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Correlations between excitons, that is, electron-hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe1-xSx/ZnS alloyed core/shell quantum dots. We selectively map the electronic structure of multiexcitons and their correlations by using two- and three-quantum 2D spectroscopy, conducted in a simultaneous measurement. Our experiments reveal the characteristics of biexcitons and triexcitons such as transition dipole moments, binding energies, and correlated transition energy fluctuations. We determine the binding energies of the first six biexciton states by simulating the two-quantum 2D spectrum. By analyzing the line shape of the three-quantum 2D spectrum, we find strong correlations between biexciton and triexciton states. Our method contributes to a more comprehensive understanding of multiexcitonic species in quantum dots and other semiconductor nanostructures.

We introduce a new approach to transient absorption spectroscopy based on measurement at specific excitation intensities. A combination of datasets isolates individual perturbative nonlinear orders, allowing systematic high-intensity measurement of clean single- and multi-particle dynamics.

We derive a generic, quantitative relationship between the n-quantum signals observable in 2DES experiments. We use this relationship to isolate individual nonlinear orders free from higher-order contamination, with demonstration on squaraine copolymers.

Optical two-dimensional electronic spectroscopy (2DES) is now widely utilized to study excitonic structure and dynamics of a broad range of systems, from molecules to solid state. Besides the traditional experimental implementation using phase matching and coherent signal field detection, action-based approaches that detect incoherent signals such as fluorescence have been gaining popularity in recent years. While incoherent detection extends the range of applicability of 2DES, the observed spectra are not equivalent to the coherently detected ones. This raises questions about their interpretation and the sensitivity of the technique. Here we directly compare, both experimentally and theoretically, four-wave mixing coherently and fluorescence-detected 2DES of a series of squaraine dimers of increasing electronic coupling. All experiments are qualitatively well reproduced by a Frenkel exciton model with secular Redfield theory description of excitation dynamics. We contrast the spectral features and the sensitivities of both techniques with respect to exciton energies, delocalization, coherent and dissipative dynamics, and exciton-exciton annihilation. Discussing the fundamental and practical differences, we demonstrate the degree of complementarity of the techniques.

Exciton transport and exciton–exciton interactions in molecular aggregates and polymers are of great importance in natural photosynthesis, organic electronics, and related areas of research. Both the experimental observation and theoretical description of these processes across time and length scales, including the transition from the initial wavelike motion to the following long-range exciton transport, are highly challenging. Therefore, while exciton dynamics at small scales are often treated explicitly, long-range exciton transport is typically described phenomenologically by normal diffusion. In this work, we study the transition from wavelike to diffusive motion of interacting exciton pairs in squaraine copolymers of varying length. To this end we use a combination of the recently introduced exciton–exciton-interaction two-dimensional (EEI2D) electronic spectroscopy and microscopic theoretical modelling. As we show by comparison with the model, the experimentally observed kinetics include three phases, wavelike motion dominated by immediate exciton–exciton annihilation (10–100 fs), sub-diffusive behavior (0.1–10 ps), and excitation relaxation (0.01–1 ns). We demonstrate that the key quantity for the transition from wavelike to diffusive dynamics is the exciton delocalization length relative to the length of the polymer: while in short polymers wavelike motion of rapidly annihilating excitons dominates, in long polymers the excitons become locally trapped and exhibit sub-diffusive behavior. Our findings indicate that exciton transport through conjugated systems emerging from the excitonic structure is generally not governed by normal diffusion. Instead, to characterize the material transport properties, the diffusion presence and character should be determined., We measure excitation energy transport in conjugated polymers using direct observation of exciton pair dynamics.

Correlations between excitons, that is, electron-hole pairs, have a great impact on the optoelectronic properties of semiconductor quantum dots and thus are relevant for applications such as lasers and photovoltaics. Upon multiphoton excitation, these correlations lead to the formation of multiexciton states. It is challenging to observe these states spectroscopically, especially higher multiexciton states, because of their short lifetimes and nonradiative decay. Moreover, solvent contributions in experiments with coherent signal detection may complicate the analysis. Here we employ multiple-quantum two-dimensional (2D) fluorescence spectroscopy on colloidal CdSe

We use the technique of exciton–exciton-interaction two-dimensional spectroscopy to investigate exciton transport in squaraine copolymers. We reveal the influence of the polymer chain conformation by comparing the exciton dynamics in H-and J-type polymers.

We directly compare nonlinear coherently and fluorescence-detected two-dimensional electronic spectra of electronically coupled squaraine heterodimers. By identifying signatures of multi-exciton nonequilibrium dynamics and delocalization, we establish grounds for interpretation of these nonlinear spectroscopy implementations.

We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 nm in our sample. Further, we report two-color pump–probe experiments using two independent NOPA beamlines. We extract local variations of the excited-state dynamics of a monolayered 2D material (WSe2) that we correlate with low-energy electron microscopy (LEEM) and reflectivity measurements. Finally, we demonstrate the in situ sample preparation capabilities for organic thin films and their characterization via spatially resolved electron diffraction and dark-field LEEM.We describe a setup for time-resolved photoemission electron microscopy with aberration correction enabling 3 nm spatial resolution and sub-20 fs temporal resolution. The latter is realized by our development of a widely tunable (215–970 nm) noncollinear optical parametric amplifier (NOPA) at 1 MHz repetition rate. We discuss several exemplary applications. Efficient photoemission from plasmonic Au nanoresonators is investigated with phase-coherent pulse pairs from an actively stabilized interferometer. More complex excitation fields are created with a liquid-crystal-based pulse shaper enabling amplitude and phase shaping of NOPA pulses with spectral components from 600 to 800 nm. With this system we demonstrate spectroscopy within a single plasmonic nanoslit resonator by spectral amplitude shaping and investigate the local field dynamics with coherent two-dimensional (2D) spectroscopy at the nanometer length scale (“2D nanoscopy”). We show that the local response varies across a distance as small as 33 n...

Plasmonic resonators can be designed to support spectrally well-separated discrete modes. The associated characteristic spatial patterns of intense electromagnetic hot-spots can be exploited to enhance light-matter interaction. Here, we study the local field dynamics of individual hot-spots within a nanoslit resonator by detecting characteristic changes of the photoelectron emission signal on a scale of ∼12 nm using time-resolved photoemission electron microscopy (TR-PEEM) and by excitation with the output from a 20 fs, 1 MHz noncollinear optical parametric amplifier (NOPA). Surprisingly, we detect apparent spatial variations of the

Unravelling the nature of energy transport in multi-chromophoric photosynthetic complexes is essential to extract valuable design blueprints for light-harvesting applications. Long-range exciton transport in such systems is facilitated by a combination of delocalized excitation wavefunctions (excitons) and remarkable exciton diffusivities. The unambiguous identification of the exciton transport, however, is intrinsically challenging due to the system's sheer complexity. Here we address this challenge by employing a novel spectroscopic lab-on-a-chip approach: A combination of ultrafast coherent two-dimensional spectroscopy and microfluidics working in tandem with theoretical modelling. This allowed us to unveil exciton transport throughout the entire hierarchical supramolecular structure of a double-walled artificial light-harvesting complex. We show that at low exciton densities, the outer layer acts as an antenna that supplies excitons to the inner tube, while under high excitation fluences it protects the inner tube from overburning. Our findings shed light on the excitonic trajectories across different sub-units of a multi-layered supramolecular structure and underpin the great potential of artificial light-harvesting complexes for directional excitation energy transport., Comment: Submitted to Nature Communications; main manuscript 37 pages (incl. references) and 5 figures. SI 59 pages (incl. references) and 25 Figures

Exciton-exciton-interaction two-dimensional (EEI2D) spectroscopy is a fifth-order variant of 2D electronic spectroscopy. It can be used to probe biexciton dynamics in molecular systems and to observe exciton diffusion in extended systems such as polymers or light-harvesting complexes. The exciton transport strongly depends on the geometrical and energetic landscape and its perturbations. These can be of both local character, such as molecular orientation and energetic disorder, and long-range character, such as polymer kinks and structural domains. In the present theoretical work, we investigate the anisotropy in EEI2D spectroscopy. We introduce a general approach for how to calculate the anisotropy by using the response-function formalism in an efficient way. In numerical simulations, using a Frenkel exciton model with Redfield-theory dynamics, we demonstrate how the measurement of anisotropy in EEI2D spectroscopy can be used to identify various geometrical effects on exciton transport in dimers and polymers. Investigating a molecular heterodimer as an example, we demonstrate the utility of anisotropy in EEI2D spectroscopy for disentangling dynamic localization and annihilation. We further calculate the annihilation in extended systems such as conjugated polymers. In a polymer, a change in the anisotropy provides a unique signature for exciton transport between differently oriented sections. We analyze three types of geometry variations in polymers: a kink, varying geometric and energetic disorder, and different geometric domains. Our findings underline that employing anisotropy in EEI2D spectroscopy provides a way to distinguish between different geometries and can be used to obtain a better understanding of long-range exciton transport.

Two-dimensional electronic spectroscopy (2DES) can be realized in increasing nonlinear orders of interaction with the electric field, bringing new information about single- and multi-particle properties and dynamics. Furthermore, signals can be detected both coherently (C-2DES) and by fluorescence (F-2DES), with fundamental and practical differences. We directly compare the simultaneous measurements of four- and six-wave mixing C-2DES and F-2DES on an excitonic heterodimer of squaraine molecules. Spectral features are described in increasing orders of nonlinearity by an explicit excitonic model. We demonstrate that the four-wave-mixing spectra are sensitive to one-exciton energies, their delocalization and dynamics, while the six-wave-mixing spectra include information on bi-exciton and higher excited states including the state energies, electronic coupling, and exciton-exciton annihilation. We focus on the possibility to extract the dynamics arising from exciton-exciton interaction directly from the six-wave-mixing spectra. To this end, in analogy to previously demonstrated fifth-order coherently detected exciton-exciton-interaction 2DES (EEI2D spectroscopy), we introduce a sixth-order fluorescence-detected EEI2D spectroscopy variant.

We discriminate different field dynamics across distances as small as 33 nm within a plasmonic slit resonator using aberration-corrected photoemission electron microscopy and a tunable broadband optical parametric amplifier at 1 MHz repetition rate.

We present a new method for the measurement of the stimulated Raman spectrum based on time-dependent spatial modulation of a laser beam as it passes through a Raman active medium. This effect is similar to the instantaneous Kerr lensing and Kerr deflection yet involves resonant vibrations which result in a time-dependent refractive index change. We use sub-nanojoule pulses together with a sensitive pump-probe measurement apparatus to excite and detect the fine (10−5–10−4) temporal and spatial variations in intensity resulting from the Raman-induced Kerr effect. We demonstrate the effect by changing the spatial overlap between the pump and probe at the sample and measuring the time-dependent deformation of the probe beam’s cross section. This method is particularly useful for detection of low-frequency Raman lines, as we demonstrate by measuring the Raman spectrum of neat liquids in a cuvette.