a low threshold (30 μJ/cm2), a high optical gain above 4000 cm-1 at 77 K, and ASE operation up to room temperature.
Abstract: Two-dimensional (2D) perovskites have been proposed as materials capable of improving the stability and surpassing the radiative recombination efficiency of three-dimensional perovskites. However, their luminescent properties have often fallen short of what has been expected. In fact, despite attracting considerable attention for photonic applications during the last two decades, lasing in 2D perovskites remains unclear and under debate. Here, we were able to improve the optical gain properties of 2D perovskite and achieve optically pumped lasing. We show that the choice of the spacer cation affects the defectivity and photostability of the perovskite, which in turn influences its optical gain. Based on our synthetic strategy, we obtain PEA2SnI4 films with high crystallinity and favorable optical properties, resulting in amplified spontaneous emission (ASE) with a low threshold (30 μJ/cm2), a high optical gain above 4000 cm–1 at 77 K, and ASE operation up to room temperature.
TL;DR: In this article , the authors reported fast and bright luminescence by coupling gap plasmon modes to nanoparticle emitters, achieving a 220-fold increase in the PL intensity.
Abstract: Graphene quantum dots (GQDs) are quasi-zero-dimensional, carbon-based luminescent nanomaterials that possess desirable physical properties, such as high photostability, low cytotoxicity, good biocompatibility, and excellent water solubility; however, their long radiative lifetimes significantly limit their use in, e.g., light emitting devices where a fast spontaneous emission rate is essential. Despite a few reports on GQD fluorescence enhancements using metal nanostructures, studies of enhanced spontaneous emission rate remain outstanding. Here, we report fast and bright luminescence by coupling gap plasmon modes to nanoparticle emitters. Through precise control over the nanoparticle's local density of states (LDOS), we achieved a 220-fold increase in the PL intensity. The shortest radiative lifetime obtained was below 8.0 ps and limited by the instrument response, which is over 288-fold shorter than the lifetime of uncoupled GQDs. These findings may benefit the future development of rapid displays and open the possibility of constructing high-frequency classical or quantum telecommunication systems.
TL;DR: In this paper , the authors performed a comparative study on the photoluminescence quantum yield (PLQY) of five specimens of 1L-WS2 prepared on two different substrates of (i) multilayer hBN and (ii) SiO2 to investigate the impact of dielectric environment on the PLQY.
Abstract: The photoluminescence quantum yield (PLQY) is a critical factor of monolayer transition metal dichalcogenides (1L-TMDs) for optoelectronic applications. However, the PLQY of 1L-TMD is severely affected by strong exciton–exciton annihilation (EEA) in two dimensions as the exciton density increases. Therefore, it is very important to suppress EEA or to improve radiative exciton recombination for ensuring a high PLQY. In this work we performed a comparative study on the PLQYs of five specimens of 1L-WS2 prepared on two different substrates of (i) multilayer hBN and (ii) SiO2 in order to investigate the impact of dielectric environment on the PLQY. Although the PL lifetime was observed to notably decrease in each of the 1L-WS2 on hBN, the specimens on both substrate types displayed similar PLQYs at low exciton densities, indicating that the lifetime reduction factors are quite similar for the radiative and nonradiative lifetimes of excitons. In our sample geometry with no encapsulation, we confirmed that the EEA rate constant does not alter for the two different substrates. Intriguingly, however, the 1L-WS2 on hBN exhibited a much-suppressed PLQY drop with increasing exciton density. The observed correlation between the reduced PL lifetime and the persisting PLQY under high excitation indicates that dense excitons bypass EEA via fast radiative recombination. Our results demonstrate that the intrinsic PLQY drop in 1L-TMDs at high exciton densities can be significantly suppressed by simple dielectric engineering with hBN, thereby promoting their practical functionality toward highly luminescent two-dimensional light-emitting devices.
TL;DR: In this article , the authors analyzed low-temperature photoluminescence (PL) and the band edge excitonic emission in bulk halide perovskite CsPbCl3 single crystals.
TL;DR: In this article , an anti-solvent treatment strategy was proposed to regulate phase components and surface morphology of quasi-2D PEA2(CsPbBr3)n−1pbBr4 thin films and an additional poly(methyl methacrylate) coating was introduced to reduce the surface defects of perovskite films and suppress the nonradiative recombination.
Abstract: Solution‐processed quasi‐2D Ruddlesden‐Popper perovskites are being considered as a promising optical gain medium in lasing applications, owing to their outstanding optoelectronic properties and inherent stability. However, the development of quasi‐2D perovskites for lasers with low threshold and high optical gain is still lagging far behind 3D‐perovskites. This work proposes an anti‐solvent treatment strategy to regulate the phase components and surface morphology of quasi‐2D PEA2(CsPbBr3)n−1PbBr4 thin films. Furthermore, an additional poly(methyl methacrylate) coating is introduced to reduce the surface defects of perovskite films and suppress the non‐radiative recombination. Due to the phase engineering and surface passivation, excellent amplified spontaneous emission (ASE) is obtained with a low threshold of 11.7 µJ cm−2 and a high net model gain of 622 cm−1. In addition, under 800 nm femtosecond laser excitation, two‐photon‐pumped ASE is also successfully observed with a low threshold of 7.2 mJ cm−2.
TL;DR: In this paper , localized excitonic electroluminescence has been reported in carbon nanodot (CD) based LEDs with a sub-bandgap turn-on voltage of 2.4 V and a maximum luminance of 60.2 cd m-2.
Abstract: Localized excitons are expected to achieve high-performance electroluminescence and have been widely investigated in GaN-based light-emitting diodes (LEDs). Although carbon nanodot (CD) based LEDs have been achieved with the radiative recombination of electrons and holes, localized excitonic electroluminescence has been not reported before. In this Letter, localized excitonic electroluminescent devices have been fabricated using fluorescent CDs as an active layer. The CDs show strong localized excitonic yellow emission with a fluorescence quantum yield of 76% and Stokes shift of 2.1 eV. The CD-based LEDs present a sub-bandgap turn-on voltage of 2.4 V and a maximum luminance of 60.2 cd m-2, which is the lowest driving voltage among the CD-based electroluminescent devices. Localized centers trap carriers effectively, resulting in sub-bandgap light emission. The current results manifest that localized excitons may furnish a promising approach to boost the development of CD-based LEDs.
TL;DR: In this paper , an interface-based approach was proposed to enhance the self-trapped exciton emission via the interfacial energy transfer in 2D/quantum dots (QDs) perovskite heterostructures.
Abstract: Self-trapped excitons, which often occurs in materials with soft lattice and strong electron–phonon coupling, have attracted a lot of attention owing to their unique broadband emission and promising applications in persistent white light sources. However, the emission of self-trapped excitons is usually weak in some two-dimensional (2D) and three-dimensional (3D) perovskites because of their low radiative recombination rate. The existing strategies of enhancing the emission efficiency of self-trapped excitons such as metal cation doping and organic ligands modification often entail complex chemical synthetic processes. Here, we report a new approach to significantly boost the self-trapped exciton emission via the interfacial excitonic energy transfer in 2D/quantum dots (QDs) perovskite heterostructures. The self-trapped exciton emission in the heterostructures could be enhanced more than two orders of magnitude compared with the constituent 2D perovskite crystals. Temperature-, excitation power- and thickness-dependent photoluminescence (PL) studies reveal that the enhanced self-trapped exciton emission in the heterostructure can be ascribed to Dexter energy transfer taking place at the interface of the heterostructure. Our study provides a simple and practical interface-based strategy to improve the emission efficiency of self-trapped excitons for photoelectric devices.
01 Jan 2022-Journal of Materials Chemistry C. Materials for Optical, Magnetic and Electronic Devices
TL;DR: Light-emitting diodes (LEDs) based on metal halide perovskites have shown great promise for next-generation display technology as they offer high color purity, satisfy Rec.2020, and have low-cost solution processability as discussed by the authors .
Abstract: Light-emitting diodes (LEDs) based on metal halide perovskites have shown great promise for next-generation display technology as they offer high color purity, satisfy Rec.2020, and have low-cost solution processability. Moreover,...
18 Mar 2022
TL;DR: In this paper , the spontaneous emission limited linewidth of a semiconductor laser consisting of hybrid or heterogeneously integrated, silicon and III-V intracavity components is calculated.
Abstract: This article describes a calculation of the spontaneous emission limited linewidth of a semiconductor laser consisting of hybrid or heterogeneously integrated, silicon and III–V intracavity components. Central to the approach are a) description of the multi-element laser cavity in terms of composite laser/free-space eigenmodes, b) use of multimode laser theory to treat mode competition and multiwave mixing, and c) incorporation of quantum-optical contributions to account for spontaneous emission effects. Application of the model is illustrated for the case of linewidth narrowing in an InAs quantum-dot laser coupled to a high- Q $Q$ SiN cavity.
TL;DR: In this article, a simple method chemical bath deposition (CBD) under optimum conditions was used to from vertically aligned ZnO NAs on glass substrate at room temperature to enhance UV random lasing emissions.
Abstract: In this paper, we report SiO2 capped Zinc Oxide nanorod array (SiO2 capped-ZnO NAs) for enhancing UV random lasing emissions at room temperature. A simple method chemical bath deposition (CBD) under optimum conditions was used to from vertically aligned ZnO NAs on glass substrate at room temperature. The random lasing emission intensity increased 30 times compared to bare ZnO NAs (without SiO2) at the same pump power. The best sample with 100 nm of SiO2 capping showed superior lasing, whereby the random lasing peaks almost suppressed the spontaneous emission peak completely. This enhancement was attributed to improved light confinement inside the ZnO NAs resonators which increases the performance of the stimulated light emission within the gain medium. To further understand the mechanism behind the enhanced lasing properties, a numerical simulation using finite-element method was performed. The simulation results showed SiO2 capped-ZnO nanorod lead to an enhancement of light confinement within the resonators.
TL;DR: In this paper , the authors reported the design strategy of facet engineering to reduce the gain threshold of amplified spontaneous emission by manyfold in CsPbBr3 NCs of the same concentration and edge length.
Abstract: Auger recombination and thermalization time are detrimental in reducing the gain threshold of optically pumped semiconductor nanocrystal (NC) lasers for future on-chip nanophotonic devices. Here, we report the design strategy of facet engineering to reduce the gain threshold of amplified spontaneous emission by manyfold in NCs of the same concentration and edge length. We achieved this hallmark result by controlling the Auger recombination rates dominated by processes involving NC volume and thermalization time to the emitting states by optimizing the number of facets from 6 (cube) to 12 (rhombic dodecahedron) and 26 (rhombicuboctahedrons) in CsPbBr3 NCs. For instance, we demonstrate a 2-fold reduction in Auger recombination rates and thermalization time with increased number of facets. The gain threshold can be further reduced ∼50% by decreasing the sample temperature to 4 K. Our systematic studies offer a new method to reduce the gain threshold that ultimately forms the basis of nanolasers.
TL;DR: In this article , the authors considered the theory of spontaneous emission for a random medium of stationary two-level atoms and investigated the dynamics of the field and atomic probability amplitudes for a one-photon state of the system.
Abstract: We consider the theory of spontaneous emission for a random medium of stationary two-level atoms. We investigate the dynamics of the field and atomic probability amplitudes for a one-photon state of the system. At long times and large distances, we show that the corresponding average probability densities can be determined from the solutions to a pair of kinetic equations.
TL;DR: In this article , the InGaN based LEDs on InyGa1−yN templates with varying In-content of 8% ≤ y ≤ 12% were studied for the same emission wavelength.
Abstract: InGaN templates have recently attracted interest due to their ability to reduce strain in the quantum wells and to induce a red shift in the emission wavelength. For such technology to be competitive, it should outperform the traditional technology for LEDs grown on GaN substrates and offer improved output characteristics. InGaN based LEDs on InyGa1−yN templates with varying In-content of 8% ≤ y ≤ 12% are studied for the same emission wavelength. The electroluminescence, optical output power, and external quantum efficiency of the LEDs are investigated as a function of the In-content in the templates. LEDs on InGaN templates with In-content of 8–10% show better performance than LEDs grown on GaN. This enhancement is attributed to improved radiative recombination as a result of the reduced strain in the quantum wells. However, templates with In-content of ∼10.5% and ∼11% show inferior performance to the LEDs on GaN because the deterioration from the increased defects from the template is stronger than the improvement in the radiative recombination. It can be concluded that the InGaN templates with 8–10% offer a technology for LEDs that is outperforming the traditional GaN technology.
TL;DR: In this article , all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative were analyzed, and the change in the fluorescence lifetime was assigned to the Purcell effect.
Abstract: Controlling the radiative rate of emitters with macromolecular photonic structures promises flexible devices with enhanced performances that are easy to scale up. For instance, radiative rate enhancement empowers low-threshold lasers, while rate suppression affects recombination in photovoltaic and photochemical processes. However, claims of the Purcell effect with polymer structures are controversial, as the low dielectric contrast typical of suitable polymers is commonly not enough to provide the necessary confinement. Here we show all-polymer planar microcavities with photonic band gaps tuned to the photoluminescence of a diketopyrrolopyrrole derivative, which allows a change in the fluorescence lifetime. Radiative and nonradiative rates were disentangled systematically by measuring the external quantum efficiencies and comparing the planar microcavities with a series of references designed to exclude any extrinsic effects. For the first time, this analysis shows unambiguously the dye radiative emission rate variations obtained with macromolecular dielectric mirrors. When different waveguides, chemical environments, and effective refractive index effects in the structure were accounted for, the change in the radiative lifetime was assigned to the Purcell effect. This was possible through the exploitation of photonic structures made of polyvinylcarbazole as a high-index material and the perfluorinated Aquivion as a low-index one, which produced the largest dielectric contrast ever obtained in planar polymer cavities. This characteristic induces the high confinement of the radiation electric field within the cavity layer, causing a record intensity enhancement and steering the radiative rate. Current limits and requirements to achieve the full control of radiative rates with polymer planar microcavities are also addressed.
TL;DR: In this paper , the sensitivity of the Indium molar fraction in InGaN QWs is explored for near-ultraviolet (UV) LEDs, and the theoretically calculated results show that as the indium composition increases, the radiative recombination increases along with an increase in carrier injection efficiency.
Abstract: InGaN-based quantum wells (QWs) have higher threading dislocation density (TDD) in InGaN Light-emitting diode (LED). Despite of higher TDD, variation of Indium (In) molar fraction in the QW generate localized excitons with higher Indium composition, thus preventing bound carriers from non-radiative recombination. In this work, the sensitivity of the Indium molar fraction in InGaN QWs is explored for near-ultraviolet (UV) LEDs. The theoretically calculated results show that as the Indium composition increases in InGaN QWs, the radiative recombination increases along with an increase in carrier injection efficiency. The reduced non-radiative recombination for higher Indium composition leads to the enhanced spontaneous emission rate and internal quantum efficiency (IQE). For lowered Indium composition, the peak emission wavelength of the InGaN LEDs shift toward the shorter wavelength and the performances degrade drastically. Hence for shorter UV LEDs, the AlGaN-based device structure should be a suitable choice.
TL;DR: In this article , the InGaN-based green micro-light-emitting diodes (µLEDs) with different active areas are investigated; results are as follows: Reverse and forward leakage currents of µLED increase as emission area is reduced owing to the non-radiative recombination process at the sidewall defects; this is more prominent in smaller µLED because of larger surface-to-volume ratio.
Abstract: Electrical and optical characteristics of InGaN-based green micro-light-emitting diodes (µLEDs) with different active areas are investigated; results are as follows. Reverse and forward leakage currents of µLED increase as emission area is reduced owing to the non-radiative recombination process at the sidewall defects; this is more prominent in smaller µLED because of larger surface-to-volume ratio. Leakage currents of µLEDs deteriorate the carrier injection to light-emitting quantum wells, thereby degrading their external quantum efficiency. Reverse leakage current originate primarily from sidewall edges of the smallest device. Therefore, aggressive suppression of sidewall defects of µLEDs is essential for low-power and downscaled µLEDs.