Reconfigurable phase-change optical metasurfaces: novel design concepts to practicable devices

Optical metasurfaces have been proven to be capable of controlling amplitude, phase and polarization of optical beams without the need of bulky geometries, making them really attractive for the development of compact photonic devices. Recently, their combination with chalcogenide phase-change materials (traditionally employed in non-volatile optical and electrical memories), whose refractive index can be reversibly and repeatedly controlled, has been proposed to yield low power consumption tunable metasurfaces having several functionalities in a single device. However, despite phase-change memories are commercially available since various decades now, the unification of phase-change materials with metasurfaces towards real life applications is becoming a formidable task, mainly due to the several engineering branches involved in this technology, which sometimes compromise each other in a non-trivial way. This includes thermo/optical, thermo/electric, and chemical incompatibilities which are typically not taken into account by researchers working in the field, resulting in devices having exciting reconfigurable properties, but at the same time, lack of practicability. This thesis is therefore dedicated to the development of novel phase-change metasurface architectures which could partially or totally address such engineering problems. Particular emphasis has been put in the realization of reconfigurable metasurfaces for active wavefront control, as such a functionality remains relatively unexplored. The first part of this thesis focuses in the first experimental demonstration of active, reconfigurable non-mechanical beam steering devices working the near-infrared. This was achieved via integration of ultra-thin films of chalcogenide phase-change materials (in this case, the widely employed alloy Ge2Sb2Te5) within the body of a dielectric spacer in a plasmonic metal/insulator/metal metasurface architecture. Active, and optically reversible beam steering between two different angles with efficiencies up to 40% were demonstrated. The second part of this work shows the work carried out in metal-free metasurfaces as a way to manipulate optical beams with high efficiency in both transmission and/or reflection. This was achieved via combination of all-dielectric silicon nanocylinders with deeply-subwavelenght sized Ge2Sb2Te5 inclusions. By strategic placement of the phase-change inclusions in the regions of high electric field density, independent and active control of the metasuface resonances is demonstrated, with modulations depths as high as 70% and 65% in reflection and transmission respectively. Multilevel, and fully reversible optically-induced switching of the phasechange layer is also reported, with up to 11 levels of tunability over 8 switching cycles. Finally, the last section of this thesis introduces the concept of hybrid dielectric/plasmonic phase-change metasurfaces having key functional benefits when compared to both purely dielectric and plasmonic approaches. The proposed architectures showed great versatility in terms of both active amplitude and phase control, offering the possibility of designing devices for different purposes (i.e. such as active absorbers/modulators or beam steerers with enhanced efficiency) employing the same unit-cell configuration with minor geometry re-optimizations. Initial device experimental demonstrations of such an approach are discussed, as well as their potential in terms of delivering in-situ electrical switching capabilities using a metallic ground plane as a resistive heater.Engineering and Physical Sciences Research Council (EPSRC

The Fate of Nephrons in Congenital Obstructive Nephropathy: Adult Recovery is Limited by Nephron Number Despite Early Release of Obstruction

Urinary tract obstruction and reduced nephron number often occur together as a result of maldevelopment of kidneys and urinary tract. We wished to determine the role of nephron number on the adaptation of remaining nephrons of mice subjected to neonatal partial unilateral ureteral obstruction (UUO) and followed through adulthood

Phase-change metasurfaces for dyamic beam steering and beam shaping in the infrared

This is the author accepted manuscript. The final version is available from the publisher via the DOI in this recordWe present novel phase-change material based metasurfaces for dynamic, recnofigurable and efficient wavefront shaping in the infrared spectrum. Dynamic control and reconfigurability was obtained by incorporating an ultra-thin layer of the widely-used phase change material Ge2Sb2Te5. Our approach exploits hybrid dielectic/plasmonic resonances to achieve local (subwavelength) phase control of light with low losses. A full 2π optical phase coverage was achieved with this approach, which allows for a wide flexibility in terms of realizable designs. To illustrate this concept, dynamic beam steering devices and reconfigurable planar focusing mirrors (both operating at optical telecommunications wavelengths) and their performance investigated. Absolute efficiencies up to 65% are achieved, significantly higher than the efficiencies of more commonly reported plasmonic-based phase-change metasurfaces.CDW acknowledges funding via the US Naval Research Laboratories ONRG programme (#N62909-16-1-2174) and the EPSRC ChAMP and WAFT grants (EP/M015130/1 and EP/M015173/1). CRdeG acknowledges funding via the EPSRC CDT in Metamaterials (EP/L015331/1). CRdeG Acknowledges Joaquin Faneca-Ruedas and Dr Anna Baldycheva

Infrared Phase-Change Meta-Devices with In-Situ Switching

This is the author accepted manuscript. The final version is available from the European Phase Change and Ovonics Symposium via the link in this recordWe describe a possible device design approach and an experimental test platform suitable for the realization and characterization of phase-change based meta-devices incorporating in-situ switching and operating at infrared wavelengths. Measurements on such a prototype device working at 1.55 µm are presented.US Naval Research LaboratoriesEngineering and Physical Sciences Research Council (EPSRC

All-dielectric hybrid silicon/Ge2Sb2Te5 optical metasurfaces for tunable and switchable light control in the near infrared

This is the final version.We report a novel reconfigurable metasurface based on the combination of all-dielectric arrays of silicon meta-atoms, with deeply subwavelength (< λ0/150) Ge2Sb2Te5 layers. Our approach allows to selectively and individually control electric and magnetic resonances.Engineering and Physical Sciences Research Council (EPSRC

Lensfree optofluidic plasmonic sensor for real-time and label-free monitoring of molecular binding events over a wide field-of-view

We demonstrate a high-throughput biosensing device that utilizes microfluidics based plasmonic microarrays incorporated with dual-color on-chip imaging toward real-time and label-free monitoring of biomolecular interactions over a wide field-of-view of >20 mm^2. Weighing 40 grams with 8.8 cm in height, this biosensor utilizes an opto-electronic imager chip to record the diffraction patterns of plasmonic nanoapertures embedded within microfluidic channels, enabling real-time analyte exchange. This plasmonic chip is simultaneously illuminated by two different light-emitting-diodes that are spectrally located at the right and left sides of the plasmonic resonance mode, yielding two different diffraction patterns for each nanoaperture array. Refractive index changes of the medium surrounding the near-field of the nanostructures, e.g., due to molecular binding events, induce a frequency shift in the plasmonic modes of the nanoaperture array, causing a signal enhancement in one of the diffraction patterns while suppressing the other. Based on ratiometric analysis of these diffraction images acquired at the detector-array, we demonstrate the proof-of-concept of this biosensor by monitoring in real-time biomolecular interactions of protein A/G with immunoglobulin G (IgG) antibody. For high-throughput on-chip fabrication of these biosensors, we also introduce a deep ultra-violet lithography technique to simultaneously pattern thousands of plasmonic arrays in a cost-effective manner

Field-portable optofluidic plasmonic biosensor for wide-field and label-free monitoring of molecular interactions

We demonstrate a field-portable optofluidic plasmonic sensing device, weighing 40 g and 7.5 cm in height, which merges plasmonic microarrays with dual-wavelength lensfree on-chip imaging for real-time monitoring of protein binding kinetics

Phase-change meta-photonics

We combine phase-change materials and metamaterial arrays (metasurfaces) to create new forms of dynamic, tuneable and reconfigurable photonic devices including ‘perfect’ absorbers, infra-red light modulators, optical beam steerers and enhanced phase-change optoelectronic displays

Overcoming diffusion issues in hybrid phase-change metasurfaces

This is the final version.The optical performance of phase-change metasurfaces can be irreversibly degraded by the effect of thermally activated diffusion during device switching. Here we present a systematic case-study of this effect in metasurfaces designed for modulation in the O and C telecommunication bands and a method for addressing such issues using ultra-thin Si3N4 barrier layers.Engineering and Physical Sciences Research Council (EPSRC