Judson D. Ryckman

Label-free porous silicon membrane waveguide for DNA sensing

We report a label-free porous silicon membrane waveguide biosensor based on a 1μm thick freestanding porous silicon film with 100nm diameter pores. The sensor operates in the Kretschmann configuration. A formvar polymer film provides robust adhesion of the porous silicon membrane to a rutile prism and enables confinement of guided modes in the porous silicon membrane. Attenuated total reflectance measurements are performed, along with theoretical calculations, to fully characterize the waveguide. The sensitivity of the sensor is investigated through DNA hybridization in the porous silicon membrane. A detection limit of 42nM was demonstrated for 24-base pair DNA oligonucleotides.

Patterned nanoporous gold as an effective SERS template

We demonstrate large area two-dimensional arrays of patterned nanoporous gold for use as easy-to-fabricate, cost-effective, and stable surface enhanced Raman scattering (SERS) templates. Using a simple one-step direct imprinting process, subwavelength nanoporous gold (NPG) gratings are defined by densifying appropriate regions of a NPG film. Both the densified NPG and the two-dimensional grating pattern are shown to contribute to the SERS enhancement. The resulting substrates exhibit uniform SERS enhancement factors of at least 10(7) for a monolayer of adsorbed benzenethiol molecules.

Ultra-compact silicon photonic devices reconfigured by an optically induced semiconductor-to-metal transition

Vanadium dioxide (VO(2)) is a promising reconfigurable optical material and has long been a focus of condensed matter research owing to its distinctive semiconductor-to-metal phase transition (SMT), a feature that has stimulated recent development of thermally reconfigurable photonic, plasmonic, and metamaterial structures. Here, we integrate VO(2) onto silicon photonic devices and demonstrate all-optical switching and reconfiguration of ultra-compact broadband Si-VO(2) absorption modulators (L < 1 μm) and ring-resonators (R ~ λ(0)). Optically inducing the SMT in a small, ~0.275 μm(2), active area of polycrystalline VO(2) enables Si-VO(2) structures to achieve record values of absorption modulation, ~4 dB μm(-1), and intracavity phase modulation, ~π/5 rad μm(-1). This in turn yields large, tunable changes to resonant wavelength, |Δλ(SMT)| ~ 3 nm, approximately 60 times larger than Si-only control devices, and enables reconfigurable filtering and optical modulation in excess of 7 dB from modest Q-factor (~10(3)), high-bandwidth ring resonators (>100 GHz). All-optical integrated Si-VO(2) devices thus constitute platforms for reconfigurable photonics, bringing new opportunities to realize dynamic on-chip networks and ultrafast optical shutters and modulators.

Porous silicon structures for low-cost diffraction-based biosensing

We present a strategy for label-free biosensing using porous silicon diffraction gratings. The gratings are fabricated using a cost-effective, high-throughput stamping technique. Unlike traditional diffraction-based biosensors that rely on microcontact printing or lithography to create gratings for the localization of analytes on the top surface of the grating, in our structure analytes are free to infiltrate the porous network and increase the effective refractive index of the grating. The large surface area of porous silicon available for molecular binding offers the potential for enhanced diffraction response compared to nonporous gratings with limited surface area. Small molecule detection of 3-aminopropyltriethoxysilane is demonstrated.

Photothermal optical modulation of ultra-compact hybrid Si-VO 2 ring resonators

We demonstrate photothermally induced optical switching of ultra-compact hybrid Si-VO₂ ring resonators. The devices consist of a sub-micron length ~70 nm thick patch of phase-changing VO₂ integrated onto silicon ring resonators as small as 1.5 μm in radius. The semiconductor-to-metal transition (SMT) of VO₂ is triggered using a 532 nm pump laser, while optical transmission is probed using a tunable cw laser near 1550 nm. We observe optical modulation greater than 10dB from modest quality-factor (~10³) resonances, as well as a large -1.26 nm change in resonant wavelength Δλ, resulting from the large change in the dielectric function of VO₂ in the insulator-to-metal transition achieved by optical pumping.

Low mode volume slotted photonic crystal single nanobeam cavity

We demonstrate a low mode volume slotted photonic crystal single nanobeam cavity in silicon. Ultra-small effective mode volumes, ~0.025(λ/n)³, an order of magnitude smaller than traditional nanobeam cavities, are achieved while maintaining Q-factors near 10⁴.

Localized Field Enhancements in Guided and Defect Modes of a Periodic Slot Waveguide

We present a periodic slot waveguide for achieving enhanced light-matter interaction that provides significant localized field and power density enhancements over traditional slot waveguides. The basic structure is based on a slot waveguide with 1-D periodic holes. The slot effect provides strong field enhancement and subwavelength confinement, and the periodicity of the structure is exploited to locally magnify or “pinch” the electric field distribution, resulting in additional enhancements. Characteristics of the modes presented by this structure are examined by finite-difference time-domain (FDTD) modeling. Distinct optical gradients and localized enhancements, which are up to 4-5 times greater than comparable slot waveguides, can be achieved. Potential application of the periodic slot waveguide structure to fields, including optical manipulation, sensing, and nonlinear or active material integration, is discussed.

A size selective porous silicon grating-coupled Bloch surface and sub-surface wave biosensor

A porous silicon (PSi) grating-coupled Bloch surface and sub-surface wave (BSW/BSSW) biosensor is demonstrated to size selectively detect the presence of both large and small molecules. The BSW is used to sense large immobilized analytes at the surface of the structure while the BSSW that is confined inside but near the top of the structure is used to sensitively detect small molecules. Functionality of the BSW and BSSW modes is theoretically described by dispersion relations, field confinements, and simulated refractive index shifts within the structure. The theoretical results are experimentally verified by detecting two different small chemical molecules and one large 40 base DNA oligonucleotide. The PSi-BSW/BSSW structure is benchmarked against current porous silicon technology and is shown to have a 6-fold higher sensitivity in detecting large molecules and a 33% improvement in detecting small molecules. This is the first report of a grating-coupled BSW biosensor and the first report of a BSSW propagating mode.

Three-dimensional patterning and morphological control of porous nanomaterials by gray-scale direct imprinting

We present a method for direct three-dimensional (3D) patterning of porous nanomaterials through the application of a premastered and reusable gray-scale stamp. Four classes of 3D nanostructures are demonstrated for the first time in porous media: gradient profiles, digital patterns, curves and lens shapes, and sharp features including v-grooves, nano-pits, and ‘cookie-cutter’ particles. Further, we demonstrate this technique enables morphological tuning and direct tailoring of nanomaterial properties, including porosity, average pore size, dielectric constant, and plasmonic response. This work opens a rapid and low-cost route for fabricating novel nanostructures and devices utilizing porous nanomaterials, with promising applications spanning diffractive and plasmonic sensing, holography, micro- and transformation optics, and drug delivery and imaging.

Controlling surface enhanced Raman scattering using grating-type patterned nanoporous gold substrates

We investigate the effect of grating design on surface enhanced Raman scattering (SERS) intensity using patterned nanoporous gold (P-NPG) films. The SERS response is systematically engineered by tuning the parameters of gratings imprinted on the nanoporous gold films, including grating period, duty cycle, and height. Compared to conventional NPG films, where the localized surface plasmon dominates the strong SERS response, the significantly enhanced SERS response from our P-NPG structures primarily arises from efficient activation of a surface plasmon polariton and its coupling with the localized surface plasmon mode. The P-NPG SERS substrates exhibit large area uniform enhancement factors near 108.