Enhancement in optically induced ultrafast THz response of MoSe2MoS2 heterobilayer.

THz conductivity of large area MoS2 and MoSe2 monolayers as well as their vertical heterostructure, MoSe2MoS2 is measured in the 0.3-5 THz frequency range. Compared to the monolayers, the ultrafast THz reflectivity of the MoSe2MoS2 heterobilayer is enhanced many folds when optically excited above the direct band gap energies of the constituting monolayers. The free carriers generated in the heterobilayer evolve with the characteristic times found in each of the two monolayers. Surprisingly, the same enhancement is recorded in the ultrafst THz reflectivity of the heterobilayer when excited below the MoS2 bandgap energy. A mechanism accounting for these observations is proposed.

Conical versus Gaussian terahertz emission from two-color laser-induced air plasma filaments.

In this Letter, we demonstrate that the far-field terahertz (THz) beam generated from a Ti:Sapphire two-color laser-induced filament can exhibit a conical or Gaussian distribution, depending on the filtering experimental conditions. Using both an incoherent Golay cell detector and a two-dimensional coherent electro-optic detection covering the 0.2-2.6 THz spectral range, in our experimental conditions, we provide evidence that the conical emission is due to photo-induced carriers in the silicon filter, typically used to block the remaining pump laser light. Moreover, the low-frequency THz beam retrieves an almost $ {{\rm TEM}_{00}} $TEM00 Gaussian spatial distribution when the silicon filter is preceded by a large bandgap ceramic filter, which stops the pump beam, thus preventing the carrier generation in the silicon filter.

Study of the Photoswitching of a Fe(II) Chiral Complex through Linear and Nonlinear Ultrafast Spectroscopy.

Photoswitching the physical properties of molecular systems opens large possibilities for driving materials far from equilibrium toward new states. Moreover, ultrashort pulses of light make it possible to induce and to record photoswitching 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] [Δ-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 gives only 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.

Geometric phase shaping of terahertz vortex beams.

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.

Development of a wavefront sensor for terahertz pulses.

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.

Terahertz-field-induced second harmonic generation through Pockels effect in zinc telluride crystal.

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.

Mechanism for optical switching of the spin crossover [Fe(NH(2)-trz)(3)](Br)(2).3H(2)O compound at room temperature.

This paper reports on phase transition photo-induced by a nanosecond laser pulse in the molecular spin crossover material [Fe(NH(2)-trz)(3)] (Br)(2).3H(2)O (with NH(2)trz = 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.

Tuning and focusing THz pulses by shaping the pump laser beam profile in a nonlinear crystal.

Spatially shaped femtosecond laser pulses are used to generate and to focus tunable terahertz (THz) pulses by Optical Rectification in a Zinc Telluride (ZnTe) crystal. It is shown analytically and experimentally that the focusing position and spectrum of the emitted THz pulse can be changed, in the intermediate field zone, by controlling the spatial shape of the near-infrared (NIR) femtosecond (fs) laser pump. In particular, if the pump consists of concentric circles, the emitted THz radiation is confined around the propagation axis, producing a THz pulse train, and focusing position and spectrum can be controlled by changing the number of circles and their diameter.

Realization of a time-domain Fresnel lens with coherent control.

Perturbative chirped pulse excitation leads to oscillations of the excited state amplitude. These coherent transients are governed by interferences between resonant and off-resonant contributions. Control mechanisms in both frequency and time domain are used to modify these dynamics. First, by applying a phase step in the spectrum, we manipulate the phase of the oscillations. By direct analogy with Fresnel zone lenses, we then conceive highly phase-amplitude modulated pulse shapes that slice destructive interferences out of the excitation time structure and enhance the final population.