Andrew J. Gallant

Terahertz frequency bandpass filters

The design, measurement, and analysis of a range of artificial materials for use at terahertz frequencies are described. The chosen structures consist of arrays of cylindrical gold-plated pillars with period comparable to the wavelength of incident radiation. An ultraviolet (UV) micromachining approach to the fabrication of these high aspect-ratio pillars is described using the negative epoxy-based resin SU8. Lattice fence structures are also realized using the same method. Terahertz (THz) frequency time domain spectroscopy is performed on these structures in the range 200 GHz to 3.0 THz and the relative transmission of the structures is determined. The pass and stop bands are observed with peak transmission of up to 97%. Finite difference time domain simulations and complex photonic band structure calculations are shown to provide good descriptions of the electromagnetic properties of the structures and are used to interpret the observed transmission spectra.

Passband filters for terahertz radiation based on dual metallic photonic structures

This paper reports on the development of two-dimensional metallic microstructures for the filtering of terahertz radiation. These structures are fabricated using ultraviolet-based processing of thick SU8 resist. This micromachining technique enables the array patterns, dimensions, and consequently the filter characteristics to be readily defined. In particular, we demonstrate that a filter with an isolated near-square-shaped passband can be realized on the basis of a combination of two different metallic photonic arrays of optimized design.

Silicon based microfluidic cell for terahertz frequencies

We present a detailed analysis of the design, fabrication and testing of a silicon based, microfluidic cell, for transmission terahertz time-domain spectroscopy. The sensitivity of the device is tested through a range of experiments involving primary alcohol/water mixtures. The dielectric properties of these solutions are subsequently extracted using a Nelder–Mead search algorithm, and are in good agreement with literature values obtained via alternative techniques. Quantities in the order of 2 μmol can be easily distinguished for primary alcohols in solution, even with the subwavelength optical path lengths used. A further display of the device sensitivity is shown through the analysis of commercial whiskeys, where there are clear, detectable differences between samples. Slight absorption variations were identified between samples of the same commercial brand, owing to a 2.5% difference in their alcoholic content. Results from data taken on subsequent days after system realignment are also presented, confirming the robustness of the technique, and the data extraction algorithm used. One final experiment, showing the possible use of this device to analyze aqueous biological samples is detailed; where biotin, a molecule known for its specific terahertz absorptions, is analyzed in solution. The device sensitivity is once again displayed, where quantities of 3 nmol can be clearly detected between samples.

Terahertz generation by GaAs nanowires

The investigation of terahertz generation by the surface of n-InAs layers and by GaAs nanowires grown on n-GaAs (111) substrates is presented. It was observed that the terahertz radiation power efficiency is significantly higher in the second case.

The modelling and fabrication of widely tunable capacitors

Coventorware 2001.1® has been used to model a widely tunable, two-gap capacitor structure. This paper explores the use of the Coventorware® to achieve a design specification and confirms device operation. Tunable capacitors fabricated to this specification have achieved a tuning ratio of 5.1:1, the best reported to date.

Single and double bosonic stimulation of THz emission in polaritonic systems.

The influence of the surrounding cavity on the efficiency of different types of polaritonic emitters of THz radiation has been analysed. It is demonstrated that THz lasing threshold in realistic structures cannot be achieved without a THz cavity, due to destruction of polaritons via excitonic Mott transition. Even modest values of cavity quality factor (not exceeding 50) provide significant quantum efficiency.

A Versatile Nanopatterning Technique Based on Controlled Undercutting and Liftoff

A new low-cost top-down nanolithography technique based on controlled undercutting and liftoff is reported. The method is applicable to a wide selection of inorganic materials (those that can be patterned by dry etching or lift-off) and can create 100-nm sized structures over wafer-sized areas. The method requires only conventional microfabrication processes and is ideal for producing nanowires, rings, and dots. A proof-of-concept experiment is also described for the fabrication of gold-nanowire transparent conducting electrodes, which show excellent optoelectronic properties.

Surface micromachined membranes for wafer level packaging

Novel surface micromachined membranes have been developed for wafer level packaging and polymer encapsulation. Laterally unconstrained electroforming allows small etch holes to be included in thick membranes whilst only requiring a thin, low stress, mould. These holes have been shown to have a sufficiently small aperture (<10 µm) and depth (17 µm) to block a high viscosity encapsulant (AZ4562 photoresist). However, they enable the low viscosity sacrificial release etchant (acetone) to pass through. Etch hole inclusion provides a rapid and clean release. In order to route interconnect to the membranes, side port flow channels have been developed which effectively block spin-on encapsulants from entering the protected cavity. The influence of the membranes on the RF performance of micromachined meander inductors is reported.

Manipulation of exposure dose parameters to improve production of high aspect ratio structures using SU-8

The detailed consideration of the chemistry occurring during the fabrication process is very important for SU-8 compared to other photoresists. In SU-8, the solvent does far more than just act as a carrier for the photoactive compounds: it plays a prominent role in the processing chemistry. In this paper we explain how the change of solvent from old to new formulations of SU-8 has an effect on the processing of this resist. We then show how, and why, in the modern 2000 series formulations the manipulation of the polymerization rate via variation of the exposure dose allows dark field features on a mask design to be reduced, in a controlled manner, by up to 30% while still maintaining good sidewall profiles in features.

Optimizing MOM diode performance via the oxidation technique

This work presents a study of the effect of a simple oxidation technique on the electrical performance of Ti/TiOx/Pt MOM (metal-oxide-metal) diodes. A fabrication process has been designed to produce devices with a high yield. The I–V characteristics show good diode behavior: subsequent mathematical analysis to extract the key parameters of curvature coefficient and resistance at zero bias demonstrate how these numbers depend on the curve fitting method. Nevertheless, diodes with high curvature (typically 5.5 V−1 unbiased, 15 V−1 biased) represent results among the best to date. Complimentary physical information from the structures has been obtained via AFM and RBS analysis.