Targeted self-assembly of nanocrystal quantum dot emitters using smart peptide linkers on light emitting diodes

Ankara : The Department of Electrical and Electronics Engineering and the Institute of Engineering and Sciences of Bilkent University, 2008.Thesis (Master's) -- Bilkent University, 2008.Includes bibliographical references leaves 68-79.Semiconductor nanocrystal quantum dots find several applications in nanotechnology. Particularly in device applications, such quantum dots are typically required to be assembled with specific distribution in space for enhanced functionality and placed at desired spatial locations on the device which commonly has several diverse material components. In conventional approaches, self-assembly of nanocrystals typically takes place nonspecifically without surface recognition of materials and cannot meet these requirements. To remedy these issues, we proposed and demonstrated uniform, controlled, and targeted self-assembly of quantum dot emitters on multi-material devices by using cross-specificity of genetically engineered peptides as smart linkers and achieved directed immobilization of these quantum dot emitters decorated with peptides only on the targeted specific regions of our color-conversion LEDs. Our peptide decorated quantum dots exhibited 270 times stronger photoluminescence intensity compared to their negative control groups.Zengin, GülisM.S

Interactions Between Localized Surface Plasmons and Molecular Resonances

Molecular plasmonics is the study of interactions between plasmonic nanostructures and molecules. It has been basis for fundamental understanding of light-matter interactions and development of many technological applications, such as biological and chemical sensing, plasmon-enhanced spectroscopies, optical switches, and plasmon-enhanced energy harvesting. When the plasmon energy of a metal nanostructure is degenerate with the absorption energy of a nearby molecule, the interaction becomes resonant. Strong and confined electromagnetic field induced by the metal nanoparticle polarizes the molecule. This consecutively modifies the electron oscillations in the metal nanostructures. As a result, optical responses of both the molecules and the plasmonic nanostructures are changed by the interaction of the molecule and plasmons. Organic chromophores interacting with plasmonic nanostructures constitutes an important part in the molecular plasmonics field. Rhodamine 6G is one of the organic chromophores that has been widely studied with the interaction of silver nanostructures in the context of single molecule surface-enhanced Raman spectroscopy. In this thesis, spectral dips in the Rayleigh scattering of single silver nanoparticles interacting with Rhodamine 6G have been shown for the first time. This was achieved by using a novel way of adsorbing Rhodamine 6G on silver surface. Similar observations have been reported between single plasmonic nanoparticles and chromophores. However, the mechanism behind was only explained by strong coupling or plasmon resonant energy transfer. Mie theory calculations suggest that surface-enhanced absorption significantly contributes to these spectral modifications as well as the coupling. The strength of molecule-plasmon interactions is strongly affected by the properties of plasmonic nanoparticles and chromophores. By decreasing the radiative damping of plasmonic particles, and using J-aggregates of a cyanine dye, which exhibit high oscillator strength and narrow transition linewidth, it is possible to approach strong coupling regime. In this thesis we observed 50% transparency in the scattering spectra of single silver nanorods. To our knowledge, this is the strongest modification reported up to date at single particle level

Realizing strong light-matter interactions between single nanoparticle plasmons and molecular excitons at ambient conditions

Realizing strong light-matter interactions between individual 2-level systems and resonating cavities in atomic and solid state systems opens up possibilities to study optical nonlinearities on a single photon level, which can be useful for future quantum information processing networks. However, these efforts have been hampered by the unfavorable experimental conditions, such as cryogenic temperatures and ultrahigh vacuum, required to study such systems and phenomena. Although several attempts to realize strong light-matter interactions at room-temperature using so-called plasmon resonances have been made, successful realizations on the single nanoparticle level are still lacking. Here, we demonstrate strong coupling between plasmons confined within a single silver nanoprism and excitons in molecular J-aggregates at ambient conditions. Our findings show that the deep subwavelength mode volumes, $V$, together with high quality factors, $Q$, associated with plasmons in the nanoprisms result in strong coupling figure-of-merit -- $Q/\sqrt{V}$ as high as $\sim6\times10^{3}$~$\mu$m$^{-3/2}$ -- a value comparable to state-of-art photonic crystal and microring resonator cavities, thereby suggesting that plasmonic nanocavities and specifically silver nanoprisms can be used for room-temperature quantum optics

Approaching the strong coupling limit in single plasmonic nanorods interacting with J-aggregates

We studied scattering and extinction of individual silver nanorods coupled to the J-aggregate form of the cyanine dye TDBC as a function of plasmon - exciton detuning. The measured single particle spectra exhibited a strongly suppressed scattering and extinction rate at wavelengths corresponding to the J-aggregate absorption band, signaling strong interaction between the localized surface plasmon of the metal core and the exciton of the surrounding molecular shell. In the context of strong coupling theory, the observed "transparency dips" correspond to an average vacuum Rabi splitting of the order of 100 meV, which approaches the plasmon dephasing rate and, thereby, the strong coupling limit for the smallest investigated particles. These findings could pave the way towards ultra-strong light-matter interaction on the nanoscale and active plasmonic devices operating at room temperature

Directional Light Extinction and Emission in a Metasurface of Tilted Plasmonic Nanopillars

Plasmonic optical antennas and metamaterials with an ability to boost light-matter interactions for particular incidence or emission angles could find widespread use in solar harvesting, biophotonics, and in improving photon source performance at optical frequencies. However, directional plasmonic structures have generally large footprints or require complicated geometries and costly nano-fabrication technologies. Here, we present a directional metasurface realized by breaking the out-of-plane symmetry of its individual elements: tilted subwavelength plasmonic gold nanopillars. Directionality is caused by the complex charge oscillation induced in each individual nanopillar, which essentially acts as a tilted dipole above a dielectric interface. The metasurface is homogeneous over a macroscopic area and it is fabricated by a combination of facile colloidal lithography and off-normal metal deposition. Fluorescence excitation and emission from dye molecules deposited on the metasurface is enhanced in specific directions determined by the tilt angle of the nanopillars. We envisage that these directional metasurfaces can be used as cost-effective substrates for surface-enhanced spectroscopies and a variety of nanophotonic applications

Evaluating Conditions for Strong Coupling between Nanoparticle Plasmons and Organic Dyes Using Scattering and Absorption Spectroscopy

Interactions between surface plasmons in metal nano particles and electronic excitations in organic chromophores have resulted in many notable findings, including single-molecule Raman scattering, nanoscale lasing, and enhanced fluorescence. Recently, plasmon-exciton interactions have been shown to reach the strong coupling limit, a nonperturbative regime in which a coupled plasmon-exciton system should be treated as a unified hybrid. Strong coupling effects could open up exciting possibilities for manipulating nano particle plasmons via molecular degrees of freedom, or vice versa. Optical properties of such hybrid systems can differ drastically from those of noninteracting components. Specifically, optical spectra of a strongly coupled system are expected to exhibit mode splitting due to Rabi oscillations of excitation energy between the system components. However, the interpretation of optical spectra in terms of strong coupling is not a straightforward matter. Here we clarify the nature of plasmon-exciton coupling for the case of rhodamine-6G (R6G) interacting with localized surface plasmons in silver nanodisks using scattering and absorption spectroscopy. We show that this system is only marginally able to reach the strong coupling limit, even for very high molecular concentrations and despite the appearance of obvious mode splitting in scattering. For lower molecular concentrations, the mode splitting we observe should be interpreted as being due to surface-enhanced absorption rather than strong coupling. These results allow us to evaluate the critical concentration necessary for reaching the strong coupling limit and propose conditions for observing strong coupling between single-particle plasmons and organic dyes, such as R6G