John Eric Bower

Optical transmission through double-layer metallic subwavelength slit arrays

We present measurements of transmission of infrared radiation through double-layer metallic grating structures. Each metal layer contains an array of subwavelength slits and supports transmission resonance in the absence of the other layer. The two metal layers are fabricated in close proximity to allow coupling of the evanescent field on individual layers. The transmission of the double layer is found to be surprisingly large at particular wavelengths, even when no direct line of sight exists through the structure as a result of the lateral shifts between the two layers. We perform numerical simulations using rigorous coupled wave analysis to explain the strong dependence of the peak transmission on the lateral shift between the metal layers.

Imaging nanostructures with coherent phonon pulses

We demonstrate submicron resolution imaging using picosecond acoustic phonon pulses. High-frequency acoustic pulses are generated by impulsive thermoelastic excitation of a patterned 15-nm-thick metal film on a crystalline substrate using ultrafast optical pulses. The spatiotemporal diffracted acoustic strain field is measured on the opposite side of the substrate, and this field is used in a time-reversal algorithm to reconstruct the object. The image resolution is characterized using lithographically defined 1-micron-period Al structures on Si. Straightforward technical improvements should lead to resolution approaching 45nm, extending the resolution of acoustic microscopy into the nanoscale regime.

Monolithic fringe-field-activated crystalline silicon tilting-mirror devices

A new approach is presented for fabricating monolithic crystalline silicon tilting-mirror microoptoelectromechanical systems (MOEMS) devices. The activation electrodes, etched from a thick silicon layer deposited over insulating oxide onto the top surface of a silicon-on-insulator (SOI) wafer, are displaced from the mirrors and interact with these tilting elements via electrostatic fringing fields. In contrast to the more usual parallel-plate activation, the rotation angle saturates at high voltages. This paper discusses concept, design, and processing, and also compares modeling and measured performance of a specific 9/spl deg/ tilt range device array.

Controlling the phase delay of light transmitted through double-layer metallic subwavelength slit arrays

We demonstrate that the phase of light transmitted through double-layer subwavelength metallic slit arrays can be controlled through lateral shift of the two layers. Our samples consist of two aluminum layers, each of which contains an array of subwavelength slits. The two layers are placed in sufficient proximity to allow coupling of the evanescent fields at resonance. By changing the lateral shift between the layers from zero to half the period, the phase of the transmitted electromagnetic field is increased by pi, while the transmitted intensity remains high. Such a controllable phase delay could open new capabilities for nanophotonic devices that cannot be achieved with single-layer structures.

Realization of tellurium-based all dielectric optical metamaterials using a multi-cycle deposition-etch process

Tellurium (Te) dielectric resonator metamaterials for thermal infrared applications were fabricated using a multi-cycle deposition-etch process that circumvents pinch-off issues during deposition. Deposition and etching of Te were studied in detail. Metamaterial samples with varying resonator dimensions were fabricated using this technique. All the samples showed two transmission minima corresponding to magnetic and electric dipole resonances. Longer resonant wavelengths were observed as the resonator dimension was increased. Observation of spectral overlap between magnetic and electric resonances gives us the potential opportunity to realize a negative refractive index material.

Spatial light modulator for maskless optical projection lithography

Spatial light modulators (SLMs) designed to replace photomasks for optical lithography have been designed, fabricated, and tested. These microelectromechanical devices are fabricated with alternating polycrystalline Si and sacrificial SiO2 layers that are patterned by a 193nm wavelength scanner to dimensions as small as 150nm. Aerial image simulations were used to define the mechanical requirements of the devices. Piston motion of electrically actuated devices was measured with an optical profilometer. The measurements were fit to a simple equation to within 1nm precision, which is adequate for defining 50nm features lithographically. Transient response measurements show that one version of the SLM responds to actuation as quickly as 20μs, fast enough for current 193nm wavelength excimer laser sources.

Transmission enhancement in an array of subwavelength slits in aluminum due to surface plasmon resonances

The coupling of light to surface plasmons through periodic subwavelength metallic structures could strongly modify the optical properties of a metal film. We demonstrate that the optical transmission through an array of subwavelength slits is as high as 80% at resonance, even though the width of each slit is almost 10 times smaller than the wavelength and the slits occupy only 25% of the area of the metal. Numerical calculations suggest that the field intensity is strongly enhanced near the metal surface. The field enhancement could be used for generating nonlinear optical effects and for high sensitivity detection of nanomechanical displacement.

Optical transmission through double-layer, laterally shifted metallic subwavelength hole arrays

We measure the transmission of IR radiation through double-layer metal films with periodic arrays of subwavelength holes. When the two metal films are placed in sufficiently close proximity, two types of transmission resonances emerge. For the surface plasmon mode, the electromagnetic field is concentrated on the outer surface of the entire metallic layer stack. In contrast, for the guided mode, the field is confined to the gap between the two metal layers. Our measurements indicate that, as the two layers are laterally shifted from perfect alignment, the peak transmission frequency of the guided mode decreases significantly, while that of the surface plasmon mode remains largely unchanged, in agreement with numerical calculations.

MICROSTRUCTURE OF SPUTTERED TIN ON AL

We use electron microscopy and x-ray diffraction to study the microstructure of TiN deposited on Al. In contrast to previous work, we show that the TiN has a large (≈1 μm) grain size arising from its epitaxial orientation on the underlying Al. Within a single grain, the TiN has a heavily voided columnar structure that closely mimics the appearance of fine grains. The within-grain columnar structure arises from the usual shadowing mechanism for sputtered films, and has a weak dependence on the deposition temperature.

Optical MEMS devices for telecom systems

As telecom networks increase in complexity there is a need for systems capable of manage numerous optical signals. Many of the channel-manipulation functions can be done more effectively in the optical domain. MEMS devices are especially well suited for this functions since they can offer large number of degrees of freedom in a limited space, thus providing high levels of optical integration. We have designed, fabricated and tested optical MEMS devices at the core of Optical Cross Connects, WDM spectrum equalizers and Optical Add-Drop multiplexors based on different fabrication technologies such as polySi surface micromachining, single crystal SOI and combination of both. We show specific examples of these devices, discussing design trade-offs, fabrication requirements and optical performance in each case.