Light matter interaction in nanocomposites and nano hybrid system
The aim of the proposed research program is to advance both the basic science and practical applications of light-matter interactions in nanocomposite materials. This will be achieved through theoretical design, modeling, simulation, and collaborative research to synthesize the required functional nanocomposite architectures. Nanocomposite materials are engineered materials fabricated by combining two or more semiconductor, biological or metallic nanostructures. By using various combinations of these nanostructures an enormous number of nanocomposites can be fabricated with varying physical and optical properties. The most prominent examples of nanostructures which can be used to build nanocomposites are quantum dots, graphene, carbon nanotubes, semiconductor nanowires, and metallic nanospheres, nanorods or nanowires.
In the presence of external electromagnetic fields, optical excitations occur in the constituent nanoparticles within a nanocomposite system. Optical excitations in semiconductor nanostructures are electron-hole pairs (excitons). Due to quantum confinement, the energy levels of excitons in these systems are discrete and are dictated by the size and shape of the nanostructure. Excitations in noble metal or graphene nanostructures are surface plasmons, which are collective oscillations of electrons, and their energies are also quantized and controlled by the shape and size of the nanostructure. The attraction of combining semiconductor and noble metal/graphene nanostructures stems from their complementary optical properties, which are long-lived excitations in semiconductor nanostructures and localized photon modes in noble metal/graphene nanostructures. The former gives rise to high emission yields and light-harvesting capabilities, while the latter contains the concentration of electromagnetic energy and enhances the local electric field. Therefore, the combination of two or more nanostructures can provide attractive opportunities to modify, design, and control the light-matter interaction and to observe new phenomena which are based on exciton-plasmon and exciton-photon interactions. Optical properties such as absorption, emission, photoluminescence, extinction cross-sections and energy transfer rates will be calculated.
We investigate the quantum optics of nanocomposite-doped photonic band gap materials (i.e. polaritonic or photonic crystals, photonic heterostructures). These photonic band gap materials would act as an engineered photonic reservoir for excitons in semiconductor nanostructures and surface plasmons in metallic or graphene nanostructures and allow for control of their radiative decay rates. Photonic states in photonic heterostructures, such as waveguides, fibers and cavities made using two or more photonic band gap materials, are quantized and localized in one-, two- and three-dimensions, respectively. These structures will also influence exciton and plasmon decay through band gap engineering and control over the photonic density of states. When used as a substrate, photonic band gap materials can play a significant role in controlling the light-matter interactions in nanocomposite systems. Nanocomposites are also of particular interest for studying nonlinear optics. Since the intensity of the local field produced by surface plasmons in a metal or graphene nanostructure is very intense, the local field can give rise to nonlinear optical effects such as second harmonic generation, the Kerr effect, multi-photon emission and gap solitons in nanocomposites. Furthermore, due to the strong coupling between excitons and plasmons, new particles called dressed excitons and dressed plasmons are created whose properties are different than that of their parent particles.
Our research will focus on transient photoluminescence and energy transfer rates of quantum dots embedded as donors in biological materials. Recently, quantum dots have been used for fluorescent labeling of cellular proteins, embryonic cell labeling, and tumor cell extravasation and seeding. In contrast to conventional organic dyes, quantum dots possess a high quantum yield, narrow and stable fluorescence, and size-dependent absorption and emission. Nanocomposites containing metamaterials will be also studied. Metamaterials (or negative-index materials) are engineered materials which possess a negative refractive index (i.e. metallic split ring resonators). In noble metals the dielectric constant is frequency dependent and the permeability is a constant, positive quantity. Metallic nanoparticles support surface plasmon modes because for certain electromagnetic frequencies they have a negative dielectric constant. In metamaterials both the dielectric constant and permeability are frequency-dependent and can both have negative values as well as support surface plasmons. Therefore the theory developed in this research program for nanocomposites containing a metallic component can be adapted to the study of nanocomposites with a metamaterial component.
Bragg Scattering in Plasmonic Nanohybrids
In this work, the Bragg Scattering for metallic nanohybrid made of an ensemble of metallic nanorods doped in a substrate. Such substrate can be any suitable gas, liquid and solid. Moreover, a theory was developed to describe the relation between an external incident laser intensity and Bragg scattered light intensity. When the external laser was applied to the metallic nanohybrids, the photons from the laser will interact with the surface polaritons in the nanorods and produced surface plasmon polaritons (SPPs). On the other hand, the incident photon induced dipoles in the ensembled nanorods, so the nanorods can interact with each other via dipole-dipole interactions (DDI). The developed theory involved the coupled-mode formulism based on Maxwell’s equation with the presence of SPP and DDI fields and analytical expressions for the SPP/DDI coupling constants were obtained in a similar manner. It is found that, the intensity of Bragg scattering would depend on the susceptibility induced by SPP and DDI field. The susceptibility was calculated by the quantum mechanical density matrix method. Combining these methods, an analytical expression for the Bragg scattering intensity as a function of incident laser intensity. Next, the theory was used to compare with the experimental data for a nanohybrids made of doping Au-nanorods into water. A decent agreement between the theoretical model and experimental data was observed. Later, several numerical simulations were performed to investigate the effects of SPP/DDI coupling, laser detuning and phase factor. The theoretical model was used to predict the Bragg intensity due to different parameters. The Bragg scattering intensity was found to be enhanced by higher SPP/DDI coupling constant. Such an enhancement can be interpreted by the extra coupling mechanism from the SPP and DDI polaritons with acoustic phonons. On the other hand, the peaks for the Bragg scattering intensity can be split into many peaks due to SPP/DDI coupling and the phase constant. Such a splitting of the peaks can be explained by the Bragg factor in the theory. In conclusion, the enhancement effect can be used to fabricate new nano-sensors, and the splitting effect can be used to design new nano-switches where the peaks can be interpreted as the ON position.
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.
