arXiv subjects

Matthew P. Halsall

Publications and source records attributed to Matthew P. Halsall.

4 recordsLinked to original sources

arXiv2026

Phase-controlled super-modes in a `lossy' tri-waveguide coupled micro-ring resonator system

In coupled micro-ring resonator (MRR) systems, the coupling process plays a critical role in determining the optical response, yet models of these structures typically assume this process to be either lossless, or dissipative with either purely real or purely imaginary coupling rate, respectively. Dissipative coupling is contingent on having appropriately phase engineered structures with intricate coupling geometries, but systems exhibiting `lossy coupling', i.e., characterized by complex-valued coupling rates, have received comparatively little attention. In this work, we investigate the effect of such `lossy' coupling, by modelling the optical response of a tri-waveguide coupled MRR system, using Temporal Coupled-Mode Theory (TCMT). The `lossy' coupling occurs via a common bus waveguide, positioned between a pair of MRRs, leading to the emergence of a pair of intrinsic resonances, whose frequency and linewidth may be tailored by controlling the respective coupling rates from either MRR. We develop these ideas, and by connecting the input (driving) waveguides, using a 3dB y-splitter, demonstrate a novel, coherently driven, lossy-coupled MRR, which exhibits either an absorptive or transparent resonance peak on demand by phase-controlled super-modes.

physics.optics↗

arXiv2026

Generalized Temporal Coupled Mode Theory (g-TCMT) applied to Coupled Resonator Optical Waveguides with Exchange Symmetry (CROWe)

In this paper, we extend our generalized Temporal Coupled Mode Theory (g-TCMT) model, developed earlier [1], from a simple, dual-coupled micro-ring resonator (MRR) system to higher-order, n-serially coupled MRRs. By treating each pair of adjacent MRRs as a single `equivalent resonator', we demonstrate excellent agreement in spectral resonance position, between the g-TCMT and numerical results obtained using the transfer matrix method (TMM), for systems up to and including order n=4. The validity of this approach hinges on the existence, or otherwise, of exchange symmetry, and so we refer to these structures as Coupled Resonator Optical Waveguides with exchange symmetry (CROWe's). We explore the limitations of our approach with illustrative examples of strongly coupled systems of order n \geq 5. Finally, we explore the properties of such higher order CROWe's for applications in non-Hermitian photonics, i.e., in which gain in some of the component MRRs and loss in the others leads to Parity-Time (PT) symmetry.

physics.optics↗

arXiv2025

On the origin of the E1 electron trap level in GaN and dilute AlxGa1-xN films

The results of high-resolution Laplace deep-level transient spectroscopy (L-DLTS) measurements applied to the E1 and FeGa electron traps in dilute AlxGa1-xN films (x = 0.063), grown by metal-organic vapor phase epitaxy (MOVPE) on Ammono-GaN substrates, are presented. It is shown that the electron emission signals associated with the E1 donor and the FeGa acceptor levels split into individual components due to the aluminium fluctuations in the nearest neighbour shells around the E1 and FeGa defects. The splitting patterns observed in the L-DLTS spectra are nearly identical for both signals. Furthermore, the ratios of peak magnitudes determined from the L-DLTS analysis for both the E1 and E3 traps are consistent with calculated probabilities of finding a given number of aluminium atoms in the second nearest neighbour shell around a Ga lattice site in AlxGa1-xN with x = 0.063. These findings provide strong evidence that both the E1 and the FeGa trap states in dilute AlxGa1-xN are related to defects located in the Ga sublattice. To elucidate the origin of the E1 trap in AlxGa1-xN, we have performed a comprehensive scan of possible impurities and defects in GaN and AlxGa1-xN using hybrid density functional calculations of transition levels and their associated shifts upon substitution of Ga neighbour atoms by Al. From analysis of the results, we find that the E1 electron trap in GaN and AlxGa1-xN is most likely related to a donor transition from a carbon or molybdenum impurity atom at the gallium site, respectively.

cond-mat.mtrl-sci↗

arXiv2009

Graphene under hydrostatic pressure

In-situ high pressure Raman spectroscopy is used to study monolayer, bilayer and few-layer graphene samples supported on silicon in a diamond anvil cell to 3.5 GPa. The results show that monolayer graphene adheres to the silicon substrate under compressive stress. A clear trend in this behaviour as a function of graphene sample thickness is observed. We also study unsupported graphene samples in a diamond anvil cell to 8 GPa, and show that the properties of graphene under compression are intrinsically similar to graphite. Our results demonstrate the differing effects of uniaxial and biaxial strain on the electronic bandstructure.

cond-mat.mtrl-sci↗