Second harmonic generation principle

Figure 8.4 Principle of autocorrelation. S, second harmonic generator crystal or sample cell M, movable mirror F, filter or monochromator D photomultiplier...

Figure 2 shows the tradeoffs between crystal and polymer organic NLO materials for device applications. Although cither type of materials could in principle be used for both applications, crystals are best for second harmonic generation, and poled polymers are best for electro-optic waveguide devices such as modulators and switches. 

Two of the most important nonlinear optical (NLO) processess, electro-optic switching and second harmonic generation, are second order effects. As such, they occur in materials consisting of noncentrosymmetrically arranged molecular subunits whose polarizability contains a second order dependence on electric fields. Excluding the special cases of noncentrosymmetric but nonpolar crystals, which would be nearly impossible to design from first principles, the rational fabrication of an optimal material would result from the simultaneous maximization of the molecular second order coefficients (first hyperpolarizabilities, p) and the polar order parameters of the assembly of subunits. (1)... 

Y.R. Shen, The Principles of Nonlinear Optics, Wiley, (1984) Optical Second Harmonic Generation at Interfaces, Ann. Rev. Phys. Chem. 40 (1989) 327. (Theoretical basis of SHG, experimental set-up, examples of applications to show what types of information are obtained from SHG measurements.)... 

Time-Resolved Second Harmonic Generation 19.2.2.1 Principles and Brief History... 

Second harmonic generation has been recognized as a powerful probe to study the electronic states at surfaces and interfaces [16]. Under the electric dipole approximation, second-order nonlinear processes are forbidden in centrosymmetric systems. This principle makes the phenomena surface-specific in many cases. Indeed, the capability of SHG spectroscopy to explore surface electronic states has been demonstrated on various systems, dye molecules at solid/liquid interfaces [17], organic molecules at liquid/air interfaces [18], semiconductor surface states [19], organic molecules at metal surfaces [20], and so on. 

Shen YR (1984) The Principles of Nonlinear Optics, John Wiley, New York, N.Y. Shen YR (1989) Surface properties probed by second-harmonic generation and sum-frequency generation. Nature 337 519-525... 

Second harmonic generation (SHG) sjjectroscopy is based on the same principles (two photons of frequency v are converted to one photon of frequency 2v). SFG has appeared to be the more successful and versatile technique to date. 

Second-harmonic generation for nonlinear optics, ferroelectricity, and piezoelectricity are all properties that are dependent on the pre.sence, magnitude, and orientation of bulk polarity in crystals and films. Therefore, the issue of how to design a polar solid from basic principles remains a challenge that has immense potential relevance to materials science. Obviously, a polar solid is guaranteed if a pure enantiomer is used as a component of a compound. However, the presence of polarity does not in any way imply that optimal packing will occur and, further-... 

Of these, SEG vibrational spectroscopy is a surface-specific technique it is quite useful for high-pressure studies based on the principle of second harmonic generation (see Fig. 1.8a). One or both laser frequencies are tuned, then spatially and temporally overlapped. By scanning one of the lasers in the infrared frequency... 

Figure 2 Sketch of second-harmonic generation from an interface between two isotropic media. The interfacial layer of thickness d is specified by a linear dielectric constant S2 and a second-order surface nonlinear susceptibility Reproduced with permission of John Wiley and Sons from Shen YR (1984). The Principles of Nonlinear Optics. New York 1984 John Wiley and Sons.

The argument Rp implies structure relaxation in the field, and P" means the nuclear relaxation part of P, while the subscript oc oo invokes the so-called infinite optical frequency (lOF) approximation. In principle, this procedure allows one to obtain most of the major dynamic vibrational NR contributions in addition to the purely static ones of Eqs.4.5. 7. The linear term in the electric field expansion of Eq. (4) gives the dc-Pockels effect the quadratic term gives the optical Kerr Effect and the linear term in the expansion of beta yields dc-second harmonic generation (all in the lOF approximation). For laser frequencies in the optical region it has been demonstrated that the latter approximation is normally quite accurate [29-31]. In fact, this approximation is equivalent to neglecting terms of the order with respect to unity (coy is a vibrational frequency). In terms of Bishop and Kirt-man perturbation theory [32-34] all vibrational contributions through first-order in mechanical and/or electrical anharmonicity, and some of second-order, are included in the NR treatment [35]. 

Discuss the principles of Fourier transform infrared spectroscopy and nonlinear laser optics [74, 75], and of second harmonic and sum frequency generation. Compare the relative advantages of both techniques for the study of the bonding, orientation, and location of adsorbed molecules either on metal or on insulator surfaces. 

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