Currently there is considerable interest in the study of nonlinear semiconductor and metallic nanoparticles as new light sources in the nanoscale regime. For example, nanoparticles consisting of non-centrosymmetric metallic nanoparticles (MNPs) exhibit two-photon second-harmonic generation (SHG) which can be used for nonlinear optical microscopy. Nanoparticles made of pure noble metals have high electron polarizabilities and produce a giant enhanced local electric field. Strong local fields are particularly important for nonlinear optical processes, such as surface-enhanced Raman scattering and SHG which scale with the power of the applied field. Two-photon photoluminescence has been studied in nanostructured noble metals, and was found to be more sensitive to the local field than single-photon luminescence. SHG in semiconductor nanoparticles such as quantum dots has also been investigated experimentally and theoretically. Two-photon excitation has many advantages over one-photon excitation including higher spatial resolution, deeper penetration and less photo-damage. Nonlinear nanoparticles have applications nanoscale antennae nanoscale lenses and photolithography and two-photon microscopy .
In this project we study the SHG in a quantum dot (QD) and metallic nanoparticle (MNP) hybrid system. The QD-MNP hybrid system is embedded in a host dielectric medium (see figure 1). In this work, silica is used however many other materials could be substituted such as photonic crystals. A schematic diagram for the present system is shown in Figure. Interactions between the MNP and QD are strong when they are in close proximity and their optical excitation frequencies are resonant with each other. Optical excitations in the QD are electron-hole pairs, which are called excitons. The optical excitations in the MNP are the collective oscillations of conduction band electrons, and are called surface plasmon polaritons (SPPs). We consider that the QD in the hybrid system has three discrete excitonic states. Similar QDs have been studied in the literature where coherent population trapping and electromagnetically induced transparency have been observed.
A strong probe field is applied to the hybrid system, which leads to two-photon absorption in the quantum dot and metallic nanoparticle. SHG photons and surface plasmon polaritons are emitted by the quantum dot and metallic particle, respectively. Induced dipoles are created in the QD and MNP due to the two-photon nonlinear effect. Hence both systems are interacting with each other via the dipole-dipole interaction. The density matrix method has been used to evaluate the SHG intensity and the dipole-dipole interaction. It is found that SHG signals from the QD are enhanced by the dipole-dipole interaction and the SHG electric field produced by the MNP. A similar enhancement is also found in SHG signals from the MNP. It has also been found that the SHG signals can be switched on and off by applying a external control field (see figure 2). There is a good agreement between our theory and the findings from recent experimental studies. The present hybrid system can be used to fabricate all optical nano-switching devices.