Multiphonon Process in Mn-Doped ZnO Nanowires.

The multiphonon process plays an essential role in understanding electron-phonon coupling, which significantly influences the optical and transport properties of solids. Multiphonon processes have been observed in many materials, but how to distinguish them directly by their spectral characteristics remains controversial. Here, we report high-order Raman scattering up to 10 orders and hot luminescence involving 11 orders of phonons in Mn-doped ZnO nanowires by selecting the excitation energy. Our results show that the intensity distribution of high-order Raman scattering obeys an exponential decrease as the order number increases, while hot luminescence is fitted with a Poisson distribution with a resonance factor. Their linewidth and frequency can be well explained by two different transition models. Our work provides a paradigm for understanding the multiphonon-involved decay process of an excited state and may inspire studies of the statistical characteristics of excited state decay.

Efficient Upper-Excited State Fluorescence in an Organic Hyperbolic Metamaterial.

Upper-excited state emission is not usually observed from molecules owing to competition with much faster nonradiative relaxation pathways; however, it can be made more efficient by modifying the photonic density of states to enhance the radiative decay rate. Here, we show that embedding the small molecule zinc tetraphenylporphyrin (ZnTPP) in a hyperbolic metamaterial enables an ∼18-fold increase in fluorescence intensity from the second singlet excited state ( S2) relative to that from the lowest singlet excited state ( S1). By varying the number of periods in the HMM stack, we are able to systematically tune the ZnTPP fluorescence spectrum from red (dominated by emission from S1) to blue (dominated by emission from S2) with an instrument-limited decay lifetime <10 ps. Our results are consistent with a broadband Purcell enhancement in the radiative rate of both transitions predicted via transfer matrix modeling and point to a general opportunity to harness upper-excited states for spectrally tunable, ultrafast fluorescence via radiative decay engineering.

Multiband Hot Photoluminescence from Nanocavity-Embedded Silicon Nanowire Arrays with Tunable Wavelength.

Besides the well-known quantum confinement effect, hot luminescence from indirect bandgap Si provides a new and promising approach to realize monolithically integrated silicon optoelectronics due to phonon-assisted light emission. In this work, multiband hot photoluminescence is generated from Si nanowire arrays by introducing trapezoid-shaped nanocavities that support hybrid photonic-plasmonic modes. By continuously adjusting the geometric parameters of the Si nanowires with trapezoidal nanocavities, the multiband hot photoluminescence can be tuned in the range from visible to near-infrared independent of the excitation laser wavelength. The highly tunable wavelength bands and concomitant compatibility with Si-integrated electronics enable tailoring of silicon-based light sources suitable for next-generation optoelectronics devices.

Unraveling the Radiative Pathways of Hot Carriers upon Intense Photoexcitation of Lead Halide Perovskite Nanocrystals.

The slowdown of carrier cooling in lead halide perovskites (LHP) may allow the realization of efficient hot carrier solar cells. Much of the current effort focuses on the understanding of the mechanisms that retard the carrier relaxation, while proof-of-principle demonstrations of hot carrier harvesting have started to emerge. Less attention has been placed on the impact that the energy and momentum relaxation slowdown imparts on the spontaneous and stimulated light-emission process. LHP nanocrystals (NCs) provide an ideal testing ground for such studies as they exhibit bright emission and high optical gain, while the carrier cooling bottleneck is further pronounced compared to their bulk analogues due to confinement. Herein, the luminescent properties of CsPbBr3, FAPbBr3, and FAPbI3 NCs in the strong photoexcitation regime are investigated. In the former two NC systems, amplified spontaneous emission is found to dominate over the radiative recombination at average carrier occupancy per nanocrystal larger than 5-10. On the other hand, under the same photoexcitation conditions in the FAPbI3 NCs, a longer lived population of hot carriers results in a competition between hot luminescence, stimulated emission, and defect recombination. The dynamic interplay between the aforementioned three emissive channels appears to be influenced by various experimental and material parameters that include temperature, material purity, film morphology, and excitation pulse width and wavelength.