Dependent Second Harmonic Generation Research Articles

Two-dimensional ferroelectricity and phase transitions in PVDF Langmuir-Blodgett films probed by second harmonic generation

Ultrathin electrode-free ferroelectric Langmuir-Blodgett (LB) films of polyvinylidene fluoride (PVDF) with trifluoroethylene are experimentally studied by means of optical second harmonic generation (SHG) in a temperature range from 140 K to 380 K. The temperature behavior of spontaneous polarization induced SHG reveals distinct features in the vicinity of 355 K and 295 K which can be attributed to ferroelectric-paraelectric phase transitions in the bulk and at the surface of this two-dimensional structures. Lattice-stiffening transition recently observed in PVDF LB films at 160 K does not show remarkable peculiarities of temperature dependence of SHG that indicates nonpolar changes in LB film structure under this transition.

Controlling the noncentrosymmetry of azodye-doped polymers by nonresonant dual-frequency coherent excitation

All-optical control of the noncentrosymmetry of azodye-doped polymer films was demonstrated by nonresonant dual-frequency coherent excitation. The tensor analysis of the photoinduced noncentrosymmetry was performed by monitoring second harmonic generation (SHG) of the samples. The quadratic dependence of SHG on the film thickness showed that a χ(2) grating satisfying the phase-matching condition for SHG was optically induced in the polymer films, indicating this nonresonant all-optical poling can be used to tailor the symmetry of bulk samples.

Nonlinear optical diagnosis of oxide traps formed during reactive ion etching

Oxide traps generated by reactive ion etching are studied using a pulsed femtosecond laser. The second harmonic generation (SHG) signal from the Si/SiO2 interface is sensitive to charged traps in the oxide. The time evolution of the SHG signal indicates that positive traps predominate. The angular dependence of the polarized signal shows that the electric field generated by the oxide traps alters the symmetry of the sample. The damage is greatest for an oxide thickness of 13 nm (for a plasma dc bias of 300 V). Thicker oxides have smaller SHG signals, presumably because the Fowler–Nordheim tunneling currents induced by plasma charging of the oxide surface are smaller. Very thin oxides also exhibit reduced damage. The time dependent SHG signals depend on the temperature of the samples; these data provide information on the trapping and detrapping of substrate electrons by oxide holes.

Photoalignment of liquid crystals by liquid crystals

We observed light-induced alignment of the nematic liquid crystal, 4, 4(')-n-pentylcyanobiphenyl (5CB), on a fused quartz. Irradiation of the adsorbed layer with polarized ultraviolet light produced homogeneous alignment in a 5CB-filled liquid crystal cell with the easy orientation axis perpendicular to the polarization direction. The measured anchoring energy increases with illumination up to 10(-4) erg/cm (2). Phase retardation and pretilt measurements confirmed near homogeneous alignment of the LC in a cell, while the polarization dependence of second harmonic generation suggested a near normal alignment of the adsorbed layer. We believe that light-induced rearrangement or phototransformation of the adsorbed layer causes the observed phenomenon.

Second-harmonic generation in Ce:BaTiO_3 nanocrystallites grown by pulsed laser deposition

Ce:BaTiO3 nanocrystallites were prepared on fused quartz substrates by pulsed laser deposition. The structural properties of the nanocrystallites were studied by scanning electron microscopy and x-ray diffraction. The refractive indices and absorption edges were obtained from the optical transmission spectra with an approximate formula. The incidence and polarization angle–dependent second-harmonic generation (SHG) were measured. The effective SHG coefficient was reduced by decreasing the deposition temperature. Some possible mechanisms of the correlation between the SHG coefficient and the deposition temperature are discussed.

Second Harmonic Generation in Langmuir-Blodgett Films of a Novel Phenylhydrazone Dye

The LB films of a new phenylhydrazone dye, 4-hexadecanoxynaphthyl-4′-nitro phenylhydrazone, were prepared. The second harmonic generation (SHG) from monolayer and multilayers of the dye was measured. The dependence of SHG on number of layers is weaker than the expected square law.

Molecular orientation of oxacyanine dye in an LB film determined by second harmonic generation and polarized absorption techniques

The molecular orientation of an oxacyanine dye (DOOC) in an LB film has been determined based on the second harmonic generation (SHG) using a fs-pulsed Ti-sapphire laser and the polarization absorption spectrometry. Both polarization and wavelength dependence of SHG varied with surface concentration of the dye. The dye was either in a monomer, a dimer or a J-aggregate form depending on the surface concentration. The tilt angle to the surface normal was 34° for the monomer, 26–29° for the dimer and 16–23° for the J-aggregate. The J-aggregate stayed more flat on surface and showed larger SHG intensity than the monomer did.

PERTURBATION THEORY AND EXPERIMENTAL STUDIES ON ANISOTROPY OF ORGANIC MOLECULAR FILMS

The optical anisotropy of organic molecular films induced by molecular reorientation was investi-gated by using the perturbation method.We presented theoretical formulae about the dependence of second harmonic generation (SHG) intensity on the polarization of fundamental beam.SHG beam and the dipping direc tion.The experimental studies and discussion were given under some special conditions.The expermental results agreed well with the theoretical calculation.

Second harmonic generation in order-disorder ferroelectrics

We investigate the temperature dependence of second harmonic generation in order-disorder ferroelectrics. We obtain the qualitative temperature dependence of the second harmonic amplitude; the amplitude goes to zero when the temperature reaches the Curie point. We take into account the disruption of wave number harmony when the temperature is changed. We show the second harmonic is largest at the temperature where the wave numbers are synchronized and there is no detuning. The theoretical conclusions reached here are confirmed by experimental data.

Second-harmonic generation in BaTiO3 films doped with cerium

Second-harmonic generation of 1.064 μm incident light was measured on BaTiO3 thin films, both a-axis and c-axis oriented, undoped and doped with cerium, prepared by pulsed laser deposition. The dependence of second-harmonic generation (SHG) coefficients of these films on the incidence angle and the polarization direction of the fundamental beam were investigated. The effective SHG coefficients was enhanced greatly when the films were doped with cerium.

Linear and nonlinear FEL–SEW spectroscopic characterization of nanometer-thick films

The conditions of the existence and transformations of surface electromagnetic waves (SEWs) on metals (surface plasmons) and dielectrics (phonon–polaritons) are discussed. Interferometric SEW experiments provide the possibility for the direct determination of the real and imaginary parts of the dielectric constants ε at the frequencies in the tuning range of a free electron laser (FEL) without any preliminary ε models. The important role of the outstanding facilities of the FEL, namely the broad tuning range, high power, narrow bandwidth of emission, well-collimated beam, in SEW experiments is outlined. It is demonstrated by the examples of the infrared absorption spectra of a Langmuir–Blodgett film on metal, of a metal oxide film and of polymeric films of nanometer thickness on metals. The free surfaces of some single crystals (CaF 2, LiNbO 3) and thin polymeric films were studied by an interferometric experiment for the optical constant determination. The nonlinear spectroscopy applications of the SEW–FEL technique to studies of a second harmonic generation (SHG) are described (the frequency dependence of efficiency, the angle dependence of SHG, and the influence of a thin film deposition on a quartz surface).

Second Harmonic Generation from Electrically Poled PbO-SiO<sub>2</sub> Glasses

Clear second harmonic generation (SHG) was observed from xPbO⋅(100-x)SiO2 glasses after electrical poling. PbO can be considered to be responsible for the SHG center since no SHG was detected from electrically poled PbO-free SiO2 glass. SHG was seen from the glasses poled above 100°C, and the intensity increased as the poling temperature increased. The minimum dc voltage necessary for SHG was 1kV. SHG intensity increased first and then decreased with the increase of PbO content, showing a maximum at 34.8 mol% PbO. This dependence of SHG seems to relate the microstructure associated with Pb2+, probably Si-O-Pb2+ units.

Shg-Active Films Formed from Uniaxial and Biaxial Nematic Liquid Crystals with Polar Ordering

We found aromatic copolymers comprising 4′-hydroxy 4-biphenyl carboxylic acid and 6-hydroxy 2-naphthoic acid with high second-harmonic-generation (SHG) activity. The polarization dependence of SHG in the solid film formed from the nematic phase reveals uniaxial and biaxial polar orderings.

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.

Symmetry Analysis of the Nonlinear Optical Response: Second Harmonic Generation at Surfaces of Antiferromagnets.

Using symmetry arguments we show how optical second harmonic generation (SHG) can be used to detect antiferromagnetism at surfaces and in thin films. Based on the group theoretical analysis of the nonlinear electric susceptibility we propose a new nonlinear magneto-optical effect, which allows even in the presence of unit-cell doubling for the unambiguous discrimination of antiferromagnetic surface spin configurations from ferromagnetic or paramagnetic ones. As an example for this effect we discuss the polarization dependence of SHG from the fcc (001) surface of NiO in some detail.

Doping dependence of second harmonic generation from native oxide/Si(111) interfaces

Optical second harmonic generation (SHG) was studied on native oxide/Si(111) substrates with both n- and p-type dopings of various doping concentrations. With the rotation about the (111) azimuth, six peaks were observed in SHG intensity at every 60° with C 3v symmetry for all substrates. However, the strong peaks appeared at 0, 120, and 240° on p-type Si substrates, whereas these peaks were small on n-type Si substrates. We found that the reduction of the peaks at 0, 120 and 240° had a correlation with the change of the doping type and the doping concentration from the p + to the n + region. Analysis of the azimuthal rotation dependence of the SH intensity indicates that the observed change is caused by the change of the 2nd order non-linear susceptibility tensor components, χ zxx. We think that this is due to the change of the interface dangling bond occupation and the resulting change of the interface dipole with the dopings.

Nonlinear Effects in the Symmetric Phase of Layered Ferroelectric Materials

The layered ferroelectric \rm {TlInS_{2}} possessed a space group of symmetry \rm {C_{2h}^6} at room temperature undergoes succession of phase transitions below T_i=216 K with decreasing temperature [1]. In the present communication the results of nonlinear behaviour of pyroelectric effect and second harmonic generation observation in the symmetric phase of \rm {TlInS_2} were reported. Above of T_i=216K in the paraelectric phase, the unusual behaviour of pyrocurrent as found. The pyrocurrent undergo the maximum at T=238K with abruptly changes its sign with decreasing temperature. The influence of beas field increases the maximum of pyrocurrent considerably. Chang in the sign of applyed field induces the swiching of pyrocurrent peak at T=238K. Besides that, near the above mentioned temperature the appearance of second harmonic generation signal have been founded out. The temperature dependence of second harmonic generation in paraelectric phase exhibits a rather unusual behaviour: the second harmonic intensity undulation is observed with decreasing temperature in the range T_i

In-Situ Observation of SHG and its Origin in Vacuum Deposited Copper Phthalocyanine Film

Abstract In situ observation system of second harmonic generation (SHG) during vacuum deposition has been constructed. It allows us to observe SHG from transmitted and reflected directions. The advantages of the system is summarized in the following two points: (1) Detailed dependence of SHG on film-thickness can be easily obtained. (2) The measurement can be achieved without exposing films to the atmosphere. This system was applied to vacuum evaporated films of copper phthalocyanine (CuPc) to clarify the origin of SH activity in such a centrosymmetric molecule. We suggest the importance of quadrupolar contribution on the basis of the thickness dependence of SHG and the comparison with the theoretical analysis.

Oxide-thickness dependence of second harmonic generation from thick thermal oxides on Si(111)

We show here that the oxide-thickness dependence of the s-polarized SHG from Si(111) covered with a thick thermal oxide is completely described by multiple reflections in the oxide film. For the p-polarized response, a strong enhancement with thickness is observed, which cannot be explained in this way. These measurements show that one should be cautious in analyzing the SHG from a buried interface, and carefully take into account the linear optics involved.

Polarization dependence of second-harmonic generation in thin-film planar waveguides.

The effect of light polarization on the growth of second-harmonic generation (SHG) in germanium-doped silica thin-film planar waveguides was investigated. Film-generated SHG produced by input coupled light with transverse magnetic polarization was measured to be over three orders of magnitude larger than that for light with transverse electric polarization. A phenomenological model, based on charge transport induced by photoionization, was used to compute the SHG and was found to agree with the experimental results.