Ultrafast Radiative Decay Research Articles

Revealing the ultrafast spontaneous emission in plasmon-enhanced monolayer semiconductor nano-light sources

Nanoscale spontaneous light sources are promising alternatives to lasers for high-speed optical communications and interconnections through energy-efficient integrated circuits. Yet, developing the spontaneous light sources faster than lasers is hampered by the detection means (e.g., time-resolved fluorescence spectroscopy). Here, by coupling monolayer WSe2 to individual plasmonic nanocavities, we achieved an efficient spontaneous light source with potential ultrafast modulation bandwidth and superior brightness. The ultrafast radiative decay rates can be determined and derived solely from the experimental parameters by combining the coupling strength and the photoluminescence enhancement in a single nanocavity-WSe2 hybrid. As a result, the hybrid light source has a radiative lifetime down to 350 fs, indicating a potential modulation bandwidth up to 440 GHz, which is 10 times of the traditional semiconductor lasers. Furthermore, the quantum yield is enhanced by a factor of over 300-folds up to 20.8% through making full use of the highly confined nanocavity mode. The nanocavity-WSe2 hybrid we built provides a promising approach for constructing high-speed light-emitting devices.

Superfluorescence of Sub-Band States in C-Plane In0.1Ga0.9N/GaN Multiple-QWs.

Superfluorescence is a collective emission from quantum coherent emitters due to quantum fluctuations. This is characterized by the existence of the delay time ( for the emitters coupling and phase-synchronizing to each other spontaneously. Here we report the observation of superfluorescence in c-plane In0.1Ga0.9N/GaN multiple-quantum wells by time-integrated and time-resolved photoluminescence spectroscopy under higher excitation fluences of the 267 nm laser and at room temperature, showing a characteristic from 79 ps to 62 ps and the ultrafast radiative decay (7.5 ps) after a burst of photons. Time-resolved traces present a small quantum oscillation from coupled In0.1Ga0.9N/GaN multiple-quantum wells. The superfluorescence is attributed to the radiative recombination of coherent emitters distributing on strongly localized subband states, Ee1→Ehh1 or Ee1→Elh1 in 3nm width multiple-quantum wells. Our work paves the way for deepening the understanding of the emission mechanism in the In0.1Ga0.9N/GaN quantum well at a higher injected carrier density.

Perovskite-type superlattices from lead halide perovskite nanocubes.

Atomically defined assemblies of dye molecules (such as H and J aggregates) have been of interest for more than 80 years because of the emergence of collective phenomena in their optical spectra1-3, their coherent long-range energy transport, their conceptual similarity to natural light-harvesting complexes4,5, and their potential use as light sources and in photovoltaics. Another way of creating versatile and controlled aggregates that exhibit collective phenomena involves the organization of colloidal semiconductor nanocrystals into long-range-ordered superlattices6. Caesium lead halide perovskite nanocrystals7-9 are promising building blocks for such superlattices, owing to the high oscillator strength of bright triplet excitons10, slow dephasing (coherence times of up to 80 picoseconds) and minimal inhomogeneous broadening of emission lines11,12. So far, only single-component superlattices with simple cubic packing have been devised from these nanocrystals13. Here we present perovskite-type (ABO3) binary and ternary nanocrystal superlattices, created via the shape-directed co-assembly of steric-stabilized, highly luminescent cubic CsPbBr3 nanocrystals (which occupy the B and/or O lattice sites), spherical Fe3O4 or NaGdF4 nanocrystals (A sites) and truncated-cuboid PbS nanocrystals (B sites). These ABO3 superlattices, as well as the binary NaCl and AlB2 superlattice structures that we demonstrate, exhibit a high degree of orientational ordering of the CsPbBr3 nanocubes. They also exhibit superfluorescence-a collective emission that results in a burst of photons with ultrafast radiative decay (22 picoseconds) that could be tailored for use in ultrabright (quantum) light sources. Our work paves the way for further exploration of complex, ordered and functionally useful perovskite mesostructures.

Polymer Nanoreactors Shield Perovskite Nanocrystals from Degradation.

Halide perovskite nanocrystals (NCs) have shown impressive advances, exhibiting optical properties that outpace conventional semiconductor NCs, such as near-unity quantum yields and ultrafast radiative decay rates. Nevertheless, the NCs suffer even more from stability problems at ambient conditions and due to moisture than their bulk counterparts. Herein, we report a strategy of employing polymer micelles as nanoreactors for the synthesis of methylammonium lead trihalide perovskite NCs. Encapsulated by this polymer shell, the NCs display strong stability against water degradation and halide ion migration. Thin films comprising these NCs exhibit a more than 15-fold increase in lifespan in comparison to unprotected NCs in ambient conditions and even survive over 75 days of complete immersion in water. Furthermore, the NCs, which exhibit quantum yields of up to 63% and tunability of the emission wavelength throughout the visible range, show no signs of halide ion exchange. Additionally, heterostructures of MAPI and MAPBr NC layers exhibit efficient Förster resonance energy transfer (FRET), revealing a strategy for optoelectronic integration.

Synergetic Enhancement of Light-Matter Interaction by Nonlocality and Band Degeneracy in ZnO Thin Films.

This study aims to reveal the full potential of ZnO as an ultrafast photofunctional material. Based on nonlocal response theory to incorporate the spatially inhomogeneous quality of the samples coupled with experimental observations of linear and nonlinear optical responses, we establish the ultrafast radiative decay of excitons in ZnO thin films that reaches the speed of excitonic dephasing at room temperature in typical semiconductors at a couple tens of femtoseconds. The consistency between the observed delay-time dependence of the transient-grating signals and the theoretical prediction reveals that the ultrafast radiative decay is due to the synergetic effects of the giant light-exciton interaction volume and the radiative coupling between multicomponent excitons.

Strong Single- and Two-Photon Luminescence Enhancement by Nonradiative Energy Transfer across Layered Heterostructure.

The strong light-matter interaction in monolayer transition metal dichalcogenides (TMDs) is promising for nanoscale optoelectronics with their direct band gap nature and the ultrafast radiative decay of the strongly bound excitons these materials host. However, the impeded amount of light absorption imposed by the ultrathin nature of the monolayers impairs their viability in photonic applications. Using a layered heterostructure of a monolayer TMD stacked on top of strongly absorbing, nonluminescent, multilayer SnSe2, we show that both single-photon and two-photon luminescence from the TMD monolayer can be enhanced by a factor of 14 and 7.5, respectively. This is enabled through interlayer dipole-dipole coupling induced nonradiative Förster resonance energy transfer (FRET) from SnSe2 underneath, which acts as a scavenger of the light unabsorbed by the monolayer TMD. The design strategy exploits the near-resonance between the direct energy gap of SnSe2 and the excitonic gap of monolayer TMD, the smallest possible separation between donor and acceptor facilitated by van der Waals heterojunction, and the in-plane orientation of dipoles in these layered materials. The FRET-driven uniform single- and two-photon luminescence enhancement over the entire junction area is advantageous over the local enhancement in quantum dot or plasmonic structure integrated 2D layers and is promising for improving quantum efficiency in imaging, optoelectronic, and photonic applications.

Interaction-induced photon blockade using an atomically thin mirror embedded in a microcavity

Narrow dark resonances associated with electromagnetically induced transparency play a key role in enhancing photon-photon interactions. The schemes realized to date relied on the existence of long-lived atomic states with strong van der Waals interactions. Here, we show that by placing an atomically thin semiconductor with ultra-fast radiative decay rate inside a \textcolor{black}{0D} cavity, it is possible to obtain narrow dark or bright resonances in transmission whose width could be much smaller than that of the cavity and bare exciton decay rates. While breaking of translational invariance places a limit on the width of the dark resonance width, it is possible to obtain a narrow bright resonance that is resilient against disorder by tuning the cavity away from the excitonic transition. Resonant excitation of this bright resonance yields strong photon antibunching even in the limit where the interaction strength is arbitrarily smaller than the non-Markovian disorder broadening and the radiative decay rate of the bare exciton. Our findings suggest that atomically thin semiconductors could pave the way for realization of strongly interacting photonic systems in the solid-state.

Self-Assembled Microdisk Lasers of Perylenediimides.

Organic solid-state lasers (OSSLs) have been a topic of intensive investigations. Perylenediimide (PDI) derivatives are widely used in organic thin-film transistors and solar cells. However, OSSLs based on neat PDIs have not been achieved yet, owing to the formation of H-aggregates and excimer trap-states. Here, we demonstrated the first PDI-based OSSL from whispering-gallery mode (WGM) hexagonal microdisk (hMD) microcavity of N,N'-bis(1-ethylpropyl)-2,5,8,11-tetrakis(p-methyl-phenyl)-perylenediimide (mp-PDI) self-assembled from solution. Single-crystal data reveal that mp-PDI molecules stack into a loosely packed twisted brickstone arrangement, resulting in J-type aggregates that exhibit a solid-state photoluminescence (PL) efficiency φ > 15%. Moreover, we found that exciton-vibration coupling in J-aggregates leads to an exceptional ultrafast radiative decay, which reduces the exciton diffusion length, in turn, suppresses bimolecular exciton annihilation (bmEA) process. These spectral features, plus the optical feedback provided by WGM-hMD microcavity, enable the observation of multimode lasing as evidenced by nonlinear output, spectral narrowing, and temporal coherence of laser emission. With consideration of high carrier-mobility associated with PDIs, hMDs of mp-PDI are attractive candidates on the way to achieve electrically driven OSSL.

Femtosecond-class radiative decay of multinode-type excitons in nano-to-bulk crossover regime

Transient grating (TG) spectrum and its delay-time dependence have been investigated for CuCl ultrathin films in a crossover regime of nano-to-bulk scale. The CuCl films whose crystalline quality is extremely improved by electron-beam irradiation at the start of deposition show anomalous mode structures in the TG spectrum. We succeeded in demonstrating that the photon energy at each peak in the TG spectrum is in good agreement with the theoretically predicted size-dependent eigenenergy including the radiative shift. Ultrafast radiative decay of excitons in a few hundred femtoseconds is observed for several eigenmodes, in which the radiative widths are extremely large due to a long-range coherent coupling between the weakly confined exciton and the radiation field.

Quantum-confined biexcitons in CuCl quantum dots and their unconventional opticalproperties

The optical properties of quantum-confined biexcitons in CuCl quantum dots (QDs) underthe novel method of two-photon resonance excitation of biexcitons are reviewed. First, webriefly overview the optical properties of excitons and biexcitons in bulk CuCl singlecrystals and also those in quantum dots, and then we cover the biexcitonic properties ofCuCl quantum dots in NaCl matrices in detail: realization of two-photon resonantexcitation of confined biexcitons with different angular momenta, studies on theenergy-level structure of biexcitonic states by size-selective two-photon resonant excitationof biexcitons, on subsequent efficient lasing of biexcitonic luminescence even inlow-Q cavity, on ultrafast radiative decay of biexcitons, and, finally, the observation of excitedstates of confined biexcitons studied by infrared transient absorption. All theseunconventional features are inherent in the biexcitons in CuCl QDs affected by quantumconfinement effects.

Ultrafast radiative decay of polaritons in an interface layer with strong excitonic response

We have demonstrated theoretically that radiative polaritons in an interface layer having strong excitonic response exhibit appreciable frequency dependence on an in-plane wave vector, k ∥, in the region of the ω–k ∥ dispersion plane between the “light-lines” of the media to either side of the layer. We predict ultrafast radiative decay of such polaritons with lifetimes as low as 10 fs in slabs of organic–inorganic PbI-based layered perovskite-type semiconductors. We confirm an unusual physical feature of the radiative polaritons by determining the linear electromagnetic response of the excitonic layer to a current δ-pulse.

Ultra-Fast Radiative Decay of Spatially Extended Non-Dipole Type Excitonic State

By means of the non-local theory of the pulse response and the analysis of the exciton-radiation coupled modes, we elucidate a recently observed short response time of degenerate four-wave mixing (DFWM) of weakly confined excitons in a thin film. Especially, it is expected that the excitons with non-dipole type of center-of-mass wavefunctions exhibit a peculiar thickness dependence owing to a spatial structure of the radiation field. The experimental results of the three-beam DFWM of GaAs thin layers strongly support our theoretical explanation of the radiative decay that is beyond the long wavelength approximation.

Ultra fast decay of enhanced degenerate-four-wave mixing of multiple GaAs thin layers

By means of the nonlocal theory of the pulse response and the analysis of the exciton–radiation coupled modes, it is elucidated that a nondipole type excitonic state confined in a thin film strongly couples with a radiation field in a particular thickness region, which leads to a large nonlinear response combined with the ultra fast radiative decay in the case of GaAs. Such strong coupling can be further enhanced by the multiple layered structure, where the radiative width concentrates in a particular collective mode keeping the optimum condition for the enhanced nonlinear response.

Double quantum well in a semiconductor microcavity: Three-oscillator model and ultrafast radiative decay.

A complete study of excitons in double quantum wells embedded in a semiconductor microcavity is presented. In the strong coupling regime we predict the possibility of observing three peaks in optical spectra when the two quantum well excitons have different resonance energies, and discuss the best structure parameters for observing this phenomenon. In the weak coupling regime we show that a radiative lifetime as short as \ensuremath{\approxeq}100 fs can be obtained by taking advantage of the coherence between the wells in addition to the microcavity effect. This ultrashort radiative lifetime is shown to be observable even in the presence of disorder.