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Ruth Oulton

Publications and source records attributed to Ruth Oulton.

At least 19 recordsLinked to original sources
arXiv2025

Aspheric lens design proposal for near-perfect mode-matching of a broadband quantum dot micropillar to a single-mode fibre

Quantum dots in micropillars are one of the most promising options for a bright, deterministic single photon source. While highly efficient devices (>95%) have been designed, there remains a significant bottleneck that impacts the overall system efficiency: the large numerical aperture of the output mode. This leads to inefficient coupling of emitted photons into single-mode fibre, thus limiting practical integration into quantum computing and communication architectures. We show that with the addition of a well designed aspheric SiO2 microlens we can decrease the mode-matching losses to a SMF from 83.1% to <0.1(0.1)%. This can result in a single photon source design with 96.4(0.1)% end-to-end efficiency, paving the way for scalable photonic quantum technologies.

quant-ph↗
arXiv2025

Room temperature spin injection into commercial VCSELs at non-resonant wavelengths

Spin lasers leverage electron spin-polarisation to control photon polarisation, offering the potential for lower thresholds, rapid modulation, and all-optical data processing. We report successful spin injection into a commercial Vertical Cavity Surface Emitting Laser (VCSEL) using optical pumping at wavelengths of 794 nm and 810 nm. Maximum circular polarisation achieved was about 20% at 794 nm, whereas this falls to 5% with exciting at 810 nm. We attribute this to reduced spin injection due to the longer excitation wavelength in line with previous measurements on quantum wells under optical orientation. We extend the Spin-Flip Model (SFM) to account for realistic excitation conditions and find our theoretical analysis accurately reproduces experimental trends.

physics.optics↗
arXiv2024

Nanoring Tamm Cavity in the Telecommunications O band

Quantum and classical telecommunications require efficient sources of light. Semiconductorsources, owing to the high refractive index of the medium, often exploit photonic cavities to enhance the external emission of photons into a well-defined optical mode. Optical Tamm States (OTS), in which light is confined between a distributed Bragg reflector and a thin metal layer have attracted interest as confined Tamm structures are readily manufactureable broadband cavities. Their effficiency is limited however by the absorption inherent in the metal layer. We propose a nanoring Tamm structure in which a nanoscale patterned annular metasurface is exploited to reduce this absorption and thereby enhance emission efficiency. To this end, we present designs for a nanoring Tamm structure optimised for the telecommunications O band, and demonstrate a near doubling of output efficiency (35%) over an analogous solid disc confined Tamm structure (18%). Simulations of designs optimised for different wavelengths are suggestive of annular coupling between the Tamm state and surface plasmons. These designs are applicable to the design of single photon sources, nanoLEDs, and nanolasers for communications.

physics.optics↗
arXiv2023

Design principles for >90% efficiency and >99% indistinguishability broadband quantum dot cavities

Quantum dots have the potential to be the brightest deterministic single photon source with plausible high end applications in quantum computing and cluster state generation. In this work, we re-examine the design of simple micropillars by meticulously examining the structural effects of the decay into leaky channels beyond the atom-like cavity estimation. We show that precise control of the side losses with the diameter and avoidance of propagating Bloch modes in the DBR structure can result in easy to manufacture broadband (Q$\approx750-2500$) micropillars and demonstrate extremely high internal efficiency ($90.5\%-96.4\%$). We also demonstrate that such cavities naturally decouple from the phonon sideband, with the phonon sideband reducing by a factor of $5-33$ allowing us to predict that the photons should show $99.2\%-99.8\%$ indistinguishability.

quant-ph↗
arXiv2023

Coherent scattering from coupled two level systems

We study the resonance fluorescence properties of an optically active spin 1/2 system, elucidating the effects of a magnetic field on the coherence of the scattered light. We derive a master equation model for this system that reproduces the results of a two level system (TLS) while also being applicable to a spin system with ground state coupling. This model is then solved analytically in the weak excitation regime. The inclusion of spin dynamics in our model alters the properties of the coherently scattered light at a fundamental level. For a TLS the coherence properties are known to be determined by the input laser. We show that spin scattered light inherits the coherence properties of the spin. This mapping allows us to measure spin dynamics and coherence time through direct measurement of the scattered fields. Furthermore, we show the ability to resolve sub-natural linewidth zeeman splittings. Along with representing an invaluable tool for spin spectroscopy understanding the coherence properties of the spin-scattered field will be vital for spin-photon based quantum technologies.

quant-ph↗
arXiv2023

Reflection by two level system: phase singularities on the Poincar\'e hypersphere

We consider the reflection of a photon by a two-level system in a quasi-one-dimensional waveguide. This is important in part because it forms the backdrop for more complicated proposals where many emitters are coupled to the waveguide: leading to super and subradiant coupling even when the emitters are distant. The incorporation of chiral effects, for example unidirectional emission of dipole emitters, has already led to rich physics such as dimer coupling. However, chirality is not the only effect of the dipole, as we explore from a phase singularity perspective. We demonstrate that control of the dipole allows a rich variety of control of the phase and amplitude of the scattered light in both directions. This expands the scope for the physics of 1D chains of emitters.

physics.optics↗
arXiv2022

Bio-inspired polymers with polaritonic properties from visible to infrared: a material playground to mimic purple bacteria light-harvesting resonators

Light-harvesting complexes in natural photosynthetic systems, such as those in purple bacteria, consist of photo-reactive chromophores embedded in densely packed "antenna" systems organized in well-defined nanostructures. In the case of purple bacteria, the chromophore antennas are composed of natural J-aggregates such as bacteriochlorophylls and carotenoids. Inspired by the molecular composition of such biological systems, we create a library of organic materials composed of densely packed J-aggregates in a polymeric matrix, in which the matrix mimics a protein scaffold. This library of organic materials shows polaritonic properties which can be tuned from the visible to the infrared by choice of the model molecule. Inspired by the molecular architecture of the light-harvesting complexes of \textit{Rhodospirillum molischianum} bacteria, we study the light-matter interactions of J-aggregate-based nanorings with similar dimensions to the analogous natural nanoscale architectures. Electromagnetic simulations show that these nanorings of J-aggregates can act as resonators, with subwavelength confinement of light while concentrating the electric field in specific regions. These results open the door to bio-inspired building blocks for all-organic metamaterials while offering a new perspective on light-matter interactions at the nanoscale in densely packed organic matter in biological organisms including photosynthetic organelles.

physics.optics↗
arXiv2021

High extraction efficiency source of photon pairs based on a quantum dot embedded in a broadband micropillar cavity

The generation of photon pairs in single quantum dots is based on a process that is, in its nature, deterministic. However, an efficient extraction of these photon pairs from a high-index semiconductor host material requires engineering of the photonic environment. We report on a micropillar-based device featuring an extraction efficiency of 69.4(10)$\%$ that is achieved by harnessing a broadband operation suitable for extraction of photon pairs emitted from a single quantum dot. Opposing the approaches that rely solely on Purcell enhancement to realize the enhancement of the extraction efficiency, our solution exploits a suppression of the emission into the modes other than the cavity mode. Our technological implementation requires modest fabrication effort enabling higher device yields that can be scaled up to meet the growing needs of quantum technologies. Furthermore, the design of the device can be further optimized to allow for an extraction efficiency of 85$\%$.

quant-ph↗
arXiv2021

Perfect Chirality with imperfect polarisation

Unidirectional (chiral) emission of light from a circular dipole emitter into a waveguide is only possible at points of perfect circular polarisation (C points), with elliptical polarisations yielding a lower directional contrast. However, there is no need to restrict engineered systems to circular dipoles and with an appropriate choice of dipole unidirectional emission is possible for any elliptical polarization. Using elliptical dipoles, rather than circular, typically increases the size of the area suitable for chiral interactions (in an exemplary mode by a factor $\sim 30$), while simultaneously increasing coupling efficiencies. We propose illustrative schemes to engineer the necessary elliptical transitions in both atomic systems and quantum dots.

physics.optics↗
arXiv2019

Telecommunication wavelength confined Tamm plasmon structures containing InAs/GaAs quantum dot emitters at room temperature

We experimentally demonstrate gold microdisc structures that produce confined Tamm plasmons (CTPs), interface modes between a metal layer and a distributed Bragg reflector, resonant around 1.3 {\mu}m. Quantum dots grown within the structures show an order of magnitude increase in the photoluminescence emitted at room temperature. Varying the disc diameter, we show spectral tuning of the resonance and measure the dispersion relation as evidence of mode confinement. The simplicity of fabrication and tuneability of these structures make CTPs an ideal platform for making scalable telecom devices based on quantum dots.

cond-mat.mes-hall↗
arXiv2019

Stabilisation of an optical transition energy via nuclear Zeno dynamics in quantum dot-cavity systems

We investigate the effect of nuclear spins on the phase shift and polarisation rotation of photons scattered off a quantum dot-cavity system. We show that as the phase shift depends strongly on the resonance energy of an electronic transition in the quantum dot, it can provide a sensitive probe of the quantum state of nuclear spins that broaden this transition energy. By including the electron-nuclear spin coupling at a Hamiltonian level within an extended input-output formalism, we show how a photon scattering event acts as a nuclear spin measurement, which when rapidly applied leads to an inhibition of the nuclear spin dynamics via the quantum Zeno effect, and a corresponding stabilisation of the optical resonance. We show how such an effect manifests in the intensity autocorrelation $g^{(2)}(\tau)$ of scattered photons, whose long-time bunching behaviour changes from quadratic decay for low photon scattering rates (weak laser intensities), to ever slower exponential decay for increasing laser intensities as optical measurements impede the nuclear spin evolution.

quant-ph↗
arXiv2018

A model for confined Tamm plasmon devices

It is shown that cavities formed between a multilayer quarter-wave Bragg reflector and a metal mirror which support Tamm plasmons can be modelled by using a hard-mirror approximation including appropriate penetration depths into the mirrors. Results from this model are in excellent agreement with those found by numerical methods. In addition Tamm modes that are laterally confined by the presence of a metallic disc deposited on the Bragg reflector can be described by the effective index model that is commonly used for vertical-cavity surface-emitting lasers (VCSELs). This enables the lateral modes confined by a circular disc to be found from conventional weakly-guiding waveguide theory similar to that used for optical fibres. The resonant wavelengths of these linearly-polarised (LP) guided modes are calculated as functions of disc diameter and other parameters.

physics.optics↗
arXiv2018

Optimal simultaneous measurements of incompatible observables of a single photon

Joint or simultaneous measurements of non-commuting quantum observables are possible at the cost of increased unsharpness or measurement uncertainty. Many different criteria exist for defining what an "optimal" joint measurement is, with corresponding different tradeoff relations for the measurements. Understanding the limitations of such measurements is of fundamental interest and relevant for quantum technology. Here, we experimentally test a tradeoff relation for the sharpness of qubit measurements, a relation which refers directly to the form of the measurement operators, rather than to errors in estimates. We perform the first optical implementation of the simplest possible optimal joint measurement, requiring less quantum resources than have previously often been employed. Using a heralded single-photon source, we demonstrate quantum-limited performance of the scheme on single quanta.

quant-ph↗
arXiv2016

Time reversal constraint limits unidirectional photon emission in slow-light photonic crystals

Photonic crystal waveguides are known to support C-points - point-like polarisation singularities with local chirality. Such points can couple with dipole-like emitters to produce highly directional emission, from which spin-photon entanglers can be built. Much is made of the promise of using slow-light modes to enhance this light-matter coupling. Here we explore the transition from travelling to standing waves for two different photonic crystal waveguide designs. We find that time-reversal symmetry and the reciprocal nature of light places constraints on using C-points in the slow-light regime. We observe two distinctly different mechanisms through which this condition is satisfied in the two waveguides. In the waveguide designs we consider, a modest group-velocity of $v_g \approx c/10$ is found to be the optimum for slow-light coupling to the C-points.

physics.optics↗
arXiv2015

A charged quantum dot micropillar system for deterministic light matter interactions

Quantum dots (QDs) are semiconductor nanostructures in which a three dimensional potential trap produces an electronic quantum confinement, thus mimicking the behaviour of single atomic dipole-like transitions. However unlike atoms, QDs can be incorporated into solid state photonic devices such as cavities or waveguides that enhance the light-matter interaction. A near unit efficiency light-matter interaction is essential for deterministic, scalable quantum information (QI) devices. In this limit, a single photon input into the device will undergo a large rotation of the polarization of the light field due to the strong interaction with the QD. In this paper we measure a macroscopic ($\sim6^o$) phase shift of light as a result of the interaction with a negatively charged QD coupled to a low quality-factor (Q$\sim290$) pillar microcavity. This unexpectedly large rotation angle demonstrates this simple low Q-factor design would enable near deterministic light-matter interactions.

quant-ph↗
arXiv2015

The stability of polarisation singularities in disordered photonic crystal waveguides

The effects of short range disorder on the polarisation characteristics of light in photonic crystal waveguides were investigated using finite difference time domain simulations with a view to investigating the stability of polarisation singularities. It was found that points of local circular polarisation (C-points) and contours of linear polarisation (L-lines) continued to appear even in the presence of high levels of disorder, and that they remained close to their positions in the ordered crystal. These results are a promising indication that devices exploiting polarisation in these structures are viable given current fabrication standards.

physics.optics↗
arXiv2015

Evidence of near-infrared partial photonic bandgap in polymeric rod-connected diamond structures

We present the simulation, fabrication, and optical characterization of low-index polymeric rod-connected diamond (RCD) structures. Such complex three-dimensional photonic crystal structures are created via direct laser writing by two-photon polymerization. To our knowledge, this is the first measurement at near-infrared wavelengths, showing partial photonic bandgaps for this structure. We characterize structures in transmission and reflection using angular resolved Fourier image spectroscopy to visualize the band structure. Comparison of the numerical simulations of such structures with the experimentally measured data show good agreement for both P- and S-polarizations.

physics.optics↗