R. Niall Tait

Surface plasmon waveguide Schottky detector

A surface plasmon polariton detector is demonstrated at infra-red wavelengths. The device consists of a metal stripe on silicon forming a Schottky contact thereon and supporting surface a plasmon polariton mode that is strongly confined and localised to the metal-semiconductor interface. Detection of optical radiation below the bandgap of silicon (at infrared wavelengths) occurs through internal photoemission. Responsivities of 0.38 and 1.04 mA/W were measured via end-fire coupling to a tapered optical fibre, at room temperature and at a wavelength of 1280 nm, for gold and aluminium stripes on n-type silicon, respectively. The device can be integrated with other structures used in nano-plasmonics, nano-photonics or silicon-based photonics, and it holds promise for short-reach optical interconnects and power monitoring applications.

Biosensing using straight long-range surface plasmon waveguides

Straight long-range surface plasmon waveguides are demonstrated as biosensors for the detection of cells, proteins and changes in the bulk refractive index of solutions. The sensors consist of 5 μm wide 22 nm thick Au stripes embedded in polymer (CYTOP™) with microfluidic channels etched into the top cladding. Bulk sensing is demonstrated by sequentially injecting six solutions of different refractive indices in 2 × 10(-3) RIU increments; such index steps were detected with a signal-to-noise ratio of ~1000. Selective capture of cells is demonstrated using Au waveguides functionalized with antibodies against blood group A, and red blood cells of group A and O in buffer as positive and negative analyte. Bovine serum albumin in buffer was used to demonstrate protein sensing. A monolayer of bovine serum albumin physisorbed on a carboxyl-terminated self-assembled monolayer on Au was detected with a signal-to-noise ratio of ~300. Overall, the biosensor demonstrated a good capability for detecting bulk changes in solution and for sensing analyte over a very wide range of mass (from cells to proteins). The biosensors are compact, inexpensive to fabricate, and may find use over a wide range of cost-sensitive sensing and detection applications.

Fabrication of surface plasmon waveguides and devices in Cytop with integrated microfluidic channels

Long range surface plasmon-polariton waveguides and devices suitable for biosensing were fabricated and characterized physically and optically. The structures consist of thin (∼35 nm) patterned Au stripes embedded in thick Cytop claddings (∼8 μm each). Portions of Au stripes were exposed by patterning and etching though the top Cytop cladding using an O2 plasma etch. The etched Cytop cavities act as microfluidic channels to contain and direct the sensing fluid. Intermediate process steps were verified through physical characterization as were fully fabricated structures. Optical testing was performed on Cytop-embedded structures and on channel-filled (with sensing fluid) structures. The structures were excited through end-fire coupling to optical fibers.Long range surface plasmon-polariton waveguides and devices suitable for biosensing were fabricated and characterized physically and optically. The structures consist of thin (∼35 nm) patterned Au stripes embedded in thick Cytop claddings (∼8 μm each). Portions of Au stripes were exposed by patterning and etching though the top Cytop cladding using an O2 plasma etch. The etched Cytop cavities act as microfluidic channels to contain and direct the sensing fluid. Intermediate process steps were verified through physical characterization as were fully fabricated structures. Optical testing was performed on Cytop-embedded structures and on channel-filled (with sensing fluid) structures. The structures were excited through end-fire coupling to optical fibers.

Optical selection, manipulation, trapping, and activation of a microgear structure for applications in micro-optical-electromechanical systems.

The optical processes involved in laser trapping and optical manipulation are explored theoretically and experimentally as a means of activating a micrometer-size gear structure. We modeled the structure by using an enhanced ray-optics technique, and results indicate that the torque present on the gear can induce the gear to rotate about the gear-arm plane center with light as the driving energy source. We confirmed these findings experimentally by using gears manufactured with conventional semiconductor techniques and from a layer of polyimide. It is expected that such a simple gear design activated by use of light could lead to an entire new class of micro-optical-electromechanical systems.

Modeling electroosmotic and pressure-driven flows in porous microfluidic devices: Zeta potential and porosity changes near the channel walls

This work presents analytical solutions for both pressure-driven and electroosmotic flows in microchannels incorporating porous media. Solutions are based on a volume-averaged flow model using a scaling of the Navier-Stokes equations for fluid flow. The general model allows analysis of fluid flow in channels with porous regions bordering open regions and includes viscous forces, permitting consideration of porosity and zeta potential variations near channel walls. To obtain analytical solutions problems are constrained to the linearized Poisson-Boltzmann equation and a variation of Brinkmans equation [Appl. Sci. Res., Sect. A 1, 27 (1947); 1, 81 (1947)]. Cases include one continuous porous medium, two adjacent regions of different porosities, or one open channel adjacent to a porous region, and the porous material may have a different zeta potential than that of the channel walls. Solutions are described for two geometries, including flow between two parallel plates or in a cylinder. The model illustrates the relative importance of porosity and zeta potential in different regions of each channel.

Plasmonic Nanostructured Metal–Oxide–Semiconductor Reflection Modulators

We propose a plasmonic surface that produces an electrically controlled reflectance as a high-speed intensity modulator. The device is conceived as a metal-oxide-semiconductor capacitor on silicon with its metal structured as a thin patch bearing a contiguous nanoscale grating. The metal structure serves multiple functions as a driving electrode and as a grating coupler for perpendicularly incident p-polarized light to surface plasmons supported by the patch. Modulation is produced by charging and discharging the capacitor and exploiting the carrier refraction effect in silicon along with the high sensitivity of strongly confined surface plasmons to index perturbations. The area of the modulator is set by the area of the incident beam, leading to a very compact device for a strongly focused beam (∼2.5 μm in diameter). Theoretically, the modulator can operate over a broad electrical bandwidth (tens of gigahertz) with a modulation depth of 3 to 6%, a loss of 3 to 4 dB, and an optical bandwidth of about 50 nm. About 1000 modulators can be integrated over a 50 mm(2) area producing an aggregate electro-optic modulation rate in excess of 1 Tb/s. We demonstrate experimentally modulators operating at telecommunications wavelengths, fabricated as nanostructured Au/HfO2/p-Si capacitors. The modulators break conceptually from waveguide-based devices and belong to the same class of devices as surface photodetectors and vertical cavity surface-emitting lasers.

Broadside excitation of surface plasmon waveguides on Cytop

Long-range surface plasmon waveguides consisting of a Au stripe on Cytop, covered with an index-matched aqueous solution, are described and characterized. The waveguides are tested using a broadside coupling technique, whereby tapered single-mode fibers are positioned in direct contact with the stripe such that the slow mode of the fiber couples through partial modal overlap to the long-range surface plasmon. Attenuation measurements obtained at λ0=1310 nm agree well with theory, thus validating the waveguide fabrication and experimental techniques. The waveguides are useful for (bio)chemical sensing and the broadside coupling technique is useful for on-wafer optical probing.

Activation of microcomponents with light for micro-electro-mechanical systems and micro-optical-electro-mechanical systems applications

We examine the light-activation properties of micrometer-sized gear structures fabricated with polysilicon surface micromachining techniques. The gears are held in place on a substrate through a capped anchor post and are free to rotate about the post. The light-activation technique is modeled on photon radiation pressure, and the equation of motion of the gear is solved for this activation technique. Experimental measurements of torque and damping are found to be consistent with expected results for micrometer-scale devices. Design optimization for optically actuated microstructures is discussed.

Near infrared amplified spontaneous emission in a dye-doped polymeric waveguide for active plasmonic applications

Near-infrared amplified spontaneous emission (ASE) from an optically-pumped dye-doped polymeric slab waveguide, consisting of IR-140 in PMMA on a glass substrate, has been characterised. The ASE gain was measured using the variable stripe length method. Linewidth narrowing with increasing pump intensity was observed, indicating ASE gain in this material. The effects of the dye concentration and pump intensity on the gain were investigated under linear operation. The maximum achieved gain coefficient is γ ~68 cm(-1) for a film with 0.8 wt % of IR-140 to PMMA for a pump intensity of 43.4 mJ/cm(2). The polarisation dependence of the ASE gain was also investigated by measuring the gain coefficient of orthogonal TE and TM modes and varying the pump polarisation relative to the amplifier length. It was observed that there is some degree of gain anisotropy when the pump polarisation is aligned perpendicular to the length, but that the gain was isotropic when the pump polarisation is aligned parallel the length. The applicability of IR-140 doped PMMA for active plasmonic applications is discussed.

Fabrication of surface plasmon waveguides on thin CYTOP membranes

The fabrication of a membrane supported long-range surface plasmon polariton waveguide for use as a biosensor is described. This type of device can be completely immersed in the sensing environments to create symmetrical waveguide surroundings. The membrane is created from the fluoropolymer CYTOP which has strong physical properties and a low index of refraction. The fabrication steps of this device are described along with examples of completed structures.