Second-order non-linear optics

Unlike linear optical effects such as absorption, reflection, and scattering, second order non-linear optical effects are inherently specific for surfaces and interfaces. These effects, namely second harmonic generation (SHG) and sum frequency generation (SFG), are dipole-forbidden in the bulk of centrosymmetric media. In the investigation of isotropic phases such as liquids, gases, and amorphous solids, in particular, signals arise exclusively from the surface or interface region, where the symmetry is disrupted. Non-linear optics are applicable in-situ without the need for a vacuum, and the time response is rapid. 

O—H bonds, hydrogen atom abstraction from, 9,127 Organic materials for second-order non-linear optics, 32,121 Organic reactivity, electron-transfer paradigm for, 35, 193 Organic reactivity, structure determination of, 35,67... 

Organic Materials for Second-Order Non-Linear Optics... 

Second order non-linear optical properties have been reported for a variety of TTF donor-acceptor compounds <02T7463> and the palladium complex 84 is a room-temperature semiconductor <02CL936>. Preparation of the zinc and cadmium compounds 85 has been reported <02CC1474> and aromatic fused TTFs such as 86 form thin films with useful electrical properties <02JAP265466>. A ferromagnetic interaction occurs in the salt of a TTF... 

Second-order non-linear optics continne to be an area of research becanse of its tremendous potential in the design of photon-based new materials for optical switching, data manipulation, and information processing [12]. 

Vibrational sum-frequency spectroscopy (VSFS) is a second-order non-linear optical technique that can directly measure the vibrational spectrum of molecules at an interface. Under the dipole approximation, this second-order non-linear optical technique is uniquely suited to the study of surfaces because it is forbidden in media possessing inversion symmetry. At the interface between two centrosymmetric media there is no inversion centre and sum-frequency generation is allowed. Thus the asynunetric nature of the interface allows a selectivity for interfacial properties at a molecular level that is not inherent in other, linear, surface vibrational spectroscopies such as infrared or Raman spectroscopy. VSFS is related to the more common but optically simpler second harmonic generation process in which both beams are of the same fixed frequency and is also surface-specific. 

Butcher and Cotter (1990), Boyd (2003) and Sutherland (2003) provide overviews of non-linear optics. Prasad and Williams (1991) have discussed the non-linear optics of molecules and polymers. Detailed accounts on this topic can be found in Bosshard et al. (1995), Kajzar and Swalen (1996) and Kuzyk and Dirk (1998). More specialised reviews are those of Yesodha et al. (2004), on non-linear polymers with azobenzene pendent groups, and Sioncke et al. (2003), who describe the second order non-linear optical properties of chiral molecules and polymers. 

Second-order non-linear optical techniques have been applied for monitoring the adsorption of soluble surfactants at the air/water interface and to characterize the resulting (sub)monolayers ). For Langmuir monolayers SHG allows the observ-... 

Spectroscopic studies of liquid interfaces provide important information about the composition and structure of the interfacial region. Early work was mainly carried out at the solid liquid interface and involved techniques such as neutron and X-ray diffraction, and reflection FTIR spectroscopy. More recently, powerful techniques have been developed to study the liquid liquid and liquid gas interfaces. These studies are especially important because of their relevance to biological systems such as cell membranes. The techniques described here are second-harmonic generation (SHG) and vibrational sum frequency spectroscopy (VSFS). They are both second-order non-linear optical techniques which are specific to the interfacial region. Since the second-order effects involve signals of low intensity, they rely on high-power lasers. 

Sum frequency generation is a second-order non-linear optical technique that has unique advantages for probing the vibrational spectrum of molecules adsorbed at a surface. The vibrational SFG process occurs when two laser beams, one in the visible spectral region and one in the infrared spectral region, are incident on the sample so that a third beam at the sum frequency of the incident beams is emitted, as shown in (1). 

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