Excitonic Quantum Coherence in Light Emission from CsPbBr3 Metal-Halide Perovskite Nanocrystals.

The decay of excited states via radiative and nonradiative paths is well understood in molecules and bulk semiconductors but less so in nanocrystals. Here, we perform time-resolved photoluminescence (t-PL) experiments on CsPbBr3 metal-halide perovskite nanocrystals, with a time resolution of 3 ps, sufficient to observe the decay of both excitons and biexcitons as a function of temperature. The striking result is that the radiative rate constant of the single exciton increases at low temperatures with an exponential functional form, suggesting quantum coherent effects with dephasing at high temperatures. The opposing directions of the radiative and nonradiative decay rate constants enable enhanced brightening of PL from excitons to biexcitons due to quantum effects, promoting a faster approach to the quantum theoretical limits of light emission. Ab initio quantum dynamics simulations reproduce the experimental observations of radiation controlled by quantum spatial coherence enhanced at low temperatures.

Light Emission from CsPbBr3 Metal Halide Perovskite Nanocrystals Arises from Dual Emitting States with Distinct Lattice Couplings.

Metal halide perovskite nanocrystals are under intense investigation for their outstanding optical and electronic properties. The presence of higher fine structure states, let alone nonequilibrium processes within the fine structure, and multiexcitonic fine structure remains poorly understood due to a lack of experimental probes. Here, we use time-resolved photoluminescence (t-PL) spectroscopy with an improvement from 100 to 3 ps resolution which reveals previously unobserved spectral dynamics from excitons to multiexcitons in 15 nm CsPbBr3 nanocrystals. The simple and immediate observation from temperature dependence is a previously unobserved fine structure to the multiexcitons. Further insight is gleaned from t-PL spectral bandwidth trajectories at extrema in temperature and exciton density. The bandwidth trajectories reveal the presence of a previously unobserved fine structure in excitons as well as multiexcitons. The bandwidth trajectories reveal a complex history, from multiexciton recombination to exciton thermalization to Auger heating to lattice thermalization. Whereas the amplitude of these spectral effects is large, ∼60 meV, modeling suggests that the spectral effects are mostly phonon based illustrating the importance of the lattice on light emission from metal halide perovskite nanocrystals.

Observing strongly confined multiexcitons in bulk-like CsPbBr3 nanocrystals.

We monitor the time-resolved photoluminescence (t-PL) from CsPbBr3 perovskite nanocrystals with a time resolution of 3 ps, which is fast enough to resolve emission from potential multiexcitonic states. Being 15 nm in length and twice the Bohr length, these nanocrystals are either weakly confined or bulk-like. In contrast to this expectation of weak confinement, emission from multiexcitons is observed with binding energies consistent with strongly confined quantum dots. In addition to emission from biexcitons, emission from triexcitons is observed. The triexciton emission includes both S and P recombination channels. Excitation with different amounts of excess energy yields the same PL spectral dynamics, indicating that there are no hot carrier effects, and the electronic structure of the absorbing states is the same. The kinetics of the multiexciton populations are presented in two ways. The kinetics are first shown in a spectrally integrated form, showing faster t-PL at higher fluences independent of excitation excess energy. Both excess energies show the same saturation response. In the second way of presenting the kinetics, the multiexciton populations are decomposed and presented as transients and saturation curves. These decomposed spectra into exciton, biexciton, and triexciton populations enable further insight into their kinetics and fluence dependence.

Exciton-polaron interactions in metal halide perovskite nanocrystals revealed via two-dimensional electronic spectroscopy.

Metal halide perovskite nanocrystals have been under intense investigation for their promise in optoelectronic devices due to their remarkable physics, such as liquid/solid duality. This liquid/solid duality may give rise to their defect tolerance and other such useful properties. This duality means that the electronic states are fluctuating in time, on a distribution of timescales from femtoseconds to picoseconds. Hence, these lattice induced energy fluctuations that are connected to polaron formation are also connected to exciton formation and dynamics. We observe these correlations and dynamics in metal halide perovskite nanocrystals of CsPbI3 and CsPbBr3 using two-dimensional electronic (2DE) spectroscopy, with its unique ability to resolve dynamics in heterogeneously broadened systems. The 2DE spectra immediately reveal a previously unobserved excitonic splitting in these 15 nm NCs that may have a coarse excitonic structure. 2D lineshape dynamics reveal a glassy response on the 300 fs timescale due to polaron formation. The lighter Br system shows larger amplitude and faster timescale fluctuations that give rise to dynamic line broadening. The 2DE signals enable 1D transient absorption analysis of exciton cooling dynamics. Exciton cooling within this doublet is shown to take place on a slower timescale than within the excitonic continuum. The energy dissipation rates are the same for the I and Br systems for incoherent exciton cooling but are very different for the coherent dynamics that give rise to line broadening. Exciton cooling is shown to take place on the same timescale as polaron formation, revealing both as coupled many-body excitation.

Perturbed free induction decay obscures early time dynamics in two-dimensional electronic spectroscopy: The case of semiconductor nanocrystals.

Two-dimensional electronic spectroscopy (2DES) has recently been gaining popularity as an alternative to the more common transient absorption spectroscopy due to the combination of high frequency and time resolution of 2DES. In order to advance the reliable analysis of population dynamics and to optimize the time resolution of the method, one has to understand the numerous field matter interactions that take place at an early and negative time. These interactions have historically been discussed in one-dimensional spectroscopy as coherent artifacts and have been assigned to both resonant and non-resonant system responses during or before the pulse overlap. These coherent artifacts have also been described in 2DES but remain less well-understood due to the complexity of 2DES and the relative novelty of the method. Here, we present 2DES results in two model nanocrystal samples, CdSe and CsPbI3. We demonstrate non-resonant signals due to solvent response during the pulse overlap and resonant signals, which we assign to perturbed free induction decay (PFID), both before and during the pulse overlap. The simulations of the 2DES response functions at early and negative time delays reinforce the assignment of the negative time delay signals to PFID. Modeling reveals that the PFID signals will severely distort the initial picture of the resonant population dynamics. By including these effects in models of 2DES spectra, one is able to push forward the extraction of early time dynamics in 2DES.

Atomic fluctuations in electronic materials revealed by dephasing.

The microscopic origin and timescale of the fluctuations of the energies of electronic states has a significant impact on the properties of interest of electronic materials, with implication in fields ranging from photovoltaic devices to quantum information processing. Spectroscopic investigations of coherent dynamics provide a direct measurement of electronic fluctuations. Modern multidimensional spectroscopy techniques allow the mapping of coherent processes along multiple time or frequency axes and thus allow unprecedented discrimination between different sources of electronic dephasing. Exploiting modern abilities in coherence mapping in both amplitude and phase, we unravel dissipative processes of electronic coherences in the model system of CdSe quantum dots (QDs). The method allows the assignment of the nature of the observed coherence as vibrational or electronic. The expected coherence maps are obtained for the coherent longitudinal optical (LO) phonon, which serves as an internal standard and confirms the sensitivity of the technique. Fast dephasing is observed between the first two exciton states, despite their shared electron state and common environment. This result is contrary to predictions of the standard effective mass model for these materials, in which the exciton levels are strongly correlated through a common size dependence. In contrast, the experiment is in agreement with ab initio molecular dynamics of a single QD. Electronic dephasing in these materials is thus dominated by the realistic electronic structure arising from fluctuations at the atomic level rather than static size distribution. The analysis of electronic dephasing thereby uniquely enables the study of electronic fluctuations in complex materials.

OPA-driven hollow-core fiber as a tunable, broadband source for coherent multidimensional spectroscopy.

Despite the impressive abilities of coherent multi-dimensional spectroscopy (CMDS), its' implementation is limited due to the complexity of continuum generation and required phase stability between the pump pulse pair. In light of this, we have implemented a system producing sub-10 fs pulses with tunable central wavelength. Using a commercial OPA to drive a hollow-core fiber, the system is extremely simple. Output pulse energies lie in the 40-80 µJ range, more than sufficient for transmission through the pulse shaping optics and beam splitters necessary for CMDS. Power fluctuations are minimal, mode quality is excellent, and spectral phase is well behaved at the output. To demonstrate the strength of this source, we measure the two-dimensional spectrum of CdSe quantum dots over a range of population times and find clean signals and clear phonon vibrations. This combination of OPA and hollow-core fiber provides a substantial extension to the capabilities of CMDS.

Fifth-order two-quantum absorptive two-dimensional electronic spectroscopy of CdSe quantum dots.

Two-quantum variants of two-dimensional electronic spectroscopy (2DES) have previously been used to characterize multi-exciton interactions in molecules and semiconductor nanostructures though many implementations are limited by phasing procedures or non-resonant signals. We implement 2DES using phase-cycling to simultaneously measure one-quantum and two-quantum spectra in colloidal CdSe quantum dots. In the pump-probe geometry, fully absorptive spectra are automatically acquired by measuring the sum of the rephasing and nonrephasing signals. Fifth-order two-quantum spectroscopy allows for direct access to multi-exciton states that may be obscured in excited state absorption signals due to population relaxation or third-order two-quantum spectra due to the non-resonant response.

Seeing Multiexcitons through Sample Inhomogeneity: Band-Edge Biexciton Structure in CdSe Nanocrystals Revealed by Two-Dimensional Electronic Spectroscopy.

The electronic structure of multiexcitons significantly impacts the performance of nanostructures in lasing and light-emitting applications. However, these multiexcitons remain poorly understood due to their complexity arising from many-body physics. Standard transient-absorption and photoluminescence spectroscopies are unable to unambiguously distinguish effects of sample inhomogeneity from exciton-biexciton interactions. Here, we exploit the energy and time resolution of two-dimensional electronic spectroscopy to access the electronic structure of the band-edge biexciton in colloidal CdSe quantum dots. By removing effects of inhomogeneities, we show that the band-edge biexciton structure must consist of a discrete manifold of electronic states. Furthermore, the biexciton states within the manifold feature distinctive binding energies. Our findings have direct implications for optical gain thresholds and efficiency droop in light-emitting devices and provide experimental measures of many-body physics in nanostructures.

Simple fiber-based solution for coherent multidimensional spectroscopy in the visible regime.

We report on a setup for coherent multidimensional spectroscopy based on visible continuum generation obtained by propagating 130 fs, <600 µJ pulses centered at 800 nm in a 2.5 m long hollow-core fiber. We find that with these modest input pulse requirements, the fiber can produce a stable, high brightness continuum spanning the 520-900 nm region, moreover in a single propagation step. The fiber exhibits 80% transmission, and the continuum features excellent spatial mode quality. In addition, spectral phase measurements suggest the possibility of a significantly self-compressed output in the visible, which simplifies aspects of the 2D spectrometer. The applicability of this simple, low-requirement source for 2D spectroscopy is demonstrated by performing a control experiment on the molecular dye Nile Blue.

Coherent multi-dimensional spectroscopy at optical frequencies in a single beam with optical readout.

Ultrafast coherent multi-dimensional spectroscopies form a powerful set of techniques to unravel complex processes, ranging from light-harvesting, chemical exchange in biological systems to many-body interactions in quantum-confined materials. Yet these spectroscopies remain complex to implement at the high frequencies of vibrational and electronic transitions, thereby limiting their widespread use. Here we demonstrate the feasibility of two-dimensional spectroscopy at optical frequencies in a single beam. Femtosecond optical pulses are spectrally broadened to a relevant bandwidth and subsequently shaped into phase coherent pulse trains. By suitably modulating the phases of the pulses within the beam, we show that it is possible to directly read out the relevant optical signals. This work shows that one needs neither complex beam geometries nor complex detection schemes in order to measure two-dimensional spectra at optical frequencies. Our setup provides not only a simplified experimental design over standard two-dimensional spectrometers but its optical readout also enables novel applications in microscopy.

The Effect of Exciton-Delocalizing Thiols on Intrinsic Dual Emitting Semiconductor Nanocrystals.

The emissive properties of thiol-capped CdSe nanocrystals (NCs) with intrinsic dual emission are investigated through temperature-dependent photoluminescence (PL) measurements. We demonstrate the influence of thiols on the relative PL intensities of the core and surface emissive states, as well as on the observed Stokes shifts. A redshift of both the core and surface PL in comparison with phosphonate-capped NCs is consistent with recent work exploring the effect of thiols as excitonic hole-delocalizing ligands. This observation is consistent with prior reports suggesting that surface excitons originate from electrons bound to cadmium trap states.

Linking surface chemistry to optical properties of semiconductor nanocrystals.

The intricate chemistry occurring at the surface of semiconductor nanocrystals is crucial to tailoring their optical properties to a myriad of applications. This perspective aims to re-evaluate long held ideas in semiconductor nanocrystal surface science in the light of a body of new and rich research. We start by reviewing recent developments in ligand chemistry, followed by a discussion of spectroscopic and computational approaches used for advancing the poorly-understood electronic structure of the surface. With the insights gained, we show how the surface impacts emissive behaviour and we summarize strategies to increase fluorescent quantum yield. This discussion is followed by a review of experimental approaches for quantitative analysis of the surface chemistry at concentrations relevant to spectroscopic measurements. We end by highlighting some new directions in ligand chemistry, namely all-inorganically passivated semiconductor nanocrystals and new applications of surface emission.

Terahertz bandwidth all-optical modulation and logic using multiexcitons in semiconductor nanocrystals.

Optical pumping of semiconductor nanocrystals with femtosecond pulse sequences was performed in order to modulate multiexciton populations. We show for the first time that control of multiexciton populations produces high speed modulation of stimulated emission. Upon the basis of the speed of multiexcitonic processes in nanocrystals, we show modulation rates approaching 1 THz by virtue of strong quantum confinement effects. Employing femtosecond optical pulse sequences, we demonstrate all-optical logic using these nanocrystals in two forms: an AND gate, and an inverter, a key step toward all optical signal processing.

Unraveling the excitonics of light emission from metal-halide perovskite quantum dots.

Metal halide semicondictor perovskites have been under intense investigation for their promise in light absorptive applications like photovoltaics. They have more recently experienced interest for their promise in light emissive applications. A key aspect of perovskites is their glassy, ionic lattice that exhibits dynamical disorder. One possible result of this dynamical disorder is their strong coupling between electronic and lattice degrees of freedom which may confer remarkable properties for light emission such as defect tolerance. How does the system, comprised of excitons, couple to the bath, comprised of lattice modes? How does this system-bath interaction give rise to novel light emissive properties and how do these properties give insight into the nature of these materials? We review recent work from this group in which time-resolved photoluminescence spectroscopy is used to reveal such insights. Based upon a fast time resolution of 3 ps, energy resolution, and temperature dependence, a wide variety of insights are gleaned. These insights include: lattice contributions to the emission linewidths, multiexciton formation, hot carrier cooling, excitonic fine structure, single dot superradiance, and a breakdown of the Condon approximation, all due to complex structural dynamics in these materials.

Two-Dimensional Electronic Spectroscopy Reveals Dynamics within the Bright Fine Structure of CdSe Quantum Dots.

Semiconductor quantum dots are characterized by a discrete excitonic structure featuring coarse as well as fine structure. The lowest fine structure states have splittings into bright-dark states which are now well confirmed by single dot spectroscopy. In contrast, the splitting of the lowest coarse exciton into bright-bright fine structure states has not been observed nor the dynamics between these states. Here, we use the unique combination of time and energy resolution of two-dimensional electronic spectroscopy to directly observe the fine structure splittings into a bright-bright doublet. These splittings are strongly size dependent, with population relaxation on the <100 fs time scale.

Hot Excitons Cool in Metal Halide Perovskite Nanocrystals as Fast as CdSe Nanocrystals.

The idea of phonon bottlenecks has long been pursued in nanoscale materials for their application in hot exciton devices, such as photovoltaics. Decades ago, it was shown that there is no quantum phonon bottleneck in strongly confined quantum dots due to their physics of quantum confinement. More recently, it was proposed that there are hot phonon bottlenecks in metal halide perovskites due to their physics. Recent work has called into question these bottlenecks in metal halide perovskites. Here, we compare hot exciton cooling in a range of sizes of CsPbBr3 nanocrystals from weakly to strongly confined. These results are compared to strongly confined CdSe quantum dots of two sizes and degrees of quantum confinement. CdSe is a model system as a ruler for measuring hot exciton cooling being fast, by virtue of its efficient Auger-assisted processes. By virtue of 3 ps time resolution, the hot exciton photoluminescence can now be directly observed, which is the most direct measure of the presence of hot excitons and their lifetimes. The hot exciton photoluminescence decays on nearly the same 2 ps time scale on both the weakly confined perovskite and the larger CdSe quantum dots, much faster than the 10 ps cooling predicted by transient absorption experiments. The smaller CdSe quantum dot has still faster cooling, as expected from quantum size effects. The quantum dots of perovskites show extremely fast hot exciton cooling, decaying faster than detection limits of <1 ps, even faster than the CdSe system, suggesting the efficiency of Auger processes in these metal halide perovskite nanocrystals and especially in their quantum dot form. These results across a range of sizes of nanocrystals reveal extremely fast hot exciton cooling at high exciton density, independent of composition, but dependent upon size. Hence these metal halide perovskite nanocrystals seem to cool heavily following quantum dot physics.

Two-color two-dimensional electronic spectroscopy using dual acousto-optic pulse shapers for complete amplitude, phase, and polarization control of femtosecond laser pulses.

We demonstrate a dual pulse-shaper setup capable of independent polarization, phase, and amplitude control over each pulse. By using active phase stabilization, we achieve a phase stability of ~λ/314 between the two pulse shapers, making the dual-shaper setup suitable for both two-quantum and one-quantum measurements. The setup is compact and easily switchable between pump-probe and collinear geometries. We further illustrate the functionality of the dual-shaper setup by performing two-color 2D visible spectroscopy on colloidal CdSe quantum dots in pump-probe geometry.

A microscopic picture of surface charge trapping in semiconductor nanocrystals.

Several different compositions of semiconductor nanocrystals are subjected to numerous spectroscopic techniques to elucidate the nature of surface trapping in these systems. We find a consistent temperature-dependent relationship between core and surface photoluminescence intensity and marked differences in electron-phonon coupling for core and surface states based on ultrafast measurements and Resonance Raman studies, respectively. These results support a minimal model of surface charge trapping applicable to a range of nanocrystal systems involving a single surface state in which the trapped charge polarization leads to strong phonon couplings, with transitions between the surface and band edge excitonic states being governed by semiclassical electron-transfer theory.

A Brief Discussion of Chemical Kinetics versus Chemical Dynamics.

Chemical kinetics and chemical dynamics are distinct but related topics. They arise again in contemporary physical chemistry often in terms of charge carrier processes in new materials which interconvert light and electrical energy on a distribution of time scales. With this recent rise in the application of concepts in kinetics and dynamics to new problems, there has also arisen confusion about the differences and connections between the two. Here, we briefly review the relationship between kinetics and dynamics in chemical processes, with particular emphasis on the photochemical and photophysical response of a system.