Enhanced photoelectric and photothermal responses on silicon platform by plasmonic absorber and omni-schottky junction

Recent progresses in plasmon-induced hot electrons open up the possibility to achieve photon harvesting beyond the fundamental limit imposed by band-to-band transitions in semiconductors. To obtain high efficiency, both the optical absorption and electron emission/collection are crucial factors that need to be addressed in the design of hot electron devices. Here, we demonstrate a photoresponse as high as 3.3mA/W at 1500nm on a silicon platform by plasmonic absorber (PA) and omni-Schottky junction integrated photodetector, reverse biased at 5V and illuminated with 10mW. The PA fabricated on silicon consists of a monolayer of random Au nanoparticles (NPs), a wide-band gap semiconductor (TiO2) and an optically thick Au electrode, resulting in broadband near-infrared (NIR) absorption and efficient hot-electron transfer via an all-around Schottky emission path. Meanwhile, time and spectral-resolved photoresponse measurements reveal that embedded NPs with superior absorption resembling plasmonic local heating sources can transfer their energy to electricity via the photothermal mechanism, which until now has not been adequately assessed or rigorously differentiated from the photoelectric process in plasmon-mediated photon harvesting nano-systems

Controlling light-matter interaction with resonant semiconductor nanostructure

This thesis aims to bridge dielectric materials' optical and electronic properties to obtain full control of light-matter interaction at the nanoscale. The outcomes may open the way for tunable, ultra-thin, cost-effective, and energy-saving optoelectronic devices. This research first studies the optical modes of dielectric nanostructures, including toroidal dipole (TD) excitation under illuminations of structured light. The quantitative comparison between the structured light and plane wave illuminations shows a lot of promise for exciting dominant toroidal response in the geometrically simple photonic systems. The tightly focused radially polarised illumination shows a near-pure excitation of the TD in dielectric nanodisk. Additionally, it will be shown that the focused doughnut pulse could be a promising tool for the resonant excitation of toroidal response in photonic structures. Toroidal excitations are a potential way of increasing light-harvesting and boosting nonlinear light-matter interactions. This thesis is then involved in pioneering research in light detection by utilising nontrivial optical modes of dielectric nanostructures to improve the electrical characteristics of conventional photodetectors. It would open the way for all-dielectric nanophotonics to be at the same level of consumer products as electronics. We study the realisation of the high-speed and highly efficient photodetectors using germanium (Ge) metasurfaces. Semiconductors such as Ge are materials that are compatible with the complementary metal–oxide–semiconductor process and thus are the proper building material for the high-volume foundry process of photonic integrated circuits (PICs). The optical properties and steady-state and transient electric behaviours will be studied to analyse the electrical response at the telecommunication C-band, a major spectral choice for optical communication and signal processing in PICs. We also propose a polarisation-independent metasurface superabsorber by exploring the quasi-bound state in the continuum (QBIC) to improve photodetectors’ electrical characteristics, including their responsivity. As the asymmetry parameter mostly governs the Q-factor of QBICs, it gives a straightforward and efficient way of optimising the light absorption using critical coupling. The metasurface is designed to operate at the C-band, but it can be tuned for other bands in the telecommunication frequency range. Two designs boosting the light collection efficiency up to 50% in the transmission and up to 100% in the reflection modes will be proposed in this thesis. Despite the symmetry-broken nature of QBICs, our metasurface is insensitive to the polarisation of incoming light and thus provides great flexibility in the practical applicability of QBIC-based metasurfaces

Antiferromagnetic nanodiscs with perpendicular magnetic anisotropy for biological applications

Abstract Magnetic particles have been widely implemented across research areas in liquid and biological environments, such as cancer therapy, drug delivery and cell sorting. Perpendicularly magnetised (PM) synthetic antiferromagnetic (SAF) particles exhibit a range of desirable properties that make them strong candidates for applications in fluid. These include a zero remanence state, sharp and tunable switching, and a high and variable saturation magnetisation. Abstract This thesis will first continue to explore the flexibility of the SAF particle design. Through engineering of the SAF thin film, the basis of the particles, this work demonstrates the ability to tune the SAFs towards a suite of applications. In addition, a novel take on the fabrication of thin film based magnetic nanoparticles is presented. This method is capable of efficient and effective production of particularly high yields of well-defined nanodiscs with robust magnetic properties. Abstract The magnetic behaviour of the PM SAF particles, and the thin films they are created from, is analysed with particular focus on the characteristics displayed in a fluidic environment. This leads to the discovery of novel magneto-mechanical transitions of SAF particles in liquid and continues to demonstrate the applicability of SAFs across a spectrum of applications. Abstract Application of the PM SAF particles is examined in the context of cancer therapy. Previous studies into SAFs in cancer therapy utilised them in the magneto-mechanical destruction of tumour cells. Leading on from this work, the concept of integrating SAF microdiscs with iron oxide nanoparticles (IONPs) in a ‘magnetic combination therapy’ is explored. This preliminary study reveals interesting inter-particle interactions between the SAFs and IONPs and shows the potential for a synergistic effect in the combined therapy. Abstract This work provides a robust toolbox for the fabrication of tailored nanodiscs for use in a range of fluidic and biological applications

Strong modulation of plasmons in Graphene with the use of an Inverted pyramid array diffraction grating

An optical device configuration allowing efficient electrical tuning of surface plasmon wavelength and absorption in a suspended/conformal graphene film is reported. An underlying 2-dimensional array of inverted rectangular pyramids greatly enhances optical coupling to the graphene film. In contrast to devices utilising 1D grating or Kretchman prism coupling configurations, both s and p polarization can excite plasmons due to symmetry of the grating structure. Additionally, the excited high frequency plasmon mode has a wavelength independent of incident photon angle allowing multidirectional coupling. By combining analytical methods with Rigorous Coupled-Wave Analysis, absorption of plasmons is mapped over near infrared spectral range as a function of chemical potential. Strong control over both plasmon wavelength and strength is provided by an ionic gel gate configuration. 0.04eV change in chemical potential increases plasmon energy by 0.05?eV shifting plasmon wavelength towards the visible, and providing enhancement in plasmon absorption. Most importantly, plasmon excitation can be dynamically switched off by lowering the chemical potential and moving from the intra-band to the inter-band transition region. Ability to electrically tune plasmon properties can be utilized in applications such as on-chip light modulation, photonic logic gates, optical interconnect and sensing applications

Development of a high brightness ultrafast Transmission Electron Microscope based on a laser-driven cold field emission source

We report on the development of an ultrafast Transmission Electron Microscope based on a cold field emission source which can operate in either DC or ultrafast mode. Electron emission from a tungsten nanotip is triggered by femtosecond laser pulses which are tightly focused by optical components integrated inside a cold field emission source close to the cathode. The properties of the electron probe (brightness, angular current density, stability) are quantitatively determined. The measured brightness is the largest reported so far for UTEMs. Examples of imaging, diffraction and spectroscopy using ultrashort electron pulses are given. Finally, the potential of this instrument is illustrated by performing electron holography in the off-axis configuration using ultrashort electron pulses.Comment: 23 pages, 9 figure

Review on Spintronics : Principles and Device Applications

Spintronics is one of the emerging fields for the next-generation nanoelectronic devices to reduce their power consumption and to increase their memory and processing capabilities. Such devices utilise the spin degree of freedom of electrons and/or holes, which can also interact with their orbital moments. In these devices, the spin polarisation is controlled either by magnetic layers used as spin-polarisers or analysers or via spin-orbit coupling. Spin waves can also be used to carry spin current. In this review, the fundamental physics of these phenomena is described first with respect to the spin generation methods as detailed in Sections 2 ~ 9. The recent development in their device applications then follows in Sections 10 and 11. Future perspectives are provided at the end

Design and optimisation of a novel magnetic detection scheme for encoded magnetic information carriers.

Previous work in the field has outlined a method to create micron-sized, tuneable encoded magnetic information carriers that can be redeposited through a liquid suspension. This thesis aims to build on this work, further characterising the information carriers and presenting a possible novel detection technique. The magnetic information carriers in this work use synthetic antiferromagnetic (SAF) particles with perpendicular magnetic anisotropy (PMA), attenuating the coupling strength between the magnetic layers using a platinum interlayer. This provides a controllable magnetic parameter which is used as the basis for the magnetic encoding. These particles can be lifted off the substrate into a solution for redeposition onto a surface which provides a magnetic ‘tag’. The particles are presented and characterised, including statistical distributions of switching events to better understand their detectable properties. A novel detection scheme for these particles is then proposed using inductive sensing and a rotating permanent magnet as a drive field source. Device efficacy is evaluated using computational simulations, allowing for the optimisation of the parameter space before physical building. The efficacy of different input parameters is evaluated using a figure of merit – the number of possible channels the detector can measure. The simulations begin with an idealised model of the detector and particle set, with zero coercivity SAF particles and perfect alignment. The different methods that the detector can be used in are assessed, as well as exploring the possible input geometries. Real-world constraints are later built into the model including the switching distributions of particles and the effects of misalignment. From these, the build constraints and electronic requirements of the system can be characterised. The detector is finally presented virtually through computer-aided design, which would be used to create a prototype model of the device

Exploring the ferromagnetic behaviour of a repulsive Fermi gas via spin dynamics

Ferromagnetism is a manifestation of strong repulsive interactions between itinerant fermions in condensed matter. Whether short-ranged repulsion alone is sufficient to stabilize ferromagnetic correlations in the absence of other effects, like peculiar band dispersions or orbital couplings, is however unclear. Here, we investigate ferromagnetism in the minimal framework of an ultracold Fermi gas with short-range repulsive interactions tuned via a Feshbach resonance. While fermion pairing characterises the ground state, our experiments provide signatures suggestive of a metastable Stoner-like ferromagnetic phase supported by strong repulsion in excited scattering states. We probe the collective spin response of a two-spin mixture engineered in a magnetic domain-wall-like configuration, and reveal a substantial increase of spin susceptibility while approaching a critical repulsion strength. Beyond this value, we observe the emergence of a time-window of domain immiscibility, indicating the metastability of the initial ferromagnetic state. Our findings establish an important connection between dynamical and equilibrium properties of strongly-correlated Fermi gases, pointing to the existence of a ferromagnetic instability.Comment: 8 + 17 pages, 4 + 8 figures, 44 + 19 reference