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We report a SOI ring resonator biosensor array with 1 reference ring and 4 sensing rings, using WDM as the addressing scheme. On-chip referencing and temperature shift cancellation are demonstrated, and the differential signal shows a low temperature sensitivity of ~ ± 0.8 pm/°C.

By exploiting the small bend radius achievable using high-index-contrast silicon photonic wire waveguides, we demonstrate a new low power thermo-optic switch arranged in a dense, double spiral geometry. Such a design permits the waveguide length to be extended for increased phase shift, without the need for increased heated volume. This provides an effective means to reduce the power consumption of thermo-optic switches, as well as a compact geometry desirable for the development of switch arrays. A low switching power of 6.5 mW was obtained for a spiral-path Mach-Zehnder interferometer device having a 10% - 90% rise time of 14 micros. The switching power is shown to be reduced by more than 5 times compared to a Mach-Zehnder interferometer employing a conventional straight waveguide geometry.

Silicon-on-insulator (SOI) is considered the most promising high-index-contrast waveguide platform for fabricating microphotonic devices with high integration density. However, there are fundamental challenges with this waveguide system related to the fixed values of the refractive indices of the constituent materials (Si and SiO 2 ). In this paper, we present a method that can potentially circumvent this limitation by using the sub-wavelength grating effect in SOI waveguides to effectively engineer materials with intermediate refractive indices by standard lithographic patterning. The first implementations of the SWG effect in SOI waveguides are discussed, including our latest results in fiber-chip couplers, anti-reflective and high-reflectivity waveguide facets, mode transformers between waveguides of different geometries, and microphotonic waveguide crossings.

This invited talk presents the evolution of the planar waveguide dispersive devices developed in our laboratory for spatial heterodyne spectroscopy. The basic concepts are introduced and then applied to a Mach-Zehnder interferometer array capable of a resolution of 0.1 nm over a bandwidth of 2.5 nm.

We use star couplers to measure the relative scattering losses of silicon-on-insulator (SOI) ridge waveguides of various widths over the range of 1.75 to 0.2 mum in a single measurement. The scattering loss data obtained for waveguides fabricated by different photolithography and e-beam base processes correlate well with the measured root-mean-square roughness of the waveguide sidewalls obtained using SEM image analysis, and are in qualitative agreement with the prediction of simple scattering loss theory.

We study the evolution of the excited-state photoluminescence of a single self-assembled quantum dot as a function of applied magnetic field in a regime where the cyclotron energy is comparable to the confinement energy. As expected, we observe the splitting of the angular-momentum states following the Fock-Darwin scheme, but we also observe that each state further splits into a doublet which shows an unexpected evolution with increasing magnetic field. This behavior cannot be explained by a simple Zeeman spin splitting but is rather consistent with the observation of spin-orbit coupling where the spin of electrons is coupled with the envelope function orbital angular momentum. We show that the addition of this type of spin-orbit coupling to the single-particle Fock-Darwin model can account for the observed dependence. By comparison of our data with such a model, we derive a spin-orbit intensity of 3.90.310 9 eV cm 2 . DOI: 10.1103/PhysRevB.79.165427 PACS numbers: 78.67.Hc, 73.21.La, 78.55.Cr, 81.07.Ta

SPIE OPTO: Integrated Optoelectronic Devices, January 24, 2009, San Jose, California, United States, Series: SPIE Proceedings

We review our work developing label-free molecular sensors using silicon nanophotonic waveguides. We show that the high index contrast of these waveguides provides high surface sensitivity and enables compact sensor designs to be realized. We also describe new waveguide circuit geometries that allow photonic wire waveguide sensors to be conveniently arrayed in two dimensions for compatibility with commercial spotting tools, while simultaneously providing long interaction length and improved molecular capture efficiency. These sensor designs are shown to provide a practical route to the development of label-free, microarrayed biochips for multiparameter analysis.

We present multiaperture stationary spectrometers in planar optical waveguides. The devices are based on the spatial heterodyning technique, do not require moving parts, and use Fourier transformation for spectra retrieval. The design is based on arrays of waveguide interferometers with linearly increasing optical path delay. The spectrometers have increased optical throughput due to multiple input waveguides. We discuss design, fabrication, and first experimental results for these multiaperture spectrometers implemented in silicon-on-insulator (SOI) ridge waveguides.

Single-mode laser diodes on GaSb substrates were developed using InGaAsSb/AlGaAsSb triple quantum well active regions grown by molecular beam epitaxy. The devices were fabricated using lateral Cr gratings, with a grating pitch designed to coincide with a strong absorption feature of HF gas, deposited adjacent to a dry-etched narrow ridge waveguide. High sidemode suppression was achieved, and in 20°C continuous-wave operation, devices with a 400μm-long cavity provided 4.5mW total output power at the 2396nm target wavelength. Anti-reflection and high-reflection facet coatings exhibited no deleterious effects on the laser tunability or mode quality, thus allowing the preferential extraction of output power from a single laser facet.

Photoluminescence data from single self-assembled InAs/InP quantum dots in magnetic fields up to 7 T are presented. Exciton $g$ factors are obtained for dots of varying height, corresponding to ground-state emission energies ranging from 780 to 1100 meV. A monotonic increase in the $g$ factor from $\ensuremath{-}2$ to $+1.2$ is observed as the dot height decreases. The trend is well reproduced by $s{p}^{3}$ tight-binding calculations, which show that the hole $g$ factor is sensitive to confinement effects through orbital angular-momentum mixing between the light-hole and heavy-hole valence bands. We demonstrate tunability of the exciton $g$ factor by manipulating the quantum dot dimensions using pyramidal InP nanotemplates.

Regrowth-free gain-coupled GaSb-based DFB lasers suitable for gas sensing were fabricated. Threshold currents for 2.4µm emission of 400µm-long DFB devices were 45mA with a total output power of nearly 11mW in CW operation at 20°C.

We demonstrate precise control of electron charging within a single deterministically positioned InAs/InP quantum dot emitting in the telecommunications band around 1500 nm. Photolumine-scence emission as a function of vertical electric field is used to monitor the electron number within the dot. From Stark shift spectroscopy, we extract strength and orientation for the built-in dipole moment that suggests a uniform InAs dot composition and a configuration in which the electron lies above the hole at zero electric field. The scalable gating technology we employ to electrically contact individual prepositioned quantum dots promises arrays of initialized single spins that can be used for fiber-based quantum information applications.

The spectral characteristics of a ring resonator made of Si photonic wires are modeled using mode expansion of supermodes of the directional coupler. The influence of the coupling coefficient, loss factor and waveguide dispersion on the spectral features are analyzed in detail. The model is then compared with the experimental data of a ring resonator designed for sensing purposes. The model that includes a wavelength dependence on coupling length reproduces the large variations of the envelope of the experimental spectrum, when coupling coefficient cover its full range from 0 to 1. Fitting parameters explain the details of the experimental spectrum and contribute to the sensor optimization, as well as illustrating general guidelines for ring resonator design.

We present experimental and theoretical results of label-free molecular sensing with the TM mode of a 0.22 µm thick silicon slab waveguide used in a guided mode resonance configuration. Due to the strong overlap of the evanescent field of the waveguide mode with a molecular layer attached to the surface, these sensors exhibit high sensitivity, while their fabrication and packaging requirements are minimal. Experimentally, we demonstrate a resonance wavelength shift of ~1 nm when a monolayer of streptavidin is attached to the surface, in good agreement with calculations based on rigorous coupled wave analysis.

We report a compact high-resolution arrayed waveguide grating (AWG) interrogator system designed to measure the relative wavelength spacing between two individual resonances of a tilted fiber Bragg grating (TFBG) refractometer. The TFBG refractometer benefits from an internal wavelength and power reference provided by the core mode reflection resonance that can be used to determine cladding mode perturbations with high accuracy. The AWG interrogator is a planar waveguide device fabricated on a silicon-on-insulator platform, having 50 channels with a 0.18 nm wavelength separation and a footprint of 8 mm×8 mm. By overlaying two adjacent interference orders of the AWG we demonstrate simultaneous monitoring of two widely separated resonances in real time with high wavelength resolution. The standard deviation of the measured wavelength shifts is 1.2 pm, and it is limited by the resolution of the optical spectrum analyzer used for the interrogator calibration measurements.

We show by theoretical analysis and experiments on submicrometer silicon-on-insulator channel waveguides that the optical scattering loss of high-index-contrast planar waveguides is modified significantly by a thin-film interference effect, caused by boundary reflections.

We review our work developing optical waveguide-based evanescent field sensors for the label-free, specific detection of biological molecules. Using high-index-contrast silicon photonic wire waveguides of submicrometer dimension, we demonstrate ultracompact and highly sensitive molecular sensors compatible with commercial spotting apparatus and microfluidic-based analyte delivery systems. We show that silicon photonic wire waveguides support optical modes with strong evanescent field at the waveguide surface, leading to strong interaction with surface bound molecules for sensitive response. Furthermore, we present new sensor geometries benefiting from the very small bend radii achievable with these high-index-contrast waveguides to extend the sensing path length, while maintaining compact size. We experimentally demonstrate the sensor performance by monitoring the adsorption of protein molecules on the waveguide surface and by tracking small refractive index changes of bulk solutions.

This paper reports the progress (design, fabrication and measurement) in label-free biological sensors based on ring resonator technology originally developed for telecommunication. These sensors are made in silicon-on-insulator (SOI), a high index contrast material platform that provides enhanced sensitivity and allows compact waveguide geometries by employing very small bend radii. A double spiral shape allows designing millimetre-long resonator in a compact size (100-150 mum) compatible with spotting technology used for large biochip assays. The detection limit estimated from the resonance shift measurement is ~5 fg of streptavidin adsorbed on the sensor surface functionalised with biotin.

We demonstrate Mach-Zehnder interferometer and ring resonator evanescent field sensors fabricated on the silicon-on-insulator material platform. These devices exploit the strong evanescent field of the transverse magnetic mode of a high index contrast, submicron dimension waveguide to obtain strong interaction of the guided mode with biomolecules adsorbed to the waveguide surface. We utilize the extremely small bend radius achievable with silicon photonic wire waveguides to fabricate spiral and folded waveguide sensors that maintain the high sensitivity of a long waveguide, while providing compact footprint. These devices offer a suitable geometry for the development of sensor arrays and provide compatibility with commercial spotting functionalization systems. We demonstrate sensors containing waveguides of up to 5.5 mm length that are contained within a 150 μm × 150 μm area.

We demonstrate a method of canceling the temperature dependence of ring resonator sensors using a double ring configuration. Temperature calibration curves are provided for SOI photonic wire ring sensors with air and SU8 polymer cladding.

We demonstrate by numerical simulations and experiments that highly reflective (HR) facets can be formed on silicon waveguides with high reflectivity diffraction gratings. We use an HR grating with a plane wave reflectivity exceeding 99.99%, as calculated by rigorous coupled wave analysis. Experimentally, we demonstrate the HR effect for silicon-on-insulator waveguide facets patterned lithographically with grating structures. Due to a strong mode size dependence, the maximum facet reflectivity for 1.5 microm thick waveguide is 77%, but finite difference time-domain simulations show that modal reflectivies larger than 90% can be achieved for silicon waveguides with thicknesses of 4 microm or more.

We demonstrate a new silicon photonic wire waveguide evanescent field (PWEF) sensor that exploits the strong evanescent field of the transverse magnetic mode of this high-index-contrast, submicrometer-dimension waveguide. High sensitivity is achieved by using a 2 mm long double-spiral waveguide structure that fits within a compact circular area of 150 microm diameter, facilitating compatibility with commercial spotting apparatus and the fabrication of densely spaced sensor arrays. By incorporating the PWEF sensor element into a balanced waveguide Mach-Zehnder interferometer circuit, a minimum detectable mass of approximately 10 fg of streptavidin protein is demonstrated with near temperature-independent response.

We present theoretical and experimental results on an interference effect caused by boundary reflections on the optical scattering loss in high-index-contrast planar waveguides. Analytical expressions for the polarization-dependent scattering loss are derived using a surface Green's function. For high-index-contrast waveguides of submicrometer dimensions a significant deviation from accepted theory arises, including scattering loss suppression owing to a thin-film interference effect. Our theoretical predictions are confirmed by loss measurements on silicon-on-insulator channel waveguides.

We present the novel design of a silicon modulator with low operation voltage of les3 V by employing periodically interleaved pn junctions. Simulations predict that in depletion mode it has a high modulation efficiency of better than 1.5 Vldrcm.

The functionality and performance of many planar waveguide devices, including lasers and optical amplifiers, are critically affected by the reflectivity of the waveguide facets. For high index contrast waveguide devices, the Fresnel reflection at the facet is a significant contribution to the fibre-to-chip coupling loss and the return loss. Currently, the common method to control the facet reflectivity of planar waveguides is the deposition of anti-or high-reflective coatings.

We demonstrate the capability to post-tune a photonic crystal nanocavity resonance without affecting its quality factor via the presence of an evanescently coupling tapered optical fibre. A 3 nm shift is reported.

We describe a novel non-destructive technique to measure the sidewall roughness induced scattering loss of SOI ridge waveguides using an integrated 5x17 star coupler. The accuracy of our technique is independent of the coupling efficiency. In our technique, we capture the near field images of the full output waveguides array with varying width ranging from 0.2 to 2.0 micron and use the intensity maps of these images to produce normalized intensity profiles, from which the relative scattering losses of output waveguides are extracted. Using our technique, we have studied and compared the scattering and polarization dependent losses of three different sets of SOI waveguide samples fabricated by different processes. We have determined the root-mean-square roughness and autocorrelation length of these samples using scanning electron microscopy (SEM). Relating the loss and roughness analysis, we have showed that the process utilizing negative e-beam photoresist and Cr-hardmask with inductively coupled plasma (ICP) etching produced the smoothest waveguide sidewalls and lowest scattering losses. We have also successfully modeled the measured ridge waveguide losses as a function of waveguide width and demonstrated that the theoretical sidewall roughness is in reasonable agreement with the measured roughness from SEM. Our technique is capable of studying roughness induced scattering loss and thus provides an efficient way of optimizing and monitoring process parameters that affect sidewall roughness., SPIE Photonics West Conference: Silicon Photonics II :Feb 2007, Series: Proceedings of SPIE; no. 6477

We demonstrate highly efficient evanescent coupling via a silica loop-nanowire, to ultra-small (0.5 (lambda/n)(3) ), InAs/InP quantum dot photonic crystal cavities, specifically designed for single photon source applications. This coupling technique enables the tuning of both the Q-factor and the wavelength of the cavity mode independently, which is highly relevant for single photon source applications. First, this allows for the optimization of the extraction efficiency while maintaining a high Purcell factor. Second, the cavity mode can be matched with a spectrally misaligned quantum dot without changing the structure or degrading the Q-factor: a 3 nm resonance shift is reported.

We present recent theoretical and experimental results on silicon photonic wire evanescent field (PWEF) sensors that show such PWEF sensors have the highest response to bulk index change as well as surface molecular adsorption.

A comparison is made of the results of simulation and experimental measurements on computer generated holograms fabricated by e-beam lithography and designed using a phase optimised general error diffusion algorithm.

The design, simulation, and experimental performance of mode converters for coupling from single-mode silicon-on-insulator ridge waveguides to high aspect ratio channel waveguides are described. The converters consist of a two-level adiabatic taper structure. The final channel waveguide is 1.5 mum high by 0.8 mum wide. Simulations predict that for total coupler lengths longer than 20 mum, the coupling loss from the fundamental ridge mode to the slit mode is better than -0.2 dB. The couplers and waveguides were fabricated using a two-step self-aligned process. The measured coupling loss for fabricated mode converters is -0.4 dB

Silicon substrate can be used as a highly effective pseudomask to perform selective growth of GaN by ammonia molecular beam epitaxy (MBE). This can be exploited to fabricate novel photonic and electronic device structures on Si substrates. Growth of GaN micro-pyramids as templates for fabricating InGaN quantum dots, and growth of AlGaN/GaN HFET mesa patterns using the selective growth approach is reported. The facet growth characteristics of the selectively grown GaN micro-pyramids are investigated. μ-photoluminescence (μ-PL) measurements resolving emissions from InGaN quantum dots and/or wells on a single GaN pyramid have been performed, but have found no evidence of formation of InGaN single quantum dot at the apex of the GaN pyramid. Possible improved growth approaches for achieving such single quantum dot structures are discussed. Success in selective growth of the HFET mesa pattern and especially the insulating carbon-doped GaN buffer layer is achieved, demonstrating a promising approach for potential integration of nitride devices with silicon circuits.

Magnetospectroscopy of a highly excited single $\mathrm{InAs}\text{\ensuremath{-}}\mathrm{GaAs}$ quantum dot is performed. The multiexcitonic emission related to the $s$, $p$, and $d$ shells of a zero-dimensional system is identified. Orbital Zeeman effects for the $p$ and $d$ energy shells are observed. The overall pattern of the magnetic field evolution of the emission lines resembles a single-particle Fock-Darwin diagram. Effects of interaction between the multiexcitonic configurations and the asymmetry of localizing potential are clearly visible when single-particle states become nearly degenerate: at $B=0$ and at a level crossing induced by the magnetic field.

We demonstrate efficient coupling via a tapered silica nanowire micro-loop to a photonic crystal based nanocavity containing quantum dots, specifically designed for single photon source applications.

We demonstrate efficient coupling via a tapered silica nanowire micro-loop, to a photonic crystal based nanocavity containing quantum dots, specifically designed for single photon source applications.

This paper presents the development of a microspectrometer and a surface plasmon waveguide sensor on a silicon platform derived from the expertise acquired in designing optical telecommunication components.

Individual and coupled quantum dots containing one or two electrons have been realized and are regarded as components for future quantum information circuits. In this Letter we map out experimentally the stability diagram of the few-electron triple dot system, the electron configuration map as a function of the external tuning parameters, and reveal experimentally for the first time the existence of quadruple points, a signature of the three dots being in resonance. In the vicinity of these quadruple points we observe a duplication of charge transfer transitions related to charge and spin reconfigurations triggered by changes in the total electron occupation number. The experimental results are largely reproduced by equivalent circuit analysis and Hubbard models. Our results are relevant for future quantum mechanical engineering applications within both quantum information and quantum cellular automata architectures.

We present an experimental investigation of the localised optical defect modes in hexagonal symmetry, single missing hole defect, InP membrane, photonic crystal microcavities. An InAs/InP quantum dot ensemble is located at the midpoint of the InP membrane to allow characterisation of the optical modes in emission. Simple modifications to the cavity configuration are used to split the degeneracy of the orthogonally polarised x- and y-dipole cavity modes and to introduce a higher energy mode into the cavity with cavity quality factor Q up to 2,800.

We demonstrate a new, highly sensitive evanescent field sensor using silicon-on-insulator (SOI) photonic wire waveguides. Theoretical analysis shows that thin SOI waveguides can provide higher sensitivity over devices based in all other common planar waveguide material systems for the probing of both thin adsorbed biomolecular layers and bulk homogeneous solutions. A Si photonic wire waveguide was incorporated into a Mach-Zehnder interferometer based sensor, configured to monitor the index change of a homogeneous solution. High effective index change of 0.31 per refractive index unit (RIU) change of the solution was measured, confirming theoretical predictions

Coupled few-electron dots with incorporated quantum point-contact charge detectors have been successfully fabricated and studied both experimentally and theoretically. In this paper we focus on charge detection measurements for the regimes most relevant to spin and charge qubit applications. We first characterize the degree of coherence of a single electron trapped in the double dot by measuring the average occupation of one of the dots at equilibrium. We find that the spin degree of freedom dramatically affects the charge states when a second electron is added to the system, resulting in novel charge transfer patterns. The patterns evolve as a function of tunnel coupling and magnetic field. A nonequilibrium occupation of ground and excited states as a possible origin of the observed phenomena is investigated theoretically through extensive numerical and CI calculations of 1–3 electron states for realistic confining potentials.

A few electron double electrostatic lateral quantum dot can be transformed into a few electron triple quantum dot by applying a different combination of gate voltages. Quadruple points have been achieved at which all three dots are simultaneously on resonance. At these special points in the stability diagram four occupation configurations are possible. Both charge detection and transport experiments have been performed on this device. In this short paper we present data and confirm that transport is coherent by observing a π phase shift in magneto-conductance oscillations as one passes through the quadruple point. © 2007 American Institute of Physics., 28th International Conference on the Physics of Semiconductors, ICPS 2006, 24 July 2006 through 28 July 2006, Vienna

We examine the optical mode structure of two-dimensional, photonic crystal microcavities based on triple missing hole defects in hexagonal symmetry, etched air-hole, suspended InP membranes. Polarisation dependent photoluminescence from InAs/InP quantum dots embedded within the cavities is used to explore mode energies and quality factors, Q, as a function of cavity design parameters. Optimised Q values are in excess of 7,500 for the modified y-dipole mode. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We present optical spectra from numerous, single, self-assembled InAs/InP quantum dots. More than 50 individual dots are studied that emit in the 1.1-1.6 mm wavelength range. The dots are of high optical quality as judged by the clean, single exciton emission line at low power, the resolution limited linewidth, and the brightness. Each dot exhibits similar trends in the power evolution spectra, despite large variations in height and diameter. The level splittings in the p -shell increase with decreasing height, which we interpret to be from dot elongation along the [01] direction. The evolution of the spectra with increasing power agrees well with predictions from effective bond orbital calculations. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Recent developments in single and double electrostatically confined quantum dots have shown that these devices are promising candidates for the implementation of quantum computation schemes in solid state systems. One of the obstacles for such an accomplishment is scalability. We present here the first realization of a triple lateral quantum dot with a few number of electrons, going down to fundamental configurations such as the vacuum state (no electrons in the system) or one electron in each quantum dot. New phenomena unseen in single or double dots are revealed involving charge and spin reconfigurations within the system, reminiscent of quantum cellular automata (QCA) functionalities. More importantly, it is necessary to understand these phenomena for they will be present in any three or more coupled quantum dots system. An equivalent circuit analysis of the system is used to reproduce these new features and to highlight some differences between experiments and the model, possibly arising from quantum phenomena not included in the circuit analysis. (© 2006 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

A silicon-on-insulator based evanescent field sensor using silicon photonic wire waveguides is reported. The unique properties of high index contrast, submicron structures are exploited to enhance electric field magnitude and localization near the sensor surface. The evanescent field sensor is demonstrated in a Mach Zehnder configuration.

We have studied switching (telegraph) noise at low temperature in $\mathrm{Ga}\mathrm{As}∕{\mathrm{Al}}_{x}{\mathrm{Ga}}_{1\ensuremath{-}x}\mathrm{As}$ heterostructures with lateral gates and introduced a model for its origin, which explains why noise can be suppressed by cooling samples with a positive bias on the gates. The noise was measured by monitoring the conductance fluctuations around ${e}^{2}∕h$ on the first step of a quantum point contact at around $1.2\phantom{\rule{0.3em}{0ex}}\mathrm{K}$. Cooling with a positive bias on the gates dramatically reduces this noise, while an asymmetric bias exacerbates it. Our model is that the noise originates from a leakage current of electrons that tunnel through the Schottky barrier under the gate into the conduction band and become trapped near the active region of the device. The key to reducing noise is to keep the barrier opaque under experimental conditions. Cooling with a positive bias on the gates reduces the density of ionized donors. This builds in an effective negative gate voltage so that a smaller negative bias is needed to reach the desired operating point. This suppresses tunneling from the gate and hence the noise. The reduction in the density of ionized donors also strengthens the barrier to tunneling at a given applied voltage. Further support for the model comes from our direct observation of the leakage current into a closed quantum dot, around ${10}^{\ensuremath{-}20}\phantom{\rule{0.3em}{0ex}}\mathrm{A}$ for this device. The current was detected by a neighboring quantum point contact, which showed monotonic steps in time associated with the tunneling of single electrons into the dot. If asymmetric gate voltages are applied, our model suggests that the noise will increase as a consequence of the more negative gate voltage applied to one of the gates to maintain the same device conductance. We observe exactly this behavior in our experiments.

We report charge detection studies of a lateral double quantum dot with controllable charge states and tunable tunnel coupling. Using an integrated electrometer, we characterize the equilibrium state of a single electron trapped in the doubled-dot (artificial H2+ molecule) by measuring the average occupation of one dot. We present a model where the electrostatic coupling between the molecule and the sensor is taken into account explicitly. From the measurements, we extract the temperature of the isolated electron and the tunnel coupling energy. It is found that this coupling can be tuned between 0 and 60 micro electron-volt in our device., 5 pages, 4 figures. Revised version with added material. To be published in Physical Review B

We have studied the origin of switching (telegraph) noise at low temperature in lateral quantum structures defined electrostatically in GaAs/AlGaAs heterostructures by surface gates. The noise was measured by monitoring the conductance fluctuations around $e^2/h$ on the first step of a quantum point contact at around 1.2 K. Cooling with a positive bias on the gates dramatically reduces this noise, while an asymmetric bias exacerbates it. We propose a model in which the noise originates from a leakage current of electrons that tunnel through the Schottky barrier under the gate into the doped layer. The key to reducing noise is to keep this barrier opaque under experimental conditions. Bias cooling reduces the density of ionized donors, which builds in an effective negative gate voltage. A smaller negative bias is therefore needed to reach the desired operating point. This suppresses tunnelling from the gate and hence the noise. The reduction in the density of ionized donors also strengthens the barrier to tunneling at a given applied voltage. Support for the model comes from our direct observation of the leakage current into a closed quantum dot, around $10^{-20} \mathrm{A}$ for this device. The current was detected by a neighboring quantum point contact, which showed monotonic steps in time associated with the tunneling of single electrons into the dot. If asymmetric gate voltages are applied, our model suggests that the noise will increase as a consequence of the more negative gate voltage applied to one of the gates to maintain the same device conductance. We observe exactly this behaviour in our experiments., Comment: 8 pages, 7 figures

Two transport techniques are described for performing charge redistribution spectroscopy at a fixed electron number. In the first method we simply apply the well established 'Cambridge' QPC charge detector technique to a dot with no leads to external reservoirs. In the second technique we use two dots in series in the regime where the electrochemical potential of only one of the dots is aligned with the external leads. In this case the transport involves co-tunneling processes through the second dot. Charge rearrangements occurring in one dot are picked up through shifts in the electrochemical potential of the other. We perform proof of concept demonstrations using well characterised transitions in the vicinity of filling factor 2 to confirm that both techniques can be used for charge redistribution spectroscopy. We discuss why such spectroscopy can provide new information about complex many-body spin phenomena in quantum dots. © 2005 American Institute of Physics., PHYSICS OF SEMICONDUCTORS: 27th International Conference on the Physics of Semiconductors, ICPS-27, 26 July 2004 through 30 July 2004, Flagstaff, AZ, Series: AIP Conference Proceedings; no. 772