Thin harmonic generation

Figure 5. Vibration diagrams depicting the effect of air on optical third harmonic generation from a thin solid. (See text for further discussion.)...

In the following sections we will first in Section 2 briefly discuss the necessary background to understand optical activity effects in linear and nonlinear optics and to illustrate the similarities and differences between both types. In Section 3 we present a more thorough analysis of nonlinear optical effects in second-harmonic generation, both from a theoretical and an experimental point of view. Section 4 deals with experimental examples that illustrate the usefulness of nonlinear optical activity in the study of chiral thin films and surfaces. Finally, in Section 5 we give an overview of the role of chirality in the field of second-order nonlinear optics and show that chiral molecules can be useful for applications in this field. 

Without going into too much detail, it is relatively easy to intuitively understand optical rotation in second-harmonic generation from a chiral thin film by simply considering the nonvanishing polarization components generated in a chiral and achiral film. For example, for an achiral thin film with CXA, symmetry and for the experimental situation shown in Figure 9.5, the nonvanishing components of the polarization can be written as ... 

Figure 9.8 Second-harmonic generation from thin film (or surface). Fundamental beam at frequency > i p or. v polarized) is incident on film at angle 0. The p- and. s-polarized second-harmonic fields (frequency 2 co) are generated in reflection and transmission. 

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."" ... 

A number of special optical techniques such as the study of second harmonic generation by irradiation of non-centrosymmetric systems by high intensity laser light will be discussed in relation to particular materials and problems. However, one optical technique having a general applicability, namely ellipsometry, must be discussed here. It is one of the best techniques available to determine the thickness of a thin organic film. Such determinations are important as they allow one to have an independent check on the number of layers deposited, given that the thickness of one layer has been determined by X-ray diffraction. 

The nonlinear optical properties of thin films of the polymers were investigated through measurement of second-harmonic generation, and exhibit second-harmonic coefficients, d33, in the range 4.1-34 pm/V. 

By simultaneously injecting both beams into a DR1 PMMA film, within hours a self-written QPM nonlinearity of 90 pm V1 was permanently inscribed, allowing further use of the film as a second harmonic generator. The limitation is that the harmonic is generated near its absorption peak and hence only very thin films with a small net conversion efficiency can be used [116]. Detuned from resonance, the nonlinearity is naturally significantly decreased. Nonetheless this method remains potentially very attractive. 

Another type of organic dye was introduced through co-condensation of TMOS and precursor 107. For a thin film (4.2 pm thick), a very high second-harmonic generation signal was obtained (d33 = 150 pm V 1, X = 1.34 pm). Interestingly, this study demonstrates the importance of the processing parameters used for the elaboration of the material... 

Loucif-Saibi, R., Nakatani, K., Delaire, J. A., Dumont, M., and Sekkat, Z. (1993). Influence of photoisomerization of azobenzene derivatives in polymeric thin films on second harmonic generation towards applications in molecular electronics. Mol. Cryst. Liq. Cryst, 235. 53.3-540. 

Churikov, V. M., and Hsu, C. C. (2000). Optical control of third harmonic generation in azo-doped polymethylmethacrylate thin films. Appl. Phys. Lett. 77, 2095-2097. 

The second harmonic generation is a coherent technique giving the fast, electronic in origin, second-order NLO susceptibility (-2co co,co) at a given, measurement frequency co. Here, we limit the discussion to poled films, with °o mm symmetry, which exhibit two nonzero x tensor components the diagonal A zzz( 2co co,co) and the off diagonal xzz( 2co co,co), where Z is the poling (preferential orientation) direction. Usually, thin films are deposited on one side of substrate only (thin film deposited on both sides is discussed in Swalen and Kaj-... 

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