Second Harmonic In-situ Investigation of Photoisomerization

Second harmonic is a convenient in situ probe of the dynamics of photoisomers in a polymer environment if the photoisomers have a second-order optical nonlinearity. A corona discharge is used to apply a large electrostatic field to align the molecules. When a change in the orientation of the photoisomers is induced through photoisomerization, second harmonic generation is used to follow the changes in the state and orientation of the photoisomers. 

The photoisomerization is induced by irradiation with green light (532 nm) from either a continuous laser or pulsed laser (frequency doubled Nd.YAG Q-switched laser operated in single pulse mode). The second harmonic is generated by a pulsed optical source such as produced by NdrYLF (1053 nm) or Nd YAG (1064 nm) Q-switched nanosecond laser. The probe 

FIGURE 9.6 All-optical light modulation of the electric field enhanced second harmonic signal produced by PI-1.The arrows indicate when the green irradiating pump light is on or off. HV Off indicates when the electric field was turned off. 

FIGURE 9.10 Chemical structure of side-chain azo-poly(methyl methacrylate) DM - PMMA), 

The polymer environment plays an important role in the mobility of isomers. In this chapter, it was showm that second harmonic generation can provide detailed information of photoisomerization in a polymer film in the presence of a large electrostatic field. Photoisomerization can control the optical non-linearities of materials that have applications in optical switching and optical storage applications. 

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