Energy transport in emerging materials is an important emergent property to characterize at the nanoscale, especially since they often contain nanoscale heterogeneities. I will therefore share recent advances in detecting, tracking, and discerning the spatiotemporal evolution of charge carriers, excitons, heat and ions as they interconvert and explore emerging materials’ structure and heterogeneity on multiple scales. I will share our development of sub-picosecond and single-digit nanometer sensitivity stroboscopic optical scattering microscopy (stroboSCAT) through a series of examples of increasing complexity. Beginning with charge carrier transport in solution-processed semiconductors and extending the approach to thermal transport in a wide range of materials opens the possibility to study these different forms of energy simultaneously. Building on these capabilities to also incorporate multiple photogenerated electronic species in transition metal oxide photoelectrodes provides an unique opportunity to elucidate the role of transport in promoting artificial photosynthesis. I will also show a similar approach to follow mass transport associated with electrochemical CO₂ reduction and the most direct measurements to-date of exciton transport in natural photosynthesis.

This symposium is focused on the ultrafast nanoscale spatio-temporal transport of excitons in energy materials and photosynthetic systems [1-4]. The natural sunlight illumination of ~1kW/m2 corresponds to about 10 photons/sec on an organic molecule cross-section. Yet, typical micro-spectroscopy, pump-probe, experiments require many orders of magnitude higher light level, at which saturation, annihilation, non-linear response and dissociation play important roles. To address any relevant function of photosynthesis one to needs to operate at natural conditions, far below shut-down: the light level of One Sun.

Here we present two strategies to study energy transfer at One-Sun and, despite the scarcity of photons, preserve the nanometer resolution to track excitons in photosynthetic and photovoltaic architectures, together with the crucial fs-ps response:

1. Structured Excitation Energy Transfer (StrEET): We push the required light levels down by at least 4 orders of magnitude, while preserving the nanoscale diffusion resolution, by encoded periodic spatial excitation close to diffraction limit. We determine the effective diffusivity D over 6 orders of magnitude excitation fluence range, revealing apparent increase at too high fluences due to onset of non-linear exciton-exciton annihilation as confirmed by lifetime decrease at higher fluence, above ~10 sun levels. [5]

2. Wide field SPAD array detection: for the first time we will explore SPAD array cameras for spatio-temporal imaging. For the periodic excitation, the full periodic distribution is captured and resolved, and no mask is needed. A 2D Fourier transform of the time-resolved image allows to easily compute the DC and the excitation frequency amplitudes, whose ratio isolates the diffusivity contribution to the amplitude decay. The APD array provides free choice of super-resolution illumination strategies, simplification of the encoding, and to speed up 100-1000 times by the parallel detection. [6]

This work is part of the project ERC Advanced Grant 101054846 FastTrack.

In an operating battery, injected electrons hybridize with the electrode lattice to form lattice-charge coupled species known as polarons [1]. The mobility of these polarons is critical to the overall conductivity of the electrode, often limiting material performance. While conventional 4-point probe measurements provide equilibrium conductivities, they fail to capture the intrinsically out-of-equilibrium dynamics during battery operation. To unravel these ultrafast, nanoscale processes, techniques capable of resolving polaron dynamics on sub-100 ps timescales and sub-10 nm spatial scales are required.
Here, we address this challenge using quantitative transient reflection microscopy (TRM) with picosecond temporal resolution and wide-field optical access [2,3]. We apply this technique to pulsed laser deposited thin films of LiₓMn₂O₄ (LMO) (100 facet), across a range of lithium contents (~0.2 < x < ~0.9). By capturing pump-probe images at various time delays, we visualize polaron transport across multiple lithiation states.
Our imaging reveals spatially resolved transient reflectivity changes, from which we can extract carrier diffusion profiles and model mean squared displacement (MSD) dynamics. This is enabled by an optical model accounting for transient changes in the dielectric function in the reflection geometry. 
We establish links between the lithiation state and the transport dynamics, and establish transport timescales. Notably, we observe differences between the early (< 1 ns) and late-time (> 10 ns) behaviour, suggesting a dynamic evolution of the transport regime. We rationalise our results using electronic structure calculations.
Altogether, our results demonstrate the potential of ultrafast spectroscopy to probe nonequilibrium charge dynamics in battery materials, offering new pathways.

Chirality is a property which is widely observed in nature and refers to the characteristic that an object cannot be superimposed on its mirror image. Recently, chiral solution-processable semiconductors have seen a surge of interest for their potential applications in emerging photonic, optoelectronic and spintronic technologies [1]. Yet, the underlying chiral light-matter interactions, especially in the excited state, which lead to spin and light polarization remain poorly understood [2].

Here, we present our preliminary results on employing transient chiroptical spectroscopy to understand the consequences of morphological (hierarchical) chirality for charge, spin and light polarization. We compare achiral chromophores in solution and a solid-state matrix with their arrangement into chiral twisted molecular crystal films with chiral domains and discuss the complex photophysics we observe [3].

By studying chiral light-matter interactions transiently and with high sensitivity we hope to aid in understanding the microscopic origin of chiral-induced spin-selectivity (CISS) observations and other mechanistic approaches that could enable efficient spin and light polarization control using cheap solution- processable semiconductors.

[1] Nature Reviews Materials 8, 365 (2023)

[2] Nature Reviews Chemistry 9, 208 (2025)

[3] Unpublished (2025)

Lead halide perovskites (LHPs) constitute a vast and highly diverse library of energy materials, which can be tailored by their organic cation composition, halide alloying, dimensionality, or chiral ligands to the specific needs of contemporary optoelectronic devices. So far, to optimize material properties, the material science community mainly focused on changing the static design of the perovskite lattice by tuning the chemical composition or morphology. Meanwhile, the full potential for dynamic phonon-driven ultrafast material control, as successfully applied for oxide perovskites, has not been exploited yet.

The advent of high-field terahertz (THz) sources enabled coherent control of fundamental low energy excitations, such as phonons. Recently, nonlinear driving schemes have extend these methods of contemporary IR-spectroscopy even to non-IR-active modes. Here, we employ these driving schemes to obtain coherent control over LHP lattice modes via the THz-induced Kerr effect (TKE). In 3D bulk LHPs, we find the TKE to be dominated by coherent octahedral twist modes, which are coupled to the electronic bandgap and the strong nonlinear THz polarizability [1]. After establishing coherent lattice control in lead bromides APbBr3 with either purely inorganic (A=Cs) or organic (A=MA, Methylammonium) A-site cations, we move to more complex stoichiometries hosting up to four cation species (Cs, MA, Guanidinium, and Formamidinium). For a specific mixing ratio of four cations, we counterintuitively find that the lattice coherence is restored and even doubled in time compared to the MAPbBr3 parent compound [2]. These dynamic lattice properties are accompanied by a stabilization of the cubic phase down to 80 K, higher photoluminescence, increased electron mobility, and thus indicate a delicate interplay of the static and dynamic lattice contributions to optoelectronic performance.

To investigate the impact of confinement and dimensionality of these lattice dynamics and their coherences, we extend our studies to 2D-layered Ruddlesden-Popper (PEA)₂MA_n-1PbnI_3n+1 compounds with n=1,2,3 inorganic octahedra layers forming periodic multiple quantum wells, separated by phenethylamine (PEA) organic spacer molecules [3]. Already at room temperature, we strikingly witness enhanced lattice coherences with a clear dependence on the degree of confinement. By the mode-selective azimuthal symmetry of the TKE, we identify simultaneous IR- and Raman-activity of specific inorganic cage modes, representing fingerprints of hidden inversion symmetry breaking despite the globally centrosymmetric crystal structure [3]. This transient or local breaking of inversion symmetry might contribute to previous indications of the Rashba-Dresselhaus effect, paving the way for spintronic applications in quasi-2D LHPs. Eventually, we further reduce the dimensionality to chain-like (1D) hybrid metal halide structures given by α-Ethylbenzylamine lead bromide compounds, which additionally open our studies to chiral properties.

Bismuth halide semiconductors like Cs₃Bi₂X₉ (X = Br or I), have emerged as environmentally stable alternatives to lead halide perovskites in photovoltaics and photocatalysis. However, their wide, indirect bandgaps and strong exciton binding energies (E_b) hinder efficient exciton dissociation and charge transport, limiting their potential in optoelectronic applications. Halide alloying in Cs₃Bi₂(Br_1-yI_y)₉ offers a strategy to tune electronic properties such as bandgap and E_b, yet the excitonic behavior of these alloys remains insufficiently understood.

In this study, we systematically investigate the influence of halide composition on excitonic properties and dynamics in Cs₃Bi₂(Br_1-yI_y)₉ film. Steady-state UV–vis absorption spectroscopy combined with Elliott fitting reveals a non-linear dependence of both bandgap and E_b: decreasing from ~3.6 eV and 780 meV for y = 0 to ~2.8 eV and 380 meV at y = 0.6 and subsequently increasing to ~3.0 eV and 460 meV at y = 0.9-1.0. This bandgap bowing behavior may be attributed to a structural phase transition, as evidenced by the emergency of two distinct excitonic peaks at y = 0.8, indicating phase coexistence. Transient absorption spectroscopy further probes exciton dynamics over nanosecond to microsecond timescales. The photobleaching signals align with steady-state excitonic features, and kinetic analysis reveals two decay components: a fast (~100 nanoseconds) process and a slower (up to several microseconds) one. Both lifetimes peak at y = 0.6, reaching 170 ns and 4.3 μs, respectively. These findings demonstrate that halide alloying enables effective tuning of bandgap and excitonic properties in bismuth halide semiconductors, offering a pathway to improved optoelectronic performance via compositional engineering.

Metal halide perovskites are highly promising materials for next-generation photovoltaics due to their exceptional optoelectronic properties and cost-effective fabrication. ^[1] Various ion substitution strategies have been explored to enhance the photovoltaic performance of methylammonium lead iodide (MAPI) films, including A-site ion substitution, B-site ion substitution, and X-site ion substitution. ^[2] In contrast to the tolerance and stability tailoring or electronically tuned modifications brought about by A-site and X-site ion substitution, the main change of B-site ion substitution is its influence on the fundamental vibrational and structural properties of the inorganic [PbI6]⁴⁻ octahedral network. ^[3] Compared to the isovalent ion substitution, heterovalent ion substitution is found to influence the sign of majority charge carriers. ^[4] In particular, the incorporation of antimony has been shown to improve the performance of MAPI devices by enhancing their optoelectronic properties, ^[5] while the underlying mechanism of such modification remains underexplored. Our study investigates the effect of antimony ion substitution on MAPI films using a combination of optical pump-terahertz probe spectroscopy (OPTP) and transient absorption spectroscopy (TA). We find that OPTP photoconductivity transients exhibit a fluence-dependent crossover that is absent in pristine MAPI. Conversely, TA results do not show this same crossover behavior. This difference strongly indicates a mobility-driven phenomenon, which we attribute to enhanced charge carrier localization with antimony ion substitution. This is also verified by the increase of radiative recombination rate and the reduction of charge carrier mobility, as the concentration of antimony ions grows. Our findings pave the way for understanding the effect of antimony ion substitution on the interaction between charge carrier localization and recombination, which is crucial for optimizing optoelectronic properties of perovskite thin films.

Semiconducting nanocrystals (NCs) with perovskite crystal structure have recently emerged as superstar light emitters with exceptional properties inspiring novel applications. The best known compounds are lead halide perovskites. However, concerns related to inherent toxicity and poor environmental stability of these materials have inspired a quest for alternatives. One of the most interesting of them are double perovskite (DP) metal chlorides. The most widely studied compounds are Cs₂AgBiCl₆, which is an indirect bandgap semiconductor and Cs₂AgInCl₆, which exhibits a direct gap, but the inter-band optical transition is parity-forbidden. As a consequence, these materials are poor light emitters and the photoluminescence quantum yields (PL QYs) usually are below 1%. It was discovered that the way to increase the PL QY of DP NCs was to fabricate alloyed structures with a low bismuth content. Alloying breaks the wave-function symmetry (breaking the parity constraints) and low bismuth content assures the direct bandgap. As a result, Cs₂(KNaAg)(InBi)Cl₆ NCs exhibit PL QYs reaching 70%, achieved for doping-level (i.e., below 1%) Bi contents. These results make DPs exciting materials for applications in optoelectronics, in particular in white-light LEDs and transparent photovoltaics.

Despite these achievements, the nature of the luminescent excited state is not well known. There is an agreement in the community that Bi³⁺ ions introduce a narrow absorption band in the near-UV associated with the inter-atomic transition S → P transition. However, it is not well understood what happens to electrons and holes after photoexcitation. The results of density functional theory (DFT) calculations indicate that the photoexcited hole becomes localized at a single [AgCl₆] octahedron, owing to a localized nature of the Ag d orbitals. However, regarding the fate of photoexcited electrons there are contradicting reports. 

In this talk, I will present results of temperature and magnetic field dependent photoluminescence dynamics, which reveal the fine structure of the luminescent state and show how the structure can be tuned with the NC chemical composition. I will then discuss optical transient absorption results, which show that carrier trapping occurs on a sub-ps timescale. Finally, I will present femtosecond x-ray absorption results that enable element-specific tracking of photocarrier dynamics, providing direct insight into the role of Bi ions in the carrier relaxation process. Together, our results create a comprehensive picture of the emission process of these highly emissive materials.

Individual quantum dots (QDs) serve as inherent sources for quantum states of light such as single photons and entangled photon pairs. Ideally, under cryogenic conditions, their radiation is fully coherent – maintaining a constat phase relation throughout the emission lifetime. Often, however, the interaction of excited electrons with the lattice limits the coherence lifetime to the picosecond timescale. Experimentally, such a timescale necessitates the use of pump-probe measurements with ultrashort laser pulses. While the generation of short pulses is hardly a novelty, applying them to the sensitive spectroscopy of single nano objects is a highly challenging task. Here, we present the first transient-transmission experiments directly measuring the coherent dynamics of excitons in an individual epitaxial CdSe QD.

Using a highly stable femtosecond Er:fiber laser source, we generate the pump and probe beams that are focused through a high-numerical-aperture objective within an optical cryostat (T=1.6 K). Exciting well above the lowest-energy transition (ΔE=105 meV) temporal oscillations of the spectral lines are a fingerprint of coherence, occurring due to an evolving superposition of two nearly degenerate exciton. To explore higher-energy excitonic state, we tune our pump laser to a narrow absorption line (ΔE=21 meV). This reveals a quasi-stable excitonic state which also maintains coherence for hundreds of picoseconds. This result is especially surprising considering the frequent scattering of phonons by states with excess energy and is indicative of a robust phonon bottleneck.

The unprecedented capability of our setup to expose the full energy landscape of a single confined exciton and its dynamics following excitation promises to bring to light new physics in the realm of nano and quantum optics.

For perovskite solar cells, the fullerene molecule (C₆₀) is one of the best performing electron transport layers (ETLs) that selectively extracts electrons from the perovskite layer, due to its suitable band alignment, good surface contact and high mobility. However, studies have shown that the perovskite-fullerene interface suffers from unwanted non-radiative recombination, which limits the device performance [1,2]. Ultrathin interlayers inserted between the perovskite surface and fullerene have been shown to improve the device performance [3], suggesting they prevent the unwanted interface recombination. In this work, we used ultrafast and nanosecond spectroscopy to investigate how interlayers affect the charge-carrier recombination and transfer at the interface.

The samples were formed by depositing C₆₀ layers on top of triple-cation ((FA_0.79MA_0.16Cs_0.05)Pb(I_0.83Br_0.17)₃) perovskite layers, with or without interlayers deposited in between. They were studied using two different versions of optical-pump terahertz-probe (OPTP) spectroscopy, which measured the carrier density in the perovskite layer as a function of time after injection. One version was the standard OPTP setup using a femtosecond laser to pump the sample and a mechanical stage to vary the pump-probe delay over a range of 3ns with sub-picosecond resolution. The other version was an electronically delayed OPTP (E-OPTP) setup [4] that used a separate electronically-triggered laser to pump the sample, giving an unlimited delay range with nanosecond resolution. Together these techniques allowed the carrier population to be studied from sub-picosecond to microsecond timescales.

The carrier dynamics were interpreted by comparison to a numerical model of the spatially and temporally varying carrier density, crucially including the Poisson equation to account for Coulombic effects of charge separation across the interface. A significant separation of charge should leave a fingerprint in the decay dynamics, and its absence implies recombination across the interface. The experimental results indicate that whilst the perovskite-C₆₀ interface suffers from rapid cross-interface recombination, the interlayers slow the extraction of electrons into the C₆₀.

Colloidal indium phosphide (InP) quantum dots have emerged as the leading material for a wide range of commercial applications, particularly as bright luminescent colour converters in displays and lighting technologies. However, despite significant advances in InP-based nanocrystal synthesis and surface passivation, the community has yet to demonstrate robust optical gain under strong excitation conditions—a milestone routinely achieved with other quantum-dot materials. Here, we investigate the fundamental photophysical processes that limit the performance of state-of-the-art InP quantum dots as a gain medium. Using a multi-scale approach combining ensemble-based transient absorption and time-resolved photoluminescence spectroscopy spanning femtoseconds to microseconds with single-dot fluorescence-lifetime measurements, we uncover an ultrafast hot-carrier trapping mechanism unique to InP systems. Following high-energy photoexcitation, hot electrons are captured by traps on sub-picosecond timescales, resulting in charge-carrier losses during cooling. This rapid channel significantly reduces the net population inversion and, consequently, the achievable optical gain. Intriguingly, this hot-carrier trapping delays but does not quench photoluminescence, consistent with the high brightness observed under low-intensity illumination. A comparative analysis with CdSe, lead–halide perovskite, and CuInS2 quantum dots highlights the distinct hot-carrier dynamics of InP and shows that trap engineering is a critical next step for future performance improvements in high-power applications.

Colloidal lead halide perovskites (LHP) nanocrystals (NCs) are popular light-emissive materials for optoelectronic devices, of interest for LEDs, LCDs, lasers and quantum light sources. Most studies on LHP NCs focus on relatively large NCs exceeding 10 nm in size, exhibiting weak to no quantum confinement effects. Recently, we showed that perovskite quantum dots (pQDs) can be synthesized using a newly developed synthesis route, resulting in pQDs that are tunable between 3 and 13 nm range.[1]

Spectroscopy has always played a key role in understanding both the chemical and physical properties of quantum dots, as is also the case for perovskite QDs. Particularly, the soft and ionic nature of perovskites strongly influences many properties. In this talk, I will discuss how we use both in situ spectroscopy and transient absorption spectroscopy ranging from measurements on the minute scales all the way to the femtosecond time scales. This includes the growth and nucleation of the pQDs,[1] their ionic halide exchanges,[2] excitons in these QDs,[3] exciton-exciton interactions,[4] as well as exciton phonon interactions. This highlights the many unique structural and optical properties of pQDs

Quantum-dot based photodiodes (QDPDs) are important candidates for low-cost light detectors in the short-wave infrared (SWIR) wavelength range. PbS-based QDPDs in particular are well-established and have been used in commercial SWIR imagers. In recent years, significant progress has been made in terms of reducing dark current, reducing response time, miniaturization and using more sustainable heavy-metal free materials [1] . However, the photocurrent of these devices tends to saturate for input powers above ~100 mW/cm2 [2]. This behaviour limits the use of QDPDs in integrated photonic devices, e.g., for interconnects or LIDAR, where the optical power density is high due to the confinement of light in waveguides with small cross sections [2]. The mechanism leading to detector saturation remains unknown to date, with indications of Auger-type processes playing a role despite the low overall excitation density powers of 100 mW/cm2 generated in these devices.

Here, we use in-operando transient absorption spectroscopy to study the charge decay PbS QDPDs designed for ns response times [3]. We combine measurements on the QDs in solution, on the individual (doped) device layers and for the first time in-situ measurements on the full device. Comparing the transient absorption decay of QDs in solution with the ligand-exchanged film confirmed that the ligand exchange passivates the surface and reduces the trap density. However, n/p doping (by the ligand exchange) induces another fast decay channel with a decay rate that scales with the square of the excitation density. This can be attributed to a trion Auger decay of the photo-generated electron-hole pair with an extrinsic background charge due to doping. The intrinsic disorder in the energy landscape brings photo-induced and extrensic charges together, resulting in a fast decay [4]. Inside a working device, this recombination mechanism outpaces the charge extraction already at a density of 0.02 excitations per QD. We measured the extraction efficiency as a function of pump power in these diodes and found a saturation at the same excitation density.

Our results add to the fundamental understanding of QD-based photo-detection devices and allow for better engineering of these devices. In particular, a better understanding of saturation in relation to doping levels could lead to an increased dynamic range.

Control over the electron’s spin (up or down) within the field of spintronics introduces an additional degree of freedom compared to traditional electronics, opening up new opportunities for energy-efficient quantum information technologies. Such applications require a spin-polarized population on timescales ranging from nano- to microseconds. Recent studies on solution-processable lead-halide perovskite semiconductors demonstrated spin lifetimes on the order of a few picoseconds at room temperature — still too short for applications.[1] The short spin lifetime in lead-based halide perovskites is often attributed to the strong spin–orbit coupling that electrons experience due to the presence of heavy elements, like lead and iodide. Moreover, the use of lead-based materials is widely forbidden due to toxicity concerns.

In this talk, I will discuss how the substitution of lead with lighter, less-toxic elements with reduced spin–orbit coupling may be used to manipulate and potentially increase the spin lifetime in halide-perovskite semiconductors using ultrafast polarization-resolved optical spectroscopy. I will discuss the challenges faced when developing lead-free halide-perovskites thin layers and demonstrate the influence of the crystal structure and composition on the observed spin lifetime.

Recently, nanocrystals (NCs) in the regime of vanishing quantum confinement—referred to as bulk nanocrystals (BNCs)—have demonstrated remarkable optical gain characteristics.[1,2] While bulk semiconductor models have successfully explained their high-power lasing behavior, the validity of these models in the low-density regime—where the number of charge carriers per nanocrystal volume becomes discrete—remains an open question. In this study, we investigate the dynamics and energetics of well-defined excited states containing 1 or 2 holes and up to 4 electrons using single-photon avalanche diode (SPAD) array technology. We observe emission from two discrete energy levels for each of the six isolated excited states, indicative of thermal excitation. To validate this thermal behavior, we investigate the temperature-dependent emission characteristics of dozens of individual quantum dots, simultaneously, using multi-particle spectroscopy. This analysis reveals a quantitative match with Boltzmann statistics. By combining the particle-in-a-box approach with Boltzmann population distributions, we develop a model that captures the dynamics and energetics of nanocrystals across the full range from strong quantum confinement to the bulk limit. This work thereby provides a framework for understanding the optical behavior of NCs in the transitional regime between quantum confined and bulk-like, which becomes increasingly relevant as these materials are gaining more prominence in optoelectronic applications.

Besides conventional optoelectronic devices (LEDs and laser), colloidal quantum dots (QDs) are pursued as non-classical light sources (i.e. single photon emitters) that might play a pivotal role in future quantum technologies, such as quantum computing, quantum cryptography and quantum sensing. Due to strongly reduced charge trapping on surface states and their defect-tolerant character, perovskite QDs become attractive as alternative quantum light sources. Indeed, very stable, blinking-free emission¹ has been observed at cryogenic temperatures with ultrafast radiative lifetime^{2, 3} and long exciton dephasing time^{4, 5}. In addition, perovskite QDs exhibit remarkably optical properties even at room temperature^{6, 7}. Their emission, however, is still affected by photoluminescence (PL) blinking, a random switching between bright and dark periods. In this work, we investigate individual QDs and demonstrate the critical role of surface chemistry in determining emission quality. We report on a new class of structurally diverse sulfonium ligands that provide robust surface passivation of perovskite nanocrystals (NCs), achieving photoluminescence quantum yields exceeding 90% (manuscript under review). Our results address a fundamental, long-standing challenge in colloidal chemistry and could pave the way toward the generation of ultra-pure, blinking-free single-photon emitters.

Lanthanides have transformed the world of lighting in the past 40 years. Presently, almost all artificial light sources rely on emission of light by lanthanide ions. In many luminescent materials, also known as phosphors, one-to-one photon conversion downshifts one high energy photon to one lower energy photon in the desired spectral region. However, recently, there is a significant increase of attention for multi-photon phosphors relying on multi-photon conversion processes, either upconversion or downconversion. Insight in the multi-photon processes is not trivial but is needed to understand the mechanism and improve the efficiency of spectral conversion processes in multi-photon phosphors which is crucial for applications, including solar cells to reduce spectral mismatch losses.

In this presentation a short historical introduction to single- and multi-photon conversion phosphors will be followed by an overview of recent developments of efficient up- and downconversion materials. Next it will be discussed how insight can be obtained in the mechanism and efficiency of up- and downconversion processes. An important aspect involves modelling of energy transfer and ligand quenching. For both up- and downconversion examples will be given on how modelling of luminescence decay curves can provide quantitative insight. A new ligand-quenching model will be presented and applied to understand multi-phonon vibrational quenching in NaYF₄:Er,Yb upconversion nanocrystals. Finally a new method will be presented that provides direct proof for downconversion. Correlated emission of photons in photon cutting materials can serve as a fingerprint for the occurrence of downconversion and can even be used to quantify the downconversion efficiency.

Solar spectrum conversion has the potential to enhance solar cell efficiencies, by shifting short-wavelength photons to longer wavelengths where the photovoltaic response is stronger. Realizing these benefits of spectral conversion requires the process of quantum cutting, where two longer-wavelength photons are emitted by a material following the absorption of one shorter-wavelength photon. This type of color conversion can approach 100% energy efficiency, thus using the high-energy part of the solar spectrum with maximum effectivity

Quantum cutting has been claimed for various materials over the past two decades, but follow-up research often disproved initial claims. Typical techniques used to prove quantum cutting are integrating-sphere quantum yield measurements, time-resolved emission or transient absorption spectroscopy. These techniques are complex and not always conclusive.

In this presentation, we show that the photon correlation analysis[1] is a universal strategy to unambiguously reveal quantum cutting. We have tested two materials, YPO₄ co-doped with Tb³⁺ and Yb³⁺ and YAG co-doped with Ce³⁺ and Yb³⁺. Both are reported in the literature to perform quantum cutting via absorption of blue light followed by cooperative energy transfer to near-infrared-emitting Yb³⁺.[2,3] We find that YPO₄:Tb³⁺, Yb³⁺ shows bunched emission, characteristic of quantum cutting. In contrast, YAG:Ce³⁺, Yb³⁺ shows regular Poissonian emission statistics. This reveals that YAG:Ce³⁺,Yb³⁺, despite various claims,[3,4] is not a quantum-cutting material.

A detailed understanding of the microscopic structure at the semiconductor nanoparticle–liquid interface is essential for optimizing surface-mediated chemical reactions. In particular, both the electrical double layer structure and the surface protonation state can significantly influence photocatalytic reactions by modulating reactant adsorption and electron transfer rates. However, investigating the electrical double layer (EDL) structure and surface acid sites under conditions that closely mimic actual catalytic operation for semiconductor nanoparticles is a technical challenge that requires to probe selectively the few molecular layers of the solid/water interface of a colloidal suspension.

Here, we show that polarimetric angular-resolved second harmonic scattering (AR-SHS) offers a fully optical, non-invasive method to determine surface potential values as well as interfacial water orientation of nanosized metal-oxide particles dispersed in aqueous solutions. By comparison of the surface potential to the zeta potential at different salt concentrations, we are able to establish a microscopic picture of the electrical double layer structure, and follow its evolution with increasing salt concentration. Then, AR-SHS measurements as a function of pH on TiO₂ nanoparticles indicate a reversal in water orientation for specific pH values. We demonstrate that the reversal in water orientation indicate transitions between predominantly protonated and deprotonated surface states, providing a direct, optical means to determine surface pKa values from surface susceptibility versus pH data.

Our findings enable a new in situ approach to investigate the structure of the electrical double layer and surface acidity of colloidal nanoparticles, offering deeper insights into fundamental mechanisms affecting photo(electro)chemical processes.

Plants and algae provide a natural example of how solar energy can be converted into chemical energy in the presence of oxygen while preventing photodamage. It has now been established that plants and algae prevent photooxidation by activating a rapidly inducible and reversible photoprotective mechanism at the level of their light-harvesting complexes. However, the precise activation process of this photoprotective mechanism remains unknown. I will here introduce our current understanding of how light-harvesting is regulated in plants and algae and, more generally, in oxygenic photosynthetic organisms. I will then highlight the spectroscopic and computational tools under development by our recently-established group at ICFO, aimed at unraveling the molecular mechanisms governing the activation of photoprotection in photosynthetic organisms – in real time.

Understanding both the mechanism and the rate at which plants can activate or deactivate photoprotection may provide answers to long-standing open questions in the fields of biophysics and physical chemistry. This knowledge will also be instrumental in inspiring new studies focused on maximizing plant productivity through the optimization of photoprotective responses.

The functionalities of photoactive materials ranging from optoelectronics, plasmonics, catalysis and phase-switching applications require not only control over the photoexcited charges but also heat generation, transport and dissipation. Controlling nanoscale thermal transport is fundamental to virtually all such applications, as they either inherently generate heat as a byproduct or deliberately harness it for operation. While pump–probe spectroscopy signals are typically attributed to electronic energy carriers (i.e., electrons, holes, excitons), there is increasing recognition that laser induced heating can also lead to transient spectral signals in semiconductor films. On one hand there is a need to better understand the contributions of heating to pump–probe measurements for accurate assignments, and at the same time this presents opportunities to investigate microscopic thermal transport and dissipation. Recent advances in thermoreflectance have enabled critical temporal and spatial thermophysical characterization to probe the mechanistic impact of nanoscale structuring on heat propagation. In this presentation I will describe how pump–probe optical measurements and modeling of thermal transport provide access to nanosecond dynamical information with local, sub-micron specificity. I will highlight examples including metallic nanocrystal superlattices,[1],[2] semiconductor nanocrystal films,[3] and insulator-to-metal phase transition thin films. We will touch on questions including: How do heterogeneous environments and interfaces impact microscopic energy transport? How can we access information about energy carriers that traditionally do not have clear spectroscopic signals? How can we control the directionality of energy carrier flow?

Tantalum nitride (Ta₃N₅) is a highly studied semiconductor for solar-driven water splitting. However, experimentally achieved efficiencies remain far below theoretical limits due to the formation of native and impurity defect states that impact charge carrier dynamics by facilitating trapping and recombination processes. In this study, we investigate the influence of different defect states in Ta₃N₅ thin films on ultrafast photocarrier dynamics using femtosecond transient absorption spectroscopy, as well as complementary photoluminescence and photoluminescence excitation measurements. Ta3N5 Photoelectrodes containing tailored shallow/deep defect state concentrations and structural disorder were synthesized by first sputtering TaOx, TaNx, and metallic Ta precursor films, followed by NH3 annealing. Through comparative studies of these samples, we identify and distinguish the important roles of both nitrogen vacancies and oxygen-related defects in shaping the charge carrier dynamics. Our results reveal that these defects function as efficient trapping and recombination centers for free carriers. The correlation with complementary measurements link shallow and deep defect properties with charge carrier dynamics and photoelectrochemical performance, enabling the tailored development of design strategies to overcome current limitations.