ITMO University Saint Petersburg National Research University of Information Technologies, Mechanics and Optics

Optics of strong coupled resonance systems: an exciton in an epitaxial semiconductor quantum dot and a plasmon in a metallic nanoparticle

Nanostructures and metamaterials incorporating semiconductor quantum dots (SQDs) and metal nanoparticles (MNPs) have been intensively studied. Such hybrid structures take advantage of the physical properties of different materials which can be useful for chemical and biological sensors, solar cells, and optical and optoelectronic devices. For instance, self-assembled InAs SQDs are excellent light emitters due to localized excitons, and are suitable for lasing, single photon sources and other applications. On the other hand, silver MNP can enhance the local electromagnetic field and provide good coupling of electro-magnetic waves with localized plasmon excitations.

In this project, a coupled plasmon-exciton system of As MNPs and InAs SQDs has been realized. The InAs SQDs were produced using the Stranski-Krastanov growth mode on GaAs (001) substrates by molecular-beam epitaxy. Five layers of InAs SQDs separated by 5nm thick GaAs spacers were formed. As a result, we obtained vertical stacks of InAs QDs with lateral size of about 20 nm.

The upper layer of the SQDs was capped by 3 a nm-GaAs/3 nm-AlAs/4 nm-GaAs layer sequence. A thin silver layer was deposited in a vacuum chamber on the top of SQD-containing wafer. Subsequent annealing transformed the Ag film into a system of isolated silver nanoparticles with size and density depending on the annealing conditions. We expect a spatial correlation between stacks of InAs QDs and Ag MNPs. So, systems of Ag MNP with sizes from 20 to 100nm were formed above the layer of buried InAs QDs.

Optical reflection, absorption and photoluminescence spectra were studied to document the optical properties of plasmons in Ag MNP and excitons in InAs and reveal their interaction. The optical plasmon resonance appeared to be centered near 1.4 eV. It showed a large inhomogeneous broadening due to substantial size dispersion in the MNP system. The InAs SQDs showed a strong emission near 1.1 eV with 0.07 eV in width. After formation of the Ag MNPs, the emission intensity increased by the factor of 2.4. The enhancement and spectral transformation of this emission are clear evidence of the interaction of the SQD excitons with the MNP plasmons.