Niek. F. van Hulst

Action-detected two-dimensional electronic spectroscopy (A-2DES) has emerged as a powerful alternative to coherent 2DES, offering unique advantages, including freedom from geometrical constraints imposed by directional phase-matching requirements, lower demands on sample quality, operando capability, and seamless integration with microscopy. By measuring incoherent observables, such as fluorescence, photocurrent, photoelectrons, or photoions from excited-state populations, action detection enables studies of systems that might be difficult to access using traditional coherent methods. However, this approach introduces fundamental challenges, including incoherent mixing, additional Liouville pathways, and detection nonlinearities that require careful consideration. This article provides the theoretical foundations, experimental implementations, key advantages, and critical limitations of action-detected spectroscopy, providing guidance for optimal application of this rapidly evolving technique across diverse systems ranging from single molecules to working photovoltaic devices.

Action-detected two-dimensional electronic spectroscopy (A-2DES) provides valuable insights into ultrafast dynamics within functional materials and devices by measuring incoherent signals like photocurrent. This work details the implementation and optimization of a pulse-shaper-based A-2DES setup, focusing on methodological strategies crucial for acquiring high-fidelity data. We present a comprehensive analysis of phase modulation routines, elucidating the critical interplay between pattern parameters (N, n _i ), pattern repetitions (N _rep ), laser repetition rate, and acousto-optic pulse shaper constraints (e.g., streaming rate, radio frequency generator nonlinearities). Utilizing a perovskite solar cell as a model system, we systematically identify and characterize significant inaccuracies inherent to A-2DES measurements. These include distortions originating from Fourier transform processing of improperly trimmed time-domain data (phase leakage), signal accumulation effects due to insufficient sample response discharge between pulse sequences at high repetition rates, and shortcomings induced by pulse shaper operation at elevated streaming powers. Crucially, we demonstrate robust data post-processing strategies, including precise data point selection for Fourier analysis and a phase correction routine, to mitigate these imperfections and retrieve accurate 2D spectra effectively. This rigorous methodological investigation and anomalous features characterization provide essential guidelines for optimizing pulse-shaper-based A-2DES experiments, ensuring data integrity and enabling reliable extraction of complex photophysical information in complex systems.

Understanding the ultrafast transport properties of charge carriers in transition metal dichalcogenides is essential for advancing technologies based on these materials. Here, we study MoSe₂ crystals with thicknesses down to the monolayer, combining ultrafast spatiotemporal microscopy and quantitative microscopic modelling. Crucially, we obtain the intrinsic ultrafast transport dynamics by studying suspended crystals that do not suffer from detrimental substrate effects. In mono- and bilayer crystals, we identify four sequential transport regimes. The first two regimes involve high-energy non-thermalized and quasi-thermalized carriers that propagate rapidly with diffusivities up to 1000 cm²/s. After ~1.5 ps, a remarkable third regime occurs with apparent negative diffusion, finally followed by exciton propagation limited by trapping into defect states. Interestingly, for trilayer and thicker crystals, only the first and last regimes occur. This work underscores the role of traps and dielectric environment in electron transport, offering valuable insights for the development of (flexible) (opto)electronic applications.

While it has long been accepted that the role of momentum dark excitons in the photoresponse of transition metal dichalcogenides (TMDs) is critical, their weak optical signature inhibits their study through conventional means. Here we expose the room-temperature contributions of both bright and dark excitons to the behavior of a TMD, WSe₂, from monolayer to multilayer to bulk. To do so, we present dual action spectroscopy, a photocurrent- and luminescence-detected Fourier-transform excitation spectroscopy scheme, to microscopically map the energy landscape of WSe₂. While bright excitons naturally dominate the luminescence response of the material, dark excitons dominate the current response. Notably, the dark KK' exciton is more accessible than the ground state KΛ, while current maps reveal a disparity in the diffusivity of the two states. This work provides the basis for a new, current-detected approach to study the dynamics of dark exciton states across different materials.

Spatiotemporal microscopy plays an important role in the quest for highly efficient light harvesting materials as it allows direct tracking of the nanoscale transport of excitons, the carriers of the photon energy. Unfortunately, achieving high resolution in both space and time often requires scanning beam spots or delay lines, limiting these techniques to specialized research groups. To overcome this problem, we introduce a novel implementation of photoluminescence-detected exciton tracking using a camera composed of an array of single-photon avalanche diodes (SPADs), gated with ∼150 ps temporal accuracy. The use of such a SPAD camera drastically simplifies the experiment, is free of moving parts, and provides at least 1 order of magnitude increase in photon collection efficiency due to the parallel multipixel acquisition. Moreover, the camera allows one to implement different super-resolution excitation strategies. Here we show both point and structured excitation in the same device by simply changing the optical element. The structured illumination allows direct retrieval of the diffusion from a single time-resolved imaging without fitting, even at fluences far below exciton-exciton annihilation conditions. We tested the SPAD camera effectiveness by studying the exciton diffusion properties of the organic photovoltaic material PM6, where we measured an exciton diffusion length of 45 nm. Certainly our new implementation, boosted by rapid advances in SPAD technology, will extend the range of both users and applications of spatiotemporal microscopy.

Optical diffraction tomography (ODT) is a powerful noninvasive 3D imaging technique, but its combination with broadband light sources is difficult. In this study, we introduce ultrabroadband ODT, covering over 150 nm of visible spectral bandwidth with a lateral spatial resolution of 150 nm. Our work addresses a critical experimental gap by enabling the measurement of broadband refractive index changes in 3D samples, crucial information that is difficult to assess with existing methodologies. We present broadband, spectrally resolved ODT images of HeLa cells, obtained via pulse-shaping-based Fourier transform spectroscopy. The spectral observations enabled by ultrabroadband ODT, combined with material-dependent refractive index responses, allow for precise three-dimensional identification of nanoparticles within cellular structures. Our work represents a crucial step toward time and spectrally resolved tomography of complex 3D structures with implications for life and materials science applications.

We present a nonlinear spectroelectrochemical technique to investigate photosynthetic protein complexes. The PEC2DES setup combines photoelectrochemical detection (PEC) that selectively probes the protein photogenerated charges output with two-dimensional electronic spectroscopy (2DES) excitation that spreads the nonlinear optical response of the system in an excitation-detection map. PEC allows us to distinguish the contribution of charge separation (CS) from other de-excitation pathways, whereas 2DES allows us to disentangle congested spectral bands and evaluate the exciton dynamics (decays and coherences) of the photosystem complex. We have developed in operando phase-modulated 2DES by measuring the photoelectrochemical reaction rate in a biohybrid electrode functionalized with a plant photosystem complex I-light harvesting complex I (PSI-LHCI) layer. Optimizing the photoelectrochemical current signal yields reliable linear spectra unequivocally associated with PSI-LHCI. The 2DES signal is validated by nonlinear features like the characteristic vibrational coherence at 750 cm^-1. However, no energy transfer dynamics is observed within the 450 fs experimental window. These intriguing results are discussed in the context of incoherent mixing resulting in reduced nonlinear contrast for multichromophoric complexes, such as the 160 chlorophyll PSI. The presented PEC2DES method identifies generated charges unlike purely optical 2DES and opens the way to probe the CS channel in multichromophoric complexes.

Time-domain spectroscopy encompasses a wide range of techniques, such as Fourier-transform infrared, pump–probe, Fourier-transform Raman, and two-dimensional electronic spectroscopies. These methods enable various applications, such as molecule characterization, excited state dynamics studies, or spectral classification. Typically, these techniques rarely use sampling schemes that exploit the prior knowledge scientists typically have before the actual experiment. Indeed, not all sampling coordinates carry the same amount of information, and a careful selection of the sampling points may notably affect the resulting performance. In this work, we rationalize, with examples, the various advantages of using an optimal sampling scheme tailored to the specific experimental characteristics and/or expected results. We show that using a sampling scheme optimizing the Fisher information minimizes the variance of the desired parameters. This can greatly improve, for example, spectral classifications and multidimensional spectroscopy. We demonstrate how smart sampling may reduce the acquisition time of an experiment by one to two orders of magnitude, while still providing a similar level of information.

With the increasing demand for new materials for light-harvesting applications, spatiotemporal microscopy techniques are receiving increasing attention as they allow direct observation of the nanoscale diffusion of excitons. However, the use of pulsed and tightly focused laser beams generates light intensities far above those expected under sunlight illumination, leading to photodamage and nonlinear effects that seriously limit the accuracy and applicability of these techniques, especially in biological or atomically thin materials. In this work, we present a novel spatiotemporal microscopy technique that exploits structured excitation in order to dramatically decrease the excitation intensity, up to 10,000-fold when compared with previously reported spatiotemporal photoluminescence microscopy experiments. We tested our method in two different systems, reporting the first exciton diffusion measurement at illumination conditions below sunlight, both considering average power and peak exciton densities in an organic photovoltaic sample (Y6), where we tracked the excitons for up to five recombination lifetimes. Next, nanometer-scale energy transport was directly observed for the first time in both space and time in a printed monolayer of the light-harvesting complex 2 from purple bacteria.

This study elucidates the information content that is extracted from action-2D electronic spectroscopy (A-2DES) when the output intensity is not proportional to the number of excitations generated. Such a scenario can be realized in both fluorescence and photocurrent detection because of direct interaction like exciton-exciton annihilation or indirect effects in the signal generation or detection. By means of an intuitive probabilistic model supported by nonlinear response theory, the study concludes that in molecular assemblies the ground-state bleaching contribution can dominate the nonlinear signal and partially or completely hide the stimulated emission. In this case, the spectral effect resembles incoherent mixing, even in the absence of exciton-exciton annihilation, implying reduced information about the excited-state dynamics with an increasing number of chromophores. This finding has important implications for the selection of samples for A-2DES as well as for its interpretation.