Xingze Lu

Second-harmonic generation in interleaved hemicyanine/nitrogencrown Langmuir - Blodgett multilayers

A novel disc-like amphiphilic molecule, with two-chain nitrogencrown was adopted as a spacer material for fabrication of optical nonlinear Langmuir - Blodgett (LB) multilayers. The intensity of second-harmonic generation (SHG) increased quadratically with the bilayer number (up to 116 bilayers) in Y-type hemicyanine/nitrogencrown interleaving LB multilayers due to improvement of the structural properties by insertion of the long hydrophobic tail of hemicyanine molecules between two chains of nitrogencrown molecules. The second-order susceptibility pM (or esu) was obtained by measuring SHG from the hemicyanine samples.

Second harmonic generation investigation of indium tin oxide thin films

Abstract Second harmonic generation (SHG) from indium tin oxide (ITO) films on glass substrates has been investigated in detail for the first time. Significant SHG intensity and non-linear optical anisotropy was observed from ITO films due to orientation of the crystal cells. The SHG reached a maximum in intensity at an incident angle of 60°. The effective second order non-linear optical susceptibility χ2eff was about 2.4×10−10 esu.

Second harmonic generation investigation on electric poling effects in fused silica

Abstract The influence of electric of poling conditions on the optical nonlinearity of fused silica has been investigated by the second harmonic generation (SHG) technique. The SHG intensity of the poled fused silica increased monotonically with increasing the poling voltage or poling time, and reached a maximum at a poling temperature of T = 250°C, but the SHG intensity decreased quickly as the thermal erasure time increased. The experimental results have been explained using an electric field induced dipole orientation model.

Ferroelectricity of a hemicyanine dye in ultrathin organized molecular films: hysteresis and pyroelectric effects

Pyroelectricity of the optical nonlinear hemicyanine dye in organized molecular films has been demonstrated by using the electric polarization and second harmonic generation measurements. The typical hysteresis loop of the samples confirmed existence of the remnant polarization of hemicyanine molecules which is 600 μC m−2. The pyroelectric coefficient of hemicyanine was found to be 12 μC m−2 K−1 at room temperature and enhanced with increasing temperature monotonically which was well explained by using a molecular dipole reorientation model.

Second harmonic generation investigation on molecular orientation in Langmuir-Blodgett films poled by an electric field

Abstract DC electric field induced molecular reorientation in hemicyanine Langmuir-Blodgett multilayers has been investigated by rotation-angle second harmonic generation (SHG) technique. The hemicyanine molecules could align their chromophore axes towards the field direction along the substrate normal, leading to reduction of the dipping-induced in-plane anisotropy and enhancement of the macroscopic optical nonlinearity upon poling at room temperature. With increasing poling field, the SHG intensity increased quadratically and reached an enhancement of an order of magnitude at a field of 1.33×10 6 V/m.

Influence of temperature on hemicyanine Langmuir–Blodgett multilayer films

Abstract Molecular structure and optical nonlinearity in interleaving hemicyanine/arachidic acid or cadmium arachidate multilayers, and Z-type pure hemicyanine multilayers were investigated as a function of temperature by means of second harmonic generation (SHG) and linear absorption. The SHG intensity shows a maximum at about 45°C in the multilayers. An inflection was observed in the temperature dependence of SHG intensity in the interleaving hemicyanine/arachidic acid and hemicyanine/cadmium arachidate multilayers, and was attributed to the melting of arachidic acid or cadmium arachidate. The absorption spectra show that the molecules in the multilayers formed H-aggregates which could be dissociated by heating.

Dipping-induced anisotropy in hemicyanine Langmuir - Blodgett multilayers

Polarized UV - visible absorption and rotation-angle second-harmonic generation techniques were used to investigate the in-plane symmetry in Y-type Langmuir - Blodgett multilayers of a hemicyanine dye interleaved with the inert material arachidic acid. Dipping-induced anisotropy of the molecular orientation with a unique reflection symmetry was enhanced with increasing dipping speed.

Structural and optical non-linear characters of hemicyanine derivatives in Langmuir-Blodgett monolayers

Abstract Structural and optical non-linear properties of four hemicyanine derivatives with similar molecular compositions but different donor-acceptor groups in Langmuir-Blodgett (LB) films have been studied by optical second-harmonic generation, UV-visible absorption and π − A isotherm measurements. The magnitude of the molecular second-order polarizability β in LB films was affected significantly by types of donor-acceptor group, and aggregation formations through a second-harmonic resonant effect. Selection of donor-acceptor also affects the molecular amphiphilicity and structural degree of order in LB films.

Electric field induced molecular reorientation in stilbazolium salt Langmuir-Blodgett monolayers

Abstract Electric field induced molecular reorientation in stilbazolium salt Langmuir-Blodgett monolayers have been investigated by using rotation-angle second harmonic generation technique. The stilbazolium salt molecules were preferentially oriented along the dipping direction and this anisotropic feature was enhanced by increasing the dipping speed. An external DC electric field (6.7 × 10 5 V/m) forced the molecules to reorient along the field direction, and caused dissociation of aggregate species, as demonstrated by the steady state and time resolved fluorescence measurements.

Effects of subphase additions on optical non-linearity of hemicyanine derivative Langmuir—Blodgett monolayers

Abstract UV - visible absorption spectroscopy and second-harmonic generation were used to examine the effects of potassium iodide (KI) in the subphase on the optical and structural properties of pure and mixed hemicyanine derivative (DAEP) monolayers. It was found that with increasing KI concentration in the subphase, an enhancement of the effective molecular hyperpolarizability β was always associated with a red shift of the absorption peak in both pure and mixed DAEP monolayers. Three mechanisms (“spacer” effect, charge distribution change and structural improvement) were responsible for the observed β enhancement. The first two contributions were comparable and explained by a 2ω resonance effect.