Photonic-plasmonic linear and non-linear coupling

Glass is transparent. Metals are shiny. A piece of wood is opaque. Each material has a different relationship with light. Some materials let light go through; others reflect back; others absorb it. This is true also at the nanoscale. In addition, optical properties at the nanoscale are even more interesting. Imagine if your gold ring would change its color if you break it in half, or if you change its size anyhow. In fact, this is what happens for nano-sized metals and semiconductors. In particular, metals can act as an antenna for light. Gold nanopads have the ability to redirect light in specific directions and locations. Semiconductors change their response to light depending on its size and shape. What about coupling the two systems?

Within this research field, I investigated the optical properties of new semiconductor architectures (V-shaped membranes). Fabrication of these structures is very challenging and we did it in collaboration with Prof. Fontcuberta group at EPFL. I studied the their optical properties, analyzing the change in the scattering spectra as the size is changed. I used second harmonic generation to probe the non-linear properties of such nanostructures, finding very interesting correlation between structural properties and optical behavior.

Second harmonic excitation spectroscopy is a very powerful too to investigate material properties. I used it to characterize substoichiometric silicon nitride thin films, to elucidate size-dependent effects on gold nanoparticles, and to obtain polarization-controlled multispectral nanofocusing of metal nanoantennas.

Moreover, I investigated the coupling between photonic and plasmonic properties. In collaboration with EPFL, we designed gold nanoantenna arrays coupled with gallium arsenide nanowires. By using second harmonic excitation spectroscopy, we elucidated all the coupling effects in these systems and we showed that new modes emerge at expenses of the expected structural resonances.

These projects have been funded by the Air Force Office of Scientific Research, and performed during my experience at Boston University.

Publication output

Conference proceedings

  • Integration of metallic nanostructures on nanowires for modification of their optical properties
    A. Casadei, E. Alarcon-Llado, E. F. Pecora, J. Trevino, C. Forestiere, D. Ruffer, E. Russo-Averchi, F. Matteini, G. Tutuncuoglu, M. Heiss, L. Dal Negro, A. Fontcuberta i Morral
    Frontiers in Nanophotonics, CSF Conference 2015
  • Second harmonic excitation spectroscopy in studies of Fano-type coupling in plasmonic arrays
    G. F. Walsh, J. Tervino, E. F. Pecora, L. Dal Negro
    SPIE Optics + Photonics 2015
  • Engineering light coupling in single nanowire with metal nano-antennas
    A. Casadei, J. Trevino, E. F. Pecora, E. Alarcò- Lladò, D. Ruffer, E. Russo-Averchi, G. Tutuncuoglu, F. Matteini, C. Forestiere, L. Dal Negro, A. Fontcuberta i Morral
    International Conference on One dimensional Nanomaterials ICON 2013
  • Second-harmonic generation from plasmonic nanoantennas and arrays
    A. Capretti, C. Forestiere, E. F. Pecora, G. Walsh, J. Trevino, S. Minissale, L. Dal Negro, G. Miano
    The International Conference on Surface Plasmon Photonics SPP6
  • Second-harmonic generation in substoichiometric silicon nitride layers
    E. F. Pecora, A. Capretti, G. Miano, L. Dal Negro
    Bulletin of the American Physical Society, vol. 58, V1.00119

Photonic-Plasmonic Coupling of GaAs Single Nanowires to Optical Nanoantennas

We successfully demonstrate the plasmonic coupling between metal nanoantennas and individual GaAs nanowires (NWs). In particular, by using dark-field scattering and second harmonic excitation spectroscopy in partnership with analytical and full-vector FDTD modeling, we demonstrate controlled electromagnetic coupling between individual NWs and plasmonic nanoantennas with gap sizes varied between 90 and 500 nm. The significant electric field enhancement values (up to 20×) achieved inside the NW-nanoantennas gap regions allowed us to tailor the nonlinear optical response of NWs by engineering the plasmonic near-field coupling regime. These findings represent an initial step toward the development of coupled metal–semiconductor resonant nanostructures for the realization of next generation solar cells, detectors, and nonlinear optical devices with reduced footprints and energy consumption.

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Generation of second harmonic radiation from sub-stoichiometric silicon nitride thin films

Enhancing second-order optical processes in Si-compatible materials is important for the demonstration of innovative functionalities and nonlinear optical devices integrated on a chip. Here, we demonstrate significantly enhanced Second-Harmonic Generation (SHG) by silicon-rich silicon nitride materials over a broad spectral range, and show a maximum conversion efficiency of 4.5 x 10-6 for sub-stoichiometric samples with 46 at. % silicon. The SHG process in silicon nitride thin films is systematically investigated over a range of material stoichiometry and thermal annealing conditions. These findings can enable the engineering of innovative Si-based devices for nonlinear signal processing and sensing applications on a Si platform.

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Vertical III-V V-shaped membranes epitaxially grown on a patterned Si[001] substrate and their enhanced light scattering

We report on a new form of III–V compound semiconductor nanostructures growing epitaxially as vertical V-shaped nanomembranes on Si(001) and study their light-scattering properties. Precise position control of the InAs nanostructures in regular arrays is demonstrated by bottom-up synthesis using molecular beam epitaxy in nanoscale apertures on a SiO2 mask. The InAs V-shaped nanomembranes are found to originate from the two opposite facets of a rectangular pyramidal island nucleus and extend along two opposite 111 B directions, forming flat {110} walls. Dark-field scattering experiments, in combination with light-scattering theory, show the presence of distinctive shape-dependent optical resonances significantly enhancing the local intensity of incident electromagnetic fields over tunable spectral regions. These new nanostructures could have interesting potential in nanosensors, infrared light emitters, and nonlinear optical elements.

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Rare-earth doped Si-rich ZnO for multiband near-infrared light emitting devices

We demonstrate a light emitting material platform based on rare-earth doping of Si-rich ZnO thin films by magnetron sputtering, and we investigate the near-infrared emission properties under both optical and electrical injection. Er and Nd radiative transitions were simultaneously activated due to energy transfer via the ZnO direct bandgap and its luminescent defect centers. Moreover, by incorporating Si atoms, we demonstrate Si-mediated enhancement of photoluminescence in Er-doped ZnO and electroluminescence. These results pave the way to novel Si-compatible light emitters that leverage the optically transparent and electrically conductive ZnO matrix for multiband near-IR telecom and bio-compatible applications.

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Nanopatterning of silicon nanowires for enhancing visible photoluminescence

Silicon Nanowires prepared by Metal-Assisted Chemical Etching have been nanopatterned into periodic and aperiodic array geometries displaying functionality at visible wavelengths using top-down planar processing techniques. Broadband photoluminescense enhancement up to approximately one order of magnitude is measured from golden-angle spiral arrays over a wide parameter space.

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Sub-250nm room-temperature optical gain from AlGaN/AlN multiple quantum wells with strong band-structure potential fluctuations

Deep-UV optical gain has been demonstrated in Al0.7Ga0.3N/AlN multiple quantum wells under femtosecond optical pumping. Samples were grown by molecular beam epitaxy under a growth mode that introduces band structure potential fluctuations and high-density nanocluster-like features within the AlGaN wells. A maximum net modal gain value of 118 cm-1 has been measured and the transparency threshold of 5 microJ/cm2 was experimentally determined, corresponding to 1.4 x 1017 cm-3 excited carriers. These findings pave the way for the demonstration of solid-state lasers with sub-250 nm emission at room temperature.

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