Daniel Ratchford

Photoinduced tunability of the Reststrahlen band in 4H-SiC

Materials with a negative dielectric permittivity (e.g., metals) display high reflectance and can be shaped into nanoscale optical resonators exhibiting extreme mode confinement, a central theme of nanophotonics. However, the ability to actively tune these effects remains elusive. By photoexciting free carriers in

Fabrication of metallic nanodisc hexagonal arrays using nanosphere lithography and two-step lift-off

4Hensuremath{-}mathrm{SiC}

Active tuning of surface phonon polariton resonances via carrier photoinjection

, we induce dramatic changes in reflectance near the ``reststrahlen band where the permittivity is negative due to charge oscillations of the polar optical phonons in the midinfrared. We infer carrier-induced changes in the permittivity required for useful tunability

ZnO nanowire and silicon nanocrystal heterostructures for photocatalytic applications

(ensuremath{sim}40phantom{rule{0.16em}{0ex}}mathrm{c}{mathrm{m}}^{ensuremath{-}1})

Pitch Control of Hexagonal Non-Close-Packed Nanosphere Arrays Using Isotropic Deformation of an Elastomer

in nanoscale resonators, providing a direct avenue towards the realization of actively tunable nanophotonic devices in the midinfrared to terahertz spectral range.

Silicon nanowire arrays for the preconcentration and Joule heating-based desorption of trace vapors

Nanosphere lithography (NSL) has been widely used as an inexpensive method to create periodic arrays of metallic nanoparticles or nanodiscs on substrates. However, most nanodisc arrays derived from a NSL template are restricted to hexagonally-ordered triangular arrays because the metal layer is deposited onto the interstices between the nanospheres. Metallic nanodisc arrays with the same arrangement as the original nanosphere array have been rarely reported. Here, we demonstrate a facile, low-cost method to fabricate large-area hexagonal arrays of metallic nanodiscs using an NSL template combined with a two-step lift-off process. We employ a bi-layer of two dissimilar metals to create a re-entrant sidewall profile to undercut the sacrificial layer and facilitate the final lift-off of the metallic nanodiscs. The quality of the nanodisc pattern and the array periodicity is determined using statistical image analysis and compared to the original nanosphere array in terms of size distribution, surface smoothness, and array pitch. This nanodisc array is used as an etch mask to create a vertically-aligned Si nanowire array. This combined approach is a scalable and inexpensive fabrication method for creating relatively large-area, ordered arrays of various nanostructures.

Probing carrier dynamics in wide-bandgap semiconductor-metal nanoparticle hybrids(Conference Presentation)

Surface phonon polaritons (SPhPs) are attractive alternatives to infrared plasmonics for subdiffractional confinement of infrared light. Localized SPhP resonances in semiconductor nanoresonators are narrow, but that linewidth and the limited extent of the Reststrahlen band limit spectral coverage. To address this limitation, we report active tuning of SPhP resonances in InP and 4H-SiC by photoinjecting free carriers into nanoresonators, taking advantage of the coupling between the carrier plasma and optic phonons to blueshift SPhP resonances. We demonstrate state-of-the-art tuning figures of merit upon continuous-wave excitation (in InP) or pulsed excitation (in 4H-SiC). Lifetime effects cause the tuning to saturate in InP, and carrier redistribution leads to rapid (<50u2009ps) recovery of the resonance in 4H-SiC. This work demonstrates the potential for this method and opens a path towards actively tuned nanophotonic devices, such as modulators and beacons, in the infrared, and identifies important implications of coupling between electronic and phononic excitations.Infrared surface phonon polariton tuning is achieved by photoinjecting free carriers into resonators.

Understanding carrier injection effects upon the Reststrahlen band of SiC using transient infrared spectroscopy (Presentation Recording)

This work reports the synthesis and materials characterization of novel heterogeneous nanostructures consisting of zinc oxide (ZnO) nanowires and silicon (Si) nanocrystals. Hydrothermal and non-thermal plasma synthesis techniques were utilized to create ZnO nanowires and Si nanocrystals, respectively. The heterostructures of ZnO nanowires and Si nanocrystals were formed and characterized using scanning electron microscopy, transmission electron microscopy, the Raman spectroscopy, and photoluminescence. The heterostructures have been evaluated for the photodegradation performance, demonstrating that the addition of Si nanocrystals to ZnO nanostructures improves the degradability under visible light.

Decoupling Diameter and Pitch in Silicon Nanowire Arrays Made by Metal-Assisted Chemical Etching

Self-assembly of colloidal nanospheres combined with various nanofabrication techniques produces an ever-increasing range of two-dimensional (2D) ordered nanostructures, although the pattern periodicity is typically bound to the original interparticle spacing. Deformable soft lithography using controlled deformation of elastomeric substrates and subsequent contact printing transfer offer a versatile method to systematically control the lattice spacing and arrangements of the 2D nanosphere array. However, the anisotropic nature of uniaxial and biaxial stretching as well as the strain limit of solvent swelling makes it difficult to create well-separated, ordered 2D nanosphere arrays with large-area hexagonal arrangements. In this paper, we report a simple, facile approach to fabricate such arrays of polystyrene nanospheres using a custom-made radial stretching apparatus. The maximum stretchability and spatial uniformity of the poly(dimethylsiloxane) (PDMS) elastomeric substrate is systematically investigated. A pitch increase as large as 213% is demonstrated using a single stretching-and-transfer process, which is at least 3 times larger than the maximum pitch increase achievable using a single swelling-and-transfer process. Unlike the colloidal arrays generated by the uniaxial and biaxial stretching, the isotropic expansion of radial stretching allows the hexagonal array to retain its original structure across the entire substrate. Upon radial strain applied to the PDMS sheet, the nanosphere array with modified pitch is transferred to a variety of target substrates, exhibiting different optical behaviors and serving as an etch mask or a template for molding.

Influence of inhomogeneous porosity on silicon nanowire Raman enhancement and leaky mode modulated photoluminescence

Silicon nanowire (SiNW) arrays are demonstrated as a suitable substrate for the preconcentration of trace nitroaromatic compounds and subsequent desorption via Joule heating of the array. Arrays are fabricated from Si wafers containing epitaxially grown layers of high conductivity p-type Si, with a relatively low conductivity intrinsic Si layer. Arrays are fabricated using a combination of nanosphere lithography (NSL) and metal-assisted chemical etching (MACE). The resulting arrays consist of ordered Si nanowires. The individual wires have a diameter of approximately 350 nm (center-to-center spacing of 500 nm) and lengths of ∼5 μm. Each SiNW array chip has approximately 108 individual wires. The temperature-dependent shift of the crystalline Si Raman single phonon line was used to estimate the temperature of the array during desorption, with temperatures in excess of 200°C observed. The effectiveness of the adsorption and desorption of trace 2,4-dinitrotoluene (2,4-DNT) was evaluated by coupling the array to a mass spectrometer.