Abstract
We develop scenarios for orientational ordering of an in-plane system of small flat octupolar molecules at the low-concentration limit, aiming towards nonlinear-optical (NLO) applications at room temperatures. The octupoles interact with external electric poling fields and intermolecular interactions are neglected. Simple statistical-mechanics models are used to analyze the orientational order in the very weak poling limit, sufficient for retrieving the NLO signals owing to the high sensitivity of NLO detectors and measurement chains. Two scenarios are discussed. Firstly, the octupolar poling field is imparted by a system of point charges; the setup is subject to cell-related constraints imposed by mechanical strength and dielectric breakdown limit. The very weak octupolar order of benchmarking TATB molecules is shown to emerge at Helium temperatures. The second scenario addresses the dipoling of octupolar molecules with a small admixture of electric dipolar component. It requires a strong field regime to become effective at Nitrogen temperature range. An estimation of the nonlinear susceptibility coefficient matrix for both scenarios is done in the high-temperature (weak interaction) limit formalism. We argue that moderate modifications of the system like, e.g., an increase of the size of the octupole, accompanied by dipole-assisted octupoling, can increase the poling temperature above Nitrogen temperatures.
Highlights
In order to increase the poling temperature the cell-related factor Q/R4, see Eq (5), has to be maximized; we look for small values of R and large values of Q
We have studied two scenarios of orientational ordering of small flat octupolar and slightly modified octupolar molecules by external electric field in the context of quadratic non-linear optics phenomena with emphasis on second harmonic generation
While we have used a simple statistical mechanics modeling, a new methodological approach was applied to the study of the very weak poling regime, when the emerging overall orientational order is much smaller than thermal fluctuations
Summary
We develop scenarios for orientational ordering of an in-plane system of small flat octupolar molecules at the low-concentration limit, aiming towards nonlinear-optical (NLO) applications at room temperatures. The practical implementation of very weak octupolar poling conditions at temperature T is limited by the restrictions imposed on the cell setup by the acceptable magnitude of the accompanying electric field E(Qv.w.) .
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