Jiabiao Zheng

Interference second harmonic generation investigation of the chromophore orientation of hemicyanine dye molecules in a Langmuir-Blodgett monolayer

Abstract A novel method designed to determine the chromophore orientation in a Langmuir-Blodgett (LB) monolayer of hemicyanine dye by second harmonic generation (SHG) is reported. A hemicyanine LB monolayer was deposited on one side of a glass substrate and a stilbazium salt monolayer, whose chromophore orientation was known, was deposited on the other side as a reference. We have observed the interference patterns of the transmitted SHG light fields from both layers when the substrate was rotated. We found that the second-order non-linear polarizability β of hemicyanine dye is in the opposite sense in relation to that of stilbazium salt. Our experimental outcome confirms that the −N(CH 3 ) 2 group of hemicyanine dye points to the hydrophilic glass substrate.

Surface enhanced SHG study on Langmuir-Blodgett mono-molecular layers

Abstract Surface enhanced second harmonic generation (SHG) of reflection at a silver surface covered with Langmuir-Blodgett mono-molecular layers of a fatty acid is studied under surface plasmon excitations. The experimental results of the SHG signals are in good agreement with theoretical calculations and the non-linear polarizability coefficient d 33 for a single arachidic molecule is estimated to be 0.32 × 10 -29 esu.

Second harmonic generation in hemicyanine Langmuir-Blodgett monolayers deposited from aqueous subphase of halide salt solution

Abstract Langmuir-Blodgett (LB) monolayers of hemicyanine dye were deposited onto glass substrates from aqueous subphase. The concentration of potassium iodide (KI) in the subphase was found to have a remarkable effect on both the isotherms and second harmonic generation (SHG) signals of the monolayers. SHG from mixed hemicyanine-arachidic acid LB monolayers prepared with and without KI in the subphase, and SHG from stilbazium salt (which has the same chromophore as hemicyanine except for substitution of the I- ion for Br- in the latter) LB monolayer were also carefully studied. The results of these experiments confirm that the I- ions in the subphase play an important role not only in disintegrating the formation of H aggregates in hemicyanine LB monolayers but also in increasing the monomeric hyperpolarizability β as a result of changing the charge distributions in dye molecules.

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.

Second-harmonic generation in mixed stilbazium salt/arachidic acid Langmuir-Blodgett films

Abstract A stilbazium salt was synthesized and its second-order molecular polarizability was deduced to be 1.2×10 −27 esu. Measurements of second-harmonic generation and small-angle X-ray diffraction on Langmuir-Blodgett films of the stilbazium salt/arachide acid mixtures showed that the mixed compounds with molar ratios of 1:2 and 1:5 could form multilayers with large second- order optical nonlinearity. Second harmonic generation study on the alternate multilayers of stilbazium salt/arachide acid and arachidic acid showed that the second-harmonic signals were increasing monotonously up to 80 bilayers, but the increment was lower than the value predicted theoretically by the quadratic law. Possible reasons are discussed.

Angular dependence of second-harmonic generation in a Langmuir–Blodgett monolayer under a full 360° rotation

A theoretical expression is summarized for second-harmonic generation in both reflection and transmission with different polarization combinations. By measuring the angular dependence of a transmitted second-harmonic generation signal of a stilbazium salt Langmuir–Blodgett monolayer, we found that the molecular tilt angle could be conveniently deduced by a single polarization combination (p→p) in the case that the sample was rotated under a full 360° rotation. This gives us a simple and accurate method to deduce the molecular second-order polarizability and molecular tilt angle. We also show that the multiple reflection in the substrate gives an important contribution for transmitted second-harmonic generation with a large angle of incidence. Consequently, the multiple reflection must be considered if a transmitted second-harmonic generation study is made with a variable angle of incidence, especially with s polarization and fixed detector position.

Temperature dependence of second harmonic generation from Si(100)(2×1) surfaces

Our experiments on reconstructed Si(100)(2×1) surfaces revealed that the reflected second harmonic (SH) signals at 532 nm originated from the contribution of the electrons in surface states. The SH intensity was inversely proportional to the square of the sample temperature. A preliminary explanation for this temperature dependence was proposed.

FABRICATION AND CHARACTERIZATION OF ALST3/AL2O3 LANGMUIR-BLODGETT FILM/ALUMINA MULTILAYER STRUCTURE

Abstract The fabrication of a Langmuir-Blodgett film/alumina multilayer structure of aluminium stearate (AlSt3)/alumina (Al2O3) is reported here for the first time. Three monolayers of AlSt3 and one Al2O3 layer form a period of the multilayer system with periodicity 11–13.5 nm. The first seven orders of Bragg peaks were observed in the low angle X-ray diffraction spectrum of an 11-layer AlSt3/Al2O3 sample. A comparison between the measurements and calculations of the X-ray diffraction pattern is presented. Auger electron spectroscopy depth profile measurements showed the modulated distribution of oxygen, carbon and aluminium and revealed the ion diffusion reaction between CdSt2 and Al2O3 film.

Studies on the second harmonic generation of bilayer assembly structures

Abstract Bilayer assembled Langmuir-Blodgett (LB) films were prepared with 11-methacrylminoundecanonic acid and stearic acid as the layer materials and the magnitudes and signs of the effective non-linear optical coefficients were studied by the optical second harmonic generation technique. It was found that the polaribilities of these two compounds have opposite signs and their second-order non-linear coefficients were superposed in the bilayer assembled LB films. This simple model offers us a new way to construct LB films with high non-linear polarizabilities by alternating the assembled multilayers. Furthermore, the polymerization of the LB film of 11-methacrylaminoundecanonic acid under irradiation with UV light was also investigated by the attenuated total reflection method.

The role of cluster in surface-enhanced raman scattering (SERS)

Experimental study on differently prepared rough silver surfaces reveal the existence of stable clusters formed by the silver atoms. The clusters with certain adsorbed molecules serve as “Raman active sites”, which play an important role in SERS.