Optical second-harmonic generation, SHG

Optical second-harmonic generation (SHG) has recently emerged as a powerful surface probe [95, 96]. Second harmonic generation has long been used to produce frequency doublers from noncentrosymmetric crystals. As a surface probe, SHG can be caused by the break in symmetry at the interface between two centrosymmetric media. A high-powered pulsed laser is focused at an angle of incidence from 30 to 70° onto the sample at a power density of 10 to 10 W/cm. The harmonic is observed in reflection or transmission at twice the incident frequency with a photomultiplier tube. 

In the single-domain state, many ferroelectric crystals also exhibit high optical nonlinearity and this, coupled with the large standing optical anisotropies (birefringences) that are often available, makes the ferroelectrics interesting candidates for phase-matched optical second harmonic generation (SHG). 

Akemann W, Friedrich KA, Linke U, Stimming U. 1998. The catal)4ic oxidation of carbon monoxide at the platinum/electrolyte interface investigated by optical second harmonic generation (SHG) Comparison of Pt(l 11) and Pt(997) electrode surfaces. Surf Sci 404 571-575. 

Recently, we [13,14] evidenced by ATR-IR spectroscopy that the membrane potential of ionophore-incorporated, PVC-based liquid membranes is governed by permselective transport of primary cations into the ATR-active layer of the membrane surface. More recently, we [14 16] observed optical second harmonic generation (SHG) for ionophore-incorporated PVC-based liquid membranes, and confirmed that the membrane potential is primarily governed by the SHG active, oriented complexed cations at the... 

Optical second harmonic generation (SHG), which is the conversion of two photons of frequency u to a single photon of frequency 2co, is known to be an inherently surface-sensitive technique, because it requires a noncentrosymmetrical medium. At the interface between two centrosymmetrical media, such as the interface between two liquids, only the molecules which participate in the asymmetry of the interface will contribute to the SHG [18]. SHG has been used as an in-situ probe of chemisorption, molecular orientation, and... 

To further corroborate these potentiometric results, we again used optical second harmonic generation (SHG). The SHG measurement system was essentially the same as used in the study described in Section II, except the laser beam was first reflected by a mirror and then totally reflected by the liquid-liquid interface. 

Non-linear optical interactions occur in materials with high optical intensities and have been used to produce coherent light over a wide range of frequencies from the far infra-red to the ultra-violet. The three wave mixing process is of particular interest as it can be used for optical parametric amplification and optical second harmonic generation (SHG) and occurs in non-centrosymmetric materials. 

All spectroscopic evidence on the composition of a relatively thin layer at the membrane surface was thus found to be in agreement with the interpretation of permselectivity as being due to the exclusion of counteranions from the membrane phase. However, the depth accessible to ATR-IR is of the order of 0.1 1.0 pm and is thus too large for the observation of phenomena in the region in closest proximity to the membrane/aqueous interface in which charge separation is assumed to take place. Optical second harmonic generation (SHG), which has an even more pronounced surface sensitivity than ATR-IR, was recently shown to be very suitable for the investigation of the interface between ISE membranes and sample solutions."" ... 

The effect of the kiex2 term on the motion of Xe is written as Xeee, for example, and physically represents optical second harmonic generation (SHG), to which the polar phonons do not contribute because they are not able to drive or follow high-frequency responses. This nonlinearity... 

Stack, A.G., Higgins, S.R., and Eggleston, C.M., Point of zero charge of a corundum-water interface probed with optical second harmonic generation (SHG) and atomic force microscopy (AFM). New approaches to oxide surface charge, Geochim. Cosmochim. Acta, 65, 3055, 2001. 

An inexhaustive survey of the literature since this article was completed late in 1973 reveals continuing activity in most of the areas surveyed, and also one or two interesting new developments. Measurements of the optical second harmonic generation (SHG) tensors of several iodate crystals were interpreted2) to give information about the I-O bonds and the lone pair on the iodine. The method of analysis used has been extended to several crystals containing different M-0 bonds. It appears that these results (in most cases obtained by... 

Brewster angle ellipsometry (BAE) and surface optical second harmonic generation (SHG, see Chap. 5) were used to study the growth of 8CB films, evaporated in air onto glass (BK7) substrates, covered with a 15 nm thick film of poly(vinyl cinnamate) (PVCN) [48]. As the thickness of 8CB on PVCN layers was far below the optical wavelengths, the Drude formula for the ellipsometric coefficient at the Brewster angle, ps, (4.3), was used. The ellipticity coefficient of the 8CB adsorbate was calculated as... 

The zero-field and field-induced optical second harmonic generation (SHG) was investigated for the nematic and smectic A phases of various liquid crystals. The components of the cubic non-linear susceptibility tensor were measured for substances with different molecular structure. The phase-matched SHG was observed for all the compounds investigated. The directions of the phase synchronisms as well as the corresponding non-linear susceptibilities were determined for the ee-o and oe-o interactions. The zero-field phase-matched SHG was observed for the oe-o interaction. It was accounted for by a multipolar mechanism. 

Fifteen years have passed since the first observations of the optical second harmonic generation (SHG) from liquid crystals however, up to now, there is no final answer to the question concerning the nature of the effect. The problem is of fundamental importance because it is closely related to the symmetry properties of liquid crystals. 

In contrast to the free surface, solid substrates can induce the polar orientational order in surface nematic layers. This has been shown by the optical second harmonic generation (SHG) technique which is very sensitive to any kind of polar ordering (in the dipole approximation SHG is forbidden in centro-symmetric media). The polar order exists at the interface between cyano-substituted compounds like 8CB and glasses covered by surfactants and polymer films [22, 23]. 

The nonlinear properties of FLCs attracted considerable attention both from the fundamental and technical points of view [132-134]. Since, in ferroelectric phases the center of inversion is absent (Fig. 7.1), the amplitude of the optical second harmonic generation (SHG) ought to be fairly strong, especially in commercially available FLC mixtures with high spontaneous polarization [133]. Reference [134] shows the ways of designing new FLC substances with the increased molecular second-order hyperpolarizability comparable to that of solid electrooptical crystals such as LINbOs. 

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