Small ensembles of In 0.55 Al 0.45 As self-assembled quantum dots have been studied at low temperatures using near-field scanning optical microscopy. We observe spectral diffusion in individual quantum dot luminescence lines. The phenomenon increases in magnitude with optical power density, but is not visible at low powers. We believe the spectral diffusion to be caused by long time-scale trapping and untrapping of charges created by photoionization or thermal ionization in the immediate vicinity of the quantum dots.

We have demonstrated large enhancements of the effective second-order nonlinear susceptibility ( (2) ) of ionic self-assembled multilayer (ISAM) films, causing a film with just 3 bilayers to be optically equivalent to a 700−1000 bilayer film. This was accomplished by using nanosphere lithography to deposit silver nanoparticles on the ISAM film, tuning the geometry of the particles to make their plasmonic resonances overlap the frequency of optical excitation. An enhancement in the efficiency of second harmonic generation (SHG) by as much as 1600 times was observed. Even though this is already a large value, we suggest that further refinements of the techniques are expected to lead to additional enhancements of similar or larger magnitude.Second-order nonlinear optical (NLO) materials are at the heart of telecommunications devices such as electro-optic modulators and optical switches, and in lasers such as highpower green and blue solid-state lasers and optical parametric amplifiers.Conventional NLO materials generally consist of inorganic crystals such as KTP, LiTaO 3 , and LiNbO 3 . 1 While they are quite efficient, high-quality crystals of sufficient size are expensive and difficult to manufacture. Organic NLO materials provide an alternative with the potential to provide high nonlinear susceptibilities in an economical fabrication process. 2 NLO materials based on ionic self-assembled multilayer (ISAM) films are particularly promising because of the ease of tailoring noncentrosymmetric structures and the long-term stability. [3][4][5] These films are made by alternately immersing a substrate in two solutions, containing a polycation and a polyanion, respectively. If the substrate initially carries negative surface charges, dipping in the polycation solution will result in a nanoscale polymer layer selfassembled on the substrate, yielding a positively charged substrate. Subsequent dipping in the polyanion solution results in a second layer of the polyanion formed on top of the first layer. The process can be repeated as many times as desired, building up films to arbitrary thickness with nanoscale precision. ISAM films may have substantial (2) values, comparable to that of lithium niobate. 5 Various methods have been suggested to improve the effective (2) of these films by modifying their composition. 6,7 Here, we demonstrate a new approach to enhance the second-order NLO susceptibility by creating hybrid structures from ISAM films and noble metal nanoparticles. Nanoparticles made from noble metals such as silver or gold have recently attracted considerable attention due to their unusual optical properties which enable light to be controlled in unique new ways. [8][9][10] The interaction of light with the free electrons in such particles gives rise to collective oscillations of the conduction electrons at optical frequencies, known as localized surface plasmons resonances (LSPRs). When excited in this fashion, the particles act as nanoscale antennas, concentrating the electromagnetic (E-M) field into v...

We model the absorption enhancement in organic photovoltaic devices induced by incorporating Al, Ag, and Au nanoparticles in the active layer. We find that Al nanoparticles should yield significantly greater enhancement than Ag or Au. This is because the much higher plasma frequency of Al ensures a better overlap between plasmon resonance and absorption band of organic semiconductors. Our predictions are verified experimentally by demonstrating enhanced absorbance in a poly(3-hexylthiophene-2,5-diyl): [6,6]-phenyl C61 butyric acid methyl ester layer with embedded functionalized Al nanoparticles.

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Articles you may be interested inCharacterization of self-assembled InAs quantum dots with InAlAs ∕ InGaAs strain-reduced layers by photoluminescence spectroscopy J. Appl. Phys. 97, 083511 (2005); 10.1063/1.1886278Photoluminescence studies of type-II self-assembled In 0.55 Al 0.45 As/Al 0.5 Ga 0.5 As quantum dots grown on (311)A GaAs substrateWe introduce and demonstrate a novel operating mode in near-field optical microscopy. The tip is used to simultaneously optically probe the sample and induce a highly localized strain in the area under study by pushing the tip into the sample. From knowledge of total tip-sample compression and tip geometry, we estimate the magnitude of stress, and show that localized uniaxial-like stresses in excess of 10 kbar can be achieved. We apply this method to a sample of InAlAs self-assembled quantum dots. A blueshift of quantum dot emission lines consistent with estimates of the strain is observed, as well as a quenching of the photoluminescence with strain.