Browsing by Subject "Emission"
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Item Open Access Applications of plasmon enhanced emission and absorption(Bilkent University, 2009) Ayas, SencerThe term plasmon-polariton is used to describe coupled modes of electromagnetic waves with electronic plasma oscillations in conductors. Surface plasmon resonances have found profound interest over the last few decades in multiple fields ranging from nanophotonics to biological sensing. In this thesis, we study enhancement of absorption and emission of radiation due to the presence of a modified local electromagnetic mode density within the vicinity of metallic surfaces supporting plasmon modes. Various coupling schemes of freespace electromagnetic modes to plasmon modes are investigated theoretically and experimentally. Local mode densities and field enhancements due to plasmon modes in planar structures, gratings and optical antennas have been studied in their relation to the absorption and emission enhancement of dipoles positioned in various orientations and locations with respect to structures displaying plasmonic effects. Particularly, grating coupled plasmon resonances were analysed using Rigorously Coupled Wave Analysis and Finite Difference Time domain methods. Experimental demonstrations of absorption and emission enhancement of dielectric layers containing Rhodamine 6G on various plasmonic structures are given. Confocal Raman microscopy was used in characterization of fabricated structures. It is seen that the experimental measurements are in good agreement with theoretical predictions. Direction dependent luminescence enhancement is observed with dye molecules on grating structures. Potential applications of plasmon enhanced absorption and emission include high sensitivity absorption spectroscopy, performance enhancement in thin film solar cells and luminescent concentrators.Item Open Access Boradiazaindacene (Bodipy)-based building blocks for the construction of energy transfer cassettes(Elsevier, 2009-04-15) Barin, G.; Yilmaz, M. D.; Akkaya, E. U.Energy transfer cassettes composed entirely of boradiazaindacene (Bodipy) units were designed and synthesized to capture photonic energy and convert it to longer wavelength fluorescence emission. The new energy transfer systems obtained by simple condensation reactions are capable of elaborating efficient energy transfer from donor Bodipy units to the distyryl-Bodipy acceptor.Item Open Access Chromogenic and fluorogenic sensing of biological thiols in aqueous solutions using BODIPY-based reagents(American Chemical Society, 2013) Isik, M.; Ozdemir, T.; Turan, I. S.; Kolemen, S.; Akkaya, E. U.Judicious design of BODIPY dyes carrying nitroethenyl substituents in conjugation with the BODIPY core yields dyes that respond to biological thiols by both absorbance and emission changes. Incorporation of solubilizing ethyleneglycol units ensures water solubility. The result is bright signaling of biologically relevant thiols in the longer wavelength region of the visible spectrum and in aqueous solutions.Item Open Access Highly directive radiation from sources embedded inside photonic crystals(American Institute of Physics, 2003) Bulu, I.; Caglayan, H.; Özbay, EkmelIn this work, we have experimentally and theoretically studied the angular distribution of power emitted from a radiation source embedded inside a photonic crystal. Our results show that it is possible to obtain highly directive radiation sources operating at the band edge of the photonic crystal. Half power beam widths as small as 6degrees have been obtained. Our results also show that the angular distribution of power strongly depends on the frequency and on the size of the photonic crystal. (C) 2003 American Institute of Physics.Item Open Access Liquid-liquid diffusion ‐ assisted crystallization: a fast and versatile approach toward high quality mixed quantum dot ‐ salt crystals(Wiley-VCH Verlag, 2015) Adam, M.; Wang, Z.; Dubavik, A.; Stachowski, G. M.; Meerbach, C.; Soran-Erdem, Z.; Rengers, C.; Demir, Hilmi Volkan; Gaponik N.; Eychmuller, A.Here, a new, fast, and versatile method for the incorporation of colloidal quantum dots (QDs) into ionic matrices enabled by liquid-liquid diffusion is demonstrated. QDs bear a huge potential for numerous applications thanks to their unique chemical and physical properties. However, stability and processability are essential for their successful use in these applications. Incorporating QDs into a tight and chemically robust ionic matrix is one possible approach to increase both their stability and processability. With the proposed liquid-liquid diffusion-assisted crystallization (LLDC), substantially accelerated ionic crystallization of the QDs is shown, reducing the crystallization time needed by one order of magnitude. This fast process allows to incorporate even the less stable colloids including initially oil-based ligand-exchanged QDs into salt matrices. Furthermore, in a modified two-step approach, the seed-mediated LLDC provides the ability to incorporate oil-based QDs directly into ionic matrices without a prior phase transfer. Finally, making use of their processability, a proof-of-concept white light emitting diode with LLDC-based mixed QD-salt films as an excellent color-conversion layer is demonstrated. These findings suggest that the LLDC offers a robust, adaptable, and rapid technique for obtaining high quality QD-salts.Item Open Access Microstructural defect properties of InGaN/GaN blue light emitting diode structures(Springer US, 2014-06-28) Bas, Y.; Demirel, P.; Akin, N.; Başköse, C.; Özen, Y.; Kınacı, B.; Öztürk, M. K.; Özcelik, S.; Özbay, EkmelIn this paper, we study structural and morphological properties of metal-organic chemical vapour deposition-grown InGaN/GaN light emitting diode (LED) structures with different indium (In) content by means of high-resolution X-ray diffraction, atomic force microscopy (AFM), Fourier transform infrared spectroscopy (FTIR), photoluminescence (PL) and current-voltage characteristic (I-V). We have found out that the tilt and twist angles, lateral and vertical coherence lengths of mosaic blocks, grain size, screw and edge dislocation densities of GaN and InGaN layers, and surface roughness monotonically vary with In content. Mosaic defects obtained due to temperature using reciprocal lattice space map has revealed optimized growth temperature for active InGaN layer of MQW LED. It has been observed in this growth temperature that according to AFM result, LED structure has high crystal dimension, and is rough whereas according to PL and FTIR results, bandgap energy shifted to blue, and energy peak half-width decreased at high values. According to I-V measurements, it was observed that LED reacted against light at optimized temperature. In conclusion, we have seen that InGaN MQW structure's structural, optical and electrical results supported one another.Item Open Access Nitrogen incorporation and optical studies of GaAsSbN∕GaAs single quantum well heterostructures(A I P Publishing LLC, 2007) Nunna, K.; Iyer, S.; Wu, L.; Li, J.; Bharatan, S.; Wei, X.; Senger, R. T.; Bajaj, K. K.In this work, the effects of N incorporation on the optical properties of GaAsSbN/GaAs single quantum wells (SQWs) have been investigated using temperature, excitation, and magnetic dependencies of photoluminescence (PL) characteristics. These layers were grown in an elemental solid source molecular beam epitaxy system with a rf plasma N source. The N concentrations in the range of 0.5%-2.5% were investigated in this study. The SQW with N similar to 0.5% exhibits a behavior similar to that in an intermediate regime where the contributions from the localized states in the band gap are dominant. The temperature and excitation dependencies of the PL characteristics indicate that for the N concentration of 0.9% and above, the alloy behavior is analogous to that of a regular alloy and the changes in optical properties are only marginal. The conduction band effective mass (m(eff)) values computed from the magnetophotoluminescence spectra using a variational formalism and the band anticrossing model are in good agreement and indicate enhanced values of m(eff). However, there is no significant variation in m(eff) values of QWs for N >= 0.9%. Small redshift of about 30-50 meV for the temperature variations from 10 to 300 K in conjunction with unusually small blueshift observed in the excitation dependence of PL for N >= 0.9% indicate that this system holds a great promise for laser applications at 1.55 mu m and beyond.Item Open Access Observation of efficient transfer from Mott-Wannier to Frenkel excitons in a hybrid semiconductor quantum dot-polymer composite at room temperature(American Institute of Physics, 2010-12-29) Nizamoglu, S.; Sun, X. W.; Demir, Hilmi VolkanEfficient conversion from Mott-Wannier to Frenkel excitons is observed at room temperature. The time-resolved photoluminescence shows that the energy transfer rate and efficiency reach 0.262 ns-1 and 80.9%, respectively. The energy transfer is enabled by strong dipole-dipole coupling in a hybrid inorganic/organic system of CdSe/ZnS core/shell heteronanocrystal and poly[2-methoxy-5-(3,7-dimethyl-octyloxy)-1,4-phenylenevinylene] homopolymer composite, and the measured energy transfer efficiencies are consistent with the analytical model.Item Open Access Reordering orbitals of semiconductor multi-shell quantum dot-quantum well heteronanocrystals(American Institute of Physics, 2012-01-27) Sahin, M.; Nizamoglu, S.; Yerli, O.; Demir, Hilmi VolkanBased on self-consistent computational modeling of quantum dot-quantum well (QDQW) heteronanocrystals, we propose and demonstrate that conduction-electron and valence-hole orbitals can be reordered by controlling shell thicknesses, unlike widely known core/shell quantum dots (QDs). Multi-shell nanocrystals of CdSe/ZnS/CdSe, which exhibit an electronic structure of 1s-1p-2s-2p-1d-1f for electrons and 1s-1p-2s-2p-1d-2d for holes using thin ZnS and CdSe shells (each with two monolayers), lead to 1s-2s-1p-1d-1f-2p electron-orbitals and 1s-2s-1p-1d-2p-1f hole orbitals upon increasing the shell thicknesses while keeping the same core. This is characteristically different from the only CdSe core and CdSe/ZnS core/shell QDs, both exhibiting only 1s-1p-1d-2s-1f-2p ordering for electrons and holes.Item Open Access Resonant nonradiative energy transfer in CdSe/ZnS core/shell nanocrystal solids enhances hybrid white light emitting diodes(Optical Society of America, 2008) Nizamoglu, S.; Demir, Hilmi VolkanWe propose and demonstrate hybrid white light emitting diodes enhanced with resonant nonradiative energy transfer in CdSe/ZnS core/shell nanocrystal solids integrated on near-UV InGaN/GaN LEDs. We observe a relative quantum efficiency enhancement of 13.2 percent for the acceptor nanocrystals in the energy gradient mixed assembly, compared to the monodisperse phase. This enhancement is attributed to the ability to recycle trapped excitons into nanocrystals using nonradiative energy transfer. We present the time-resolved photoluminescence of these nanocrystal solids to reveal the kinetics of their energy transfer and their steady-state photoluminescence to exhibit the resulting quantum efficiency enhancement.