Andreas Jakobsson (Lund University)

Title: Exploiting structure in signals

Abstract: Most spectroscopic signals contain a detailed structure that can be exploited when forming estimators of their different characteristics. Depending on what is relevant for a given research question, different characteristics may be more relevant than others – and sometimes notable prior knowledge is available that can further improve how these are estimated. In this talk, I will discuss some ideas on how one may do so and also discuss how one can optimally select which information to collect given the prior information that is available. I will also include some reflections on how to best collaborate with applied mathematicians in problems such as these.

Barbara Fresch (University of Padova)

Title: Dissecting the Action Signal through Simple Theoretical Reflections: Effects of Incoherent Mixing, Disorder and Coherent Evolution 

Abstract: The action-detected signal in 2D electronic spectroscopy can, at least theoretically, be dissected into distinct components by combining perturbative and non-perturbative approaches in the solution of light-induced quantum dynamics. In this talk, I will discuss results from numerically simulated action spectra of model systems, returning a clear picture of the key mechanisms governing signal formation. This analysis provides insights into the relation between the coherent and action response, the nature of incoherent mixing signals, and the potential of action detection to probe excited-state dynamics in interacting and disordered molecular systems. Notably, the presence of a large static background in the spectrum and the potential to observe excited-state dynamics with enhanced contrast—compared to coherent detection—are two sides of the same coin, with significant implications for sample selection.

Donatas Zigmantas (Lund University)

Title: Charge carrier dynamics studied with time-resolved photoemission electron microscopy

Abstract: There is a clear need to understand photo-excited charge carrier dynamics in semiconductor and other materials used in photoactive devices. One example of relevance can be found in the new generation photovoltaic devices employing hot charge carriers, which could beat the solar cell efficiency limit (Shockley–Queisser). At the same time, because of the varying morphology of the materials on the nm-mm scale, it is important to study spatially-resolved dynamic processes. To achieve these goals, we use time-resolved photoemission electron microscopy (TR-PEEM), an action-detected spectroscopy technique which simultaneously provides femtosecond time resolution and tens of nanometers spatial resolution. A few years ago we used this method to study hot carrier dynamics in individual InAs nanowires. We have recently added a couple of additional modalities to TR-PEEM: excitation frequency resolution and transient grating excitation. Whereas the former allows for targeted investigation of hot charge carriers and surface defects such as intraband traps, the latter enables studies of charge carrier diffusion and therefore mobility in nanomaterials, as demonstrated in the experiments on InP nanoplatelets.

Edoardo Amarotti (Lund University)

Title: Perovskite solar cell dynamics probed with photocurrent action-detected 2D spectroscopy

Abstract: Over the past decade, significant efforts in the energy sector have been directed toward developing alterantive solutions to replace non-renewable energy sources. Among the various renewable energy options, solar energy has made remarkable strides, emerging as a leader in innovation and adoption. Notably, perovskite-based solar cells have continually shattered efficiency benchmarks, with recent advancements pushing efficiencies beyond 25%. Despite these impressive achievements, the ultrafast dynamics governing the photophysics of these systems remain not fully understood. In this study, we delve into the photophysics of highly efficient perovskite-based solar cells using the cutting-edge technique of action-detected 2D spectroscopy. This approach involves exciting the sample with a sequence of four phase-modulated pulses, which induces an incoherent signal—measured as photocurrent—captured via a voltage readout card. In this work, we observe relaxation dynamics for different samples of hybrid perovskite-based solar cells, in which each sample presents a different ratio of FAPbI3 and MAPbBr3.

Elisabetta Collini (University of Padova)

Title: Photocurrent-detected 2DES on QD-based Devices 

Abstract: Photocurrent-detected 2D electronic spectroscopy (PC-2DES) was employed to investigate the ultrafast behavior of electro-optical devices made by multilayers of CdSe quantum dots. The data emphasized the significant contribution of red states, which were only partially observed in the linear absorption spectrum. These results are examined in terms of the inhomogeneity of the QDs, the impact of dimerization, trap states, and the potential development of a Stark shift. 

Jennifer Ogilvie (University of Ottawa)

Title: Photosynthetic Energy Transfer: Missing in Action (Detected Spectroscopy)? 

Abstract: I will discuss our recent implementations of fluorescence-detected two-dimensional electronic spectroscopy (F-2DES) and our studies of the light-harvesting II (LH2) complex from purple bacteria as a model system to understand F-2DES’s ability to monitor energy transfer in multichromophoric systems. We demonstrate that the B800−B850 energy transfer process in LH2 is weak but observable in F-2DES, unlike in coherently detected 2DES where the energy transfer is visible with 100% contrast. We explain the weak signatures using a disordered excitonic model that accounts for experimental conditions. We find that the prominence of excited-state dynamics in action-detected spectroscopy offers a unique probe of excitonic delocalization in multichromophoric systems.

Joe Wragg (ICFO)

Title: Dual Action Spectroscopy for Diagnosing Energy Transport in Photoactive Materials

Abstract: How an exciton coverts its energy at the end of its life determines the response of a photoactive material. Understanding which states lead to what outcome can help us maximise the desired output of a material. Through dual action spectroscopy, we untangle the complex excitonic picture in WSe2, a 2d semiconductor, to understand which exciton states lead to charge production and which states recombine. Tracking the exciton from its creation in excitation, to its death in detection.

Julien Luettig (University of Michigan)

Title: Characterizing Artifacts in Action-Detected Two-Dimensional Spectroscopy Using Pulse Shapers

Abstract: Pulse shapers are widely utilized in action-detected two-dimensional (2D) spectroscopy to generate multi-pulse sequences with variable time delay. Pulse shapers offer several advantages such as inherent phase stability, the ability to operate in the rotating frame, and the possibility of phase cycling. However, under the conditions necessary for action-detected 2D spectroscopy experiments, pulse shapers are increasingly likely to exhibit nonlinear responses that can distort the 2D spectrum and obscure the molecular response. To obtain signals free from pulse shaper nonlinearities, careful use of pulse shapers is necessary. Here, we characterize the artifacts observed in action-detected 2D spectra produced by acousto-optic programmable dispersive filters (Dazzler, Fastlite). We illustrate the effects of pulse shaper nonlinearity and demonstrate how to effectively account for them in data collection and processing, enabling artifact-free action-detected 2D spectroscopy measurements under certain conditions.

Matteo Bruschi (Charles University)

Title: Taking Action (-detected Spectroscopy) to Parametrize the Multi-Exciton Frenkel Model

Abstract: Exciton-Exciton Annihilation (EEA), a process resulting in the net loss of an exciton from a pair upon encounter, represents a known source of incoherent mixing in action-detected spectroscopy. Traditionally, EEA has been described using phenomenological models that rely on several assumptions. While recent advancements in ultrafast optical spectroscopy have proven the inadequacy of these models, a comprehensive understanding of the EEA process remains elusive. To fill this gap, we are developing a multi-exciton description of multichromophoric systems by extending the Frenkel exciton model in the following ways: (i) including higher-excited states for each chromophore, (ii) accounting for the excitonic coupling between higher-excited and multi-exciton states, and (iii) introducing Exciton-Exciton Interaction (EEI) via a Hubbard-type term. However, parametrizing such a model requires detailed information about the excited states of individual chromophores, i.e., transition energies, static dipole moments, and transition dipole moments. In this regard, we will discuss how action-detected spectroscopies can be leveraged to extract these parameters.

Martin Zanni (University of Wisconsin-Madison)

Title: Incoherent Mixing and Technical Challenges in Action Detected 2D Spectroscopies

Abstract: Action spectroscopies, such as fluorescence detection, photoelectron detection, and current detection, offer unique insights that are often not accessible with standard absorption-based ultrafast 2D spectroscopy. However, these techniques face technical challenges, particularly due to the presence of incoherent mixing signals that can obscure true third-order responses. In this talk, I will discuss methods to differentiate incoherent artifacts from true signals by examining their polarization responses. Additionally, I will propose pulse sequences designed to minimize artifacts and share best practices for generating pulse trains that ensure artifact-free measurements. By addressing these technical challenges, action spectroscopies can become a more robust tool for studying ultrafast dynamics. 

Pavel Maly (Charles University)

Title: Towards fluorescence-detected spectroscopy of large systems 

Abstract: Action-detected spectroscopy faces a challenge in form of so-called incoherent mixing of excitations during the signal emission, caused by nonlinear interaction such as exciton--exciton annihilation.  This constant mixing background threatens to render excitaiton dynamics in large systems such as photosynthetic complexes invisible. I will discuss a general spectro-temporal symmetry of action-detected 2DES and PP spectra that allows system-independent elimination of stationary signals such as incoherent mixing, emphasizing excitation dynamics. Time-permitting, I will touch on other ways to suppress the EEA-induced mixing, as well as on our progress with implementing fluorescence-detected PP in a FLIM microscope. 

Stefan Müller (Julius-Maximilians-Universität Würzburg)

Title: Background-free extraction of multiexciton dynamics in higher-order fluorescence-detected nonlinear spectroscopy 

Abstract: We introduce two-quantum fluorescence-detected pump–probe spectroscopy and present a strategy to suppress undesired signal contributions using squaraine dimers and polymers as examples. These undesired contributions include static background from cross-population pathways as well as pulse-overlap-associated contributions such as false two-quantum coherence. 

Tönu Pullerits (Lund University)

Title: Action-Detected CMDS: How Did It Start and Where Is It Going?

Abstract: Action-detected CMDS has been around for more than 15 years. In this presentation, I will revisit its origins and trace its development over the years, highlighting the various flavours of the method that have emerged. I will discuss its key strengths and limitations, as well as the insights it has provided into complex molecular and nanoscale systems. Finally, I will explore the potential future directions of action-detected CMDS, considering both theoretical advancements and experimental innovations.

Xiaoji Xu (Lehigh University)

Title: Photothermal Action-based AFM-2DIR Spectroscopy 

Abstract: In this presentation, I will present the recent development of Atomic Force Microscopy-based Two-dimensional Infrared (AFM-2DIR) spectroscopy. AFM-2DIR is a photothermal action-based 2DIR spectroscopy that combines atomic force microscopy (AFM)’s mechanical detection of the photothermal action with femtosecond infrared (IR) pulse sequences. This nanoscale spectroscopy achieves a spatial precision of 10–20 nanometers, enabling access to vibrational anharmonicity and energy transfers in heterogeneous materials. AFM-2DIR bridges the 2D-IR spectroscopy with AFM-based IR microscopy, connecting these two frontiers of IR techniques. In this presentation, I will also discuss its current limitations and ideas to overcome challenges to achieve its full potential, as well as the characteristics of photothermal action detection as a route to build multidimensional action spectroscopy. 

Zachary Faitz (University of Wisconsin-Madison)

Title: Elucidating Current-Generating Exciton Dynamics in Carbon Nanotube-Based Photovoltaic Devices

Abstract: Single-walled, semiconducting carbon nanotubes are a promising material for next generation photovoltaics that offer high efficiencies and chemical stability. The dynamics of these systems are of particular interest because they contain long-lived, mobile excitons that are known to transfer between carbon nanotubes with different absorptions. We perform simultaneous acquisition of 2D white light and 2D photocurrent to measure the dynamics of all excitons in the sample and those that create photocurrent, respectively. The spectra are measured using a polarization condition that eliminates non-coherent signals in the action-detected spectra, producing background-free 2D photocurrent spectra. We observe excitons that migrate between nanotubes before dissociating to electrons and excitons that are immediately dissociated. Furthermore, we observe hole transfer, the effects of minority chiralities, and potentially trion formation. The ability to compare all excitons to current-generating excitons, gives insight into the pathways within the photoactive layers of devices that informs device design.