Optically initiated orientation of nonlinear photoisomers

Second order nonlinearities can be imposed in a disordered polymer film by poling. Electric field poling is the most common technique but poling can also be achieved optically. In optical poling, a sample is irradiated with linearly polarized light and the nonlinear molecules undergo successive isomerizations, contractions and relaxations and eventually become aligned perpendicular to the incident polarization.1 This photoisomerization can also increase the mobility of the nonlinear molecules within the polymer and permit improved alignment to an electric field.2 The previous reports of optically induced reorientation have used continuous-wave illumination and required long exposure times.3-11However, the trans-cis photoisomerization has been shown to be extremely fast.12 We are investigating the dynamics of the trans-cis isomerization and orientation of nonlinear stilbene molecules in the presence of an electric field by monitoring the changes in the second order nonlinearity. In our research we have used short optical pulses to initiate molecular reorientation by photoisomerization. The unstable cis-isomer of azobenzene is used to enable a transition between two states which both involve the stable trans-isomer. Because of the extreme speed with which the photoisomerization can be initiated, it could find an application in digital optical storage.