Valerian H Hall-Chen | University of Oxford (original) (raw)
Papers by Valerian H Hall-Chen
The minimisation of birefringence, or polarisation mode dispersion, is vital for simplifying and ... more The minimisation of birefringence, or polarisation mode dispersion, is vital for simplifying and miniaturising photonic components. In this work, we present a systematic study of the slot waveguide geometries required for having zero birefringence (ZB). We show that the rail widths required for ZB are more strongly dependent on the height of the waveguide than on the slot separation. After which, we demonstrate that the ZB geometry is significantly affected by the slanting of the waveguide walls. This paper proceeds to show that within the range studied, one can fix the height, slot, slant angle, and bend radius, and still achieve ZB by varying the widths of both of the rails. Given a fabrication tolerance of 5 nm, we show that a coherence length on the order of a hundred microns can be achieved. We finish by showing that for straight and bent ZB waveguides, having symmetric rails is preferable due to higher tolerances and lower sensitivity to bending. Since any arbitrarily shaped slot waveguide is a combination of both single mode straight and bent waveguides, we have a toolbox from which we can achieve ZB for any given slot and height.
A gyroid structure is a distinct morphology that is triply periodic and consists of minimal isosu... more A gyroid structure is a distinct morphology that is triply periodic and consists of minimal isosurfaces containing no straight lines. We have designed and synthesized amorphous silicon (a-Si) mid-infrared gyroid photonic crystals that exhibit a complete bandgap in infrared spectroscopy measurements. Photonic crystals were synthesized by deposition of a-Si/Al2O3 coatings onto a sacrificial polymer scaffold defined by two-photon lithography. We observed a 100% reflectance at 7.5 μm for single gyroids with a unit cell size of 4.5 μm, in agreement with the photonic bandgap position predicted from full-wave electromagnetic simulations, whereas the observed reflection peak shifted to 8 μm for a 5.5 μm unit cell size. This approach represents a simulation-fabrication-characterization platform to realize three-dimensional gyroid photonic crystals with well-defined dimensions in real space and tailored properties in momentum space.
The optimal geometry of silicon-organic hybrid slot waveguides is investigated in the context of ... more The optimal geometry of silicon-organic hybrid slot waveguides is investigated in the context of the efficiency of four-wave mixing (FWM), a χ(3) nonlinear optical process. We study the effect of slot and waveguide widths, as well as waveguide asymmetry on the two-photon absorption (TPA) figure of merit and the roughness scattering loss. The optimal waveguide core width is shown to be 220nm (symmetric) with a slot width of 120nm, at a fixed waveguide height of 220nm. We also show that state-of-the-art slot waveguides can outperform rib waveguides, especially at high powers, due to the high TPA figure-of-merit.
Cobalt films with in-plane anisotropy were studied using two-axis magnetometry. Uniaxial anisotro... more Cobalt films with in-plane anisotropy were studied using two-axis magnetometry. Uniaxial anisotropy was induced in Co films by linear features on the substrate. These linear features were formed by various means: self-assembly of CaF 2 grooves, nano-imprinting, and high energy atom flux or steps on vicinal surface of Si. From the analysis of components of magnetisation parallel and perpendicular to the applied field, it was found that the extrinsic switching field distribution (SFD) of all samples was an exponential function of the field. Modelling of hysteresis loops with this SFD gives good fit with experiments for linear features studied. It follows that the exponential SFD is a common property of magnetic thin films having in-plane uniaxial anisotropy and magnetisation reversal as coherent rotation.
The minimisation of birefringence, or polarisation mode dispersion, is vital for simplifying and ... more The minimisation of birefringence, or polarisation mode dispersion, is vital for simplifying and miniaturising photonic components. In this work, we present a systematic study of the slot waveguide geometries required for having zero birefringence (ZB). We show that the rail widths required for ZB are more strongly dependent on the height of the waveguide than on the slot separation. After which, we demonstrate that the ZB geometry is significantly affected by the slanting of the waveguide walls. This paper proceeds to show that within the range studied, one can fix the height, slot, slant angle, and bend radius, and still achieve ZB by varying the widths of both of the rails. Given a fabrication tolerance of 5 nm, we show that a coherence length on the order of a hundred microns can be achieved. We finish by showing that for straight and bent ZB waveguides, having symmetric rails is preferable due to higher tolerances and lower sensitivity to bending. Since any arbitrarily shaped slot waveguide is a combination of both single mode straight and bent waveguides, we have a toolbox from which we can achieve ZB for any given slot and height.
A gyroid structure is a distinct morphology that is triply periodic and consists of minimal isosu... more A gyroid structure is a distinct morphology that is triply periodic and consists of minimal isosurfaces containing no straight lines. We have designed and synthesized amorphous silicon (a-Si) mid-infrared gyroid photonic crystals that exhibit a complete bandgap in infrared spectroscopy measurements. Photonic crystals were synthesized by deposition of a-Si/Al2O3 coatings onto a sacrificial polymer scaffold defined by two-photon lithography. We observed a 100% reflectance at 7.5 μm for single gyroids with a unit cell size of 4.5 μm, in agreement with the photonic bandgap position predicted from full-wave electromagnetic simulations, whereas the observed reflection peak shifted to 8 μm for a 5.5 μm unit cell size. This approach represents a simulation-fabrication-characterization platform to realize three-dimensional gyroid photonic crystals with well-defined dimensions in real space and tailored properties in momentum space.
The optimal geometry of silicon-organic hybrid slot waveguides is investigated in the context of ... more The optimal geometry of silicon-organic hybrid slot waveguides is investigated in the context of the efficiency of four-wave mixing (FWM), a χ(3) nonlinear optical process. We study the effect of slot and waveguide widths, as well as waveguide asymmetry on the two-photon absorption (TPA) figure of merit and the roughness scattering loss. The optimal waveguide core width is shown to be 220nm (symmetric) with a slot width of 120nm, at a fixed waveguide height of 220nm. We also show that state-of-the-art slot waveguides can outperform rib waveguides, especially at high powers, due to the high TPA figure-of-merit.
Cobalt films with in-plane anisotropy were studied using two-axis magnetometry. Uniaxial anisotro... more Cobalt films with in-plane anisotropy were studied using two-axis magnetometry. Uniaxial anisotropy was induced in Co films by linear features on the substrate. These linear features were formed by various means: self-assembly of CaF 2 grooves, nano-imprinting, and high energy atom flux or steps on vicinal surface of Si. From the analysis of components of magnetisation parallel and perpendicular to the applied field, it was found that the extrinsic switching field distribution (SFD) of all samples was an exponential function of the field. Modelling of hysteresis loops with this SFD gives good fit with experiments for linear features studied. It follows that the exponential SFD is a common property of magnetic thin films having in-plane uniaxial anisotropy and magnetisation reversal as coherent rotation.