arXiv subjects
Publications and source records attributed to James Seddon.
We generate and transmit 75-GHz-bandwidth OFDM signals over the air using three mutually frequency-locked lasers, achieving minimal frequency gap between the wireless W and D bands using optical-assisted approaches, resulting in 173.5 Gb/s detected capacity.
Continuous Wave (CW) Terahertz spectroscopy enabled by photomixing is a promising high precision spectroscopic tool for the examination of a wide variety of samples including biological, chemical, and solid state. However, often it would be of interest to examine isolated samples free from bulk effects that broaden spectral features. In this form samples with low concentrations of absorbers have a reduced cross section making coupling to an external driving field a challenge. By utilising THz metamaterials one can borrow the concepts of surface enhanced Raman spectroscopy to confine and concentrate the THz fields to increase sensitivity for samples with a small cross section. Our work combines high-speed photodiode technology and THz metamaterials to offer a solution to this challenge. We present here a proof of concept on-chip THz spectrometer integrated with a metamaterial waveguide.
The first waveguide coupled phosphide-based UTC photodiodes grown by Solid Source Molecular Beam Epitaxy (SSMBE) are reported in this paper. Metal Organic Vapour Phase Epitaxy (MOVPE) and Gas Source MBE (GSMBE) have long been the predominant growth techniques for the production of high quality InGaAsP materials. The use of SSMBE overcomes the major issue associated with the unintentional diffusion of zinc in MOVPE and gives the benefit of the superior control provided by MBE growth techniques without the costs and the risks of handling toxic gases of GSMBE. The UTC epitaxial structure contains a 300 nm n-InP collection layer and a 300 nm n++-InGaAsP waveguide layer. UTC-PDs integrated with Coplanar Waveguides (CPW) exhibit 3 dB bandwidth greater than 65 GHz and output RF power of 1.1 dBm at 100 GHz. We also demonstrate accurate prediction of the absolute level of power radiated by our antenna integrated UTCs, between 200 GHz and 260 GHz, using 3d full-wave modelling and taking the UTC-to-antenna impedance match into account. Further, we present the first optical 3d full-wave modelling of waveguide UTCs, which provides a detailed insight into the coupling between a lensed optical fibre and the UTC chip.
We experimentally investigate the nucleation of surface nanobubbles on PFDTS-coated silicon as a function of the specific gas dissolved in the water. In each case we restrict ourselves to equilibrium conditions ($c=100%$, $T_{liquid} = T_{substrate}$). Not only is nanobubble nucleation a strong function of gas type, but there also exists an optimal system temperature of $\sim 35-40\mathrm{^oC}$ where nucleation is maximized, which is weakly dependent on gas type. We also find that contact angle is a function of nanobubble radius of curvature for all gas types investigated. Fitting this data allows us to describe a line tension which is dependent on the type of gas, indicating that the nanobubbles are sat on top of adsorbed gas molecules. The average line tension was $τ\sim -0.8 \mathrm{nN}$.
We provide a model for the remarkable stability of surface nanobubbles to bulk dissolution. The key to the solution is that the gas in a nanobubble is of Knudsen type. This leads to the generation of a bulk liquid flow which effectively forces the diffusive gas to remain local. Our model predicts the presence of a vertical water jet immediately above a nanobubble, with an estimated speed of $\sim3.3\,\mathrm{m/s}$, in good agreement with our experimental atomic force microscopy measurement of $\sim2.7\,\mathrm{m/s}$. In addition, our model also predicts an upper bound for the size of nanobubbles, which is consistent with the available experimental data.