Aaron L. Holsteen

Diameter and Polarization-Dependent Raman Scattering Intensities of Semiconductor Nanowires

Diameter-dependent Raman scattering in single tapered silicon nanowires is measured and quantitatively reproduced by modeling with finite-difference time-domain simulations. Single crystal tapered silicon nanowires were produced by homoepitaxial radial growth concurrent with vapor-liquid-solid axial growth. Multiple electromagnetic resonances along the nanowire induce broad band light absorption and scattering. Observed Raman scattering intensities for multiple polarization configurations are reproduced by a model that accounts for the internal electromagnetic mode structure of both the exciting and scattered light. Consequences for the application of Stokes to anti-Stokes intensity ratio for the estimation of lattice temperature are discussed.

Extraordinary dynamic mechanical response of vanadium dioxide nanowires around the insulator to metal phase transition.

Nanomechanical resonators provide a compelling platform to investigate and exploit phase transitions coupled to mechanical degrees of freedom because resonator frequencies and quality factors are exquisitely sensitive to changes in state, particularly for discontinuous changes accompanying a first-order phase transition. Correlated scanning fiber-optic interferometry and dual-beam Raman spectroscopy were used to investigate mechanical fluctuations of vanadium dioxide (VO2) nanowires across the first order insulator to metal transition. Unusually large and controllable changes in resonator frequency were observed due to the influences of domain wall motion and anomalous phonon softening on the effective modulus. In addition, extraordinary static and dynamic displacements were generated by local strain gradients, suggesting new classes of sensors and nanoelectromechanical devices with programmable discrete outputs as a function of continuous inputs.

Gap Plasmon Resonance in a Suspended Plasmonic Nanowire Coupled to a Metallic Substrate

We present an experimental demonstration of nanoscale gap plasmon resonators that consist of an individual suspended plasmonic nanowire (NW) over a metallic substrate. Our study demonstrates that the NW supports strong gap plasmon resonances of various gap sizes including single-nanometer-scale gaps. The obtained resonance features agree well with intuitive resonance models for near- and far-field regimes. We also illustrate that our suspended NW geometry is capable of constructing plasmonic coupled systems dominated by quasi-electrostatics.

Probing the electrical switching of a memristive optical antenna by STEM EELS.

The scaling of active photonic devices to deep-submicron length scales has been hampered by the fundamental diffraction limit and the absence of materials with sufficiently strong electro-optic effects. Plasmonics is providing new opportunities to circumvent this challenge. Here we provide evidence for a solid-state electro-optical switching mechanism that can operate in the visible spectral range with an active volume of less than (5 nm)3 or ∼10−6 λ3, comparable to the size of the smallest electronic components. The switching mechanism relies on electrochemically displacing metal atoms inside the nanometre-scale gap to electrically connect two crossed metallic wires forming a cross-point junction. These junctions afford extreme light concentration and display singular optical behaviour upon formation of a conductive channel. The active tuning of plasmonic antennas attached to such junctions is analysed using a combination of electrical and optical measurements as well as electron energy loss spectroscopy in a scanning transmission electron microscope.

Light-matter interaction controlled by electrochemically manufactured micro-optics

Gradient refractive index (GRIN) optical elements use spatially graded refractive index profiles to make light travel in a physically curved path.1 By introducing curved pathways for light, GRINs offer an opportunity to decouple the optical function of an element from its physical shape, enabling optical elements such as carpet cloaks2, 3 and flat lenses.4 Flat, siliconbased GRIN microlenses are useful within photonic integrated circuits,4 while the presence of birefringence affords GRIN elements the ability to perform distinct optical functions depending on the polarization of the input light field.5 Unfortunately, realization of such elements has so far relied on slow, serial nanostructuring of silicon that typically confines these GRINs to thin, 2D architectures.2–4, 6, 7 Porous silicon (PSi) represents another manifestation of nanostructured silicon. PSi—formed by electrochemically etching a flat silicon wafer in a hydrofluoric acid electrolyte— is a highly promising option for GRIN applications,8, 9 as its nanoscale porosity (i.e., void fraction), and thus effective refractive index, is defined by the etching current density and can be varied along the 1D etch path during fabrication.10, 11 A 2D or 3D GRIN can be formed by way of a spatially varied current density.12 The current-density-dependent etch rate of PSi inhibits flat elements,13, 14 however, while current spreading leaves sharp GRIN profiles inaccessible. We have found that a more versatile approach is to start with a silicon wafer that features preformed microstructures. These microstructures serve as the starting point for the lateral etching that is required to form PSi 3D GRIN micro-optics (see Figure 1, left).15, 16 Shape-defined PSi formation begins with a p-type Figure 1. Left: Schematic depicting silicon (Si) square columns (SCs) being electrochemically etched into porous silicon (PSi) micro-optics. Right: Side-view scanning electron micrograph showing that n-type silicon is unperturbed during etching and that PSi merely forms beneath the n-silicon capping layer. Pt: Platinum. HF/EtOH: Hydrofluoric acid/ethanol.