We investigate theoretically the population transfer process in a Λ-type three-level quantum system near a metallic nanosphere using the Stimulated Raman Adiabatic Passage (STIRAP) technique. We combine density matrix quantum dynamical calculations with first-principle electromagnetic calculations, which quantify the influence of the plasmonic nanoparticle on the electric field of the pump and Stokes pulses in STIRAP as well as on the spontaneous emission rates within the Λ-type system. We study the population transfer process by varying the free-space spontaneous emission rate, the distance of the quantum system from the nanosphere, the polarization direction with respect to the nanoparticle surface and the relative strength of the pump and Stokes pulses used in STIRAP. We find that when the pump and Stokes fields have tangential and radial polarizations with respect to the nanosphere surface, the transfer efficiency is improved due to the increase of the decay rate of the excited state to the target state relatively to the decay to the initial state. The optimal population transfer is achieved for small interparticle distances, moderate free space spontaneous decay rate, large values of the pump Rabi frequency and small values of the Stokes Rabi frequency. When we exchange the polarization directions of the pump and Stokes fields we can still find a range of parameters where the population transfer remains efficient, but larger Stokes Rabi frequencies are necessary to overcome the increased decay rate from the excited state back to the initial state.

We study the nonlinear optical rectification of an inversion-symmetry-broken quantum system interacting with an optical field near a metallic nanoparticle, exemplified in a polar zinc–phthalocyanine molecule in proximity to a gold nanosphere. The corresponding nonlinear optical rectification coefficient under external strong field excitation is derived using the steady-state solution of the density matrix equations. We use ab initio electronic structure calculations for determining the necessary spectroscopic data of the molecule under study, as well as classical electromagnetic calculations for obtaining the influence of the metallic nanoparticle to the molecular spontaneous decay rates and to the external electric field applied to the molecule. The influence of the metallic nanoparticle to the optical rectification coefficient of the molecule is investigated by varying several parameters of the system, such as the intensity and polarization of the incident field, as well as the distance of the molecule from the nanoparticle, which indirectly affects the molecular pure dephasing rate. We find that the nonlinear optical rectification coefficient can be greatly enhanced for particular incident-field configurations and at optimal distances between the molecule and the metallic nanoparticle.

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A theoretical investigation on the population transfer in a 𝚲-type quantum system near a spherical gold nanoparticle under application of two stimulated Raman adiabatic passage (STIRAP) shortcuts and efficiency comparison with conventional STIRAP. It combines the density matrix approach for system dynamics, with classical electromagnetic calculations used to obtain the modified electric field amplitudes of the applied pulses and the Purcell factor of the quantum system due to the presence of the nanoparticle. The efficiency of population transfer is investigated by varying the distance between the quantum system and the nanoparticle, the free-space decay rate of quantum states, the mutual polarization, and the Rabi frequencies of each STIRAP shortcut pulses. In all cases, at least one of the applied shortcuts is more efficient than conventional STIRAP, while in most cases both perform better. When the pump and Stokes fields of the shortcuts have radial and tangential polarizations with respect to the nanoparticle surface, respectively, high transfer efficiency is obtained for small distances of the quantum system to the nanoparticle, moderate free space decay rates and large Rabi frequencies of the fields, while when the pulse polarizations are interchanged, the transfer becomes highly efficient only at large distances.

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.