Petr Stepanov

Ionic Liquid Gating of Suspended MoS2 Field Effect Transistor Devices

We demonstrate ionic liquid (IL) gating of suspended few-layer MoS2 transistors, where ions can accumulate on both exposed surfaces. Upon application of IL, all free-standing samples consistently display more significant improvement in conductance than substrate-supported devices. The measured IL gate coupling efficiency is up to 4.6 × 10(13) cm(-2) V(-1). Electrical transport data reveal contact-dominated electrical transport properties and the Schottky emission as the underlying mechanism. By modulating IL gate voltage, the suspended MoS2 devices display metal-insulator transition. Our results demonstrate that more efficient charge induction can be achieved in suspended two-dimensional (2D) materials, which with further optimization, may enable extremely high charge density and novel phase transition.

A fiber-coupled quantum-dot on a photonic tip

We present the experimental realization of a quantum fiber-pigtail. The device consists of a semiconductor quantum-dot embedded into a conical photonic wire that is directly connected to the core of a fiber-pigtail. We demonstrate a photon collection efficiency at the output of the fiber of 5.8% and suggest realistic improvements for the implementation of a useful device in the context of quantum information. We also discuss potential applications in scanning probe microscopy. The approach is generic and transferable to other materials including diamond and silicon.Davide Cadeddu,1 Jean Teissier,1 Floris Braakman,1 Niels Gregersen,2 Petr Stepanov,3, 4 Jean-Michel Gérard,3, 4 Julien Claudon,3, 4 Richard J. Warburton,1 Martino Poggio,1 and Mathieu Munsch1 Department of Physics, University of Basel, Klingelbergstrasse 82, CH-4056 Basel, Switzerland DTU Fotonik, Department of Photonics Engineering, Technical University of Denmark, Building 343, DK-2800 Kongens Lyngby, Denmark Universit Grenoble Alpes, F-38100 Grenoble, France CEA, INAC-SP2M, 17 rue des Martyrs, F-38054 Grenoble, France (Dated: June 23, 2015)

Highly directive and Gaussian far-field emission from “giant” photonic trumpets

Photonic trumpets are broadband dielectric antennas that efficiently funnel the emission of a point-like quantum emitter—such as a semiconductor quantum dot—into a Gaussian free-space beam. After describing guidelines for the taper design, we present a “giant” photonic trumpet. The device features a bottom diameter of 210 nm and a 5 μm wide top facet. Using Fourier microscopy, we show that 95% of the emitted beam is intercepted by a modest numerical aperture of 0.35. Furthermore, far-field measurements reveal a highly Gaussian angular profile, in agreement with the predicted overlap to a Gaussian beam Mg=0.98. Future application prospects include the direct coupling of these devices to a cleaved single-mode optical fiber. The calculated transmission from the taper base to the fiber already reaches 0.59, and we discuss strategies to further improve this figure of merit.

Large and Uniform Optical Emission Shifts in Quantum Dots Strained along Their Growth Axis

We introduce a calibration method to quantify the impact of external mechanical stress on the emission wavelength of distinct quantum dots (QDs). Specifically, these emitters are integrated in a cross-section of a semiconductor core wire and experience a longitudinal strain that is induced by an amorphous capping shell. Detailed numerical simulations show that, thanks to the shell mechanical isotropy, the strain in the core is uniform, which enables a direct comparison of the QD responses. Moreover, the core strain is determined in situ by an optical measurement, yielding reliable values for the QD emission tuning slope. This calibration technique is applied to self-assembled InAs QDs submitted to incremental elongation along their growth axis. In contrast to recent studies conducted on similar QDs submitted to a uniaxial stress perpendicular to the growth direction, optical spectroscopy reveals up to ten times larger tuning slopes, with a moderate dispersion. These results highlight the importance of the stress direction to optimize the QD optical shift, with general implications, both in static and dynamic regimes. As such, they are in particular relevant for the development of wavelength-tunable single-photon sources or hybrid QD opto-mechanical systems.

Quantum dot spontaneous emission control in a ridge waveguide

We investigate the spontaneous emission (SE) of self-assembled InAs quantum dots (QDs) embedded in GaAs ridge waveguides that lay on a low index substrate. In thin enough waveguides, the coupling to the fundamental guided mode is vanishingly small. A pronounced anisotropy in the coupling to non-guided modes is then directly evidenced by normal-incidence photoluminescence polarization measurements. In this regime, a measurement of the QD decay rate reveals a SE inhibition by a factor up to 4. In larger wires, which ensure an optimal transverse confinement of the fundamental guided mode, the decay rate approaches the bulk value. Building on the good agreement with theoretical predictions, we infer from calculations the fraction β of SE coupled to the fundamental guided mode for some important QD excitonic complexes. For a charged exciton (isotropic in plane optical dipole), β reaches 0.61 at maximum for an on-axis QD. In the case of a purely transverse linear optical dipole, β increases up to 0.91. This optimal configuration is achievable through the selective excitation of one of the bright neutral excitons.

All-Optical Mapping of the Position of Quantum Dots Embedded in a Nanowire Antenna

Nanowire antennas embedding single quantum dots (QDs) have recently emerged as a versatile solid-state platform for quantum optics. Within the nanowire section, the emitter position simultaneously determines the strength of the light-matter interaction, as well as the coupling to potential decoherence channels. Therefore, to quantitatively understand device performance and guide future optimization, it is highly desirable to map the emitter position with an accuracy much smaller than the waveguide diameter, on the order of a few hundreds of nanometers. We introduce here a nondestructive, all-optical mapping technique that exploits the QD emission into two guided modes with different transverse profiles. These two modes are fed by the same emitter and thus interfere. The resulting intensity pattern, which is highly sensitive to the emitter position, is resolved in the far-field using Fourier microscopy. We demonstrate this technique on a standard microphotoluminescence setup and map the position of individual QDs in a nanowire antenna with a spatial resolution of ±10 nm. This work opens important perspectives for the future development of light-matter interfaces based on nanowire antennas. Beyond single-QD devices, it will also provide a valuable tool for the investigation of collective effects that imply several emitters coupled to an optical waveguide.

Integer and Fractional Quantum Hall effect in Ultrahigh Quality Few-layer Black Phosphorus Transistors

As a high mobility two-dimensional semiconductor with strong structural and electronic anisotropy, atomically thin black phosphorus (BP) provides a new playground for investigating the quantum Hall (QH) effect, including outstanding questions such as the functional dependence of Landau level (LL) gaps on magnetic field B, and possible anisotropic fractional QH states. Using encapsulated few-layer BP transistors with mobility up to 55 000 cm2/(V s), we extracted LL gaps over an exceptionally wide range of B for QH states at filling factors -1 to -4, which are determined to be linear in B, thus resolving a controversy raised by its anisotropy. Furthermore, a fractional QH state at ν ≈ -4/3 and an additional feature at -0.56 ± 0.1 are observed, underscoring BP as a tunable 2D platform for exploring electron interactions.

Raman Spectroscopy, Photocatalytic Degradation, and Stabilization of Atomically Thin Chromium Tri-iodide

As a 2D ferromagnetic semiconductor with magnetic ordering, atomically thin chromium tri-iodide is the latest addition to the family of two-dimensional (2D) materials. However, realistic exploration of CrI3-based devices and heterostructures is challenging due to its extreme instability under ambient conditions. Here, we present Raman characterization of CrI3 and demonstrate that the main degradation pathway of CrI3 is the photocatalytic substitution of iodine by water. While simple encapsulation by Al2O3, PMMA, and hexagonal BN (hBN) only leads to modest reduction in degradation rate, minimizing light exposure markedly improves stability, and CrI3 sheets sandwiched between hBN layers are air-stable for >10 days. By monitoring the transfer characteristics of the CrI3/graphene heterostructure over the course of degradation, we show that the aquachromium solution hole-dopes graphene.

Optical localization of quantum dots in tapered nanowires

In this work we have measured the far-field emission patterns of In As quantum dots embedded in a GaAs tapered nanowire and used an open-geometry Fourier modal method for determining the radial position of the quantum dots by computing the far-field emission pattern for different quantum dot locations.

Determination of radial quantum dot position in trumpet nanowires from far field measurements

A quantum dot embedded in a trumpet nanowire has been shown to be a good candidate for realising an efficient on-demand single-photon source [1, 2]. At the lateral quantum dot position the diameter of the nanowire only supports two guided modes — the HE11 and TE01 modes. These modes are excited by the quantum dot after recombination of an electron-hole pair. The slow expansion of the nanowire diameter secures an adiabatic propagation of the modes through the taper meaning that the occupation of the HE11 and TE01 modes will remain the same at the top of the trumpet nanowire (see Fig. 1 right). It is for now not possible to control the radial position of the quantum dot in the nanowires and thus it is important to experimentally determine this after fabrication.