Ferroelectrics are polar materials whose polarization can be switched by applying electric fields; they offer unique opportunities to develop performant photostrictive materials, i.e., materials that can deform under visible light illumination. Naturally devoid of inversion symmetry, they exhibit original photogalvanic effects such as the Bulk Photovoltaic Effect, which relies on “hot” photoexcited carriers. It has long been thought that the electric field generated by this effect may couple to the natural piezoelectric abilities of ferroelectrics to provide large photoinduced deformations. However, due to competing effects, such as thermal dilatation, deformation potential, polarization, or depolarizing-field screening by thermalized carriers, it remains unclear which microscopic phenomena govern the photoinduced deformations in classical ferroelectric materials. Here, we demonstrate the largest photoinduced deformation measured in a ferroelectric thin film. Reaching 1

Density Functional Theory (DFT) calculations not only allow to predict the vibrational and optical properties of solids but also to understand and disentangle the mechanisms playing a key role in the generation of coherent optical phonons. Recent experiments performed on a Bi_2Te_3 nanofilm have shown that a THz pulse launches at least a coherent A_1g^1 phonon as the transient transmittance measured using an isotropic detection scheme displays oscillations with a frequency matching the frequency of the A_1g^1 mode measured in Raman experiments. Such an observation can be explained by invoking either a sum frequency process or cubic/quartic phonon-phonon couplings as considered for Bi_2Se_3, a parent compound of Bi_2Te_3. By resorting to group theory and calculating energy surfaces from first-principles, the main phonon-phonon couplings can be identified. Furthermore, a minimal model can be built to compute the dynamics of the Raman active modes coupled to the infrared active mode driven by the experimental THz pulse. Our model firmly establishes that cubic phonon-phonon interactions are relevant as the agreement between the computed and experimental transmittance is noteworthy.

In this paper, we develop a method that combines optical birefringence properties and time-domain Brillouin scattering microscopy to determine in situ the optical axis orientation of each single micrometer size grain in a polycrystalline sample. We illustrate the method by investigating the room temperature multiferroic material BiFeO3 where the optical axis coincides with the ferroelectric polarization direction. We are able to find the grain orientation and also provide the sound velocity (longitudinal and transverse) since the method is based on the Brillouin scattering process. These advances open interesting perspectives for probing the anisotropy of a micrometer size grain with an extension to the evaluation of the ferroelastic domain orientation in a non-contact and non-destructive manner.

Spin waves (SWs) hold great potential for carrying information in future data transport and processing technologies. Generating SWs that combine high frequency with low damping using minimal energy is crucial for ultrafast and energy-efficient applications. In this paper, we investigate the laser-induced ultrafast magnetization dynamics in a coupled BiYIG/Co heterostructure and compare the results with those obtained in bare BiYIG. We demonstrate that femtosecond laser pulses can trigger high-frequency standing SWs in the BiYIG-based heterostructure with very low laser energy density, which cannot excite spin precession in bare BiYIG. We argue that the primary mechanism behind the excitation relates to ultrafast changes in the interlayer exchange field acting on BiYIG, caused by ultrafast laser heating of the adjacent Co layer. These results can open exciting perspectives for ultrafast and energy-efficient photomagnonics applications.

Mechanisms of spin/charge interconversion like the spin Hall effect can be used to generate and detect spin currents at timescales ranging from dc to subpicosecond. In the dc regime, the best candidates are Rashba interfaces showing the inverse Edelstein effect, with LaAlO3/SrTiO3 3 / SrTiO 3 presenting a record conversion efficiency. At picosecond timescales, devices relying on inverse spin Hall effect are efficient THz emitters, but this is less clear for Rashba-Edelstein systems. Here we study the conversion of angular momentum into charge at different timescales in the LaAlO3/SrTiO3 3 / SrTiO 3 interface. We show that while the effect is exceptionally large in the dc regime, it is reduced by more than four orders of magnitude at the picosecond timescale. This loss of efficiency is discussed in terms of specificities linked to the Rashba state at the interface between these two large band-gap insulators. This has strong implications for the spintronic THz emitters and our work underlines the salient features required for their optimal operation.

Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone which gives rise to additional electronic bands and phonon modes. This has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices, for example. Here, we show that multiferroic BiFeO_3/LaFeO_3 superlattices are more than just periodic chemical stacking. Using transmission electron microscopy, X-ray diffraction and first-principles calculations, we demonstrate the existence of a new order of FeO_6 octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e. a superorder. The effect of this new structural order on the lattice dynamics is studied with ultrafast optical pump-probe experiments. While a mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural description. Our work shows that multiferroic BiFeO_3/LaFeO_3 superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

Superlattices are materials created by the alternating growth of two chemically different materials. The direct consequence of creating a superlattice is the folding of the Brillouin zone, which gives rise to additional electronic bands and phonon modes. This phenomenon has been successfully exploited to achieve new transport and optical properties in semiconductor superlattices. Here, we show that multiferroic BiFeO3/LaFeO3 superlattices exhibit several structural orders parallel and perpendicular to the growth direction, not existing in individual bulk materials. Using transmission electron microscopy, x-ray diffraction, and first-principles calculations, we reveal in particular a new long-range order of tilted FeO6 octahedra, with a period along the growth direction about twice that of the chemical supercell, i.e., a superorder. The effect of this new structural order on the phonon dynamics is studied with ultrafast optical pump-probe experiments. While a folded-mode at 1.2 THz is attributed solely to the chemical modulation of the superlattice, the existence of another 0.7 THz mode seems to be explained only by a double Brillouin zone folding in agreement with the structural out-of-plane superorder. Our work shows that multiferroic BiFeO3/LaFeO3 superlattices can be used to tune the spectrum of coherent THz phonons, and potentially that of magnons or electromagnons.

The dielectric properties of Bi$_2$Te$_3$, a layered compound crystallizing in a rhombohedral structure, are investigated by means of first-principles calculations at the random phase approximation level. A special attention is devoted to the anisotropy in the dielectric function and to the local field effects that strongly renormalize the optical properties in the UV-visible range when the electric field is polarized along the stacking axis. Furthermore, both the Born effective charges for each atom and the zone center phonon frequencies and eigenvectors needed to describe the dielectric response in the infrared range are computed. Our theoretical near-normal incidence reflectivity spectras in both the UV-visible and infrared range are in fairly good agreement with the experimental spectras, provided that the free carriers Drude contribution arising from defects is included in the infrared response. The anisotropic plasmon frequencies entering the Drude model are computed within the rigid band approximation, suggesting that a measurement of the reflectivity in the infrared range for both polarizations might allow to infer not only the type of doping but also the level of doping.

Spin/charge interconversion mechanisms provide an essential handle to generate and detect spin currents. Their applications at different timescales are critical in spintronics since they cover a technologically relevant broadband spectrum. While the inverse spin Hall effect is known to be robust from quasi-static to sub-picosecond timescales, the conversion efficiency evolution of the inverse Edelstein effect has not been addressed yet. In this work, we report that while the quasi-static response of the inverse Edelstein effect can be comparable to that of the most efficient inverse spin Hall systems, a drastic drop of efficiency is observed in the terahertz (THz) regime. This behavior at the sub-picosecond timescale is qualitatively understood from the dependence of the inverse Edelstein effect on the energy distribution of spin-carrier entities, which is different between thermalized carriers in the quasi-static regime and hot carriers generated by light pulses. This finding is supported by the pump-laser wavelength dependence in the THz regime for the inverse Edelstein effect, which offers a promising route for tunability of spintronic devices.

Ultrashort light pulses induce rapid deformations of crystalline lattices. In ferroelectrics, lattice deformations couple directly to the polarization, which opens the perspective to modulate the electric polarization on an ultrafast time scale. Here, we report on the temporal and spatial tracking of strain and polar modulation in a single-domain BiFeO 3 thin film by ultrashort light pulses. To map the light-induced deformation of the BiFeO 3 unit cell, we perform time-resolved optical reflectivity and time-resolved x-ray diffraction. We show that an optical femtosecond laser pulse generates not only longitudinal but also shear strains. The longitudinal strain peaks at a large amplitude of 0.6%. The access of both the longitudinal and shear strains enables to quantitatively reconstruct the ultrafast deformation of the unit cell and to infer the corresponding reorientation of the ferroelectric polarization direction in space and time. Our findings open new perspectives for ultrafast manipulation of strain-coupled ferroic orders.

Projects of inertial confinement fusion using lasers need numerous optical components whose coatings allow the increase in their transmission and their resistance to high laser fluence. A coating process based on the self-assembly of sol-gel silica nanoparticles and a post-treatment with ammonia vapor over the surfaces of the optical components ("ammonia curing process") was developed and successfully optimized for industrial production. Manufacturing such antireflective coatings has clear advantages: (i) it is much cheaper than conventional top-down processes; (ii) it is well adapted to large-sized optical components and large-scale production; and (iii) it gives low optical losses in transmission and high resistances to laser fluence. The post-treatment was achieved by a simple exposition of optical components to room-temperature ammonia vapors. The resulting curing process induced strong optical and mechanical changes at the interface and was revealed to be of paramount importance since it reinforced the adhesion and abrasion resistance of the components so that the optical components could be handled easily. Here, we discuss how such coatings were characterized and how the initial thin nanoparticle film was transformed from a brittle film to a resistant coating from the ammonia curing process.

The understanding of the lattice dynamics in ferroic compounds driven by an ultrashort light pulse is an exciting research direction due to the exceptional non-linear properties (optical, elastic, electric and magnetic) of ferroic and multiferroic materials. Photo-induced strain in ferroic materials is driven by a complex interplay between charge, phonon and spin dynamics with microscopic mechanisms that still need to be elucidated. We present recent experiments where ultrafast photoinduced strain is evaluated in BiFeO3-based multiferroic materials, with a focus on the description of the ultrafast symmetry change of the unit-cell that appears after an ultrashort laser pulse. A combination of optical and X-ray time-resolved techniques will be presented. We show that it is possible to modulate at the picosecond time scale the ferroic order by playing with the out-of-plane and in-plane light-induced strains. These new results provide new insights for the understanding of the physics of photo-induced strain, in relation with the light-induced ferroelectric modulation in nanostructured ferroic compounds and could be the first step towards their use as on-purpose ferroic architectures in devices like actuators or modulators with ultra-short light pulses.

Bimetallic Au/Ag core–shell cuboid nanoparticles (NPs) exhibit a complex plasmonic response dominated by a dipolar longitudinal mode and higher-order transverse modes in the near-UV, which may be exploited for a range of applications. In this paper, we take advantage of the strong signature of these modes in the NP ultrafast transient optical response, measured by pump-probe transient absorption (TA) spectroscopy, to explore the NP vibrational landscape. The fast Fourier transform analysis of the TA dynamics reveals specific vibration modes in the frequency range 15–150 GHz, further studied by numerical simulations based on the finite element method. While bare Au nanorods exhibit extensional and breathing modes, the bimetallic NPs undergo more complex motions, involving the displacement of facets, edges and corners. The amplitude and frequency of these modes are shown to depend on the Ag shell thickness, as the silver load modifies the NP aspect ratio and mass. Moreover, the contributions of the vibrational modes to the experimental TA spectra are shown to vary with the probe laser wavelength at which the signal is monitored. Using the combined simulations of the NP elastic and optical properties, we elucidate this influence by analyzing the effect of the mechanisms involved in the acousto-plasmonic coupling.

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.

Co-doping BaTiO3 with (X3+, Y5+) transition metal ions improves its optical absorption properties while retaining good ferroelectricity.

Over the last decade, the development of high-power ultrafast laser systems led to the emergence of intense pioseconds terahertz (THz) pulses, which provide a new tool for studying fundamental aspects of light-matter interactions by driving out-of-equilibrium electrons, phonons or magnons at ultrafast time scale. Thanks to spectral weight in the THz frequency range, it is possible to directly couple light to infrared-active optical phonon mode in solid and it has been widely demonstrated and studied in various materials. However, only sparse and incomplete reports are available on THz-induced coherent acoustics phonons and none of them clearly demonstrate the origin of coherent acoustics phonons generation. Here, we report on the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting acoustic eigenmode in nanometric sized thin films.

We report for the first time the generation of coherent acoustic phonons in materials with terahertz ultrashort pulses. This is demonstrated in metals and topological insulators by exciting acoustic eigenmode in nanometric sized thin films.