Tolga Atay

Highly efficient resonant coupling of optical excitations in hybrid organic/inorganic semiconductor nanostructures

The integration of organic and inorganic semiconductors on the nanoscale offers the possibility of developing new photonic devices that combine the best features of these two distinct classes of material. Such devices could, for example, benefit from the large oscillator strengths found in organic materials and the nonlinear optical properties of inorganic species. Here we describe a novel hybrid organic/inorganic nanocomposite in which alternating monolayers of J-aggregates of cyanine dye and crystalline semiconductor quantum dots are grown by a layer-by-layer self-assembly technique. We demonstrate near-field photon-mediated coupling of vastly dissimilar optical excitations in the two materials that can reach efficiencies of up to 98% at room temperature. By varying the size of the quantum dots and thus tuning their optical resonance for absorption and emission, we also show how the ability of J-aggregates to harvest light can be harnessed to increase the effective absorption cross section of the quantum dots by up to a factor of ten. Combining organic and inorganic semiconductors in this way could lead to novel nanoscale designs for light-emitting, photovoltaic and sensor applications.

Optical Detection of Brain Cell Activity Using Plasmonic Gold Nanoparticles

Metal nanoparticles are being actively explored for applications that use localized surface plasmon (LSP) resonance for optical sensing. Here we report an electrostatic field sensing technique which has been applied to detection of mammalian brain cell activity, by optically measuring the cellular potential induced shift in the SP resonance mode of an adjacent planar gold nanoparticle array. An experimental scheme was first devised which enables a quantitative calibration of the field-induced plasmon resonance modulation in air. Hippocampal (brain) neural cells were then grown onto the nanoparticle template and cellular level individual transient signals were detected optically when the chemically triggered neurons switched their potential. Experimental data are compared with calculations using the Drude model for the dielectric response of gold and the Stern model for the metal-electrolyte junction, with good agreement.

Large enhancement of fluorescence from CdSe/ZnS quantum dots induced by resonant coupling to spatially controlled surface plasmons

Nanoengineered fluorescent response is reported in semiconductor core-shell (CdSe/ZnS) quantum dots in proximity to the surface plasmon field of periodic silver nanoparticle arrays. Tuning the surface plasmon resonance to the quantum dot exciton emission band results in an /spl sim/50-fold enhancement of the fluorescence.

Strongly interacting plasmon nanoparticles: from dipole interaction to conductively coupled regime

We have investigated the electromagnetic interaction of periodic arrays of metallic (gold) nanoparticles, composed of a pair of particles on each lattice site. By varying the interparticle separation from dielectric proximity to conductive contact on nm scale, we observe very large renormalization and splitting of the surface plasmon energy associated with an abrupt transition from the dipolar to a conductively coupled regime

Detection of neural cell activity using plasmonic gold nanoparticles

Metal nanoparticles have been studied intensively for their applications using localized surface plasmon polariton (SPP) resonance. We have demonstrated for the first time that using gold nanoparticles, one can detect electrical activities from the neurons.

Study of phonon propagation in water using picosecond ultrasonics

The propagation of ultra-short sound pulses in water is studied by using the picosecond ultrasonic technique. A sound pulse is generated when light is absorbed in a metal transducer film deposited onto a substrate. The sound propagates across a thin layer of water and is then reflected back to the surface at which it was generated. The efficiency of optoacoustic detection of the reflected sound is enhanced through the use of a resonant optical cavity. We show that the variation of the shape of the returning sound pulse with propagation distance agrees with that calculated by using the attenuation of sound in water that has been measured at lower frequencies.

Photovoltaic energy converter as a chipscale high efficiency power source for implanted active microelectronic devices

We report the development of a microscale photovoltaic energy converter which has been designed and implemented to deliver power to CMOS-based microelectronic chips. The design targets the delivery of voltages on the order of 3V with power levels in excess of 10 mW. The geometry of the prototype device, which has been fabricated and tested, is specifically designed for coupling to an optical fiber, to facilitate remote power delivery in implantable component environment.

Highly Efficient Resonance Energy Transfer in Ultrathin Organic-Inorganic Semiconductor Hybrid Films

We report on the optical study of efficient excitation transfer in organic-inorganic hybrid thin films composed of alternating monolayers of CdSe/ZnS QDs and J-aggregate of cyanine dyes, by steady-state and time-resolved photoluminescence spectroscopy study.

Gallium nitride-organic semiconductor heterojunctions for optoelectronic devices

We report on study of GaN/InGaN-organic semiconductor heterostructures where electronic transport in planar junction structures shows electron or/and hole injection across the interfaces resulting e.g. light emission from the nitride quantum wells.

Ultrafast exciton response of high optical density J-aggregates from ultrathin films of cyanine dyes

Ultrathin films of cyanine dye J-aggregates show extraordinary exciton cross sections. Exciton dynamics are studied by ultrafast pump-probe and transient photoluminescence spectroscopy. An exciton delocalization length of N=18 monomers was measured at room temperature.