Since their first synthesis in 2015 [1], perovskite quantum dots (pQDs) have revealed their potential as scalable solid-state quantum emitters, combining bright emission, high-purity single photon generation at room temperature [2], and even photon indistinguishability with 50% visibility at cryogenic temperatures [3]. Coupling individual pQDs to optimized photonic structures in order to control and enhance their emission via cavity quantum electrodynamics (cQED) effects is now a crucial objective. In this context, a prerequisite is the precise characterization of key cQED metrics. The most challenging ones for solid-state emitters are the light-matter vacuum Rabi coupling strength and the homogeneous linewidth, the latter being typically blurred by spectral diffusion in standard photoluminescence measurements.

In this talk, I will present a linear cQED approach that delineates the respective contributions of pure dephasing and spectral diffusion to the emitter linewidth, without relying on quadratic photon correlation techniques, which often suffer from low signal-to-noise ratio [4]. Our method is based on a fully tunable cavity-emitter platform that enables the deterministic and reversible coupling of individual CsPbBr₃ pQDs to an open fiber Fabry-Pérot microcavity [5]. This allows us to study the very same pQD in both free space and cavity configurations while keeping its environment and excitation conditions unchanged, thereby eliminating the statistical biases inherent to monolithic photonic structures where spatial and spectral matching cannot be adjusted.
 
First, time-resolved photoluminescence experiments reveal up to a twofold increase in single photon emission rates by Purcell effect in the cavity configuration, corresponding to a Purcell factor of ~3.3. Then, by exploiting the cavity tunability, we study the cavity-induced reshaping of the multiplet excitonic fine structure and show that it provides a sensitive fingerprint of the light-matter coupling strength. Finally, combining these detailed temporal and spectral investigations enables a reliable determination of cQED parameters by crossing the constraints imposed by the two independent datasets, allowing us to extract both the homogeneous linewidth (~250 +/- 50 μeV, free of spectral diffusion) and a vacuum Rabi coupling strength up to 40 +/- 10 μeV for the smallest cavity mode volume.

Overall, I will present a robust framework for quantifying light-matter interaction in nano-emitters dominated by spectral diffusion, and I will show that the strong coupling regime is within reach with optimized pQDs and reduced cavity volumes.

Perovskite quantum dots are now established emitters of non-classical light. We present a single-emitter study using a spectrally dispersed single-photon avalanche diode (SPAD) array to perform highly multiplexed time- and frequency-resolved photon-correlation spectroscopy on individual quantum dots at low temperature.[1] This methodology reveals even complex higher-order many-body dynamics. We report biexciton binding energies and assign their charged states, which undergo fast (µs) switching dynamics. Most notably, we identify a spectral feature blue-shifted from the exciton by 7.4 ± 1.9 meV as the P-type triexciton and establish the order of its cascade emission.

Colloidal lead halide perovskite quantum dots (pQDs) have rapidly emerged as a versatile class of nanomaterials that combine the appealing optoelectronic properties of bulk perovskites with the advantages of quantum confinement [1]. At the single dot level, pQDs exhibit stable single photon emission up to room temperature [2] and a 50% photon indistinguishability at cryogenic temperatures [3]. This positions them among the very few colloidal emitters capable of combining high brightness, coherence, and quantum purity. However, to date, single pQD studies have relied almost exclusively on hot-injection synthesis [4], a process that requires stringent control of temperature and inert atmosphere, thus limiting scalability.

Here, we demonstrate that a modified ligand-assisted reprecipitation (LARP) method provides a robust and highly versatile alternative for producing single CsPbBr₃ pQDs. Using a recently introduced amine-mediated trimming strategy [5], combined with didodecyldimethylammonium bromide (DDAB) ligands for enhanced surface passivation, we obtain isolated LARP-synthesized pQDs suitable for fundamental photophysical properties studies. High-resolution micro-photoluminescence experiments at cryogenic temperatures show a stable emission with minimal spectral diffusion of the bright exciton with its characteristic fine structure and its low-energy optical phonon replicas. Power-dependent studies also reveal the trion and biexciton states contributions. Time-resolved measurements demonstrate sub-100-ps radiative lifetimes, and photon-correlation experiments under CW and pulsed excitation confirm high-purity single photon emission.

These results demonstrate that ligand-tailored LARP synthesis produces pQDs with intrinsic optical properties comparable to hot-injection counterparts, while offering great flexibility for post-synthetic ligand engineering. This versatility could be crucial for assembling pQDs into superstructures in order to investigate collective quantum phenomena.

Colloidal lead halide perovskite nanocrystals (NCs) are generally regarded as diamagnetic; however, emerging evidence suggests that quantum confinement and lattice distortion can induce unexpected magnetic responses. In this work, we present the first systematic investigation of size-dependent excitonic Zeeman splitting in colloidally synthesized CsPbX₃ (X = Cl, Br) nanocrystals. Using variable-field and temperature-dependent magnetic circular dichroism (MCD) spectroscopy, we reveal a pronounced Curie-type (1/T) temperature dependence and unusually large negative excitonic g-factors (g_eff ≈ −20) in strongly confined CsPbBr₃ NCs. The magnitude of the Zeeman splitting decreases monotonically with increasing NC size, directly correlating magnetic response with confinement-induced lattice strain. Comparative analysis shows that CsPbBr₃ exhibits a significantly stronger magnetic response than CsPbCl₃, which is attributed to its orthorhombic crystal structure and enhanced Pb–Br bond asymmetry. These structural distortions promote the formation of local paramagnetic centers associated with Pb oxidation-state fluctuations (Pb²⁺ → Pb¹⁺/Pb³⁺). In contrast, CsPbCl₃ NCs, which adopt a more symmetric cubic structure, remain relatively strain-free and display a much weaker Zeeman response despite a higher halide vacancy density. These results establish structural strain—rather than vacancy concentration alone, as a key parameter governing emergent magnetism in perovskite nanocrystals, providing new design principles for magnetically tunable quantum and spin-optoelectronic materials.

Colloidal semiconductor nanocrystals (NCs) have become a key material for next-generation electro-optical and quantum technologies. Their versatility in synthesis offers a wide range in the design of e.g. size, shape and composition. Among them, halide-perovskite NCs stand out as exceptionally bright emitters with narrow linewidths and robust photostability at cryogenic temperatures [1]. These properties render them highly attractive for single-photon generation [2, 3]. However, spectral diffusion (SD) – stochastic spectral fluctuations – critically limits the performance of perovskite NCs in quantum-optical applications.

Spectral fluctuations occur over an extremely broad temporal range, from nanoseconds to seconds, far beyond the capabilities of standard photoluminescence-spectroscopy methods that cannot simultaneously provide high time and energy resolution. In II–VI nanoparticles, such as CdSe/CdS core/shell NCs, it has been well established that spectral variations originate from local electric-field fluctuations and can be attributed to the quantum-confined Stark effect [4]. Whether a similar mechanism underlies the SD dynamics in halide-perovskite (e.g. CsPbBr_₃) NCs remains an open and highly relevant question.

To address this question, we use heralded spectroscopy, monitoring the correlations of photon energies as a function of the time between detections. This allows us to observe SD across the entire accessible time range from a single measurement. To achieve high time and energy resolutions, we replace the standard relatively slow camera at the output of a spectrometer with a linear single-photon avalanche diode (SPAD) array. Each of the 64 SPADs independently provides a sub-nanosecond temporal resolution and sub-meV spectral precision [5].

We statistically analyze the difference (δE) and average (⟨E⟩) of the energies for pairs of photons vs. the time difference (τ) between their detections. By performing this evaluation for an exponentially increasing τ we access the dynamics of SD from nanoseconds to seconds. For each time step, we quantify the distribution of δE and ⟨E⟩ across the entire measurement time of a single CsPbBr_₃ NC (a few minutes) at a temperature of 6 K.

Considering a fluctuating source, photons emitted shortly one after the other present a higher energy correlation than two photons separated by a longer time difference. As a result, the distribution of δE for short τ is significantly narrower than that of ⟨E⟩. Our measurements find a power law increase of δE with τ, consistent with a diffusive process, over the entire observed time axis and up to ~10 seconds. Interestingly, we find that both the magnitude and timescale of SD are critically sensitive to the laser excitation power. Importantly, at lower excitation powers, nearly no difference is observed between the distributions of δE with increasing τ, indicating that hardly any spectral fluctuations occur.

Our heralded spectroscopy measurements reveal, for the first time, the power-law progression of spectral fluctuations over 11 orders of magnitude in the time domain. To uncover the microscopic origin of these dynamics, we plan a systematic study across CsPbBr_₃ nanoparticles of varying dimensions and capping ligands. In order to test the influence of the quantum-confined Stark effect, we intend to perform heralded spectroscopy under external electric bias. Such insights will bring us closer to understanding and ultimately suppressing SD in perovskite quantum emitters.

Quantum dot (QD) emitters are promising candidates for next-generation display and quantum technologies. However, advancing their optical performance beyond fundamental limitations remains a key scientific challenge. In this work, we present a metal–semiconductor hybrid QD system that enables ultrabright, polychromatic polaritonic emission through nanoscale plasmonic cavities. By harnessing strong coupling between QD excitons and localized plasmons, we achieve color-tunable emission driven by ultrafast Rabi oscillations. We investigate how this interaction seamlessly transitions from weak to strong coupling and even reaches the elusive ultrastrong coupling regime. In addition, we leverage tip-enhanced gradient forces and optical torques in a liquid environment to achieve highly reproducible nano-optical trapping and self-alignment of individual QDs. The trapped and aligned QD exhibits a striking self-induced back-action (SIBA) effect, as unveiled by real-time near-field photoluminescence spectroscopy. This work provides profound insights into light–matter interactions at the nanoscale, offering a new strategy for advanced display technologies and quantum photonics in diverse environments.

Lead halide perovskites open great prospects for optoelectronics and a wealth of potential applications in quantum optical and spin-based technologies. Precise knowledge of the fundamental optical and spin properties of charge-carrier complexes at the origin of their luminescence is crucial in view of the development of these applications. We perform low temperature magneto-optical spectroscopy on single perovskite nanocrystals, to reveal their entire band-edge exciton fine structure and study the bright-dark exciton level ordering depending on materials composition. Combining spectroscopic measurements on various perovskite nanocrystal compounds, we show evidence for universal scaling laws relating the exciton fine structure splitting, the trion and biexciton binding energies to the band-edge exciton energy in lead-halide perovskite nanostructures.

We also show that tailored perovskite nanostructures can be used as robust coherent single photon sources which display two-photon quantum interference visibilities of up to 60% in the absence of any radiative enhancement or photonic architecture. These findings highlight the remarkable potential of perovskite nanocrystals as scalable, colloidal sources of indistinguishable single photons for quantum technologies.

Metal halide perovskite nanocrystals (PeNCs) represent outstanding gain materials for optically pumped lasing, providing high and tunable optical gain that can be achieved at comparatively low thresholds[1]. Amplified spontaneous emission (ASE) functions both as a performance benchmark and as a diagnostic tool for guiding the development of practical PeNC-based laser technologies.

The talk will offer an overview of the group’s investigations into the ASE properties of CsPbX₃ nanocrystals (X = Br, I), together with recent advances involving Pb-free CsSnI₃ nanocrystals. It will first examine the ASE behavior of PeNC thin films operating as simple optical waveguides, followed by a discussion of strategies for optimizing ASE. These include improved waveguiding through polymer–PeNC multilayer architectures[2] and the demonstration of stimulated emission in free-standing membranes[3], representing progress toward flexible, optically pumped PeNC lasers. Lastly, the talk will present new results on nanosecond-excited ASE in long-range ordered nanocrystal superlattices and compare their performance with that of conventional glassy PeNC thin films.

Halide perovskite semiconductors represent an attractive system for optoelectronic and photonic applications. While traditional strategies focus mainly on photovoltaics and light emitting diodes, the potential of perovskite nanocrystals in quantum light sources has attracted significant attention. In this respect the energy structure of exciton complexes, as well as their interactions and ultrafast coherent dynamics after excitation with femtosecond pulses represent particular interest. Conventional time-integrated reflectivity or photoluminescence techniques alone often fail to provide clear conclusions about the energy structure due to the inhomogeneous broadening of optical transitions and the complex dynamics of photoexcited carriers. Here, nonlinear optical techniques based on photon echoes or two-dimensional Fourier spectroscopy offer unique insights into the coherent dynamics of excitons in perovskite semiconductors.

We study the coherent dynamics of excitons in halide perovskite semiconductors of different compositions and dimensionality. Due to the inhomogeneous broadening of optical transitions, coherent optical response is represented by photon echoes even in bulk perovskite crystals. In mixed (FA,Cs)Pb(Br,I)₃ crystals, the magnitude of fluctuations of the energy bandgap is in the order of 10-20 meV. Here, we observe exceptionally long exciton coherence times up to 80 ps at a low temperature of 2 K due to the localization of excitons at the scale of tens to hundreds of nanometers [1]. In this case, the homogenous line of 16 µeV is about three orders of magnitude smaller than the inhomogeneous broadening of optical transitions. This allows us to evaluate fine structure splitting between bright and dark excitons of 0.5 meV by analyzing the quantum beats between the exciton spin states in an external magnetic field. Next, the role of exciton-exciton interactions in bulk crystals is evaluated. Here, polarization-resolved transient signals provide rich information about the biexciton binding energy and spin-dependent interactions in dense exciton ensembles [2,3]. Finally, we study coherent optical response from lead-halide nanocrystals where quantum beats in the photon echo signal are observed due to excitons fine structure and interaction with optical phonons. Specifically, we demonstrate a previously unexplored regime of coherent exciton dynamics with coherence times approaching ~300 ps in CsPbI₃ nanocrystal ensembles, revealed through quantum beats between exciton–polaron states [4].

Quantum dots (QDs) offer a tunable and technology-compatible platform for quantum light-matter interactions and the inspiring technology that comes along with it. The extent of this promise is directly related to our ability to understand and control this interaction. However, understanding the electronic dynamics is tremendously challenging, in large part due to the large range of time scales in which they occur.

Spectral diffusion (SD), the stochastic fluctuation in the energy of photons emitted from a QD, is a prime example observed on nanoseconds and up to minutes. These significantly erode the potential of QDs to produce indistinguishable photons – a critical resource for quantum technologies – even at low temperatures. In this talk, I will present how heralded spectroscopy, a novel technique based on single-photon detection, can explore SD in unprecedented details.

Relying on a single-photon avalanche detector (SPAD) array, we observe SD down to the nanosecond scale. We quantify SD over 9 orders of magnitude in the time domain from single measurement and show that it follows a power-law dynamics analogous to a random walk. Furthermore, analysing SD in single CsPbBr₃ nanocrystals at cryogenic conditions reveals an unexpected type of fluctuation. Upon increasing the laser power excitation, we observe rapid (sub millisecond) changes in the number of spectral peaks. Namely, two spectral features merge into a single one. I will discuss this discovery, explain why we suspect that phase transitions in the lattice symmetry are responsible for it and how we learn from it what is the underlying mechanism of SD in halide-perovskite nanocrystals.

Halide perovskite quantum dot nanocrystals have shown great potential as quantum emitters of single photons or entangled photon pairs due to their exceptional brightness, weak blinking or spectral diffusion and optical coherence times close to their radiative lifetimes. Key to their performances are the properties of excitonic states, strongly interacting electron-hole pairs confined inside the quantum dots. In particular, their spin degree of freedom leads to a meV-scale splitting of the excitonic ground state into four states of differing oscillator strength called the fine structure. This talk is meant as an accessible introduction and discussion of the exciton fine structure in perovskite quantum dots from a theoretical viewpoint based on semi-empirical methods. In particular, the physical origin and symmetry of the exciton fine structure, the screening of the exchange interaction and its decomposition into a short range part and a long range part will be discussed. 

Metal halide perovskite quantum dots (PQDs) continue to challenge computational chemistry due to their structural softness and highly dynamic surface chemistry. In our group, we combine high-level DFT with molecular dynamics to generate next-generation machine-learning force fields (MLFFs) capable of capturing these complexities across realistic time and length scales. By curating cross-material datasets and integrating active learning, we train transferable MLFFs that describe ligand binding, surface reconstruction, and defect dynamics with near-DFT accuracy. These models unlock nanosecond molecular dynamics for CsPbBr₃ PQDs, providing statistically meaningful insight into the mechanistic processes that occur at PQD surfaces and their associated energy landscapes. We complement this with interpretable electronic-structure workflows that directly link structural motifs to emergent optoelectronic properties. This talk will show how machine learning enhanced simulations are reshaping our understanding of halide-perovskite QDs and highlight the methodological advances needed to achieve predictive, chemically aware MLFFs for complex, technologically relevant nanomaterials.

In recent years, 3D inorganic CsPbX3 (X=Cl, Br, I) perovskite materials have gained popularity for various optoelectronic applications. Particularly as quantum dots, these materials provide an attractive platform for classical and quantum light emission because of the high photoluminescence quantum yield, high optical coherence times, tunable emission wavelength and high photoluminescence stability [1,2]. For instance, quantum optics [3,4] and quantum computing [5] are their most promising applications.

However, their lattice vibrations and electron-phonon interactions remain incompletely understood due to strong anharmonicity and phase transitions. These effects can influence basic properties, including carrier mobility and excitonic effects. Therefore, investigating lattice vibrations and electron-phonon interactions in these materials are crucial. Here we present a first-principles study of CsPbBr3 across its orthorhombic, tetragonal, and cubic phases. We analyze the impact of the structural distortions on the phonon dispersions including additional effects related to local disorder and anharmonicity. We quantify the Fröhlich electron-phonon coupling strength for individual polar phonons. Moreover, we trace their symmetry evolution using group theory and standard band unfolding approaches. Despite pronounced structural transformations and enhanced anharmonicity at elevated temperatures, the long-range polar coupling mechanism associated with the LO mode lying at high frequency remains dominant throughout the three phases. In particular, the polar phonons at the orthorhombic Γ point are, overall, governed by the Γ point contributions from the cubic phase.

In conclusion, our study establishes a microscopic foundation for understanding polaronic and excitonic effects in CsPbBr3​ and related nanostructures.

Perovskite semiconductors attract a lot of attention due to their intriguing optical and electrical properties. Ultrafast spectroscopy of coherent acoustic phonons employs excitation and detection by short femtosecond laser pulses in pump-probe techniques, representing a powerful method to investigate the lattice dynamics. This method exploits excitation of stress associated with the thermoelastic coupling in metals or with the photostriction in semiconductors. In most settings, the longitudinal acoustic (LA) phonons are generated. The methods to induce transverse acoustic (TA, shear) phonons: excitation at the crystalline surface of low symmetry [1] and the inverse piezoelectric mechanism [2] fail in centrosymmetric halide perovskites, where the mechanisms of generation of shear acoustic waves are still under debate.
In [3], a new mechanism for efficient optical generation of shear strain in perovskite semiconductors via the giant anisotropic photostriction of the tetragonal crystal lattice is revealed and elaborated. Coherent TA phonons with amplitudes comparable to LA modes are observed in the tetragonal phase of a Cs₂AgBiBr₆ crystal using time-domain Brillouin spectroscopy by femtosecond laser pulses. One of the two TA modes is excited with polarization direction given by the projection of the c-axis on the sample surface. In cubic phase only LA strain pulse is generated. The weak temperature dependence of the photogenerated coherent phonons provides evidence for the non-thermal nature of the effect, i.e., for the photostrition.
To further elaborate the microscopic origin of the giant anisotropic photostriction, the DFT calculations were performed. The calculations reproduce different sign of photostriction in teteragonal phase of CsPbI₃ [4] and predict similar effect in teteragonal double perovskite Cs₂AgBiBr₆. Calculations of deformation potentials demonstrate the essential importance of the optical deformation potential of the mode associated with octahedron rotation mode in Γ-point.

The assembly of monodisperse nanocrystals (NCs) into long-range ordered superlattices (SLs) provides a platform for engineering materials with programmable and distinct optoelectronic properties compared to those of the constituent building blocks [1]. SLs that integrate NCs of different materials can unite the different functionalities and generate new light-matter interactions [2], expanding the potential for integration into the next-generation optoelectronic devices. This work studies the novel collective optical properties in binary SLs comprising of cubic CsPbBr₃ NCs co-assembled with spherical Au nanoparticles (NPs). The study reveals modifications in the absorption and photoluminescence spectra of the binary SLs with reference to the respective single-component CsPbBr₃ SLs, attributed to electronic interactions between the nanocrystal excitons and the localized surface plasmons. The dependency of the collective optical properties to the size of the nanocrystal building blocks and the perovskite – plasmonic intercomponent distance is also presented.

Colloidal halide perovskite quantum dots (QDs) have garnered significant attention as single-photon source materials due to their scalable synthesis, solution-processability, and high brightness. Despite extensive efforts to stabilize the surface through chemical engineering of ligand binding components, individual CsPbBr₃ perovskite QDs often exhibit fluorescence intermittence (also known as “blinking”) and severe photodegradation. Here, we introduce a solid-state organic crystal passivation strategy for CsPbX₃ (X=Br, I) perovskite QDs with sizes ranging from 3.6 to 14 nm. By leveraging the low steric hindrance of phenethylammonium ligands and their attractive intermolecular interactions, we promote the epitaxial ligand crystallization on the surface of single CsPbX₃ (X=Br, I) QDs. These QDs exhibit nearly non-blinking single-photon emission with high purity (~98%) and remain photostable after 12 hours of continuous operation. Single QDs can also withstand saturated laser excitation.[1] These advantages enable the determination of size-dependent exciton radiative rates and emission linewidths of CsPbBr₃ QDs at the single-particle level. Additionally, we discuss the impact of the ligand molecular crystal on the optical properties of single perovskite QDs at room and cryogenic temperatures.

Colloidal caesium lead-halide (CsPbX₃, X = Cl, Br, I) perovskite quantum dots (QDs) increasingly gain attention as quantum light sources, owing to their fast and pure single-photon emission. The rapid radiative decay is a crucial characteristic for quantum emitters, not only because it translates into their brightness, but also for boosting single-photon indistinguishability. However, the future practical application of perovskite quantum emitters is restricted by the non-degeneracy of the bright-triplet excitons, manifesting itself in two to three, closely spaced sharp emission lines of comparable intensity and orthogonal polarization.

QD shape engineering offers an additional and powerful tool for further fine-tuning and improvement of optical properties, allowing the manipulation of features that are inaccessible by keeping the shape isotropic. In the case of perovskite QDs, shape anisotropy can enable, for example, directional emission, spatial confinement of excitons in one or two dimensions, tuning of exciton fine structure, and radiative decay. To systematically explore shape-dependent properties of one-dimensional CsPbBr₃ perovskite structures, we developed a synthetic approach toward stable, size- and shape-uniform nanorods (NRs) with tunable thickness (5-24 nm) and aspect ratio (1-16, larger for thinner nanorods). The obtained NRs are of parallelopiped shape (elongated cuboids) and expose four (110) and two (002) facets of the orthorhombic CsPbBr₃ (Pbnm space group). By exploiting the difference between {110} and {002} facets, we achieved precise control over NR morphology. The synthesis was guided and rationalized by molecular dynamics simulation of the ligand-binding to pseudo-cubic (100)-facets of the orthorhombic CsPbBr₃. The steric effect of the ligand tail and carboxylate binding group were shown to yield a small but finite difference in the ligand-binding energy for two out of four facets, driving the elongation during the growth. With the use of phosphoethanolamine-based zwitterion ligands providing sufficient stability, we performed comprehensive optical characterization, paving the way for advanced optical functionalities in perovskite nanostructures.

Lead-halide perovskite nanocrystals are emerging as a promising platform for next-generation scintillators, combining high light yield with sub-nanosecond timing and access to collective quantum-optical regimes. This talk presents a materials-by-design approach in which CsPbBr₃ nanocrystals are integrated into high-Z and mesostructured hosts to suppress defect-mediated degradation while preserving ultrafast radiative kinetics. Sensitization with heavy oxide nanoparticles such as HfO₂ enhances energy deposition and charge generation under ionising radiation, effectively decoupling absorption from emission. Weak quantum confinement yields giant oscillator strengths that reconcile brightness and speed. At higher levels of ordering, nanocrystal superlattices exhibit scintillation superfluorescence, converting stochastic ionisation cascades into deterministic picosecond light bursts. Plasmonic coupling between metallic nanoparticles and polyconjugated emitters sensitized by high-Z particles enables Purcell-enhanced scintillation with accelerated radiative decay.Together, these advances point toward nanoscintillators with light yields and timing resolutions approaching a few tens of picoseconds, with implications for time-of-flight imaging, high-energy physics and precision dosimetry.