E. Freysz

Surface vibrational spectroscopic studies of hydrogen bonding and hydrophobicity.

Surface vibrational spectroscopy by sum-frequency generation was used to study hydrophobicity at the molecular level at various interfaces: water—surfactant-coated quartz, water-hexane, and water-air. In all cases, hydrophobicity was characterized by the appearance of dangling hydroxyl bonds on 25 percent of the surface water molecules. At the water-quartz interface, packing restrictions force the water surface layer to have a more ordered, ice-like structure. A partly wettable water-quartz interface was also studied.

Nanoparticles of iron(II) spin-crossover

We report the synthesis of spin crossover 69 nm spherical nanoparticles of [Fe(NH2-trz)3](Br)2.3H2O.0.03(surfactant) (NH2trz = 4-amino-1,2,4-triazole, surfactant = Lauropal), prepared by the reverse micelle technique, which exhibit at room temperature a thermal hysteresis characterized by magnetic, diffuse reflectivity and Raman studies.

Nanoparticles of [Fe(NH2-trz)3]Br2.3H2O (NH2-trz=2-amino-1,2,4-triazole) prepared by the reverse micelle technique: influence of particle and coherent domain sizes on spin-crossover properties.

This paper describes the synthesis of iron(II) spin-crossover nanoparticles prepared by the reverse micelle technique by using the non-ionic surfactant Lauropal (Ifralan D0205) from the polyoxyethylenic family. By changing the surfactant/water ratio, the size of the particles of [Fe(NH2-trz)3]Br2.3H2O (with NH2trz=4-amino-1,2,4-triazole) can be controlled. On the macroscopic scale this complex exhibits cooperative thermal spin crossovers at 305 and 320 K. We find that when the size is reduced down to 50 nm, the spin transition becomes gradual and no hysteresis can be detected. For our data it seems that the critical size, for which the existence of a thermal hysteresis can be detected, is around 50 nm. Interestingly, the change of the particle size induces almost no change in the temperature of the thermal spin transition. A systematic determination of coherent domain size carried out on the nanoparticles by powder X-ray diffraction indicates that at approximately 30 nm individual particles consist of one coherent domain.

Experimental study of the origin of the second-order nonlinearities induced in thermally poled fused silica

Second-harmonic generation (SHG) experiments at the surfaces and in the volumes of thermally poled commercial fused-silica samples have been performed. The experimental results show, for the first time to our knowledge and in agreement with our model, that second-order nonlinearity is induced on both cathodic and anodic interfaces. No contribution from the bulk of the sample could be detected. Moreover, our data also reveal that reorientable moieties are at the origin of the induced nonlinearity. The second-order susceptibility, evaluated by a surface SHG experiment, is chi((2))(333)~1 pm/V . The estimated susceptibility associated with the dc Kerr effect is negligible.

Second-order non-linear optical response of metallo-organic compounds: towards switchable materials

A series of coordination compounds, [M(PM-L)2(NCS)2], with large aromatic ligands (PM-L) and a divalent metal ion (MII=Mn, Fe, Co, Ni and Zn) have been synthesized. The magnetic data are consistent with divalent metal ions in an octahedral ligand field environment. Hyper-Rayleigh scattering measurements show an enhancement of the molecular hyperpolarizabilities (β) from Ni compounds (with two unpaired electrons) to Mn compounds (five unpaired electrons). In the case of the Fe(II) metal ion, the influence of the ligand on the β values is reported. This work provides attractive information for the design of switchable materials based on the spin-crossover phenomenon.

Mechanism for optical switching of the spin crossover [Fe(NH2-trz)3](Br)2·3H2O 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.

Second-harmonic generation under phase-velocity and group-velocity mismatch: influence of cascading self-phase and cross-phase modulation

Temporal and spectral characteristics of pulses resulting from second-harmonic generation of 120-fs amplified Ti:sapphire laser pulses up to 0.1 mJ at a wavelength of 815 nm in type I KDP crystal were experimentally and theoretically analyzed. Widely different behaviors were observed, according to the sign of the phase mismatch. Comparison between the theoretical simulation and experimental data demonstrates that the competition between third- and second-order nonlinear phenomena strongly modifies the structure of the pulses generated.

Room temperature study of the optical switching of a spin crossover compound inside its thermal hysteresis loop

We have studied the low-spin to high spin state phase transition induced by a single or a sequence of nanosecond laser pulses within the thermal hysteresis loop of the [Fe(NH2-trz)3](NO3)2-H2O spin crossover compound. We demonstrate that the final state that is photoinduced can be finely controlled by changing the central wavelength and the energy of the laser pulses. A simple model accounts for the observed phenomena and paves the way for the practical applications to optical data storage at room temperature of spin state transition compounds.

Computational modeling of second-harmonic generation by solution of full-wave vector Maxwell equations

A numerical study of second-harmonic generation based on direct solutions of full-wave vector Maxwell equations, conducted by use of a finite-difference time-domain scheme, is reported. Although nonlinear problems have already been solved by finite-difference time-domain schemes, this is the first finite-difference time-domain computation of second-harmonic generation in a nonlinear crystal with a complete description of the electric field. Only spatially plane waves are considered, but the three components of the electric field are taken into account. The advantages and the drawbacks of this new approach are shown: On the one hand, all the spectral components of the waves are computed, but, on the other, the phase mismatch, which is imposed rather than computed, requires the use of a fine temporal mesh. The numerical results obtained for second-harmonic generation of femtosecond pulses in a thin KDP crystal are compared with those obtained by solution of the nonlinear Schrodinger equations. They illustrate the advantages of this method.

Femtosecond optical pulse shaping for tunable terahertz pulse generation

We report on the generation of phase-locked terahertz pulse pairs and tunable narrow-band terahertz pulses by means of optical pulse shaping combining a liquid crystal spatial light filter and optical rectification in ZnTe. They are generated through simple sinusoidal and/or triangular phase modulations and do not require any optimization algorithm. The corresponding spectra are tunable between 0.5 and 2.5 THz with a spectral bandwidth as narrow as 140 GHz.