This work presents findings from femtosecond time-resolved spectroscopy on LaVS3, revealing interactions between carriers and phonon modes. It elucidates the effects on thermal conductivity and the emergence of metastable states due to carrier trapping within vanadium clusters.

In this Letter, we investigate coherent phonon dynamics in the incommensurate LaVS3 crystal by femtosecond pump-probe spectroscopy. Two coherent phonon modes are systematically observed in the transient reflectivity, centered at 1.8 and 2.85 THz, respectively, while a third mode centered at 4.5 THz is observed only at high pump fluence. The experimental results obtained at two different polarization configurations as well as a comparison with recent theoretical results allow to assign the two main modes to the interlayer shearing mode and to an intralayer mode, respectively. Two possible assignments are discussed for the third mode, by invoking a possible emergence of nonlinear phonon processes.

In this article, we study electron dynamics in HgTe quantum dots with a 1.9 mu m gap, a material relevant for infrared sensing and emission, using ultrafast spectroscopy with 35 fs time resolution. Experiments have been carried out at several probing photon energies around the gap, which allows us to follow the relaxation path of the photoexcited electrons. We compare such dynamics in two kind of samples, HgTe quantum dots with long ligands and with short ligands, in order to distinguish the role of the coupling between adjacent quantum dots. Three main dynamics can be observed in the transient reflectivity on both samples, with slightly different relaxation times: two fast decays on the time scale of hundreds of femtoseconds and a few picoseconds, respectively, followed by a slower relaxation back to the unperturbed value over hundreds of picoseconds. The two fast components are associated with intraband relaxation of the photoexcited electrons within the conduction band, while the final relaxation path can be assigned to Auger relaxation mechanisms and to the slower interband exciton recombination.

As they have gained maturity, colloidal nanocrystals (NCs) have also expand the spectral range over of which they could be used for photonic and optoelectronic applications. In particular, the infrared use of NCs has become of utmost interest to develop cost-effective alternatives to current technologies. It is then critical not to let the material dictate the light-matter interaction, which is why the coupling of NCs to photonic cavities has been proposed. For infrared NCs, this approach has first been devoted to the control of absorption with in mind the increase of the signal magnitude for detectors. A Lot of efforts have been focused on the use of metallic metasurfaces. However, these generate significant optical losses and yield low quality factor. Here, this study rather focus on the coupling of infrared NCs to a dielectric mirror cavity. HgTe/CdS core-shell NCs are used and integrated into a cavity made of aperiodic dielectric mirrors. The effect of the substrate is systematically study on spectral linewidth, carrier dynamic, and emission directivity. The cavity is shown to narrow the PL by a factor 10, while focusing the emission over a 12 degrees angle. Monitoring the power dependence of the emission, this study shows that the cavity leads to 250 K increase in the effective electronic temperature. Schematic of narrow band gap HgTe nanocrystals within a dielectric mirror cavity. Inside the cavity, the photoluminescence spectrum is drastically narrowed, the effective electronic temperature of the carriers is strongly increased, while the emission is also made more directive. image

In this paper, we demonstrate the ultrafast reduction of conductivity in an incommensurate crystal structure within hundreds of femtoseconds. This phenomenon stands in stark contrast to most prior experimental investigations where incident light pulses led to increased conductivity. We achieve this by selectively targeting a specific atomic bond using near-infrared light pulses. Our investigation focuses on misfit layered chalcogenide (LaS)1.196VS2, known as LaVS3, a semimetal with incommensurability along one crystallographic direction. Our time-resolved electron dynamics investigation reveals that the conductivity decreases as photoexcited electrons are promoted into localized energy states within vanadium clusters due to the incommensurate structure. These findings offer insights into the potential for controlling electronic properties at femtosecond time scales, with implications for the development of ultrafast electronic devices.

This article reports a comparative study of bulk and surface properties in the transition metal dichalcogenide 1T-TaS$_2$. When heating the sample, the surface displays an intermediate insulating phase that persists for $\sim 10$ K on top of a metallic bulk. The weaker screening of Coulomb repulsion and stiffer Charge Density Wave (CDW) explain such resilience of a correlated insulator in the topmost layers. Both time resolved ARPES and transient reflectivity are employed to investigate the dynamics of electrons and CDW collective motion. It follows that the amplitude mode is always stiffer at the surface and displays variable coupling to the Mott-Peierls band, stronger in the low temperature phase and weaker in the intermediate one.

Intraband absorption in doped nanocrystals offers an interesting alternative to narrow-band-gap materials to explore mid-infrared optoelectronic device designs. However, the performance of such a device clearly lags behind the ones relying on intrinsic materials. Livache et al. proposed a dye-sensitized approach to overcome the limitations observed from intraband materials (high dark current, slow response, low activation energy), where an intraband absorber is coupled with an undoped material, which takes care of the charge conduction. Here, we unveil the coupling between both materials using mid-infrared transient reflectivity (TR) measurement. We show that the hybrid material displays a unique feature in the TR signal that we attribute to a charge transfer for which the dynamics matches the hopping time. Then, we developed a strategy to enhance the photodetection performances of the hybrid material by coupling the intraband absorption to a light resonator for the first time. The latter is used to enhance the absorption by a factor of 4 and enables an increase in the operating temperature by 80 K compared to the reference device. The obtained device matches the performance of the best devices relying on intraband absorption.

Phase-change alloys have seen widespread use, from rewritable optical disks to current interest in their use in emerging neuromorphic computing architectures. In spite of this enormous commercial interest, the physics of the carriers in these materials is still not fully understood. Here, we describe the time and space dependence of the coupling between photoexcited carriers and the lattice in both the amorphous and crystalline states of one phase-change material, GeTe. We study this material using a time-resolved optical technique called the picosecond acoustic method to investigate the in situ thermally assisted amorphous-tocrystalline phase transformation in GeTe. Our work reveals a clear evolution of electron-phonon coupling during the phase transformation, as the spectra of photoexcited acoustic phonons in the amorphous (alpha-GeTe) and crystalline (alpha-GeTe) phases are different. In particular, and surprisingly, our analysis of the photoinduced acoustic pulse duration in crystalline GeTe suggests that part of the energy deposited during the photoexcitation process takes place over a distance that clearly exceeds that defined by the skin depth of the pump light. Alternatively, the photoexcitation process remains localized within that skin depth in the amorphous state. We then demonstrate that this is due to supersonic diffusion of photoexcited electronhole plasma in the crystalline state. Consequently, these findings prove the existence of the nonthermal transport of energy, which is much faster than lattice heat diffusion.

Topological insulators (TIs) are promising materials for future spintronic applications such as emerging spin-to-charge conversion (SCC) devices, possibly working at GHz-THz frequency for ultrafast data processing. These devices will rely on hybrid nanostructures composed, for example, of a ferromagnetic layer deposited on the topological insulator. The efficiency of spin-to-charge conversion will depend on the quality of the interface, including chemical (interfacial chemical reactions) and physical (band bending effect, Fermi pinning) aspects. This paper presents a complete study of electronic structures and photoexcited carrier dynamics in topological insulators capped with iron and iron oxide. We combine static and time-resolved angle-resolved photoemission spectroscopies (ARPES, TR-ARPES) with time-resolved optical methods (transient optical reflectivity and transmission). Both single crystal and thin films of Bi2Te3 are studied. We show that monolayers of iron and iron oxide significantly affect the electronic band structure at the interface by shifting the Fermi level into the conduction band, which we explain by a band bending effect, and is confirmed by in situ XPS measurements. This modified interfacial electronic structure offers a new channel for relaxation of hot carriers, illustrated by a drastic decrease of their characteristic decay time after optical excitation. These results might have a potential impact in the future development of TI-based SCC devices.

Visible nanocrystal-based light-emitting diodes (LEDs) are about to become commercially available. However, their infrared counterparts suffer from two key limitations. First, III–V semiconductor technologies are strong competitors. Second, their potential for operation beyond 1.7 µm remains unexplored. The range from 1.5 to 4 µm corresponds to a technological gap in which the efficiency of interband quantum-well-based devices vanishes and quantum cascade lasers are not efficient enough. Powerful infrared LEDs in this range are needed for applications such as active imaging, organic molecule sensing and airfield lighting. Here we report the design of a HgTe nanocrystal-based LED with luminescence between 2 and 2.3 µm. With an external quantum efficiency of 0.3% and radiance up to 3 W Sr−1 m−2, these HgTe LEDs already present a competitive performance for emission above 2 µm. Near-infrared emission at around 2 µm is observed from HgTe nanocrystals. LEDs based on this material platform could prove to be a useful low-cost, convenient light source for applications in gas sensing and other tasks.

The impact of europium doping on the electronic and structural properties of the topological insulator Bi2Te3 is studied in this paper. The crystallographic structure studied by electron diffraction and transmission microscopy confirms that grown by Molecular Beam Epitaxy (MBE) system film with the Eu content of about 3% has a trigonal structure with relatively large monocrystalline grains. The X-ray photoemission spectroscopy indicates that europium in Bi2Te3 matrix remains divalent and substitutes bismuth in a Bi2Te3 matrix. An exceptional ratio of the photoemission 4d multiplet components in Eu doped film was observed. However, some spatial inhomogeneity at the nanometer scale is revealed. Firstly, local conductivity measurements indicate that the surface conductivity is inhomogeneous and is correlated with a topographic image revealing possible coexistence of conducting surface states with insulating regions. Secondly, Time of Flight Secondary Ion Mass Spectrometry (TOF-SIMS) depth-profiling also shows partial chemical segregation. Such in-depth inhomogeneity has an impact on the lattice dynamics (phonon lifetime) evaluated by femtosecond spectroscopy. This unprecedented set of experimental investigations provides important insights for optimizing the process of growth of high-quality Eu-doped thin films of a Bi2Te3 topological insulator. Understanding such complex behaviors at the nanoscale level is a necessary step before considering topological insulator thin films as a component of innovative devices.

Heralded as one of the key elements for next generation spintronics devices, topological insulators (TIs) are now step by step envisioned as nanodevices like charge-to-spin current conversion or as Dirac fermions based nanometer Schottky diode for example. However, reduced to few nanometers, TIs layers exhibit a profound modification of the electronic structure and the consequence of this quantum size effect on the fundamental carriers and phonons ultrafast dynamics has been poorly investigated so far. Here, thanks to a complete study of a set of high quality molecular beam epitaxy grown nanolayers, we report the existence of a critical thickness of around ~6 nm, below which a spectacular reduction of the carrier relaxation time by a factor of ten is found in comparison to bulk Bi2 Te3 In addition, we also evidence an A1g optical phonon mode softening together with the appearance of a thickness dependence of the photoinduced coherent acoustic phonons signals. This drastic evolution of the carriers and phonons dynamics might be due an important electron-phonon coupling evolution due to the quantum confinement. These properties have to be taken into account for future TIs-based spintronic devices.

Topological insulators (TI) are a class of materials gaining in importance due to their unique spin/electronic properties, which may allow for the generation of quasiparticles and electronic states which are not accessible in classical condensed-matter systems. Not surprisingly, TI are considered as promising materials for multiple applications in next generation electronic or spintronic devices, as well as for applications in energy conversion, such as thermo-electrics. In this study, we examined the practical challenges associated with the formation of a well-defined junction between a model 3D topological insulator, Bi2Te3, and a metal, Fe or Eu, from which spin injection could potentially be realized. The properties of multilayer systems grown by molecular beam epitaxy (MBE), with Fe or Eu thin films sandwiched between two Bi2Te3 layers, were studied in-situ using electron diffraction and photoelectron spectroscopy. Their magnetic properties were measured using a SQUID magnetometer, while the in-depth chemical structure was assessed using secondary ion mass spectroscopy. An examination of impact of Bi2Te3 structure on chemical stability of the junction area has been realized. For Fe, we found that despite room temperature growth, a reaction between the Fe film and Bi2Te3 takes place, leading to the formation of FeTe and also the precipitation of metallic Bi. For the Eu tri-layer, a reaction also occurs, but the Te chemical state remains intact.

Ultrafast lattice dynamics of few quintuple layers of topological insulator (TI) Bi$_2$Te$_3$ is studied with time-resolved optical pump-probe spectroscopy. Both optical and acoustic phonons are photogenerated and detected. Here, in order to get new insights on the out-of-equilibrium electron-phonon coupling and phonons dynamics in confined TI, different nanostructures have been investigated (single or polycrystalline QLs assemblies and nano-crystallized islands). Contrary to previous literature claims, we show that even for nanostructures containing only 10 quintuple layers (QLs), the symmetric A1g(I) coherent optical phonon is efficiently photogenerated and no restriction due to the structural confinement appears. We also observe that whatever the arrangement of the nanostructures, the A1g(I) optical phonon features are similar (lifetime). We also report the observation of confined coherent acoustic phonons propagating from QLs to QLs whose spectrum is, this time, very sensitive to the atomic arrangement. In the case of the single crystalline ultrathin film, the time of flight analysis of these acoustic phonons provides direct estimate of the elastic properties of these nanostructures as well as some estimates of Van der Waals interactions between QLs.