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, ni), pattern repetitions (Nrep), 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.

Full control of light polarization is one of the most sought-after functionalities in nanophotonics since it allows the replacement of bulky optical components like wave retarders. Here, the study reports the theoretical and experimental demonstration of an ultra-compact dual-mode frequency selective polarization controller that leverages the topological features of symmetry protected quasi-bound states in the continuum (q-BICs) supported by a silicon-based nanostructure. Thanks to q-BIC resonances, arbitrarily polarized incoming light can be converted into linearly polarized light without resorting to the local phase tuning mechanisms that characterize optical metasurfaces. Moreover, the dual-mode operating regime allows to select the transmitted polarization without modifying the device orientation, therefore overtaking the concept of the wire grid polarizer. The experimental findings show that the proposed meta-polarizer possesses an extinction ratio of approximate to 40 dB for two linear cross-polarization excitations. These results pave the way for a novel class of ultra-compact devices that can be used to compensate unwanted birefringence in optical fibers or to control polarization in complex media environments. A properly designed silicon-based metasurface that supports quasi-bound states in the continuum can convert arbitrarily polarized incoming light into either horizontal or vertical linearly polarized light without rotating or actively modifying the nanostructure. The experimental findings show that the proposed meta-polarizer possesses an extinction ratio of approximate to 40dB for two linear cross-polarization excitations.image

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.

Using ultrafa st thermo-modulation microscopy, we show that the spatio-temporal heat diffusion in gold films has an initial ps-scale, electron-dominated diffusion, followed by an unexpected negative diffusion stage, and only then a much slower phonon-dominated diffusion.

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.

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.

We present a theoretical and experimental demonstration of a single layer silicon-based metasurface able to convert any arbitrarily polarized incoming light into linearly polarized light. Our metasurface leverages the topological features of symmetry protected quasi-bound states in the continuum and shows an experimental extinction ratio of ~40dB for two linear cross-polarization excitations, paving the way for a novel class of ultra-compact multi-frequency linear polarizers.

Non-fullerene acceptors (NFAs) have delivered advancement in bulk heterojunction organic solar cell efficiencies, with a significant milestone of 20% now in sight. However, these materials challenge the accepted wisdom of how organic solar cells work. In this work we present a neat Y6 device with an efficiency above 4.5%. We thoroughly investigate mechanisms of charge generation and recombination as well as transport in order to understand what is special about Y6. Our data suggest that Y6 generates bulk free charges, with ambipolar mobility, which can be extracted in the presence of transport layers.

Since cameras are too slow for shot-to-shot acquisition, pump-probe microscopy was limited to single-point detection. We demonstrate widefield pump-probe images spanning 100×100 μm2 using an all-optical lock-in camera based on off-axis holography. We use it to study exciton diffusion in perovskites around 100 diffraction-limited spots.

Ultrafast light-induced spatiotemporal dynamics in metals in the form of electron and/or phonon heating is a fundamental physical process that has tremendous practical relevance. In particular, understanding the resulting lateral heat transport is of key importance for various (opto)electronic applications and thermal management but has attracted little attention. Here, by using scanning ultrafast thermo-modulation microscopy to track the spatiotemporal electron diffusion in thin gold films, we show that a few picoseconds after the optical pump there is unexpected heat flow from phonons to electrons, accompanied by negative effective thermal diffusion, characterized by shrinking of the spatial region with increased temperature. Peculiarly, this occurs on the intermediate time scale, between the few picosecond long thermalization stage and the many picosecond stage dominated by thermoacoustic vibrations. We accurately reproduced these experimental results by calculating the spatiotemporal photothermal response based on the two-temperature model and an improvement of the standard permittivity model for gold. Our findings facilitate the design of nanoscale thermal management strategies in photonic, optoelectronic, and high-frequency electronic devices.

Spectroscopy detected in the time domain entails many techniques, such as FTIR, pump-probe, FT-Raman, and 2DES, and applications, such as molecule characterization, excited state dynamics studies, or spectra classifications. Surprisingly, all these techniques use sampling schemes that rarely exploit the a priori knowledge the scientist has before the experiment. Indeed, not all the sampling coordinates carry the same amount of information. In this work, we rationalize with examples the various advantages of a smart sampling scheme tailored to the specific experiment characteristics and/or the expected results. The application of a Fisher information approach allows for finding the best sampling scheme to minimize the variance of a desired observable, greatly improving, for example, spectral classifications and multidimensional spectroscopy. In general, we demonstrate how a smart sampling allows reducing by one to two orders of magnitude the acquisition time of an experiment while still providing a similar level of information.

Understanding the spatial dynamics of nanoscale exciton transport beyond the temporal decay is essential for further improvements of nanostructured optoelectronic devices, such as solar cells. The diffusion coefficient (D) of the nonfullerene electron acceptor Y6 has so far only been determined indirectly, from singlet-singlet annihilation (SSA) experiments. Here, we present the full picture of the exciton dynamics, adding the spatial domain to the temporal one, by spatiotemporally resolved photoluminescence microscopy. In this way, we directly track diffusion and we are able to decouple the real spatial broadening from its overestimation given by SSA. We measured the diffusion coefficient, D = 0.017 ± 0.003 cm2/s, which gives a Y6 film diffusion length of L=Dτ≈35 nm. Thus, we provide an essential tool that enables a direct and free-of-artifacts determination of diffusion coefficients, which we expect to be pivotal for further studies on exciton dynamics in energy materials.

Diffusion is one of the most ubiquitous transport phenomena in nature. Experimentally, it can be tracked by following point spreading in space and time. Here, we introduce a spatiotemporal pump-probe microscopy technique that exploits the residual spatial temperature profile obtained through the transient reflectivity when probe pulses arrive before pump pulses. This corresponds to an effective pump-probe time delay of 13 ns, determined by the repetition rate of our laser system (76 MHz). This pre-time-zero technique enables probing the diffusion of long-lived excitations created by previous pump pulses with nanometer accuracy and is particularly powerful for following in-plane heat diffusion in thin films. The particular advantage of this technique is that it enables quantifying thermal transport without requiring any material input parameters or strong heating. We demonstrate the direct determination of the thermal diffusivities of films with a thickness of around 15 nm, consisting of the layered materials MoSe2 (0.18 cm2/s), WSe2 (0.20 cm2/s), MoS2 (0.35 cm2/s), and WS2 (0.59 cm2/s). This technique paves the way for observing nanoscale thermal transport phenomena and tracking diffusion of a broad range of species.

We present a setup capable of performing optical diffraction tomography with femtosecond pulses broader than 100nm. This will allow recording ultrafast processes in three dimensions with spectral resolution.

Here, we present and implement a new approach for producing modular inkjet-printable surface-enhanced Raman scattering (SERS) chemosensors. These sensors, combined with a rapid large field-of-view imaging system allow for fast imaging of the chemical characteristics of a sample. The performance of these materials is illustrated by printing a pH sensor on paper and interrogating aqueous solutions at different pH values. Results show single-shot images exceeding 9 mm(2) which are readily read out via SERS imaging.

Using off-axis holography, we create an all-optical lock-in camera capable of acquiring ultrafast transient images spanning 100×100 μm2. Combined with structured illumination, it enables us to study exciton diffusion around 100 diffraction-limited spots.

When thinning down silicon films toward nanometer thickness, their thermal conductivity decreases dramatically. In article number 2108352, Klaas-Jan Tielrooij and co-workers report a combined experimental– theoretical study that shows that this is not the case for the layered semiconductor MoSe2. For the thinnest MoSe2 films, the decreasing thermal conductivity is compensated by low-energy, long-mean-free-path heat-carrying modes. These thin films furthermore exhibit efficient heat dissipation to air molecules.

Pump-probe microscopy was limited to single-point detection because cameras are too slow for shot-to-shot demodulation. We use off-axis holography to create an all-optical lock-in camera, obtaining pump-probe images spanning 100×100 μm2. We use it to study exciton diffusion in perovskites around 100 diffraction-limited spots.

Surface-enhanced Raman scattering (SERS) imaging is a powerful technology with unprecedent potential for ultrasensitive chemical analysis. Point-by-point scanning and often excessively long spectral acquisition-times hamper the broad exploitation of the full analytical potential of SERS. Here, we introduce large-scale SERS particle screening (LSSPS), a multiplexed widefield screening approach to particle characterization, which is 500-1000 times faster than typical confocal Raman implementations. Beyond its higher throughput, LSSPS simultaneously quantifies both the sample's Raman and Rayleigh scattering to directly quantify the fraction of SERS-active particles which allows for an unprecedented correlation of SERS activity with particle size..

Understanding heat flow in layered transition metal dichalcogenide (TMD) crystals is crucial for applications exploiting these materials. Despite significant efforts, several basic thermal transport properties of TMDs are currently not well understood, in particular how transport is affected by material thickness and the material's environment. This combined experimental–theoretical study establishes a unifying physical picture of the intrinsic lattice thermal conductivity of the representative TMD MoSe 2 . Thermal conductivity measurements using Raman thermometry on a large set of clean, crystalline, suspended crystals with systematically varied thickness are combined with ab initio simulations with phonons at finite temperature. The results show that phonon dispersions and lifetimes change strongly with thickness, yet the thinnest TMD films exhibit an in‐plane thermal conductivity that is only marginally smaller than that of bulk crystals. This is the result of compensating phonon contributions, in particular heat‐carrying modes around ≈0.1 THz in (sub)nanometer thin films, with a surprisingly long mean free path of several micrometers. This behavior arises directly from the layered nature of the material. Furthermore, out‐of‐plane heat dissipation to air molecules is remarkably efficient, in particular for the thinnest crystals, increasing the apparent thermal conductivity of monolayer MoSe 2 by an order of magnitude. These results are crucial for the design of (flexible) TMD‐based (opto‐)electronic applications.