Nonlinear Optical Rectification in a Polar Molecule-Plasmonic Nanoparticle Structure

Abstract

The study of nonlinear optical properties of quantum systems, such as quantum dots and molecules, near plasmonic nanostructures, has attracted significant interest in the past decade. Several nonlinear phenomena have been studied in quantum systems next to plasmonic nanostructures, such as second and third harmonic generations, Kerr nonlinearity, four-wave mixing, optical bistability, and nonlinear optical rectification. The latter occurs in asymmetric quantum systems and it can be strongly influenced, enhanced, or suppressed, depending on the particular plasmonic nanostructure used. In this work, we theoretically studied the nonlinear optical rectification of a polar two-level quantum system, a specific molecule, the zinc–phalocyanine molecular complex, interacting with an optical field near a gold nanoparticle. Initially, we used the steady-state solution of the density matrix equations for determining the correct form of the nonlinear optical rectification coefficient. We then used ab initio electronic structure calculations for determining the electronic structure of the molecule under study, i.e., the necessary energy differences and the induced and permanent electric dipole moments. We also used classical electromagnetic calculations for calculating the influence of the metallic nanoparticle on the decay rates of the molecule due to the Purcell effect and on the electric field applied in the molecule in the presence of the metallic nanoparticle. We then used the above to investigate the form of the corresponding nonlinear coefficient in the absence and presence of the plasmonic nanoparticle for various parameters. We found that the nonlinear optical rectification coefficient can be enhanced for specific field polarization and for suitable distance between the molecule and the plasmonic nanoparticle. Additionally, we observed that high efficiency of this process was obtained for weak field intensity, zero pure dephasing rates, and for small values of the transition dipole moments.