arXiv subjects

Christopher J. Mellor

Publications and source records attributed to Christopher J. Mellor.

7 recordsLinked to original sources

arXiv2026

Linear and nonlinear optical tuning with Sb$_2$S$_3$-based metasurfaces

Here, we experimentally demonstrate the unique properties of Sb$_2$S$_3$ metasurfaces on monolithic and hybridised Sb$_2$S$_3$-Si platforms. Their advantages are particularly pronounced in the telecommunication spectral range, where both phases remain nearly lossless together with a large and stable refractive index contrast compared to other counterparts. The monolithic metasurface enables transmission modulation depths of up to 92\% and resonance shifts as large as $\sim$150~nm at the telecommunication wavelengths through laser-induced phase transitions from amorphous to $\sim$ 50\% polycrystalline states. Furthermore, we demonstrate that integrating a silicon overlayer introduces high-Q hybrid resonances with enhanced near-field confinement, enabling comparable modulation, experimentally, with nearly half the laser switching power required as compared to the monolithic structure. Beyond linear optical switching, the broad resonance tunability of the Sb$_2$S$_3$ metasurface, together with the strong third-order nonlinearity of these materials, provides a promising platform for tunable nonlinear light generation. By exciting the Sb$_2$S$_3$ metasurfaces with a fixed broadband pump, we experimentally demonstrate tunable third-harmonic generation emission over a $\sim$40~nm spectral range through phase-change transition. Such tunability provides a versatile route for integrating Sb$_2$S$_3$ with intrinsically highly nonlinear materials to enable the combination of large optical tunability and efficient nonlinear light generation. We have proven this concept via our hybrid Sb$_2$S$_3$-Si platform, which shows a threefold enhancement in tunable THG emission arising from the combined nonlinear responses of Sb$_2$S$_3$ and silicon, and benefiting from enhanced electromagnetic field confinement enabled by the metasurface's high-Q resonances.

physics.optics↗

arXiv2026

In-situ Silicon Doped hBN by High-Temperature Molecular Beam Epitaxy Enables Single Photon Emission

Hexagonal boron nitride (hBN) has emerged as a leading host for optically active quantum defects. Yet introduction of specific impurity species other than carbon remains unexplored. Here, we demonstrate an in-situ silicon doping of hBN grown by high-temperature molecular beam epitaxy (HT-MBE). By systematically varying the growth temperature from 900 to 1390 °C under a constant silicon flux, we establish an optimal window for Si incorporation to host a diverse range of emitters from 430-750 nm at room temperature. By transferring silicon-doped hBN film on SiO$_2$ substrate, we verified that single photon emitter activity was sustained in the hBN, demonstrating compatibility with device integration. The plausible origins of the observed optical transitions were discussed, and several potential candidates were proposed. Our results demonstrate a step toward a comprehensive understanding of in-situ doping of hBN and its utilization for quantum photonic applications.

physics.optics↗

arXiv2025

Optically detected magnetic resonance of wafer-scale hexagonal boron nitride thin films

Hexagonal boron nitride (hBN) has recently been shown to host native defects exhibiting optically detected magnetic resonance (ODMR) with applications in nanoscale magnetic sensing and imaging. To advance these applications, deposition methods to create wafer-scale hBN films with controlled thicknesses are desirable, but a systematic study of the ODMR properties of the resultant films is lacking. Here we perform ODMR measurements of thin films (3-2000nm thick) grown via three different methods: metal-organic chemical vapour deposition (MOCVD), chemical vapour deposition (CVD), and molecular beam epitaxy (MBE). We find that they all exhibit an ODMR response, including the thinnest 3nm film, albeit with different characteristics. The best volume-normalised magnetic sensitivity obtained is 30uT/sqrt(Hz um^3). We study the effect of growth temperature on a series of MOCVD samples grown under otherwise fixed conditions and find 800-900C to be an optimum range for magnetic sensitivity, with a significant improvement (up to two orders of magnitude) from post-growth annealing. This work provides a useful baseline for the magnetic sensitivity of hBN thin films deposited via standard methods and informs the feasibility of future sensing applications.

cond-mat.mtrl-sci↗

arXiv2024

Hyperpixels: Pixel Filter Arrays of Multivariate Optical Elements for Optimized Spectral Imaging

We introduce the concept of `hyperpixels' in which each element of a pixel filter array (suitable for CMOS image sensor integration) has a spectral transmission tailored to a target spectral component expected in application-specific scenes. These are analogous to arrays of multivariate optical elements that could be used for sensing specific analytes. Spectral tailoring is achieved by engineering the heights of multiple sub-pixel Fabry-Perot resonators that cover each pixel area. We first present a design approach for hyperpixels, based on a matched filter concept and, as an exemplar, design a set of 4 hyperpixels tailored to optimally discriminate between 4 spectral reflectance targets. Next, we fabricate repeating 2x2 pixel filter arrays of these designs, alongside repeating 2x2 arrays of an optimal bandpass filters, perform both spectral and imaging characterization. Experimentally measured hyperpixel transmission spectra show a 2.4x reduction in unmixing matrix condition number (p=0.031) compared to the optimal band-pass set. Imaging experiments using the filter arrays with a monochrome sensor achieve a 3.47x reduction in unmixing matrix condition number (p=0.020) compared to the optimal band-pass set. This demonstrates the utility of the hyperpixel approach and shows its superiority even over the optimal bandpass case. We expect that with further improvements in design and fabrication processes increased performance may be obtained. Because the hyperpixels are straightforward to customize, fabricate and can be placed atop monochrome sensors, this approach is highly versatile and could be adapted to a wide range of real-time imaging applications which are limited by low SNR including micro-endoscopy, capsule endoscopy, industrial inspection and machine vision.

physics.optics↗

arXiv2024

Single- and multi-layer micro-scale diffractive lens fabrication for fiber imaging probes with versatile depth-of-field

Hair-thin optical fiber endoscopes have opened up new paradigms for advanced imaging applications in vivo. In certain applications, such as optical coherence tomography (OCT), light-shaping structures may be required on fiber facets to generate needle-like Bessel beams with large depth-of-field, while in others shorter depths of field with high lateral resolutions are preferable. In this paper, we demonstrate a novel method to fabricate light-shaping structures on optical fibres, achieved via bonding encapsulated planar diffractive lenses onto fiber facets. Diffractive metallic structures have the advantages of being simple to design, fabricate and transfer, and our encapsulation approach is scalable to multi-layer stacks. As a demonstration, we design and transfer a Fresnel zone plate and a diffractive axicon onto fiber facets, and show that the latter device generates a needle-like Bessel beam with 350 mu m focal depth. We also evaluate the imaging performance of both devices and show that the axicon fiber is able to maintain focussed images of a USAF resolution target over a 150 mu m distance. Finally, we fabricate a two-layer stack of Fresnel zone plates on a fiber and characterise the modified beam profile and demonstrate good imaging performance. We anticipate our fabrication approach could enable multi-functional complex optical structures (e.g. using plasmonics, polarization control) to be integrated onto fibers for ultra-thin advanced imaging and sensing.

physics.optics↗

arXiv2020

Identifying Carbon as the Source of Visible Single Photon Emission from Hexagonal Boron Nitride

Single photon emitters (SPEs) in hexagonal boron nitride (hBN) have garnered significant attention over the last few years due to their superior optical properties. However, despite the vast range of experimental results and theoretical calculations, the defect structure responsible for the observed emission has remained elusive. Here, by controlling the incorporation of impurities into hBN and by comparing various synthesis methods, we provide direct evidence that the visible SPEs are carbon related. Room temperature optically detected magnetic resonance (ODMR) is demonstrated on ensembles of these defects. We also perform ion implantation experiments and confirm that only carbon implantation creates SPEs in the visible spectral range. Computational analysis of hundreds of potential carbon-based defect transitions suggest that the emission results from the negatively charged VBCN- defect, which experiences long-range out-of-plane deformations and is environmentally sensitive. Our results resolve a long-standing debate about the origin of single emitters at the visible range in hBN and will be key to deterministic engineering of these defects for quantum photonic devices.

physics.app-ph↗

arXiv2014

Amplification of electromagnetic waves excited by a chain of propagating magnetic vortices in YBaCuO Josephson-junction arrays at 77K and above

When a soliton propagates in a discrete lattice it excites small-amplitude linear waves in its wake. In a dc current-biased Josephson-junction (JJ) array these manifest as electromagnetic (EM) waves excited by a (magnetic field induced) chain of propagating magnetic vortices. When the vortex velocity and the phase velocity of one of the excited EM waves match, phase-locking occurs. This produces resonant steps in the current-voltage characteristics where amplification of EM radiation occurs. We report the first observation of phase-locking-induced amplification of EM radiation at 77K and above in JJ arrays made of high temperature superconductors.

cond-mat.supr-con↗