Adam J. Baragwanath

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.

Negative refraction and the spectral filtering of terahertz radiation by a photonic crystal prism

We demonstrate how micromachined photonic crystals can be used to negatively refract terahertz frequency light. The photonic crystals, which are constructed from conventional dielectric materials, manipulate the incident beam via interaction with their photonic bands. Consequently, we show that different components of a broadband beam incident on the structure may be positively or negatively refracted, depending upon its frequency and that the structure can be used as an effective spectral filter of THz radiation.

Terahertz Plasmonic Structures

Here, we review the background physics of surface plasmons and plasmonic devices. In doing so, we describe how terahertz radiation can be used to excite plasmonic effects. These effects are shown to be beneficial for applications such as terahertz waveguiding, sensing, imaging and wavefront engineering.

Negatively refracting plasmonic devices at THz frequencies

In this paper, we present experimental results showing how appropriately designed and constructed artificial materials can be used to refract negatively terahertz frequency light without the need for the materials to have both a negative permeability and a negative permittivity.

Silicon based microfluidic device for THz frequencies

In this paper we report on the design, fabrication and testing of a silicon based, microfluidic device, for transmission THz-TDS. Testing of the device and the associated data extraction algorithms is accomplished by the analysis of primary alcohol/water mixtures, where quantities of 2 µmol can be distinguished between samples. The analysis of commercial whiskeys shows clear absorption differences between brands, owing to the individual composition of the blends. Testing of biotin solutions has displayed sensitivity of the order of 3 nmol.

Micromachined Devices for Use in Terahertz Applications

Here we present results from key aspects of our interest in using micromachined devices in the THz region. First, our early work on making filters from rods of gold-coated SU8 is described. Pass (up to 97%) and stop bands can be observed which are theoretically underpinned by both FDTD and complex band structure simulations. Second, there is a discussion of how THz radiation passes through two-dimensional periodic arrays of subwavelength apertures. In particular, the geometry of the arrays has been studied with time-domain spectroscopy. A time-of-flight model is presented which can be used to provide insight into the operation of these arrays and has implications for the optimum design of THz plasmonic sensors. Finally, we report the THz ‘super’ extraordinary transmission properties of an optimised hybrid subwavelength aperture array, surrounded by subwavelength grooves.

Plasmonic apertures for THz QCL near-field imaging

In this paper we report on the fabrication and testing of sub-wavelength apertures surrounded by annular plasmonic structures, for use in near-field imaging with a QCL source. Both double corrugated structures and planarised designs have been tested, with the latter producing a spatially uniform far-field emission profile. Greatly increased transmission has been observed when using plasmonic apertures over standard plain apertures. Sub-wavelength resolution has been measured with a standard, near-field knife-edge test.

Micromachined terahertz waveguides with embedded metal rods

In this paper we report on a novel fabrication approach for the manufacture of terahertz waveguides with embedded metal rods. It is anticipated that this type of structure could be used for sub-wavelength imaging or as a method to effectively couple radiation into biological samples.