H2_2 Double Ionization with Few-Cycle Laser Pulses

International audienceThe temporal dynamics of double ionization of H$_2$ has been investigated both experimentally and theoretically with few-cycle laser pulses. The main observables are the proton spectra associated to the H$^+$ + H$^+$ fragmentation channel. The model is based on the time-dependent Schrödinger equation and treats on the same level the electronic and nuclear coordinates. Therefore it allows to follow the ultrafast nuclear dynamics as a function of the laser pulse duration, carrier-envelope phase offset and peak intensity. We mainly report results in the sequential double ionization regime above 2 x 10$^{14}$ W/cm$^{-2}$. The proton spectra are shifted to higher energies as the pulse duration is reduced from 40fs down to 10fs. The good agreement between the model predictions and the experimental data at 10fs permits a theoretical study with pulse durations down to a few femtoseconds. We demonstrate the very fast nuclear dynamics of the H$_2^+$ ion for a pulse duration as short as 1fs between the two ionization events giving respectively H$_2^+$ from H$_2$ and H$^+$ + H$^+$ from H$_2^+$. Carrier-envelope phase offset only plays a significant role for pulse durations shorter than 4fs. At 10fs, the laser intensity dependence of the proton spectra is fairly well reproduced by the model

Ultrafast electro-nuclear dynamics of H2 double ionization

The ultrafast electronic and nuclear dynamics of H2 laser-induced double ionization is studied using a time-dependent wave packet approach that goes beyond the fixed nuclei approximation. The double ionization pathways are analyzed by following the evolution of the total wave function during and after the pulse. The rescattering of the first ionized electron produces a coherent superposition of excited molecular states which presents a pronounced transient H+H- character. This attosecond excitation is followed by field-induced double ionization and by the formation of short-lived autoionizing states which decay via double ionization. These two double ionization mechanisms may be identified by their signature imprinted in the kinetic-energy distribution of the ejected protons

Auger recombination and multiple exciton generation in colloidal two-dimensional perovskite nanoplatelets: Implications for light-emitting devices

International audienceImproving the understanding of multiple exciton interactions and dynamics in semiconductor nanostructures is mandatory for their successful use as photoactive materials in light convertors such as electroluminescent diodes, lasers, or single-photon sources. Here high-fluence and high-energy excitation effects are investigated in strongly confined two-dimensional (2D) lead iodide perovskite nanoplatelets (NPLs) using time-resolved photoluminescence and femtosecond transient absorption spectroscopy. Nonradiative Auger recombination (AR) is the dominant pathway for multiexciton recombination. Its dynamics are found to be subquadratic with the exciton density. Indeed, because of the limited exciton wave-function delocalization length, AR is limited by exciton diffusion in the 2D plane at moderate excitation fluence and takes place in several hundreds of picoseconds, with typical recombination rates on the order of 10–2 cm2/s. At high excitation fluence leading to an average interexciton distance comparable with the exciton delocalization length, the measured “intrinsic” AR time is faster than 10 ps and independent of the NPL composition. The strong dependence of the AR rate on the interexciton distance allows us to identify the recombination resulting from multiple exciton generation, involving the reaction of “geminate biexcitons”, upon excitation at low fluence with high-energy photons

Multiple exciton generation and Auger recombination in strongly confined colloidal 2D perovskite nanoplatelets

International audienc

Multiple exciton generation and Auger recombination in strongly confined colloidal 2D perovskite nanoplatelets

International audienc

Charge carrier relaxation in colloidal FAPbI3_3 nanostructures using global analysis

International audienceWe study the hot charge carrier relaxation process in weakly confined hybrid lead iodide perovskite colloidal nanostructures, FAPbI$_3$ (FA = formaminidium), using femtosecond transient absorption (TA). We compare the conventional analysis method based on the extraction of the carrier temperature (Tc) by fitting the high-energy tail of the band-edge bleach with a global analysis method modeling the continuous evolution of the spectral lineshape in time using a simple sequential kinetic model. This practical approach results in a more accurate way to determine the charge carrier relaxation dynamics. At high excitation fluence (density of charge carriers above 10$^{18}$ cm$^{−3}$), the cooling time increases up to almost 1 ps in thick nanoplates (NPs) and cubic nanocrystals (NCs), indicating the hot phonon bottleneck effect. Furthermore, Auger heating resulting from the multi-charge carrier recombination process slows down the relaxation even further to tens and hundreds of picoseconds. These two processes could only be well disentangled by analyzing simultaneously the spectral lineshape and amplitude evolution

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized exciton-induced Stark and Phonon Bottleneck Effects

International audienceHybrid halide perovskites have emerged as rising materials for solution-processed photovoltaics, photodetectors, and light-emitting devices. For these applications, a deep understanding of the relaxation mechanism within the photoactive material is crucial since the rate at which hot carriers relax to the band edge will directly impact the performance of the optoelectronic devices. Several groups have investigated the cooling process in hybrid perovskite bulk materials as thin films using pump-probe spectroscopy [1–3].More recently, the development of low dimensional perovskite structures has enabled the investigation of carrier cooling in confined materials. In these systems, slower cooling is expected even at low excitation density due to the larger energy level separation. An apparent intrinsic phonon bottleneck was observed in weakly confined nanocrystals (NCs) [4]. However, contradictory results were reported in strongly confined 2D perovskites: thin films and colloidal nanoplatelets (NPLs). [5,6] Thus, a clear understanding of the confinement effect in the ultrafast relaxation dynamics is lacking.Here, using fs transient absorption spectroscopy (TA), we investigate the cooling rate in lead iodide-based perovskite 2D nanostructures. For such strongly confined systems, we propose an alternative method to characterize the cooling process by analyzing the TA spectral lineshape evolution of the first excitonic transitions. Indeed, the strong Stark signals in the TA spectra and the discrete nature of the optical transitions prevent to use of the classical analysis model of relaxation by extracting the time-dependent carrier temperatures or measuring the build-up of the band state bleaching, as applied previously in bulk and bulk-like perovskites nanocrystals. Using global data analysis, we extracted the rates of carrier relaxation after pump excitation above the band edge, at low and high excitation density. The ultrafast hot exciton relaxation in one- and three-monolayer thick NPLs confirms the absence of intrinsic phonon bottleneck effect, which was found independent of the nature of the internals cations. Remarkably, we found an enhanced delayed cooling rate at higher carrier densities known as the hot phonon bottleneck effect, in the 2D layered perovskite thin films compared to colloidal n=1 NPLs. This fact suggested a role of the ligands y/o sample superficial state in the cooling process

Exciton Cooling in 2D Perovskite Nanoplatelets: Rationalized Carrier-Induced Stark and Phonon Bottleneck Effects

International audienceUsing femtosecond transient absorption (fs-TA), we investigate the hot exciton relaxation dynamics in strongly confined lead iodide perovskite nanoplatelets (NPLs). The large quantum and dielectric confinement leads to discrete excitonic transitions and strong Stark features in the TA spectra. This prevents the use of conventional relaxation analysis methods extracting the carrier temperature or measuring the buildup of the band-edge bleaching. Instead, we show that the TA spectral line shape near the band-edge reflects the state of the system, which can be used to probe the exciton cooling dynamics. The ultrafast hot exciton relaxation in one- to three- monolayer-thick NPLs confirms the absence of intrinsic phonon bottleneck. However, excitation fluence-dependent measurements reveal a hot phonon bottleneck effect, which is found to be independent of the nature of the internal cations but strongly affected by the ligands and/or sample surface state. Together, these results suggest a role of the surface ligands in the cooling process