Combining a precise sputter etching method with subsequent AlOx growth within an atomic layer deposition chamber enables the fabrication of atomically flat lateral patterns of SiO2 and AlOx. The transfer of MoSe2 monolayers onto these dielectrically modulated substrates results in the formation of lateral heterojunctions due to the interaction with alternating regions of SiO2 and AlOx, with the flat substrate topography leading to minimal strain across the junction. Kelvin probe force microscopy measurements show significant variations in the contact potential difference (CPD) across the interface, with AlOx regions inducing a 230 mV increase in CPD. Photoluminescence spectroscopy reveals shifts in spectral weight of neutral and charged exciton species across the different dielectric regions. On the AlOx side, the Fermi energy moves closer to the conduction band, leading to a higher trion-to-exciton ratio, indicating a bandgap shift consistent with CPD changes. In addition, transient reflection spectroscopy highlights the influence of the dielectric environment on carrier dynamics, with the SiO2 side exhibiting rapid carrier decay typical of neutral exciton recombination. In contrast, the AlOx side shows slower, mixed decay behavior consistent with conversion of trions back into excitons. These results demonstrate how dielectric substrate engineering can tune 2D materials, allowing scalable fabrication of advanced junctions for novel (opto)electronics applications.

We demonstrate a new generation of composition-tuned, ternary GaAsSb nanowire lasers on silicon with emission wavelengths tuned to below the Si bandgap. By solving previous limitations in the growth of III-As-Sb based nanowires, resonator cavities with extended lengths > 7 µm and high Sb-content (~30%) are realized as a base for bulk-type or quantum-well based nanowire lasers. Bulk GaAsSb nanowire lasers with high radiative efficiency and low threshold are enabled by use of lattice-matched InAlGaAs surface passivation layers. Coaxial InGaAs multi-quantum well (MQW) active regions grown on GaAsSb nanowire templates open further scope of tailoring material gain and lasing wavelength.

Performance limiting factors for GaAs(Sb)-AlGaAs nanowire (NW) lasers are unveiled by exploring the impact of impurity-induced point defects. Lasing properties show large tolerance to these defects which, however, limit radiative efficiency and lasing threshold beyond critical densities.

Nanowire lasers can be monolithically and site-selectively integrated onto silicon photonic circuits. To assess their full potential for ultrafast opto-electronic devices, a detailed understanding of their lasing dynamics is crucial. However, the roles played by their resonator geometry and the microscopic processes that mediate energy exchange between the photonic, electronic, and phononic subsystems are largely unexplored. Here, we study the dynamics of GaAs-AlGaAs core-shell nanowire lasers at cryogenic temperatures using a combined experimental and theoretical approach. Our results indicate that these NW lasers exhibit sustained intensity oscillations with frequencies ranging from 160 GHz to 260 GHz. As the underlying physical mechanism, we identified self-induced electron-hole plasma temperature oscillations resulting from a dynamic competition between photoinduced carrier heating and cooling via phonon scattering. These dynamics are intimately linked to the strong interaction between the lasing mode and the gain material, which arises from the wavelength-scale dimensions of these lasers. We anticipate that our results could lead to new approaches for ultrafast intensity and phase modulation of chip-integrated nanoscale semiconductor lasers.

Recent progress in III-V nanowire (NW) light sources integrated onto Si (quantum) photonic circuits is presented, illustrating key results for low-threshold vertical-cavity NW-lasers and integrated NW-quantum emitters with efficient light coupling to Si waveguides.

Realizing telecom-band lasing in GaAs-based nanowires (NW) with low bandgap gain media has proven to be notoriously difficult due to the high compressive strain built up in the active regions. Here, we demonstrate an advanced coaxial GaAs-InGaAs multi-quantum well (MQW) nanowire laser that solves previous limitations by the introduction of a strain compensating InAlGaAs buffer layer between the GaAs core and the MQW active region. Using a buffer layer thickness comparable to the core diameter applies a significant tensile strain to the GaAs core which efficiently minimizes the compressive strain in the InGaAs MQW and enables large In-content without plastic relaxation. Experimental verification is shown for NW-lasers with an In-content of up to 40% in the MQW, evidencing a clear strain-relieved redshift of the lasing emission and a strong reduction of the lasing threshold compared to highly strained MQWs in state-of-the-art GaAs NW-lasers. This way we achieve optically pumped room temperature lasing operation with a threshold below 50 μJ cm−2 in the telecom O-band close to 1.3 μm.

Andreas Thurn,1, ∗ Jochen Bissinger,1 Stefan Meinecke,2 Paul Schmiedeke,1 Sang Soon Oh,3 Weng W. Chow,4 Kathy Lüdge,2 Gregor Koblmüller,1 and Jonathan J. Finley1, † Walter Schottky Institut, Technische Universität München, Am Coulombwall 4, 85748 Garching, Germany. Institut für Theoretische Physik, Technische Universität Berlin, Hardenbergstraße 36, 10623 Berlin, Germany. School of Physics and Astronomy, Cardiff University, Cardiff CF24 3AA, UK. Sandia National Laboratories, Albuquerque, New Mexico 87185-1086, USA.

We demonstrate lasing from GaAs-InGaAs-based core-multiple quantum well nanowires with lasing emission tunable from ~0.8 to ~1.1 μm. By controlling the shell growth temperature, the quantum well In-molar fraction is increased to 25% without plastic relaxation. © 2019 The Author(s)

Semiconductor nanowire (NW) lasers are attractive as integrated on-chip coherent light sources with strong potential for applications in optical communication and sensing. Realizing lasers from individual bulk-type NWs with emission tunable from the near-infrared to the telecommunications spectral region is, however, challenging and requires low-dimensional active gain regions with an adjustable band gap and quantum confinement. Here, we demonstrate lasing from GaAs-(InGaAs/AlGaAs) core-shell NWs with multiple InGaAs quantum wells (QW) and lasing wavelengths tunable from ∼0.8 to ∼1.1 μm. Our investigation emphasizes particularly the critical interplay between QW design, growth kinetics, and the control of InGaAs composition in the active region needed for effective tuning of the lasing wavelength. A low shell growth temperature and GaAs interlayers at the QW/barrier interfaces enable In molar fractions up to ∼25% without plastic strain relaxation or alloy intermixing in the QWs. Correlated scanning transmission electron microscopy, atom probe tomography, and confocal PL spectroscopy analyses illustrate the high sensitivity of the optically pumped lasing characteristics on microscopic properties, providing useful guidelines for other III-V-based NW laser systems.

We demonstrate lasing from individual GaAs-based NWs integrated onto Si ridge waveguides. In addition, proof-of-principle coupling emission to the Si WG is shown, with propagation distances of the lasing mode exceeding > 60 mu m.

Free-space optical communication links are promising channels for establishing secure quantum communication. Here we study the transmission of nonclassical light through a turbulent atmospheric link under diverse weather conditions, including rain or haze. To include these effects, the theory of light transmission through atmospheric links in the elliptic-beam approximation presented by Vasylyev et al. [D. Vasylyev et al., Phys. Rev. Lett. 117, 090501 (2016)] is further generalized. It is demonstrated, with good agreement between theory and experiment, that low-intensity rain merely contributes additional deterministic losses, whereas haze also introduces additional beam deformations of the transmitted light. Based on these results, we study theoretically the transmission of quadrature squeezing and Gaussian entanglement under these weather conditions.

In this work, we extend loop quantum gravity (LQG) both, to higher dimensions and supersymmetry (i.e. supergravity theories), thus overcoming the current limitation to 3+1 dimensions with standard model matter fields. On the one hand, this gives a proof of principle that LQG is in accordance with these two theoretical concepts, and on the other hand hopefully allows contact with superstring/M - theory, which necessarily is supersymmetric and formulated in ten or eleven spacetime dimensions. Symmetry arguments suggest that supergravity theories in the corresponding dimensions constitute the low energy effective field theory limit of superstring/M - theory. This makes a study of the loop quantisation thereof, which we start here, a promising endeavour at the border between the two approaches. In more detail, our findings are the following: firstly, a new canonical formulation for general relativity in D + 1 spacetime dimensions (D ≥ 2) on a Yang Mills theory phase space is presented for the first time, with the core properties that 1. the canonical variables encoding the metric information are a real connection and its real conjugate momentum, in particular satisfying the standard canonical Poisson bracket relations, 2. the gauge group can be chosen to be a compact group (namely SO(D + 1)) for both, Lorentzian and Euclidean signature spacetimes, and 3. the system of constraints is first class (in Dirac’s terminology). Up to now, such a formulation was only known for D = 3 (and D = 2), corresponding to Ashtekar Barbero variables, constituting the classical foundation of the loop quantisation programme. The quantisation procedure itself is formulated almost independently of the number of spacetime dimensions and the choice of compact gauge group, and therefore the lack of higher dimensional analogues of LQG only was caused by the missing classical canonical formulation satisfying 1. - 3. Thus it is not surprising and we show explicitly that the new formulation we present can be quantised using the methods developed in the loop community straightforwardly to obtain LQG theories in higher dimensions. The formulation which we present is genuinely new in that it does not reduce to the Ashtekar Barbero formulation for D = 3, and furthermore for D > 2 comes with an additional set of constraints, the so called simplicity constraints, which pose the only conceptually new challenge when quantising. Interestingly, these constraints are not at all unknown in (quantum) gravity research, and in particular are a standard ingredient in the covariant approach to LQG called spin foam models. The formulation in this sense builds a novel bridge between the covariant and canonical approaches to LQG. The quantum anomalies known for this constraint from spin foams are recovered, which lead to problems when implementing it at the quantum level. We present some new proposals of how to deal with these problems. In the second part of this work, we give an extension of the above framework to the loop quantisation of a large class of Lorentzian signature supergravities, including in particular the D + 1 = 4 N = 8, D + 1 = 11 N = 1 and D + 1 = 10 N = 1 theories. Concretely, we incorporate standard and also non-standard matter fields, which appear in supergravity theories due to the requirement of supersymmetry, into the afore developed framework of higher dimensional LQG. Coupling to standard model matter fields has already been achieved in usual LQG and the results obtained there carry over to the case at hand. The only exception is the treatment of Dirac fermions, which needs slight adjustment: coming from an action principle, the Dirac field transforms in the spinor representation of the gauge group SO(1, D) for the physically relevant Lorentzian theory, but due to the strong similarity of the Lorentzian and the Euclidean Clifford algebras, the gauge group can be exchanged for SO(D + 1) to fit in with the gravitational degrees of freedom. Typical non-standard fields appearing in supergravity theories are the spin 3/2 Rarita Schwinger field (“gravitino”) on the fermionic side, and (Abelian) higher p-form fields as novel bosonic fields (i.e. generalisations of the Maxwell field to higher form degree). The former usually is a Majorana fermion (i.e. it is its own antiparticle) and therefore belongs to a real representation space of SO(1, D). In order to formulate supergravities in terms of SO(D + 1) gauge theories, we again have to exchange the gauge group SO(1, D) with SO(D + 1), but there is no action of SO(D + 1) on these real representation spaces, which hugely complicates the passage when compared to the case of Dirac fermions. We present a solution to this problem and for the first time, to the best of the author’s knowledge, provide a background independent Hilbert space representation for the gravitino field. Concerning novel bosonic fields, we exemplarily treat the three-form field (“three index photon”) of D + 1 = 11 N = 1 supergravity. Due to an additional Chern Simons term in the action, this field is not a simple generalisation of the Maxwell field to three-forms, but actually becomes self interacting and the equivalent of the electric field is not gauge invariant. We propose a reduced phase space quantisation with respect to the equivalent of the Gaus constraint, and the background independent representation we use is given by a state of Narnhofer-Thirring type, which already has been used in the loop literature in Thiemann’s treatment of the closed bosonic string. In the third part of this work, as a first application of the new variables, we extend the isolated horizon treatment (a quasi-local notion of black holes) in LQG to higher dimensions. In D = 3, the use of Ashtekar Barbero variables induces a Chern Simons theory on the horizon and the quantisation thereof and subsequent state counting led to the derivation of the famous Bekenstein Hawking entropy formula for black holes from LQG. Here, we study (non-distorted) isolated horizons in 2(n + 1) dimensional spacetimes and find that using the new variables induces an SO(2(n+1)) Chern Simons theory thereon. Since this theory, unlike its D = 3 counterpart, has local degrees of freedom, the quantisation and finally rederivation of the entropy formula become significantly more intricate and are left for further research. We want to stress that several aspects of both, the higher dimensional as well as the supersymmetric extension, definitely deserve further study to actually catch up with the current status of usual canonical LQG. In the non-supersymmetric case, this concerns mainly the implementation of the simplicity constraint and its interplay with the dynamics. In the supersymmetric case, of course the supersymmetry constraint needs intensive study, in particular its role in the quantum super Dirac algebra. We hope that the generalisation of LQG to higher dimensions and supersymmetry achieved in this work will spark further development to clarify the mentioned open problems and finally lead to new interrelations between LQG and superstring/M - theory.