Polarizability Switching by Photoisomerization

The contribution of photo-induced nonpolar orientation through even order parameters (see Equation 8A.9 in the appendix, page 286) to the effects observed in the EFISH decrease is negligible. The same decrease is observed for a TE- or TM-polarized probe regardless of the pump polarization (not shown). The photochemically induced molecular shape change of the NLO dye blows out the strong optical field driven anharmonic movement of the 

The photochemical reduction of y is further rationalized by a theoretical model that neglects photo-orientation effects to EFISH and assumes that y of the chromophore decreases upon trans to cis molecular shape change. The model predicts an EFISH intensity at the photostationary state, which varies hyperbolically relative to the irradiating light intensity. Indeed, hyperbolic functions were adjusted to the experimental data showing the variation of the SH intensity at the steady state of the irradiation versus the irradiation intensity for both PI-1 and PI-2 (not shown). This finding demonstrates that the azo chromophores in PI-1 and PI-2 behave consistently with the model, and validates the concept of the reversible rapid photochemical erase of y oi these isomerizable NLO dyes. 

FROH MOLECULARTO MACROSCOPIC NONilt AR OPTICAL PROPERTIES 

The interaction of a nonlinear optical material with electromagnetic fields is described by the properties of the material and the powers of the applied fields. The electric polarization by unit volume at the optical frequency (O can be written formally in the dipole approximation as a series expansion in powers of the electric fields  

Summation over repeated indices is implied and is the th-order susceptibility tensor that describes the interaction between the electric fields and the material. The first two terms on the right-hand side of Equation 8.A1 give the spontaneous polarization and linear optics effects. The last two terms lead to various phenomena in nonlinear optics. They include SHG and EO Pockels and Kerr effects. The EO susceptibilities are obtained by combining optical and static fields therefore, the susceptibilities that describe the EO Pockels and Kerr effects are (-co, co, 0) and x% respectively. In a 

PROM MOLECULARTO MACROSCOPIC NONLINEAR OPTICAL PROPERTIES 

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FIG. 8.8 Nonlinear polarizability switching by reversible photoisomerization of the NLO azo chromophore in PI-2 observed by EFISH, a feature that leads to all-optlcaHight modulation of the SH signal of the Pf-2 polymer. The irradiation and sample conditions are those explained in dte caption to Figure 8,6, and the moments of turning on and off the irradiation are indicated. Adapted from reference 9, and redrawn by permission of ACS. 

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