Parinda Vasa

Mechanism of the Size Dependence of the Superconducting Transition of Nanostructured Nb

In nanocrystalline Nb films, the superconducting Tc decreases with a reduction in the average particle size below 20nm. We correlate the decrease in Tc with a reduction in the superconducting energy gap measured by point contact spectroscopy. Consistent with the Anderson criterion, no superconducting transition was observed for sizes below 8 nm. We show that the size-dependence of the superconducting properties in this intermediate coupling Type II superconductor is governed by changes in the electronic density of states rather than by phonon softening.

Adiabatic nanofocusing on ultrasmooth single-crystalline gold tapers creates a 10-nm-sized light source with few-cycle time resolution.

We demonstrate adiabatic nanofocusing of few-cycle light pulses using ultrasharp and ultrasmooth single-crystalline gold tapers. We show that the grating-induced launching of spectrally broad-band surface plasmon polariton wavepackets onto the shaft of such a taper generates isolated, point-like light spots with 10 fs duration and 10 nm diameter spatial extent at its very apex. This nanofocusing is so efficient that nanolocalized electric fields inducing strong optical nonlinearities at the tip end are reached with conventional high repetition rate laser oscillators. We use here the resulting second harmonic to fully characterize the time structure of the localized electric field in frequency-resolved interferometric autocorrelation measurements. Our results strongly suggest that these nanometer-sized ultrafast light spots will enable new experiments probing the dynamics of optical excitations of individual metallic, semiconducting, and magnetic nanostructures.

Interplay between Strong Coupling and Radiative Damping of Excitons and Surface Plasmon Polaritons in Hybrid Nanostructures

We report on the interplay between strong coupling and radiative damping of strongly coupled excitons (Xs) and surface plasmon polaritons (SPPs) in a hybrid system made of J-aggregates and metal nanostructures. The optical response of the system is probed at the field level by angle-resolved spectral interferometry. We show that two different energy transfer channels coexist: coherent resonant dipole-dipole interaction and an incoherent exchange due to the spontaneous emissions of a photon by one emitter and its subsequent reabsorption by another. The interplay between both pathways results in a pronounced modification of the radiative damping due to the formation of super- and subradiant polariton states. This is confirmed by probing the ultrafast nonlinear response of the polariton system and explained within a coupled oscillator model. Such a strong modification of the radiative damping opens up interesting directions in coherent active plasmonics.

Photoconductivity in sputter-deposited CdS and CdS-ZnO nanocomposite thin films

Nanocrystalline CdS and CdS-ZnO thin films were deposited at room temperature using high-pressure radio-frequency magnetron sputtering. The CdS-ZnO nanocomposite film was made up of particles smaller than 2 nm and showed a composite band gap of 2.72 eV. Both samples showed appreciable photoconductivity, extremely fast response and high photostability. We show that the photoresponse in these samples arises mainly from the band-gap transition and excitonic contributions are insignificant. A comparison of the photocurrent response with absorption and photoluminescence data indicates that photocurrent spectroscopy is a simple and extremely useful technique for characterizing systems (such as nanoparticles) with low radiative cross sections.

Photoluminescence enhancement in nanocomposite thin films of CdS–ZnO

We show that the photoluminescence emitted from a dense, two-component quantum dot ensemble on a thin film is significantly higher and decays much faster than that from quantum dots of either of the two pure systems (CdS and ZnO). The semiconductor nanocomposite, in which the characteristic grain size of each species was 2–3nm, was deposited directly on Si wafers by high-pressure magnetron sputtering, and exhibits a single, relatively sharp optical absorption edge.

Chemical passivation of sputter-deposited nanocrystalline CdS thin films

Due to their extremely high surface and interface area, nanocrystalline materials are extraordinarily susceptible to environmental degradation, and often need to be effectively isolated from the ambient before they can be put to applications. In addition, semiconductor nanoparticles usually require to be surface-passivated in order to enhance radiative decay cross sections. We describe an rf-sputtering technique for the synthesis of nanocrystalline CdS thin films and propose a simple and effective method for the in-situ surface passivation of such films by terminating the surface dangling bonds of CdS with hydrogen.

Supercontinuum generation in water doped with gold nanoparticles

We report enhanced supercontinuum generation in water doped with gold nanoparticles of different shapes under modest ultrafast (35 fs) laser excitation. Reasonably, flat supercontinuum spectra covering ∼1.45–2 eV (855–620 nm) are observed with as much as ∼161 meV (63 nm) increase in the visible extent compared to pure water for dopants whose surface plasmon resonance (SPR) overlaps the excitation laser spectrum. We use a phenomenological self-phase modulation model to rationalize our results, taking cognizance of plasma contributions to the third-order susceptibility of water along with SPR-induced field enhancement. Such large spectral broadening may be useful for several applications involving imaging or microscopy with modest incident intensities.

Fast and reversible excited state absorption in II-VI-based nanocomposite thin films

Nanocomposite CdS-ZnO thin films deposited directly on quartz substrates by high-pressure magnetron sputtering show a completely reversible photodarkening at a very low threshold intensity (∼1kWcm−2), and a moderately fast recovery time (<1ms). This makes them ideal in optical limiting applications for both continuous wave as well as high rep-rate pulsed lasers. The same system also shows an intensity-dependent quenching of the photoluminescence. Using a pump-probe experiment, we show that photodarkening in such a quantum-dot thin film originates from excited state absorption.

Proximity effect in Nb/Zr multilayers with variable Nb/Zr ratio

Abstract We have investigated the effect of varying the individual layer thickness on the superconducting transition temperature (TC) of Nb/Zr multilayers. These thin film multilayer structures were deposited using UHV magnetron sputtering with layer thickness ranging from 0.5 to 8 nm. In conformity with the predictions of the de Gennes equations in the Cooper limit (layer thickness small compared to coherence length), we find that the TC increases with increasing thickness of the Nb layer (when the Zr layer thickness is constant), and decreases with increasing thickness of the Zr layer (when the Nb layer thickness is constant). The possible effect of the existence of an interfacial Nb–Zr layer is discussed. We also point out the marked influence of the in-plane grain dimension on the TC in these multilayers.

Infrared emission from the substrate of GaAs-based semiconductor lasers

We report on the origin of three additional low-energy spontaneously emitted bands in GaAs-based broad-area laser diodes. Spectrally and spatially resolved scanning optical microscopy and Fourier-transform infrared spectroscopy assign the different contributions to bandtail-related luminescence from the gain region as well as interband and deep-level-related luminescences from the GaAs substrate. The latter processes are photoexcited due to spontaneous emission from the active region followed by a cascaded photon-recycling process within the substrate.