Derivation of complex cubic response function to guide development of nonlinear optical molecular materials

Theoretical simulations are important to understanding and optimizing nonlinear optical responses in applications like 3D memory chips, optical power limiting, and optical image processing. Presenting their research in The Journal of Chemical Physics, Tobias Fahleson and Patrick Norman of the KTH Royal Institute of Technology in Sweden have theoretically derived the second-order nonlinear response function based on an Ehrenfest formulation of wave function dynamics.

The authors employed the complex polarization propagator approach to derive the complex cubic response function for exact states as well as approximate single-determinant Hartree-Fock and Kohn-Sham reference states. Then, using computational methods, they found that two-photon absorption cross-sections could be efficiently calculated from this response function.

Previously, Norman and co-workers had taken a similar approach to develop the linear and quadratic complex response functions. This new study is a continuation of that work with a focus on the cubic response function. The response theory is based on the Ehrenfest equation with an additional damping term representing the rate of relaxation, which is solved for the unknown time-dependent wave function parameters. The results allow for the direct calculation of nonlinear molecular properties without explicitly addressing the excited-state manifold.

The researchers carried out a computational analysis of the intensity-dependent refractive index for para-nitroaniline and the two-photon absorption cross-section of neon using a locally modified version of the DALTON program.

Norman says the next step is to use their developed theory and program module to find synthetic design strategies for optimizing nonlinear responses in the X-ray region, which will be relevant for molecular materials studies at X-ray free electron laser facilities.

Source: “Resonant-convergent second-order nonlinear response functions at the levels of Hartree-Fock and Kohn-Sham density functional theory,” by Tobias Fahleson and Patrick Norman, The Journal of Chemical Physics (2017). The article can be accessed at https://doi.org/10.1063/1.4991616 .