Distinguishing impurity concentrations in GaAs and AlGaAs, using very shallow undoped heterostructures

We demonstrate a method of making a very shallow, gateable, undoped 2-dimensional electron gas. We have developed a method of making very low resistivity contacts to these structures and systematically studied the evolution of the mobility as a function of the depth of the 2DEG (from 300nm to 30nm). We demonstrate a way of extracting quantitative information about the background impurity concentration in GaAs and AlGaAs, the interface roughness and the charge in the surface states from the data. This information is very useful from the perspective of molecular beam epitaxy (MBE) growth. It is difficult to fabricate such shallow high-mobility 2DEGs using modulation doping due to the need to have a large enough spacer layer to reduce scattering and switching noise from remote ionsied dopants.Comment: 4 pages, 5 eps figure

Theory of the in-plane photoelectric effect in a two-dimensional electron system

A new photoelectric phenomenon, the in-plane photoelectric (IPPE) effect, has been recently discovered at terahertz (THz) frequencies in a GaAs/Al$_x$Ga$_{1-x}$As heterostructure with a two-dimensional (2D) electron gas (W. Michailow et al., Science Advances, DOI: 10.1126/sciadv.abi8398). In contrast to the conventional PE phenomena, the IPPE effect is observed at normal incidence of radiation, the height of the in-plane potential step, which electrons overcome after absorption of a THz photon, is electrically tunable by gate voltages, and the effect is maximal at a negative electron "work function", when the Fermi energy lies above the potential barrier. Based on the discovered phenomenon, efficient detection of THz radiation has been demonstrated. In this work we present a detailed theory of the IPPE effect providing analytical results for the THz wave generated photocurrent, the quantum efficiency, and the internal responsivity of the detector, in dependence on the frequency, the gate voltages, and the geometrical parameters of the detector. The calculations are performed for macroscopically wide samples at zero temperature. Results of the theory are applicable to any semiconductor systems with 2D electron gases, including III-V structures, silicon-based field effect transistors, and the novel 2D layered, graphene-related materials.Comment: 21 pages, 15 figures, substantially revised improved versio

Nuclear spin coherence in a quantum wire

We have observed millisecond-long coherent evolution of nuclear spins in a quantum wire at 1.2 K. Local, all-electrical manipulation of nuclear spins is achieved by dynamic nuclear polarization in the breakdown regime of the Integer Quantum Hall Effect combined with pulsed Nuclear Magnetic Resonance. The excitation thresholds for the breakdown are significantly smaller than what would be expected for our sample and the direction of the nuclear polarization can be controlled by the voltage bias. As a four-level spin system, the device is equivalent to two qubits.Comment: 5 pages, 5 figure

Active metamaterial polarization modulators for the Terahertz frequency range

Abstract Active control of chirality in the terahertz frequency range is of great importance in many scientific areas, which include research into fundamental optical phenomena, investigation of novel materials, spectroscopy, imaging, wireless communications and chemistry. The lack of efficient, integrated and fast-reconfigurable polarization modulators has hindered, so far, the full exploitation of applications in all the aforementioned fields. Metamaterials are artificial resonant elements possessing unique remarkable properties such as high efficiency and miniaturization capability. The interplay of metallic metamaterial arrays with electrostatically tunable monolayer graphene has been demonstrated to be a valid approach for the realization of a novel class of THz devices. In this work, the realization of active chiral graphene/metamaterial modulator is presented. The versatility of this experimental approach allowed the device integration with broadband sources such as terahertz time domain spectrometers as well as with quantum cascade lasers. A continuous rotation of the polarization plane &gt; 30° has been reported with a reconfiguration speed &gt; 5 MHz. These results pave the way to the integration of fast terahertz polarization modulators in all the applications where these devices are in great demand.</jats:p

Heterodyne receiver at 2.5 THz with quantum cascade laser and hot electron bolometric mixer

Quantum cascade lasers (QCLs) operating at 2.5 THz have been used for gas phase spectroscopy and as local oscillator in a heterodyne receiver. One QCL has a Fabry-Perot resonator while the other has a distributed feedback resonator. The linewidth and frequency tunability of both QCLs have been investigated by either mixing two modes of the QCL or by mixing the emission from the QCL with the emission from a 2.5 THz gas laser. The frequency tunability as well as the linewidth is sufficient for Doppler limited spectroscopy of methanol gas. The QCLs have been used successfully as local oscillators in a heterodyne receiver. Noise temperature measurements with a hot electron bolometer and a QCL yielded the same result as with a gas laser as local oscillator

Non-invasive detection of the evolution of the charge states of a double dot system

Coupled quantum dots are potential candidates for qubit systems in quantum computing. We use a non-invasive voltage probe to study the evolution of a coupled dot system from a situation where the dots are coupled to the leads to a situation where they are isolated from the leads. Our measurements allow us to identify the movement of electrons between the dots and we can also identify the presence of a charge trap in our system by detecting the movement of electrons between the dots and the charge trap. The data also reveals evidence of electrons moving between the dots via excited states of either the single dots or the double dot molecule.Comment: Accepted for publication in Phys. Rev. B. 4 pages, 4 figure

Linear non-hysteretic gating of a very high density 2DEG in an undoped metal-semiconductor-metal sandwich structure

Modulation doped GaAs-AlGaAs quantum well based structures are usually used to achieve very high mobility 2-dimensional electron (or hole) gases. Usually high mobilities ($>10^{7}{\rm{cm}^{2}\rm{V}^{-1}\rm{s}^{-1}}$) are achieved at high densities. A loss of linear gateability is often associated with the highest mobilites, on account of a some residual hopping or parallel conduction in the doped regions. We have developed a method of using fully undoped GaAs-AlGaAs quantum wells, where densities $\approx{6\times10^{11}\rm{cm}^{-2}}$ can be achieved while maintaining fully linear and non-hysteretic gateability. We use these devices to understand the possible mobility limiting mechanisms at very high densities.Comment: 4 pages, 3 eps figure

An in-plane photoelectric effect in two-dimensional electron systems for terahertz detection

Many mid- and far-infrared semiconductor photodetectors rely on a photonic response, when the photon energy is large enough to excite and extract electrons due to optical transitions. Toward the terahertz range with photon energies of a few milli–electron volts, classical mechanisms are used instead. This is the case in two-dimensional electron systems, where terahertz detection is dominated by plasmonic mixing and by scattering-based thermal phenomena. Here, we report on the observation of a quantum, collision-free phenomenon that yields a giant photoresponse at terahertz frequencies (1.9 THz), more than 10-fold as large as expected from plasmonic mixing. We artificially create an electrically tunable potential step within a degenerate two-dimensional electron gas. When exposed to terahertz radiation, electrons absorb photons and generate a large photocurrent under zero source-drain bias. The observed phenomenon, which we call the “in-plane photoelectric effect,” provides an opportunity for efficient direct detection across the entire terahertz range.George and Lilian Schiff Studentship, Schiff Foundation, University of Cambridge Honorary Vice-Chancellor’s Award, Cambridge Trust, University of Cambridg