Improved optical imaging of high aspect ratio nanostructures using dark-field microscopy.

Improvements to white light optical imaging of widely spaced, high aspect ratio nanostructures are demonstrated using dark-field field microscopy. 1D models of bright- and dark-field imaging are developed from rigorous modal diffraction theory by assuming that features are periodic. A simple model is developed to explain dark field results and simulated line images obtained using the two modalities are compared for different dimensions and materials. Increased contrast between etched features and the substrate is demonstrated in dark field, due to its reduced sensitivity to scattering from flat areas. The results are verified using silicon nanostructures fabricated by sidewall transfer lithography, and feature separation with improved tolerance to apparent substrate brightness is demonstrated during image segmentation using the Otsu method.

Tailored fibre waveguides for precise two-axis Lissajous scanning.

A two-axis optical imaging system using a Lissajous scan pattern with non-integer frequency ratio is presented. A waveguide with precisely tuned mechanical resonant frequencies is constructed by dip coating two fibres with a transparent polymer. Motion is achieved by mounting a waveguide cantilever at 45° on a single piezoelectric actuator with a dual-frequency drive. Confocal signal collection is achieved using a mode-stripping detector, and feedback signals needed for frequency and phase locking are derived from intermittent reflection from an apertured mirror. The first scan axis is locked to the resonance of one of the modes, while the second scan axis is locked to the correct phase at the desired frequency ratio. Accurate acquisition of two-dimensional images is demonstrated.

Resonant fiber scanner with optical feedback.

The addition of an apertured mirror before the imaging lens is proposed as a method of providing feedback in a single-axis resonant fiber scanner. Reflection at the scan extremities generates timing signals interlaced with back-scattered data, and a phase locked loop and a proportional controller then adjust the drive frequency and amplitude. The capture range and stability of the system are examined. Verification is obtained using a confocal scanner based on mechanically biaxial fiber.

Folded dipole plasmonic resonators.

A class of folded ordered plasmonic dipole nanoresonators based on insulator-metal-insulator (IMI) slab waveguides is proposed and studied. This work is motivated by the development of a novel fabrication process that avoids the need for direct write nanolithography and instead relies on accessible UV lithography and other top-down parallel fabrication techniques that result in metallic dolmen structures with nanometre sized gaps. In this context, the dolmen geometry consists of two vertical segments supporting a flat horizontal slab. It is shown using frequency domain finite element analysis that such structures, which are essentially folded dipole antennas, resonate in a similar manner to their linear unfolded counterparts. The effect of the likely fabrication features is also studied.

Distributed gain in plasmonic reflectors and its use for terahertz generation.

Semiconductor plasmons have potential for terahertz generation. Because practical device formats may be quasi-optical, we studied theoretically distributed plasmonic reflectors that comprise multiple interfaces between cascaded two-dimensional electron channels. Employing a mode-matching technique, we show that transmission through and reflection from a single interface depend on the magnitude and direction of a dc current flowing in the channels. As a result, plasmons can be amplified at an interface, and the cumulative effect of multiple interfaces increases the total gain, leading to plasmonic reflection coefficients exceeding unity. Reversing the current direction in a distributed reflector, however, has the opposite effect of plasmonic deamplification. Consequently, we propose structurally asymmetric resonators comprising two different distributed reflectors and predict that they are capable of terahertz oscillations at low threshold currents.

Multilevel self-aligned microcontact printing system.

A multilevel microcontact printing (µCP) system that avoids the use of optical alignment and precision manipulation equipment is demonstrated. Most of the complexity is transferred to the poly(dimethylsiloxane) (PDMS) stamp itself by forming the features, a mechanical self-alignment mechanism, and an elastic membrane by wafer scale replica molding on a Si master. Flexible 50-µm-thick photoetched stainless steel sheets are bonded to PDMS prior to demolding to improve the mechanical stability. The Si master itself is made using conventional MEMS fabrication tools such as photolithography, reactive ion etching, and anisotropic wet etching. Self-alignment is achieved by introducing protrusions on the stamp that mate onto corresponding grooves on a machined substrate. Complete 10 mm × 10 mm prototypes are fabricated, and six-level µCP is demonstrated with an average layer-to-layer misalignment of 5-10 µm.

Advances in microfabricated mass spectrometers.

Mass filters for miniature mass spectrometers are now being constructed using three-dimensional microfabrication techniques. Crossed-field, travelling-wave, time-of-flight, quadrupole and cylindrical ion-trap filters have all been demonstrated, with steadily increasing mass range and mass resolution. This paper reviews the range of available devices and the state of the art.

2-Dimensional MEMS dielectrophoresis device for osteoblast cell stimulation.

A fixed microelectrode device for cell stimulation has been designed and fabricated using micro-electro-mechanical systems (MEMS) technology. Dielectrophoretic forces obtained from non-uniform electric fields were used for manipulating and positioning osteoblasts. The experiments show that the osteoblasts experience positive dielectrophoresis (p-DEP) when suspended in iso-osmotic culture medium and exposed to AC fields at 5 MHz frequency. Negative dielectrophoresis (n-DEP) is obtained at 0.1 MHz. The viability of osteoblasts under dielectrophoresis has been investigated. The viability values for cells exposed to DEP are nearly three times higher than the control values, indicating that dielectrophoresis may have an anabolic effect on osteoblasts.