Resonant nanostructured metallic devices have attracted considerable recent attention through phenomena such as
extraordinary transmission and their potential application as sensing elements, metamaterials and for enhancing
nonlinear optical effects. Here we report on the investigation of the geometry and material properties on the performance
of periodic and random arrays of coaxial apertures in thin metallic films. Such apertures in perfect conductors have been
shown to resonate at a wavelength governed by the geometry of the apertures leading to enhanced transmission. This
resonant wavelength is dictated by the cutoff wavelength of the fundamental mode propagating in the corresponding
coaxial waveguide and, as a consequence, is largely independent of whether the apertures are isolated or in random or
periodic arrangements. In the case of periodic samples, however, these resonances can coherently couple to surface
waves to produce an analogue of the enhanced optical transmission seen in arrays of circular and other apertures. We
have previously shown that as the width of the rings decreases, there are substantial red-shifts in the resonant wavelength
from that predicted for perfect conductivity when the optical properties of the metal are considered. Here we report on
recent developments in fabrication, design and modelling of metallic resonant structures and their near- and far-field
optical characterisation. In particular, we consider the relationship between random and regular arrangements of
The near and far-field transmission characteristics of nanoscale annular array metamaterials fabricated using ion beam
lithography are investigated both computationally and experimentally. Experimental results in the far-field regime
demonstrate high transmission efficiencies in the near infra-red region of the electromagnetic spectrum for these devices,
in excellent qualitative agreement with a previously developed numerical model. The diffractive near-field behavior of
such structures is discussed, with a particular emphasis on the implications associated with verifying such predictions
We describe how a quantum non-demolition device based on electromagnetically-induced transparency in solidstate atom-like systems could be realized. Such a resource, requiring only weak optical nonlinearities, could potentially enable photonic quantum information processing (QIP) that is much more efficient than QIP based on linear optics alone. As an example, we show how a parity gate could be constructed. A particularly interesting physical system for constructing devices is the nitrogen-vacancy defect in diamond, but the excited-state structure for this system is unclear in the existing literature. We include some of our latest spectroscopic results that indicate that the optical transitions are generally not spin-preserving, even at zero magnetic field, which allows the realization of a Λ-type system.
In the last decade, the synthesis and characterization of nanometer sized carbon clusters have attracted growing interest within the scientific community. This is due to both scientific interest in the process of diamond nucleation and growth, and to the promising technological applications in nanoelectronics and quantum communications and computing. Our research group has demonstrated that MeV carbon ion implantation in fused silica followed by thermal annealing in the presence of hydrogen leads to the formation of nanocrystalline diamond, with cluster size ranging from 5 to 40 nm. In the present paper, we report the synthesis of carbon nanoclusters by the implantation into fused silica of keV carbon ions using the Plasma Immersion Ion Implantation (PIII) technique, followed by thermal annealing in forming gas (4% 2H in Ar). The present study is aimed at evaluating this implantation technique that has the advantage of allowing high fluence-rates on large substrates. The carbon nanostructures have been characterized with optical absorption and Raman spectroscopies, cross sectional Transmission Electron Microscopy (TEM), and Parallel Electron Energy Loss Spectroscopy (PEELS). Nuclear Reaction Analysis (NRA) has been employed to evaluate the deuterium incorporation during the annealing process, as a key mechanism to stabilize the formation of the clusters.
Single electron transistors (SET) are devices that can be used as highly sensitive electrometers, with sensitivities approaching the quantum noise level. They can be used as measurement devices for quantum systems, including quantum computers and quantum-dot cellular automata, and as logic elements in their own right as replacements for MOSFETs. For solid-state quantum computing applications it is vital to maximize the charge sensitivity to decrease readout time and increase readout fidelity. We analyze the interactions between a SET and a double dot system. The SET sensitivity is described in terms of the current variation through the SET due to a single electron's location on the two dots. We use finite element modeling to determine the capacitive coupling between all objects in our system, which in turn allows us to determine the current in the SET based on steady-state energy minimization arguments. We base our geometries on experiments involving a twin-SET device developed as a prototype for solid-state quantum computing read-out. Our model allows us to systematically vary the device geometry in order to optimize the sensitivity of the SET.
Although sub-micron structures have been fabricated with ion beam lithography using focused MeV ions, the best resolution of the method has not yet been approached. The best resolution is potentially around 10 nm which is the diameter of latent damage produced by the passage of a single fast ion through sensitive materials where the ion range could be tens of micrometres. In principle, the latent damage can be developed to create very high aspect ratio nanostructures. We call this technique single ion nanolithography. In order to approach the ultimate resolution of lithography with single ions we investigate the resist material, the exposure as a function of ion type and development parameters. To implement the technique we have developed a novel strategy that employs a resist film on an active substrate that functions as a detector sensitive to single ion impacts. Together with a focused microbeam, the precise control of ion fluence attained by counting ion impacts allows us to perform a convenient systematic study of the track formation and seek conditions where single ion tracks can be produced. We report here the current status of the investigations using PMMA and CR-39 resists which are shown to be sensitive to single ions. A key issue is also the post-development imaging method.
We report recent progress in single keV ion implantation and online detection for the controlled implantation of single donors in silicon. When integrated with silicon nanofabrication technology this forms the “top down” strategy for the construction of prototype solid state quantum computer devices based on phosphorus donors in silicon. We have developed a method of single ion implantation and online registration that employs detector electrodes adjacent to the area into which the donors are to be implanted. The implantation sites are positioned with nanometer accuracy using an electron beam lithography patterned PMMA mask. Control of the implantation depth of 20 nm is achieved by tuning the phosphorus ion energy to 14 keV. The counting of single ion implantation in each site is achieved by the detection of e-/h+ pairs produced by the implanted phosphorus ion in the substrate. The system is calibrated by use of Mn K-line x-rays (5.9 and 6.4 keV) and we find the ionization energy of the 14 keV phosphorus ions in silicon to be about 3.5-4.0 keV for implants through a 5 nm SiO2 surface layer. This paper describes the development of an improved PIN detector structure that provides more reliable performance of the earlier MOS structure. With the new structure, the energy noise threshold has been minimized to 1 keV or less. Unambiguous detection/counting of single keV ion implantation events were achieved with a confidence level greater than 98% with a reliable and reproducible fabrication process.
We report on progress towards a charge-based qubit using phosphorus atoms implanted in a silicon substrate. Prototype devices have been fabricated using standard lithographic techniques together with a new method of controlled single ion implantation using on-chip detector electrodes. Positional accuracy of the implanted ions was achieved using a nanoaperture mask defined using electron beam lithography. The two implanted phosphorus atoms are positioned ~50 nm apart, to form a qubit test device. A series of process steps has been developed to repair implant damage, define surface control gates, and to define single electron transistors used for qubit readout via the detection of sub-electron charge transfer signals. Preliminary electrical measurements on these devices show single charge transfer events that are resilient to thermal cycling.
By capactively coupling sensitive charge detectors (i.e. single-electron transistors - SETs) to nanostructures such as quantum dots and two-dimensional systems, it is possible to investigate charge transport properties in extremely low conduction regimes where direct transport measurements are increasingly difficult. Ion-implanted nano-MOSFETs coupled to aluminium SETs have been constructed in order to study charge transport between locally doped regions in Si at mK temperatures. This configuration allows for direct source-drain measurement as well as non-invasive charge detection. Of particular interest are the effects of material defects and gate control on charge transport, which is of relevance to Si-based quantum computing.
Ion beam implantation using 2.4 MeV H+ ions was used to fabricate long period gratings in multimode optical fibres. The Bragg peak from the ion beam was located in the centre of the core of the fiber. The ion beam was implanted through a mask on the fibre with a period of 1.1 mm to produce a periodic a refractive index modulation, of up to 1%, along the axis of the core of the fibre. The mode scrambling, from the established fundamental mode, was monitored in situ as the implantation was done by observation of the light transmitted through the fibre. This paper describes the experimental method used to achieve this and describes the results. Significant mode scrambling was observed to occur when 3 grating periods were written into the fibre.
The construction of micro- and nano-scale electronic devices that exploit the properties of single atoms have been proposed. A very promising device is a silicon based solid state quantum computer based on an array of single 31P atoms as qubits in a pure 28Si substrate. Operation of the device requires independent control, coupling and readout of the state of individual qubits. We have developed a construction strategy for a few qubit device based on ion implantation of the qubits into prefabricated cells. An ion energy of less than 20 keV is necessary to ensure the ion range is at the required depth in the substrate which is of the order of 20 nm. Single ion impacts are registered by the electrical transient induced in an external circuit. Electron Beam Lithography fabricated cells, containing electrodes of the required nanometre scale, have been implanted with 14 keV 31P ions and the pulse height spectrum of single ion impacts has been successfully recorded. Discrimination on the pulse height allows rejection of ions that suffer unacceptable straggling. This opens the way to the rapid construction of a two qubit device in the first instance that will test many of the essential mechanisms of a revolutionary solid state quantum computer.
The nuclear spin quantum computer proposed by Kane1 exploits as a qubit array 31P dopants embedded within a silicon matrix. Single-qubit operations are controlled by the application of electrostatic potentials via a set of metallic A gates, situated above the donors, on the silicon surface, that tune the resonance frequency of individual nuclear spins, and a globally applied RF magnetic field that flips spins at resonance. Coupling between qubits is controlled by the application of potentials via a set of J gates, between the donors, that induce an electron-mediated coupling between nuclear spins. We report the results of a study of the electric field and potential profiles arising within the Kane device from typical gate operations. The extent to which a single nuclear spin can be tuned independently of its neighbours, by operation of an associated A-gate, is examined and key design parameters in the Kane architecture are addressed. Implications for current fabrication strategies involving the implantation of 31P atoms are discussed. Solution of the Poisson equation has been carried out by simulation using a TCAD modeling package (Integrated Systems Engineering AG).
We describe a novel technique for the fabrication of nanoscale structures, based on the development of localized chemical modification caused in a PMMA resist by the implantation of single ions. The implantation of 2 MeV He ions through a thin layer of PMMA into an underlying silicon substrate causes latent damage in the resist. On development of the resist we demonstrate the formation within the PMMA layer of clearly defined etched holes, of typical diameter 30 nm, observed using an atomic force microscope employing a carbon nanotube SPM probe in intermittent-contact mode. This technique has significant potential applications. Used purely to register the passage of an ion, it may be a useful verification of the impact sites in an ion-beam modification process operating at the single-ion level. Furthermore, making use of the hole in the PMMA layer to perform subsequent fabrication steps, it may be applied to the fabrication of self-aligned structures in which surface features are fabricated directly above regions of an underlying substrate that are locally doped by the implanted ion. Our primary interest in single-ion resists relates to the development of a solid-state quantum computer based on an array of 31P atoms (which act as qubits) embedded with nanoscale precision in a silicon matrix. One proposal for the fabrication of such an array is by phosphorous-ion implantation. A single-ion resist would permit an accurate verification of 31P implantation sites. Subsequent metalisation of the latent damage may allow the fabrication of self-aligned metal gates above buried phosphorous atoms.
We describe progress in a range of nanofabrication processes for the production of silicon-based quantum computer devices. The processes are based upon single-ion implantation to place phosphorus-31 atoms in accurate locations, precisely self-aligned to metal control gates. These fabrication schemes involve multi-layer resist and metal structures, electron beam lithography and multi-angled aluminium shadow evaporation. The key feature of all fabrication schemes is a gate pattern defined in a resist structure using electron beam lithography, used in conjunction with a second pattern written in another resist layer. The locations where the two patterns overlap define channels down to the substrate through which ions can be implanted, with the remaining metal/resist structure behaving as a mask. Further processing on the resist structures allows for deposition of the control gates and read-out structures. Central to this process is a new technique which allows for control of the implantation process at a single-ion level.