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International audience; Monolayers of transition metal dichalcogenides are semiconducting materials which offer many prospects in optoelectronics. A monolayer of molybdenum disulfide (MoS2) has a direct bandgap of 1.88 eV. Hence, when excited with optical photon energies below its bandgap, no photocarriers are generated and a monolayer of MoS2 is not ofmuch use in either photovoltaics or photodetection.Here, we demonstrate that large size MoS2 monolayer sandwiched between two graphene layers makes this heterostructure optically active well below the band gap of MoS2. An ultrafast optical pump-THz probe experiment reveals in real-time, transfer of carriers between graphene and MoS2 monolayer upon photoexcitation with photon energies down to 0.5 eV. It also helps to unravel an unprecedented enhancement in the broadband transient THz response of this tri-layer material system. We propose possible mechanism which can account for this phenomenon. Such specially designed heterostructures, which can be easily built around different transition metal dichalcogenide monolayers, will considerably broaden the scope for modern optoelectronic applications at THz bandwidth.

THz conductivity of large area MoS

Terahertz time-domain, optical pump-THz probe and optical pump-optical probe spectroscopies are used to study the optoelectronic properties of two PEDOT-based conducting polymers: PEDOT/PSTFSIK and PEDOT/PSS. These experiments highlight the role played by polarons and bipolarons in the carriers' dynamics taking place within these compounds after their photoexcitation by ultrashort optical pulses.

We demonstrate that the far-field terahertz beam generated from a Ti:Sapphire two-color laser-induced filament can exhibit a conical or a Gaussian distribution depending on the filtering experimental conditions. Using a 2D electro-optic detection, we provide the first clear evidence that the conical emission in the 0.2-2.6 THz spectral range is due to photo-induced carriers in the silicon filter typically used to block the remaining pump laser light. The low-frequency terahertz beam retrieves an almost Gaussian spatial distribution when the silicon filter is preceded by a large bandgap ceramic filter, which stops the pump beam, preventing carriers generation in the silicon filter.

The evolution of the THz reflectivity of a high resistivity silicon wafer upon excitation by a femtosecond pulse centered at 800 nm is studied. Upon optical excitation, the THz reflectivity of silicon is increased within less than 1 ps and it can be tailored even beyond 90 % over a broad THz frequency range. Such variation is well accounted for by the pump pulse induced carriers within a thin layer on the surface of silicon.

The THz conductivity of MoS 2 , MoSe 2 monolayers and their heterostructure is compared and reported here. We also measured the hot carrier relaxation upon optical excitation in these samples below and above the bandgap of the monolayers. While in the heterostructure the recovery kinetics of the carriers resembles that of monolayers, the conductivity change is enhanced by one order of magnitude. A simple model accounts for this phenomenon.

The steady state AC terahertz (THz) conductivity and photo-induced carrier dynamics in a single layer of MoS 2 sandwiched in between graphene layers have been studied using sub-200 fs THz pulses. The THz conductivity of the heterostructure resembles that of the graphene. When excited with photon energy of ~1.55 eV, it is shown that about 70% of energetic carriers generated in graphene are rapidly and efficiently transferred back and forth from graphene towards MoS 2 . A mechanism accounting for this phenomenon is proposed.

Far IR and Raman spectroscopy of [Fe–(PM-BiA)2 (NCS)2] and [Fe –(PM-PEA)2 (NCS)2] are performed to record the vibration modes of these complexes and their evolutions during spin state transition. THz spectroscopy is used to measure the far IR absorption of these compounds down to 20 cm-1. Using density functional theory calculations we are able to retrieve and to visualize these vibrations in both high spin and low spin states. In both compounds, Raman spectra recorded in the 800 cm-1 2250 cm-1 wavenumber range are well accounted for by our computations. For far IR vibration modes, most of the experimental absorption peaks above 100 cm-1 are well reproduced. However, below 100 cm-1 , marked differences are recorded. For [Fe–(PM-BiA)2 (NCS)2] we demonstrate that the interactions with surrounding molecules impacts non-negligibly the computed far IR spectra and can partly correct for this difference.

We study, by terahertz time-domain and optical pump-THz probe spectroscopies, the optical and electrical properties of the PEDOT/PSTFSIK polymer. The latter is designed to get reed of some drawbacks of the well-known and widely used conducting polymer: the PEDOT/PSS. The optoelectronic properties in the THz range of both compounds are compared, as well as the relaxation mechanisms taking place after their photoexcitation by ultrashort optical pulses.

We report on the conversion of an infrared vector beam into THz vortex beams using a ZnTe cubic crystal. We provide a theoretical analysis demonstrating how an infrared vector beam with the azimuthal order l can be transformed into a THz beam endowed with an orbital angular moment content that consists of optical vortices with topological charge ±2l. Experimentally, quasi-monochromatic THz vortex beams with topological charges +2 and −2 are produced and characterized both in amplitude and phase using real-time two-dimensional imaging of the terahertz electric field.

Photo-switching the physical properties of molecular systems opens large possibilities for driving materials far from equilibrium toward new states. Moreover, ultra-short pulses of light make it possible to induce and to record photo-switching on a very short time-scale, opening the way to fascinating new functionalities. Among molecular materials, Fe(II) complexes exhibit an ultrafast spin-state transition during which the spin state of the complex switches from a low spin state (LS, S=0) to a high spin state (HS, S=2). The latter process is remarkable: it takes place within ~100 fs with a quantum efficiency of ~100%. Moreover, the spin state switching induces an important shift of the broad metal to ligand absorption band of the complex and it results in large modifications of the physical and chemical properties of the compounds. But, because most of the Fe(II) complexes crystallize in centrosymmetric space groups, this prevents them from exhibiting piezoelectric, ferroelectric as well as second-order nonlinear optical properties such as second harmonic generation (SHG). This considerably limits their potential applications. We have recently synthesized [Fe(phen)3] [$\Delta$-As2(tartrate)2] chiral complexes that crystallize in a noncentrosymmetric 32 space group. Hereafter, upon the excitation of a thin film of these complexes by a femtosecond laser pulse and performing simultaneously transient absorption (TRA) and time-resolved SHG (TRSH) measurements, we have recorded the ultrafast LS to HS switching. Whereas a single TRA measurement only gives partial information, we demonstrate that TRSH readily reveals the different mechanisms in play during the HS to LS state relaxation. Moreover, a simple model makes it possible to evaluate the relaxation times as well as the hyperpolarizabilities of the different excited states through which the system travels during the spin-state transition, Comment: Journal of Physical Chemistry Letters, American Chemical Society, 2019

International audience; Wavefront characterization of terahertz pulses is essential to optimize far-field intensity distribution of time-domain (imaging) spectrometers or increase the peak power of intense terahertz sources. In this paper, we report on the wavefront measurement of terahertz pulses using a Hartmann sensor associated with a 2D electro-optic imaging system composed of a ZnTe crystal and a CMOS camera. We quantitatively determined the deformations of planar and converging spherical wavefronts using the modal Zernike reconstruction least-squares method. Associated with deformable mirrors, the sensor will also open the route to terahertz adaptive optics.

We propose a topological beam-shaping strategy of terahertz beams using geometric phase elements made of space-variant birefringent slabs. Quasi-monochromatic terahertz vortex beams are produced and characterized both in amplitude and phase from the reconstructed real-time two-dimensional imaging of the electric field. Nonseparable superpositions of such vortex beams are also obtained and characterized by two-dimensional polarimetric analysis. These results emphasize the versatility of the spin-orbit electromagnetic toolbox to prepare on-demand structured light endowed with polarization-controlled orbital angular momentum content in the terahertz domain, which should find many uses in future terahertz technologies.

International audience; We propose a topological beam-shaping strategy of terahertz (THz) beams using geometric phase elements made of space-variant birefringent slabs. Quasi-monochromatic THz vortex beams are produced and characterized both in amplitude and phase from the reconstructed real-time two-dimensional imaging of the electric field. Nonseparable superpositions of such vortex beams are also obtained and characterized by two-dimensional polarimetric analysis. These results emphasize the versatility of the spin-orbit electromagnetic toolbox to prepare on-demand structured light endowed with polarization-controlled orbital angular momentum content in the THz domain, which should find many uses in future THz technologies.

International audience; We report on the development of a wavefront sensor for terahertz pulses using a direct two-dimensional electro-optic imaging system composed of a ZnTe crystal and a CMOS camera. By measuring the phase variation of the terahertz electric field in the crystal plane, we are able to reconstruct the terahertz (THz) wavefront in order to determine the optical aberrations of the optical system. Associated with deformable mirrors or specifically designed optics, the sensor will open the route to terahertz adaptive optics.

International audience; We compare two methods of wavefront analysis of THz pulses associated with a 2D electro-optic imaging system. The first one is the standard Hartmann sensor, and the second one is based on the well-known fact that EO sampling can record the phase variation of the THz electric field.

International audience; We have analyzed both theoretically and experimentally the spectrum of the optical pulses produced by the interaction of optical and THz pulses in a ZnTe crystal. Recorded as a function of the delay between the two pulses, the resulting spectrogram can be viewed as a frequency resolved cross-correlation between the optical and THz pulses making it possible to characterize the optical pulse.

International audience; We report on the conversion of an infrared vector beam into terahertz vortex beams using a 110-cut ZnTe cubic crystal. First, we provide a theoretical analysis demonstrating how an infrared vector beam with the azimuthal order can be transformed into a terahertz beam endowed with an orbital angular moment content that consists of optical vortices with topological charge ±2. Experimentally, quasi-monochromatic terahertz vortex beams with topological charges +2 and −2 are produced and characterized both in amplitude and phase using real-time two-dimensional imaging of the terahertz electric field. These results enrich the terahertz vortex beam toolbox via the transfer of topological information from infrared to tera-hertz domains.

International audience; A simple model makes it possible to relate the dielectric constant of spin-crossover nanoparticles to the indices of refraction and the absorption coefficients of solutions containing such nanoparticles. Using this model we show that in the visible spectral range, the switching from a diamagnetic (low spin state) to a paramagnetic (high spin state) state results in a noticeable change of the dielectric constants of the spin crossover nanoparticles containing 3, 5 and 7 iron (II) ions. Within the studied temperature range and for all the studied nanoparticles, the variation of their index of refraction is about 10%. Media whose optical dielectric constant can be tuned by different means open a wide range of possibilities in the design of optical or data storage components [1]. Among such media, optically bistable dispersions of Fe II spin-crossover nanoparticles (SCO NPs) are very promising [2–12]. Some of these SCO NPs present a large thermal hysteresis loop at room temperature. Within the hysteresis loop, the magnetic state of these NPs can be reversibly switched from a diamagnetic low spin (LS, S = 0) state toward a paramagnetic high spin (HS, S = 2) state. This spin state transition is due to electron spin pairing (S = 0) or unpairing (S = 2) within the Fe II ions. At the macroscopic scale the spin transition is associated with a phase transition. This phase transition can be induced by many means using thermal or pressure variations, laser or magnetic exci-tations. For most of the SCO NPs, the spin state transition results in marked absorption changes. For instance, a solution containing SCO NPs is almost uncolored when the particles are in the HS state and turns red or pink when the NPs are in the LS state. This pink color is related to a broad absorption band at around 520 nm, which is characteristic of the d–d transition of the Fe II ions in the LS state (1 A 1g → 1 T 1g) [13]. The d–d transition of the Fe II ions in HS state (5 T 2g → 5 E g) is expected at lower energy in the near infrared region (∼830 nm) [13]. This accounts for the absence of color of the solution containing the SCO NPs in the HS state. SCO NPs with a broad thermal hysteresis loop at room temperature and large absorption changes are therefore very good candidates for data storage. Indeed as previously shown, the spin state transition results in a change in the dielectric constant of the SCO NPs [14–19]. This should result in

International audience; We report on the wavefront analysis of THz pulses emitted by optical rectification of femtosecond laser pulses in ZnTe crystal. The system is based on a Hartmann sensor associated with a 2D electro-optic imaging system.

International audience; The spectra of the optical pulses generated during the interaction of THz and optical pulses in a ZnTe crystal show that, besides sum and difference frequency mixing, spectral components associated to cross-phase modulation are produced.

Two-dimension electro-optic imaging combined with a Hartmann sensor enables THz wavefront characterization in time- and frequency-domain without cross-section scanning. Reconstruction with Zernike polynomials method allows qualitative and quantitative analysis as additional advantages.

We report on the application of shaped femtosecond pulses on low-field chirped excitation of an atomic two-level system. The induced transient phenomena can be considered as a phase diagram of the excitation pulse. Their high sensitivity to the phase-modulated pulse is analyzed, by comparing numerical solutions of the time-dependent Schrodinger equation with experimental results. These coherent transients allow for high precision calibration of pulse-shaping setups where usual methods are less efficient. As an illustration, a comparison between 128-pixel and 640-pixel spatial light modulator pulse shapers is given.

International audience; A simple and distortion free method to sample THz pulse in zinc-blende crystal is proposed and experimentally demonstrated in a cut ZnTe. A comparison of its performances with other techniques is given.

International audience; We report on the second harmonic generation (SHG) of a near-infrared pulse in a zinc telluride crystal through the Pockels effect induced by an intense terahertz pulse. The temporal and angular behaviors of the SHG have been measured and agree well with theoretical predictions. This phenomenon, so far overlooked, makes it possible to generate second harmonic through cascading of two second-order nonlinear phenomena in the near-infrared and terahertz ranges. We also show how this cascading process can be used to sample terahertz pulses.

International audience; We consider the propagation of terahertz (THz) pulses resonant with the lowest rotational levels in ammonia vapors at room temperature. We demonstrate a quasisimultonic regime of propagation for two-color THz pulses (their lossless simultaneous propagation with equal group velocities and stable envelopes at finite distances, like coupled solitons). The quasisimultons are formed by two pulses synchronized in time, the first one being intense and two humped with an area multiple of 4π , while the another one is weaker, one humped, and with an area much less than π. Two-and one-humped pulses have equal width and copropagate in ammonia vapor at the same group velocity. Moreover, the larger the dip of the two-hump pulse is, the weaker may be the one-humped pulse. The stability of this coupled state is provided by the fact that the two-humped pulse makes the medium transparent for the one-humped pulse. In its turn, the latter prevents the breakup of the former into separate pulses. It is established as well that a multihumped pulse with an odd or even number of humps and a hyperbolic secant pulse can behave like a simulton. In this case, the area of multihumped pulse equals to 2nπ , where n is the number of humps in the powerful pulse structure.

International audience; We report on the observation of the Kerr effect induced by an intense terahertz pulse in a 〈100〉 gallium phosphide crystal. The temporal and angular behaviors of the phase retardation have been measured and agree well with theoretical predictions. From these measurements, we extracted the two nonzero tensor elements of the third-order response function of the crystal in the terahertz range.

We show how in molecular predissociation a method combining ultrafast pump-probe techniques with a measurement of the relative recoil velocity can map time-dependent neutral fragment distributions into the ionic continuum. With an appropriate probe pulse exciting a resonant transition (such as (1+1) Resonance Enhanced Multiphoton Ionisation, or excitation of ZEKE states), the temporal evolution of fragment distributions can in principle be measured. Numerical simulations on NaI predissociation are compared to a simple approximate mapping interpretation. The results are discussed in terms of the interplay between temporal and energetic resolution with respect to current experimental limitations.

International audience; We report on optimal control of the output energy of terahertz (THz) waves generated by optical rectification of femtosecond pulses in ZnTe crystals. An enhancement by a factor up to 2.4 is obtained with chirped pump pulses. The optimized THz wave also displays a spectral broadening. The influence of the optical pulse shaping on the pump pulse propagation, and consequently on the THz generation efficiency, is numerically investigated and discussed.

International audience; We demonstrate that the interaction of a CO2 laser with a host matrix, in which spin-crossover compounds are diluted, makes it possible to induce a low-spin-to-high-spin transition. The phenomenon is demonstrated for two different compounds diluted in a commercial polymer. The photo-induced transition is due to laser-induced heating of the host-matrix, which results in a localized modification of the properties of the sample. This phenomenon is reversible, and its kinetics are well accounted for by a simple model. This process makes it possible to write optical information at a high speed and to erase it in such materials.

Optimal control of the output energy of terahertz wave generated by optical rectification in ZnTe crystals is reported. An enhancement by factor up to 2.4 is reported. Two-photon absorption and spectral phase effects on the THz generation efficiency are numerically investigated and discussed.

International audience; We have studied the low spin to high spin phase transition induced by nanosecond laser pulses outside and within the thermal hysteresis loop of the [Fe(Htrz)2 trz](BF4)2-H2O spin crossover nanoparticles. We demonstrate that, whatever the temperature of the compound, the photo-switching is achieved in less than 12.5 ns. Outside the hysteresis loop, the photo-induced high spin state remains up to 100 μs and then relaxes. Within the thermal hysteresis loop, the photo-induced high spin state remains as long as the temperature of the sample is kept within the thermal loop. A Raman study indicates that the photo-switching can be completed using single laser pulse excitation.

International audience; We present and demonstrate experimentally two simple and distortion-free methods to sample terahertz pulse in zinc-blende-type crystals like ZnTe or GaP. They are based on an optical heterodyne detection scheme which makes it possible to measure the terahertz field by detecting the optical probe beam or its second harmonic. Both are compared to conventional electro-optic sampling and give results in good agreement with the latter.

International audience; The evolution of THz spectrum generated in a ZnTe crystal versus the intensity of ultra-short laser pulses is reported. Free carrier absorption and phase matching shift induced by two-photon absorption account for the observed phenomena.

International audience; We have measured the evolution of the THz spectrum of polymeric iron(II) spin crossover compounds within the Low-Spin/High-Spin thermal hysteresis loop in the ~0.6-6 THz frequency range. This study enabled to evidence the large variations of both the refractive optical index and the absorption that occur during the spin state phase transition.

International audience; We study the evolution of the energy and the spectrum of a terahertz wave generated by optical rectification of an ultrashort laser pulse in ZnTe crystals with different thicknesses. As the pump intensity increases, we observe a shift in the terahertz spectrum toward lower frequencies. Moreover, at high pump intensities, in disagreement with common sense, thin crystals have a better conversion efficiency than thicker ones. These phenomena are accounted for by the pump depletion induced by two-photon absorption, pulse broadening and the impact of the photoinduced free carriers on the complex refractive index of the crystal in the terahertz range.

International audience; This paper reports on phase transition photo-induced by a nanosecond laser pulse in the molecular spin crossover material [Fe(NH2-trz)3] (Br)2·3H2O (with NH2trz = 4-amino-1,2,4-triazole) around room temperature and in the close vicinity of the thermal hysteresis loop. The measurements are carried out using a time-resolved pump–probe experiment and by recording the reflectivity change at various temperatures and laser intensities. The dynamics of the optically induced reflectivity changes are presented and discussed. We propose a simple model that describes well the recorded phenomena. It takes into account the physical and optical properties of the sample that directly impact the amplitude and the dynamics of the laser-induced heating of the compound.

Up to now, the common media for optical data storage are the discs (blue ray technology for example). However, by definition, this technology is limited to two dimensions. The necessity for increasing data storage capacity requires the use of three-dimensional (3D) optically based systems. One of the methods for 3D optical data storage is based on volume holography. The physical mechanism is photochromism, which is defined as a reversible transformation of a single chemical species between two states that have different absorption spectra and refractive indices. This allows for holographic multiplexing recording and reading, such as wavelength (Rakuljic et al., 1992), angular (Mok, 1993), shift (Psaltis et al., 1995) and phase encoding. Another promising 3D optical data storage system is the bit-by-bit memory at the nanoscale (Li et al., 2007). It is based on the confinement of multi-photon absorption to a very small volume because of its nonlinear dependence on excitation intensity. This characteristic provides an income for activating chemical or physical processes with high spatial resolution in three dimensions. As a result there is less cross talk between neighbouring data layers. Another advantage of multi-photon excitation is the use of infrared (IR) illumination, which results in the reduction of scattering and permits the recording of layers at a deep depth in a thick material. Two-photon 3D bit recording in photopolymerizable (Strickler & Webb, 1991), photobleaching (Pan et al., 1997; Day & Gu, 1998) and void creation in transparent materials (Jiu et al., 2005; Squier & Muller, 1999) has been demonstrated with a femtosecond laser. Recording densities could reach terabits per cubic centimeter. Nevertheless, these processes suffer from several drawbacks. The index modulation associated with high bit density limits the real data storage volume due to light scattering. The fluorescence can limit the data transfer rate and the lifetime of the device. Thanks to various available compositions, ease of implementation, stability and transparency, both organic and inorganic materials are convenient for 3D data storage. To be good candidates for 3D optical data storage, these materials must satisfy several norms 3

International audience; Phase-locked THz pulse pairs and tunable narrowband THz pulses are generated by means of femtosecond optical pulse shaping combined with optical rectification in ZnTe crystal. THz spectra as narrow as 140 GHz and tunable between 0.5 and 2.5 THz are reported.

International audience; We show that the combination of a free running 80 MHz femtosecond Ti–sapphire oscillator with a nanosecond Nd:YAG laser makes it possible to record in real time, with a nanosecond time resolution, reversible or irreversible phase transitions photo-induced by a single laser pulse. This set-up is used to study the spin state transition photo-induced inside and outside the thermal hysteresis loops of a series of [Fe(NH2trz)3]X2·nH2O compounds (with X = NO3 and Br). Original dynamics are presented and discussed. The results indicate that at room temperature, it is the laser induced heating that drives the recorded photo-induced phase transition.

We demonstrate that spatiotemporal properties of Terahertz (THz) pulses generated in a ZnTe crystal are controlled in the intermediate field by shaping the transverse spatial profile of the near infrared (NIR) femtosecond (fs) optical pump.

At 150 K, we have recorded the evolution of the energy threshold versus the wavelength and the kinetic of formation and relaxation of the spin state phase transition induced by a single laser pulse in the [ Fe ( PM-BiA ) 2 ( NCS ) 2 ] compound. We demonstrate that different absorption bands of the complexes can drive the phase transition. The saturation-energy and the threshold-energy of the laser pulses are not directly related to the absorption cross-section of the sample. Both the switching and the relaxation kinetics are well fitted using two different time-constants. The switching lasts about 50 μs. The relaxation time-constants are independent of the used excitation-wavelength.

We propose and demonstrate the concept of OPCPA based on spectrally addressed amplification in a periodically poled Lithium niobate.

3 p; International audience; We use a spin crossover material as a defect inserted into a periodic structure of alternating layers of glass and air. A single defect mode of this Bragg filter designed for the submillimeter wavelength can be tuned over 15 GHz by inducing the spin transition of the defect. This shows potential applications of spin crossover materials at terahertz frequencies.

Using two complementary techniques, the relaxation of a photo-induced spin state transition in a strongly cooperative compound has been studied from 10 to 170 K. Up to 150 K, the relaxation rate increases with temperature. At higher temperature, the stabilization of the high-spin state results in a decrease of the relaxation rate. Within the hysteresis loop, as one approaches the switching temperature branch, a critical slowing down of the kinetics and a constant decrease of the cooperative contribution are recorded. This study evidences, for the first time, the complex temperature evolution of the relaxation of photo-induced spin state transition.

Two-photon excitation of a quantum ladder system by a chirped pulse leads to strong interferences in the excited state population. The interest of such scheme in the project of the artificial star is discussed.

10 pages; Two-photon excitation of a quantum ladder system by an ultrashort chirped pulse leads to interferences in the excited-state population, between direct and sequential paths. Experimental results have been obtained in atomic sodium vapor. The presence of several intermediate and final states leads to new phenomena. The interplay between the two sequential paths leads to strongly contrasted oscillations and a large enhancement of the transition probability for precise values of the chirp. A cubic spectral phase modifies significantly the behavior of the quantum interferences and can be used to enhance the two-photon absorption rate.