Graphene and carbon nanostructures

We study quantum optics and energy transfer in a graphene nanostructures and carbon nanotubes. Here the quantum dot (QD)-graphene system is embedded in a photonic crystal, which acts as a tunable photonic reservoir for the QD (see figure). Photonic crystals are engineered, periodically ordered microstructures that facilitate the trapping and control of light on the microscopic level. Applications for photonic crystals include all-optical microchips for optical information processing, optical communication networks, sensors and solar energy harvesting. In our investigation we consider a nonlinear photonic crystal, which has a refractive index distribution that can be tuned optically. The nonlinear photonic crystal surrounds the QD-graphene system and is used to manipulate the interaction between the QD and graphene nanodisk.

Surface plasmon polaritons are created in the graphene nanodisk due to the collective oscillations of conduction band electrons. They arise due to the dielectric contrast between graphene and the surrounding dielectric medium. Plasmonics is widely studied due to applications in ultrasensitive optical biosensing, photonic metamaterials, light harvesting, optical nanoantennas and quantum information processing. Generally, noble metals are considered as the best available materials for the study of surface plasmon polaritons. However, noble metals are hardly tunable and exhibit large Ohmic losses which limit their applicability to optical processing devices. Graphene plasmons provide an attractive alternative to noble-metal plasmons, as they exhibit much tighter confinement and relatively long propagation distances. Furthermore, surface plasmons in graphene have the advantage of being highly tunable via electrostatic gating. Compared to noble metals, graphene also has superior electronic and mechanical properties, which originate in part from its charge carriers of zero effective mass.¹⁹⁻²⁶ For example, charge carriers in graphene can travel for micrometers without scattering at room temperature.²¹ Graphene has also been recognized as a useful optical material for novel photonic and optoelectronic applications. For these reasons, the study of plasmonics in graphene has receieved significant attention both experimentally and theoretically. Recently, experimental research on graphene has been extended to the fabrication and study of QD-graphene nanostructures.

In the QD-graphene system considered here, energy transfer occurs due to the interaction between optical excitations in the QD and graphene nanodisk. The optical excitations in the QD are excitons, which are electron-hole pairs, while those in the graphene nanodisk are surface plasmon polaritons, which are created due to the collective oscillations of conduction band electrons. We have applied a probe laser field which is coupled with one excitonic transition and measures the energy transfer spectra of the QD and graphene. A control laser field is applied to monitor and control the energy transfer. Besides creating excitons in the QD, these fields also generate surface plasmon polaritons in graphene. The dipoles created by excitons in the QD and plasmons in the graphene nanodisk then interact via the DDI. This interaction is strong when the QD and graphene are in close proximity and their optical excitation frequencies are resonant.

We have found that the energy transfer spectrum of the QD has two peaks when the QD and graphene nanodisk are in close proximity, indicating the creation of two dressed excitons due to the DDI. These dressed excitons are transported to graphene, and produce two peaks in the graphene energy transfer spectrum. We show that the energy transfer between the QD and graphene can be switched on and off by changing the strength of the DDI coupling or by applying an intense laser field to the nonlinear photonic crystal. The intensities of peaks in the energy transfer spectra can be controlled by changing the number of graphene monolayers or by changing the distance between the QD and graphene. We have also predicted that the intensity of these peaks can be modified in the presence of biological materials. Our findings agree with the experimental on a qualitative basis. The present system can be used to fabricate nano-biosensors, all-optical nano-switches, energy transfer devices and quantum tele-transportation devices.