Superradiant emission in a high-mobility two-dimensional electron gas.

We use terahertz time-domain spectroscopy to study gallium arsenide two-dimensional electron gas samples in external magnetic field. We measure cyclotron decay as a function of temperature from 0.4 to10Kand a quantum confinement dependence of the cyclotron decay time belowT0=1.2K. In the wider quantum well, we observe a dramatic enhancement in the decay time due to the reduction in dephasing and the concomitant enhancement of superradiant decay in these systems. We show that the dephasing time in 2DEG's depends on both the scatteringrateand also on the distribution of scattering angles.

Multidimensional spectroscopy of magneto-excitons at high magnetic fields.

We perform two-dimensional Fourier transform spectroscopy on magneto-excitons in GaAs at magnetic fields and observe Zeeman splitting of the excitons. The Zeeman components are clearly resolved as separate peaks due to the two-dimensional nature of the spectra, leading to a more accurate measurement of the Zeeman splitting and the Landé g factors. Quantum coherent coupling between Zeeman components is observed using polarization dependent one-quantum two-dimensional spectroscopy. We use two-quantum two-dimensional spectroscopy to investigate higher four-particle correlations at high magnetic fields and reveal the role of the Zeeman splitting on the two-quantum transitions. The experimental two-dimensional spectra are simulated using the optical Bloch equations, where many-body effects are included phenomenologically.

Perfect absorption in GaAs metasurfaces near the bandgap edge.

Perfect optical absorption occurs in a metasurface that supports two degenerate and critically-coupled modes of opposite symmetry. The challenge in designing a perfectly absorbing metasurface for a desired wavelength and material stems from the fact that satisfying these conditions requires multi-dimensional optimization often with parameters affecting optical resonances in non-trivial ways. This problem comes to the fore in semiconductor metasurfaces operating near the bandgap wavelength, where intrinsic material absorption varies significantly. Here we devise and demonstrate a systematic process by which one can achieve perfect absorption in GaAs metasurfaces for a desired wavelength at different levels of intrinsic material absorption, eliminating the need for trial and error in the design process. Using this method, we show that perfect absorption can be achieved not only at wavelengths where GaAs exhibits high absorption, but also at wavelengths near the bandgap edge. In this region, absorption is enhanced by over one order of magnitude compared a layer of unstructured GaAs of the same thickness.

Particle-Hole Symmetry and the Fractional Quantum Hall Effect in the Lowest Landau Level.

We report on detailed experimental studies of a high-quality heterojunction insulated-gate field-effect transistor (HIGFET) to probe the particle-hole symmetry of the fractional quantum Hall effect (FQHE) states about half-filling in the lowest Landau level. The HIGFET is specially designed to vary the density of a two-dimensional electronic system under constant magnetic fields. We find in our constant magnetic field, variable density measurements that the sequence of FQHE states at filling factors ν=1/3,2/5,3/7… and its particle-hole conjugate states at filling factors 1-ν=2/3,3/5,4/7… have a very similar energy gap. Moreover, a reflection symmetry can be established in the magnetoconductivities between the ν and 1-ν states about half-filling. Our results demonstrate that the FQHE states in the lowest Landau level are manifestly particle-hole symmetric.

Enhanced stability of quantum Hall skyrmions under radio-frequency radiations.

We present in this paper the results from a recent study on the stability of the quantum Hall skyrmions state at a Landau level filling factor (ν) close to ν = 1 in a narrow GaAs quantum well. Consistent with previous work, a resonant behavior is observed in the resistively detected NMR measurements. In the subsequent current-voltage (I-V) measurements to examine its breakdown behavior under radio frequency radiations, we observe that the critical current assumes the largest value right at the 75As nuclear resonant frequency. We discuss possible origin for this unexpectedly enhanced stability.

Giant Zero Bias Anomaly due to Coherent Scattering from Frozen Phonon Disorder in Quantum Point Contacts.

We demonstrate an unusual manifestation of coherent scattering for electron waves in mesoscopic quantum point contacts, in which fast electron dynamics allows the phonon system to serve as a quasistatic source of disorder. The low-temperature conductance of these devices exhibits a giant (≫2e^{2}/h) zero bias anomaly (ZBA), the features of which are reproduced in a nonequilibrium model for coherent scattering from the "frozen" phonon disorder. According to this model, the ZBA is understood to result from the in situ electrical manipulation of the phonon disorder, a mechanism that could open up a pathway to the on-demand control of coherent scattering in the solid state.

Near-field spectroscopy and tuning of sub-surface modes in plasmonic terahertz resonators.

Highly confined modes in THz plasmonic resonators comprising two metallic elements can enhance light-matter interaction for efficient THz optoelectronic devices. We demonstrate that sub-surface modes in such double-metal resonators can be revealed with an aperture-type near-field probe and THz time-domain spectroscopy despite strong mode confinement in the dielectric spacer. The sub-surface modes couple a fraction of their energy to the resonator surface via surface waves, which we detected with the near-field probe. We investigated two resonator geometries: a λ/2 double-metal patch antenna with a 2 µm thick dielectric spacer, and a three-dimensional meta-atom resonator. THz time-domain spectroscopy analysis of the fields at the resonator surface displays spectral signatures of sub-surface modes. Investigations of strong light-matter coupling in resonators with sub-surface modes therefore can be assisted by the aperture-type THz near-field probes. Furthermore, near-field interaction of the probe with the resonator enables tuning of the resonance frequency for the spacer mode in the antenna geometry from 1.6 to 1.9 THz (~15%).

8-beam local oscillator array at 4.7 THz generated by a phase grating and a quantum cascade laser.

We present an 8-beam local oscillator (LO) for the astronomically significant [OI] line at 4.7 THz. The beams are generated using a quantum cascade laser (QCL) in combination with a Fourier phase grating. The grating is fully characterized using a third order distributed feedback (DFB) QCL with a single mode emission at 4.7 THz as the input. The measured diffraction efficiency of 74.3% is in an excellent agreement with the calculated result of 75.4% using a 3D simulation. We show that the power distribution among the diffracted beams is uniform enough for pumping an array receiver. To validate the grating bandwidth, we apply a far-infrared (FIR) gas laser emission at 5.3 THz as the input and find a very similar performance in terms of efficiency, power distribution, and spatial configuration of the diffracted beams. Both results represent the highest operating frequencies of THz phase gratings reported in the literature. By injecting one of the eight diffracted 4.7 THz beams into a superconducting hot electron bolometer (HEB) mixer, we find that the coupled power, taking the optical loss into account, is in consistency with the QCL power value.

Consequences of Spin-Orbit Coupling at the Single Hole Level: Spin-Flip Tunneling and the Anisotropic g Factor.

Hole transport experiments were performed on a gated double quantum dot device defined in a p-GaAs/AlGaAs heterostructure with a single hole occupancy in each dot. The charging diagram of the device was mapped out using charge detection confirming that the single hole limit is reached. In that limit, a detailed study of the two-hole spin system was performed using high bias magnetotransport spectroscopy. In contrast to electron systems, the hole spin was found not to be conserved during interdot resonant tunneling. This allows one to fully map out the two-hole energy spectrum as a function of the magnitude and the direction of the external magnetic field. The heavy-hole g factor was extracted and shown to be strongly anisotropic, with a value of 1.45 for a perpendicular field and close to zero for an in-plane field as required for hybridizing schemes between spin and photonic quantum platforms.

Strong Quantum Coherence between Fermi Liquid Mahan Excitons.

In modulation doped quantum wells, the excitons are formed as a result of the interactions of the charged holes with the electrons at the Fermi edge in the conduction band, leading to the so-called "Mahan excitons." The binding energy of Mahan excitons is expected to be greatly reduced and any quantum coherence destroyed as a result of the screening and electron-electron interactions. Surprisingly, we observe strong quantum coherence between the heavy hole and light hole excitons. Such correlations are revealed by the dominating cross-diagonal peaks in both one-quantum and two-quantum two-dimensional Fourier transform spectra. Theoretical simulations based on the optical Bloch equations where many-body effects are included phenomenologically reproduce well the experimental spectra. Time-dependent density functional theory calculations provide insight into the underlying physics and attribute the observed strong quantum coherence to a significantly reduced screening length and collective excitations of the many-electron system.

Exploring two-dimensional electron gases with two-dimensional Fourier transform spectroscopy.

The dephasing of the Fermi edge singularity excitations in two modulation doped single quantum wells of 12 nm and 18 nm thickness and in-well carrier concentration of ∼4 × 10(11) cm(-2) was carefully measured using spectrally resolved four-wave mixing (FWM) and two-dimensional Fourier transform (2DFT) spectroscopy. Although the absorption at the Fermi edge is broad at this doping level, the spectrally resolved FWM shows narrow resonances. Two peaks are observed separated by the heavy hole/light hole energy splitting. Temperature dependent "rephasing" (S1) 2DFT spectra show a rapid linear increase of the homogeneous linewidth with temperature. The dephasing rate increases faster with temperature in the narrower 12 nm quantum well, likely due to an increased carrier-phonon scattering rate. The S1 2DFT spectra were measured using co-linear, cross-linear, and co-circular polarizations. Distinct 2DFT lineshapes were observed for co-linear and cross-linear polarizations, suggesting the existence of polarization dependent contributions. The "two-quantum coherence" (S3) 2DFT spectra for the 12 nm quantum well show a single peak for both co-linear and co-circular polarizations.

In situ biaxial rotation at low-temperatures in high magnetic fields.

We report the design, construction, and characterization of a biaxial sample rotation stage for use in a cryogenic system for orientation-dependent studies of anisotropic electronic transport phenomena at low temperatures and high magnetic fields. Our apparatus allows for continuous rotation of a sample about two axes, both independently and simultaneously.

1D-1D Coulomb drag signature of a Luttinger liquid.

One-dimensional (1D) interacting electronic systems exhibit distinct properties when compared to their counterparts in higher dimensions. We report Coulomb drag measurements between vertically integrated quantum wires separated by a barrier only 15 nanometers wide. The temperature dependence of the drag resistance is measured in the true 1D regime where both wires have less than one 1D subband occupied. As a function of temperature, an upturn in the drag resistance is observed below a temperature T* ~ 1.6 kelvin. This crossover in Coulomb drag behavior is consistent with Tomonaga-Luttinger liquid models for the 1D-1D drag between quantum wires.

Formation of a protected sub-band for conduction in quantum point contacts under extreme biasing.

Managing energy dissipation is critical to the scaling of current microelectronics and to the development of novel devices that use quantum coherence to achieve enhanced functionality. To this end, strategies are needed to tailor the electron-phonon interaction, which is the dominant mechanism for cooling non-equilibrium ('hot') carriers. In experiments aimed at controlling the quantum state, this interaction causes decoherence that fundamentally disrupts device operation. Here, we show a contrasting behaviour, in which strong electron-phonon scattering can instead be used to generate a robust mode for electrical conduction in GaAs quantum point contacts, driven into extreme non-equilibrium by nanosecond voltage pulses. When the amplitude of these pulses is much larger than all other relevant energy scales, strong electron-phonon scattering induces an attraction between electrons in the quantum-point-contact channel, which leads to the spontaneous formation of a narrow current filament and to a renormalization of the electronic states responsible for transport. The lowest of these states coalesce to form a sub-band separated from all others by an energy gap larger than the source voltage. Evidence for this renormalization is provided by a suppression of heating-related signatures in the transient conductance, which becomes pinned near 2e(2)/h (e, electron charge; h, Planck constant) for a broad range of source and gate voltages. This collective non-equilibrium mode is observed over a wide range of temperature (4.2-300 K) and may provide an effective means to manage electron-phonon scattering in nanoscale devices.

Tuning the Fano resonance with an intruder continuum.

Through a combination of experiment and theory we establish the possibility of achieving strong tuning of Fano resonances (FRs), by allowing their usual two-path geometry to interfere with an additional, "intruder", continuum. As the coupling strength to this intruder is varied, we predict strong modulations of the resonance line shape that, in principle at least, may exceed the amplitude of the original FR itself. For a proof-of-concept demonstration of this phenomenon, we construct a nanoscale interferometer from nonlocally coupled quantum point contacts and utilize the unique features of their density of states to realize the intruder. External control of the intruder coupling is enabled by means of an applied magnetic field, in the presence of which we demonstrate the predicted distortions of the FR. This general scheme for resonant control should be broadly applicable to a variety of wave-based systems, opening up the possibility of new applications in areas such as chemical and biological sensing and secure communications.

Inducing an incipient terahertz finite plasmonic crystal in coupled two dimensional plasmonic cavities.

We measured a change in the current transport of an antenna-coupled, multigate, GaAs/AlGaAs field-effect transistor when terahertz electromagnetic waves irradiated the transistor and attribute the change to bolometric heating of the electrons in the two dimensional electron channel. The observed terahertz absorption spectrum indicates coherence between plasmons excited under adjacent biased device gates. The experimental results agree quantitatively with a theoretical model we developed that is based on a generalized plasmonic transmission line formalism and describes an evolution of the plasmonic spectrum with increasing electron density modulation from homogeneous to the crystal limit. These results demonstrate an electronically induced and dynamically tunable plasmonic band structure.

Coherent propagation of spin helices in a quantum-well confined electron gas.

We use phase-resolved transient grating spectroscopy to measure the propagation of spin helices in a high mobility n-GaAs/AlGaAs quantum well with an applied in-plane electric field. At relatively low fields helical modes crossover from overdamped excitations where the spin-precession period exceeds the spin lifetime, to a regime of coherent propagation where several spin-precession periods can be observed. We demonstrate that the envelope of a spin polarization packet reaches a current-driven velocity of 10(7) cm s(-1) in an applied field of 70 V cm(-1).

Positive and negative Coulomb drag in vertically integrated one-dimensional quantum wires.

Electron interactions in and between wires become increasingly complex and important as circuits are scaled to nanometre sizes, or use reduced-dimensional conductors such as carbon nanotubes, nanowires and gated high-mobility two-dimensional electron systems. This is because the screening of the long-range Coulomb potential of individual carriers is weakened in these systems, which can lead to phenomena such as Coulomb drag, where a current in one wire induces a voltage in a second wire through Coulomb interactions alone. Previous experiments have demonstrated Coulomb electron drag in wires separated by a soft electrostatic barrier of width ≳80 nm (ref. 12), which was interpreted as resulting entirely from momentum transfer. Here, we measure both positive and negative drag between adjacent vertical quantum wires that are separated by ∼15 nm and have independent contacts, which allows their electron densities to be tuned independently. We map out the drag signal versus the number of electron sub-bands occupied in each wire, and interpret the results both in terms of momentum-transfer and charge-fluctuation induced transport models. For wires of significantly different sub-band occupancies, the positive drag effect can be as large as 25%.

Common mode frequency instability in internally phase-locked terahertz quantum cascade lasers.

Feedback from a diode mixer integrated into a 2.8 THz quantum cascade laser (QCL) was used to phase lock the difference frequencies (DFs) among the Fabry-Perot (F-P) longitudinal modes of a QCL. Approximately 40% of the DF power was phase locked, consistent with feedback loop bandwidth of 10 kHz and phase noise bandwidth ~0.5 MHz. While the locked DF signal has ≤ 1 Hz linewidth and negligible drift over ~30 min, mixing measurements between two QCLs and between a QCL and molecular gas laser show that the common mode frequency stability is no better than a free-running QCL.

Measurement of electron-hole friction in an n-doped GaAs/AlGaAs quantum well using optical transient grating spectroscopy.

We use phase-resolved transient grating spectroscopy to measure the drift and diffusion of electron-hole density waves in a semiconductor quantum well. The unique aspects of this optical probe allow us to determine the frictional force between a two-dimensional Fermi liquid of electrons and a dilute gas of holes. Knowledge of electron-hole friction enables prediction of ambipolar dynamics in high-mobility electron systems.