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New materials for non-linear optics based on hyperpolarizable molecules

Inorganic crystals such as KDP are  often used in optoelectronic devices to double the frequency of a laser beam (SHG: second harmonic generation), thus enabling a much higher bandwith to be used in, eg., high-speed fiber-optics networks.  Inorganic crystals are stable under very intense light, but their second-order nonlinear optical (NLO) properties are not as large as those exhibited by some organic molecules with large, easily polarisable, pi-electron clouds.

Charge  transfer effects between a donor and acceptor group of a dipolar molecule enhance the polarisability, but  these molecules tend to crystalise in centrosymmetric structures that  forbid bulk SHG. This problem can be avoided by co-crystalising the organic molecule with another (chiral) organic or inorganic species.

Octupolar molecules, ie. molecules that due to their symmetry have no dipole moment but which are highly polarisable,  can circunvent the disadvantages of dipolar molecules.

CEMDRX has an on-going research project in collaboration with Centre for Computational Physics of FCTUC aiming at engineering new non-centrosymmetric NLO crystals based on such octupolar moleculesm, such as TPG (Triphenylguanidine).

TPG ab-initio calculation of HUMO

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