Alessandro Baserga

Two-dimensional van der Waals materials display rich nonequilibrium physics due to their reduced dimensionality and strong light–matter interactions. Layered metal phosphorus trichalcogenides (MPX3) feature bandgaps spanning the visible to ultraviolet range and exhibit a complex interplay between excitons, phonons, and defects. Here we use static and time-resolved photoluminescence and femtosecond broadband transient reflectivity to investigate the temperature-dependent ultrafast carrier dynamics in layered HgPSe3, a semiconductor with a quasi-direct bandgap near 2 eV. We identify distinct sub-bandgap photoluminescence features that become prominent at low temperature. Their temperature and fluence dependence strongly indicate radiative recombination involving localized states. The transient reflectivity map displays three features associated with interband optical transitions in the visible range, as corroborated by density functional theory band structure calculations. These features exhibit fluence-dependent decay dynamics, indicating a competition between carrier localization into sub-bandgap states, state-filling effects that limit the available electronic phase space for hot-carrier relaxation, and phonon-bottleneck-limited carrier cooling. Our results provide new insights into the ultrafast carrier dynamics of HgPSe3 and establish a framework for understanding the role of sub-bandgap states in MPX3 semiconductors, with potential implications for broadband optoelectronic and photodetection applications.

Electro‐ and thermochromic materials change their optical properties through the physical movement of atomic species within thin films. This movement is induced by electric fields or high temperatures and occurs on the second to minute timescale. Ultrashort laser pulses can manipulate atomic positions on the femto‐ to picosecond timescale. In this work, we modulated the optical properties of a palladium hydride nanoparticulate thin film on the picosecond timescale using femtosecond optical pulses. Density functional theory calculations confirmed that the incident photons activate interstitial hydrogen atoms and the subsequent trajectories of these atoms affect the dielectric properties of the palladium hydride thin film. These responsive interstitial hydrogen atoms provide a modulator for optical switching on the picosecond timescale and could lead to faster photochromic materials.

Electron–phonon coupling in two-dimensional Bi₂Se₃ nanoplatelets was investigated using broadband ultrafast pump–probe spectroscopy (probe 1.6 to 2.8 eV) at 293 and 78 K. The high time resolution resolves coherent phonon oscillations on picosecond time scales, specifically the out-of-plane A_1g ⁽¹⁾ (2.12 THz) and A_1g ⁽²⁾ (4.95 THz) optical modes, and a low-frequency interlayer breathing mode (∼0.4 THz), arising from standing waves defined by the nanoplatelet thickness. By mapping the probe energy dependence of the oscillation amplitude, we find that the A_1g ⁽¹⁾ mode couples most strongly to an electronic transition near 1.97 eV, which we assign using the computed band structure to transitions along the high-symmetry Γ–K line. The absence of Raman-active E_g modes is explained by a symmetry analysis based on the displacive excitation of coherent phonons. These results characterize carrier–lattice interactions in 2D Bi₂Se₃, which are relevant for optoelectronic device applications.

The ability to initialize and control long-lived spin populations in lead halide perovskites is essential to enable their use in spintronics technologies. Here, we show that the interlayer exciton observed in the photoluminescence spectrum of a (BA)₂PbI₄/WSe₂ monolayer heterostructure below the intralayer exciton transitions exhibits circular dichroism, whose sign can be modulated by tuning the excitation energy. Equilibrium and transient absorption measurements reveal an additional absorptive feature between the A and B excitons of WSe₂ with hybrid character. This resonance arises from hybridized valence band states of the two materials and plays a key role in controlling the helicity of the interlayer exciton emission. The tunable spin polarization demonstrated here, with the WSe₂ monolayer effectively acting as a tunable spin filter, represents an important step toward the use of 2D perovskites in opto-spintronics applications.

Chalcogen phosphates of transition metals make up a well-known group of antiferromagnetic semiconductors with the general formula MPX₃, where M represents a transition metal and X is a chalcogen, either sulfur or selenium. Most of these compounds adopt a similar structure; however, mercury phosphochalcogenides present an exception with their unique van der Waals layered structure. Transition metal chalcogenides are highly appealing materials for photodetectors due to their exceptional optoelectronic properties. Among them, HgPSe₃, a layered van der Waals phosphoselenide, shows promise for photodetection over a broad spectral range, from visible light to X-rays. Despite this, the electronic processes governing its photoresponse remain unclear. In this study, we demonstrate a nanosecond response time of a HgPSe₃-based photodetector to visible light and gain deeper insights into the underlying charge carrier dynamics through a comprehensive investigation using complementary time-resolved experimental techniques. Our findings on the role of carrier traps provide a potential pathway for optimizing optoelectronic device performance.

A 2.6-fold spectral broadening of mid-infrared femtosecond µJ-level pulses has been achieved using an unfolded multi-pass configuration of germanium plates and zinc selenide lenses. This method maintains a throughput higher than 60% while preserving the spatial quality and the temporal duration of the input beam. Numerical simulations match the experimental results and show the potential to tailor the parameters of the cell to obtain different spectra.