Spin exchange between confined holes and nuclei has been demonstrated for zero-dimensional quantum dots by optical techniques but has not been observed for gated planar structures. Here, enabled by strong spin-orbit interaction, and under microwave (MW) illumination, we report hyperfine interaction and dynamic polarization of the nuclei with confined heavy-holes in a GaAs/AlGaAs double quantum dot device. Distinct signatures of the resultant hyperfine field on the electron dipole spin resonance (EDSR) signal include: hysteresis on sweeping the magnetic (B-) field up and down with characteristics that are strongly dependent on both MW power and B-field sweep rate; free bidirectional dragging of the EDSR condition; stable locking on resonance on a timescale of hours; slow temporal change as the hyperfine field decays (T1 nuclear decay time ~ 100 s); and oscillations in time commensurate with Larmor precession of the 75As nuclei. We attain pumped nuclear (Overhauser) fields ~ 25 mT (~20% nuclear polarization).

There is rapidly expanding interest in exploiting the spin of valence-band holes rather than conduction-band electrons for spin qubit semiconductor circuits composed of coupled quantum dots. The hole platform offers stronger spin–orbit interaction (SOI), large difference between in-dot-plane and out-of-dot-plane g -factors, i.e. g -factor anisotropy, and a significantly reduced hyperfine coupling to nuclei in the host material. These attributes collectively can deliver fast all-electric coherent spin manipulation, efficient spin-flip inter-dot tunneling channels, a voltage tunable effective g -factor, a g -factor adjustable to nearly zero in an appropriately oriented external magnetic field, and long spin relaxation and coherence times. Here, we review our recent work on the physics of heavy holes confined in a planar GaAs/AlGaAs double quantum dot system with strong SOI. For a single-hole , we have performed resonant tunneling magneto-spectroscopy to extract spin-flip and spin-conserving tunneling strengths, implemented spin-flip Landau–Zener–Stückelberg–Majorana (LZSM) interferometry, determined the spin relaxation time T 1 as a function of magnetic field using a fast single-shot latched charge technique, electrically tuned the effective g -factor revealed by electric dipole spin resonance, and found signatures of the hyperfine interaction and dynamic nuclear polarization with holes. For two-holes , we have measured the energy spectrum in the presence of strong SOI (and so not limited by Pauli spin blockade), quantified the heavy-hole (HH) g -factor anisotropy on tilting the magnetic field, described a scheme to employ HHs whose g -factor is tunable to nearly zero for an in-plane magnetic field for a coherent photon-to-spin interface, and observed a well-defined LZSM interference pattern at small magnetic fields on pulsing through the singlet-triplet anti-crossing.

Electrical tunability of the g -factor of a confined spin is a long-time goal of the spin qubit field. Here we utilize the electric dipole spin resonance (EDSR) to demonstrate it in a gated GaAs double-dot device confining a hole. This tunability is a consequence of the strong spin-orbit interaction (SOI) in the GaAs valence band. The SOI enables a spin-flip interdot tunneling, which, in combination with the simple spin-conserving charge transport leads to the formation of tunable hybrid spin-orbit molecular states. EDSR is used to demonstrate that the gap separating the two lowest energy states changes its character from a charge-like to a spin-like excitation as a function of interdot detuning or magnetic field. In the spin-like regime, the gap can be characterized by the effective g -factor, which differs from the bulk value owing to spin-charge hybridization, and can be tuned smoothly and sensitively by gate voltages.

Long distance entanglement distribution is an important problem for quantum information technologies to solve. Current optical schemes are known to have fundamental limitations. A coherent photon-to-spin interface built with quantum dots (QDs) in a direct bandgap semiconductor can provide a solution for efficient entanglement distribution. QD circuits offer integrated spin processing for full Bell state measurement (BSM) analysis and spin quantum memory. Crucially the photo-generated spins can be heralded by non-destructive charge detection techniques. We review current schemes to transfer a polarization-encoded state or a time-bin-encoded state of a photon to the state of a spin in a QD. The spin may be that of an electron or that of a hole. We describe adaptations of the original schemes to employ heavy holes which have a number of attractive properties including a g-factor that is tunable to zero for QDs in an appropriately oriented external magnetic field. We also introduce simple throughput scaling models to demonstrate the potential performance advantage of full BSM capability in a QD scheme, even when the quantum memory is imperfect, over optical schemes relying on linear optical elements and ensemble quantum memories.

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D’ peak corresponding to a double phonon process involving both an interand intra-valley phonon. In order to both eliminate substrate effects and to probe large areas, we undertook to study Raman scattering for large scale chemical vapor deposition (CVD) grown graphene using two different isotopes (C12 and C13) so that we can effectively exclude and subtract the substrate contributions, since a heavier mass downshifts only the vibrational properties, while keeping all other properties the same.

Semiconductor nanowires offer a versatile platform for the fabrication of new nanoelectronic and nanophotonic devices. These devices will require a high level of control of the nanowire position in relation to both other components of the device and to other nanowires. We demonstrate unprecedented control of the position of InAs nanowires using selective-area vapor-liquid-solid epitaxy (VLS) on an InP ridge template. The high level of control allows us to design structures which connect individual nanowires through coalescence of their catalyst particles. The interconnection process acts as a perturbation to the geometry of the nanowire system that can contribute to the understanding of droplet dynamics in VLS growth. Postgrowth imaging reveals a complex sequence of droplet configurations, including predicted geometries that have not previously been observed.

Using very uniform large scale chemical vapor deposition grown graphene transferred onto silicon, we were able to identify 15 distinct Raman lines associated with graphene monolayers. This was possible thanks to a combination of different carbon isotopes and different Raman laser energies and extensive averaging without increasing the laser power. This allowed us to obtain a detailed experimental phonon dispersion relation for many points in the Brillouin zone. We further identified a D+D' peak corresponding to a double phonon process involving both an inter-and intra-valley phonon. In order to both eliminate substrate effects and to probe large areas, we undertook to study Raman scattering for large scale chemical vapor deposition (CVD) grown graphene using two different isotopes (C12 and C13) so that we can effectively exclude and subtract the substrate contributions, since a heavier mass downshifts only the vibrational properties, while keeping all other properties the same.

We present the electronic properties of a triple quantum dot molecule embedded inside a sub-micron mesa, made from a quadruple-barrier triple-quantum-well structure, and surrounded by a single gate electrode. We outline the design principles of the quadruple-barrier triple-quantum-well structure and calculate the energy of the three lowest states as a function of center well thickness. We observe regular and irregular shaped Coulomb diamond regions similar to those for double quantum dot devices. Variation in the Coulomb blockade region shape is introduced by fluctuation in the offset energies between the quantum dots likely associated with device processing and random impurity potential in the material. We also present Coulomb blockade patterns calculated with a constant interaction model for sequential tunneling through the three series-coupled quantum dots.

We describe a means to realize slow and fast electron channels by coherent mixing of single-particle levels in quantum dots. The underlying physics, which gives insight into state superposition, can potentially be realized in multi-dot structures with complex gate control. However, we employ vertical double dot structures and in our scheme the mixing of single-particle levels arises because of natural perturbations in the confining potential of the high-symmetry dots. Additionally, because of the intrinsic properties of a Fock–Darwin-like spectrum, we utilize a magnetic field to bring multiple single-particle energy levels into close proximity. We determine single-electron resonant tunneling times (effectively dwell times when on resonance) that are either extended in the slow channel or shortened in the fast channel. Most dramatically, for the slow channel, slow-down factors of ∼10 and single-electron resonant tunneling times extended into the µs range are demonstrated in all systems of two, three, and four mixed single-particle states investigated here.

The electroluminescence from single‐walled carbon nanotube field effect transistors is spectrally resolved, and shows two distinct modes of light emission. The vast majority of nanotubes have spectrally broad emission consistent with the spectrum of blackbody radiation. Much more rarely, superposed on the broad emission is a single narrow (<50 meV) peak which is consistent with expectation for electron–hole recombination. The narrow emission is strong even at lower biases and in general has greater peak intensity than the broadband emission. (© 2009 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Fine features in the leakage current within the two-electron spin blockade region in a weakly coupled vertical double dot device induced on application of a moderate out-of-dot-plane magnetic field (up to 2.5 T) are studied. The nature of these features is consistent with an electron-spin nuclear-spin (hyperfine) interaction origin, and furthermore, their appearance depends strongly on the bias voltage sweep direction through the spin blockade region. Applying multi-part 10 mHz bias voltage waveforms, we can program the total current response via the hyperfine interaction. We demonstrate the operation of shifting a down- (up-) bias sweep through three up- (down-) bias sweeps which leads to a unique outcome in the measured current.

A comparison is made between the conventional non-selective vapour-liquid-solid growth of InP nanowires and a novel selective-area growth process where the Au-seeded InP nanowires grow exclusively in the openings of a SiO(2) mask on an InP substrate. This new process allows the precise positioning and diameter control of the nanowires required for future advanced device fabrication. The growth temperature range is found to be extended for the selective-area growth technique due to removal of the competition between material incorporation at the Au/nanowire interface and the substrate. A model describing the growth mechanism is presented which successfully accounts for the nanoparticle size-dependent and time-dependent growth rate. The dominant indium collection process is found to be the scattering of the group III source material from the SiO(2) mask and subsequent capture by the nanowire, a process that had previously been ignored for selective-area growth by chemical beam epitaxy.

A method of rapidly identifying and imaging suspended nanotubes by scanning electron microscopy is reported. Nanotubes are visible in high contrast and even at low magnification. The contrast can be explained by considering the effect that the charge on the nanotube has on the substrate. The proposed mechanism is general and should apply to any charged nanostructure in proximity to a surface or interface. This represents a new contrast mechanism in scanning electron microscopy.

We report on photoluminescence experiments carried out at very low temperatures and with magnetic fields up to 28T performed on a high mobility two-dimensional electron gas GaAs/AIGaAs quantum well. Our experiments show that clear signatures of the v =1/3, 2/5, 3/7, 3/5, 2/3, 1 sequence of the FQHE states can also be easily visible even in raw magneto-photoluminescence spectra. Theoretical calculations of excitonic transitions suggest that an explanation of the red shift of emission energy at the Hall plateau boundary may be due to the appearance of additional free charged quasi-particles that bind to an exciton, forming a fractionally charged exciton whose emission energy is expected to be lower, in analogy to well known charged excitons in n-type semiconductors. The magnitude of the shift is a measure of the fractionally charged exciton binding energy. Emission in the insulating state of 2DEG at v = 1/3 is attributed to a neutral quasi-exciton whose complicated energy dispersion results in an emission doublet with its low energy line due to the recombination from excited excitonic states.

We studied electrostatic confinement effects on correlated electronic states in vertical quantum dots in high magnetic fields. We prepared vertical quantum dots with different lateral confinement strengths and investigated the magnetic field evolution of the electronic states. We observed that the formation of the maximum density droplet state corresponding to filling factor nu=1 shifts to lower magnetic field as the lateral confinement energy becomes weaker. In addition, we found a ground-state transition in the nu < 1 regime for the most weakly laterally confined two-electron quantum dot that is predicted to appear beyond an easily accessible magnetic field in standard vertical quantum dots more typically investigated.

In this paper, we study the behavior of a high mobility two dimensional electron gas under microwave irradiation by means of magneto-photoluminescence (PL) and absorption measurements. The high mobility sample investigated is a 15nm wide GaAs/AlGaAs quantum well with an electron concentration between 1-2×1011 cm -2, tunable by visible-light illumination. Structures in the microwave absorption at 40-60GHz are identified as geometrically confined magneto-plasmons.

We study the properties of high mobility GaAs/GaAlAs quantum well structure by monitoring the microwave induced changes in the low-temperature photoluminescence of the 2DEG as a function of the external magnetic field. The most pronounced changes are observed at cyclotron resonance conditions, but weak features are also visible at cyclotron resonance replicas. Possible observation of microwave induced shift in the Landau-level structure of magneto-photoluminescence is reported in addition to the effects of carrier heating which are conventionally bracketed together with microwave irradiation.

We demonstrate confocal, direct Raman imaging of carbon nanotubes based on the detection of the G-band using an electron multiplying charge coupled detector. Individual carbon nanotubes and bundles of nanotubes are observed in real-time over a large area using global illumination. We use the technique to show that suspended nanotubes can be manipulated selectively with a focused laser beam and describe the relevant physical mechanisms.