Thomas Grange

nextnano GmbH

In quantum nanoelectronics, numerical simulations have become a ubiquitous tool. Yet the comparison with experiments is often done at a qualitative level or restricted to a single device with a handful of fitting parameters. In this work, we assess the predictive power of these simulations by comparing the results of a single model with a large exp...

The progress of charge manipulation in semiconductor-based nanoscale devices opened up a novel route to realise a flying qubit with a single electron. In the present review, we introduce the concept of these electron flying qubits, discuss their most promising realisations and show how numerical simulations are applicable to accelerate experimental...

A significant enhancement in the output power of a GaAs-based terahertz quantum cascade laser (THz QCL) was achieved by implementing a relatively high impurity doping concentration. The QC structure was precisely designed using the nonequilibrium green function method by considering the band bending effect caused by a higher doping concentration. T...

The progress of charge manipulation in semiconductor-based nanoscale devices opened up a novel route to realise a flying qubit with a single electron. In the present review, we introduce the concept of these electron flying qubits, discuss their most promising realisations and show how numerical simulations are applicable to accelerate experimental...

In quantum nanoelectronics, numerical simulations have become an ubiquitous tool. Yet the comparison with experiments is often done at a qualitative level or restricted to a single device with a handful of fitting parameters. In this work, we assess the predictive power of these simulations by comparing the results of a single model with a large ex...

A GaAs/AlGaAs based terahertz quantum cascade laser with a peak power of 1.31 W and average power of 52 mW is demonstrated. In order to realize a high output power, an effective way is to increase the electron density at active region by using high doping level. This work aims to optimize a design enabling to use a heavy doping level, by re‐alignin...

A 1.31 W peak power and 52 mW average power THz QCL is presented by variable Al composition active structure with high doping concentration based on NEGF method design. Device has thick growth active layers and large mesa size with the consideration of heat dissipation.

We report electroluminescence originating from L-valley transitions in n-type Ge/Si0.15Ge0.85 quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of Δ f / f ≈ 0.2. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. The design is based on...

We report electroluminescence originating from L-valley transitions in n-type Ge/Si$_{0.15}$Ge$_{0.85}$ quantum cascade structures centered at 3.4 and 4.9 THz with a line broadening of $\Delta f/f \approx 0.2$. Three strain-compensated heterostructures, grown on a Si substrate by ultrahigh vacuum chemical vapor deposition, have been investigated. T...

In this manuscript, we develop a generalized theory for the scattering process produced by interface roughness on charge carriers that is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape of Ge/Ge-Si heterointerfaces obtained by atom probe tomography, we are able to d...

We develop a generalized theory for the scattering process produced by interface roughness on charge carriers and which is suitable for any semiconductor heterostructure. By exploiting our experimental insights into the three-dimensional atomic landscape obtained on Ge/GeSi heterointerfaces obtained by atom probe tomography, we have been able to de...

The waveguide losses from a range of surface plasmon and double metal waveguides for Ge/Si1-xGex THz quantum cascade laser gain media are investigated at 4.79 THz (62.6 μm wavelength). Double metal waveguides demonstrate lower losses than surface plasmonic guiding with minimum losses for a 10 μm thick active gain region with silver metal of 21 cm-1...

Exploiting intersubband transitions in Ge/SiGe quantum cascade devices provides a way to integrate terahertz light emitters into silicon-based technology. With the view to realizing a Ge/SiGe Quantum Cascade Laser, we present the optical and structural properties of n-type strain-symmetrized Ge/SiGe asymmetric coupled quantum wells grown on Si(001)...

Operating at high temperatures in the range of thermoelectric coolers is essential for terahertz quantum cascade lasers to real applications. The use of scattering-assisted injection scheme enables an increase in operating temperature. This concept, however, has not been implemented in a short-period structure consisting of two quantum wells. In th...

n-type Ge/SiGe terahertz quantum cascade lasers are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs quantum cascade laser design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling ope...

Theoretical predictions indicate that the n-type Ge/SiGe multi quantum-well system is the most promising material for the realization of a Si-compatible THz quantum cascade laser (QCL) operating at room temperature. As a key step forward for QCL design within the SiGe material platform, we studied both experimentally and theoretically asymmetric co...

Theoretical predictions indicate that the n-type Ge/Si−Ge multi-quantum-well system is the most promising material for the realization of a Si-compatible THz quantum cascade laser operating at room temperature. To advance in this direction, we study, both experimentally and theoretically, asymmetric coupled multi-quantum-well samples based on this...

n-type Ge/SiGe terahertz quantum cascade laser are investigated using non-equilibrium Green's functions calculations. We compare the temperature dependence of the terahertz gain properties with an equivalent GaAs/AlGaAs QCL design. In the Ge/SiGe case, the gain is found to be much more robust to temperature increase, enabling operation up to room t...

Terahertz quantum cascade lasers (THz QCLs) are theoretically analyzed using the non-equilibrium Green's function method. Simulations reveal a carrier leakage channel from the upper laser level to the first high-energy state in the emitting double well of the next period. This leakage channel is due to unintentional alignment of the two states, and...

Carrier transport in GaN terahertz (THz) quantum cascade laser (QCL) structures is theoretically investigated using a non-equilibrium Green's function method. Although scattering due to polar optical phonons in GaN is greatly enhanced with respect to GaAs/AlGaAs THz QCLs, the phonon-induced broadening of the laser levels is found to remain much sma...

Optical nonlinearities usually appear for large intensities, but discrete transitions allow for giant nonlinearities operating at the single-photon level. This has been demonstrated in the last decade for a single optical mode with cold atomic gases, or single two-level systems coupled to light via a tailored photonic environment. Here, we demonstr...

We show how cavity quantum electrodynamics can be used to strongly suppress phonon-assisted emission from solid-state artificial atoms such as semiconductor quantum dots. This results in substantially enhanced performances in single-photon sources.

Optical logic down to the single photon level holds the promise of data processing with a better energy efficiency than electronic devices [1]. In addition, preservation of quantum coherence in such logical components could lead to optical quantum logical gates [2--4]. Optical logic requires optical non-linearities to enable photon-photon interacti...

Using a single InAs quantum dot embedded in a GaAs photonic wire, we realize a giant non-linearity between two optical modes to experimentally demonstrate an all-optical transistor triggered by 10 photons.

Solid-state emitters are excellent candidates for developing integrated sources of single photons. Yet, phonons degrade the photon indistinguishability both through pure dephasing and through phonon-assisted emission. Here, we study the indistinguishability of photons emitted by a semiconductor quantum dot in a microcavity as a function of temperat...

The influence of electron--phonon interactions on the dynamics of a quantum dot coupled to a photonic cavity mode is investigated using a nonequilibrium Green's function approach. Within a polaron frame, the self-consistent-Born approximation is used to treat the phonon-assisted scattering processes between the quantum dot polaron and the cavity. T...

In a quantum network based on atoms and photons, a single atom should control the photon state and, reciprocally, a single photon should allow the coherent manipulation of the atom. Both operations require controlling the atom environment and developing efficient atom–photon interfaces, for instance by coupling the natural or artificial atom to cav...

Supplementary Figures 1-2, Supplementary Notes 1-2 and Supplementary References

The desiderata for an ideal photon source are high brightness, high single-photon purity, and high indistinguishability. Defining brightness at the first collection lens, these properties have been simultaneously demonstrated with solid-state sources, however absolute source efficiencies remain close to the 1% level, and indistinguishability only d...

Single photons are the natural link between the nodes of a quantum network: they coherently propagate and interact with many types of quantum bits including natural and artificial atoms. Ideally, one atom should deterministically control the state of a photon and vice-versa. The interaction between free space photons and an atom is however intrinsi...

We demonstrate the tunable enhancement of the zero phonon line of a single nitrogen-vacancy color center in diamond at cryogenic temperature. An open cavity fabricated using focused ion beam milling provides mode volumes as small as 1.24 $\mu$m$^3$. In-situ tuning of the cavity resonance is achieved with piezoelectric actuators. At optimal coupling...

Transport and gain properties of a resonant-phonon terahertz quantum cascade laser are calculated using nonequilibrium Green's functions. Impurity scattering is shown to be responsible for contrasting nonlinear effects in the transport and the gain properties. For typical doping concentrations, the current density is found to be weakly sensitive to...

Single-photons are key elements of many future quantum technologies, be it for the realisation of large-scale quantum communication networks for quantum simulation of chemical and physical processes or for connecting quantum memories in a quantum computer. Scaling quantum technologies will thus require efficient, on-demand, sources of highly indist...

Pure and bright single photon sources have recently been obtained by inserting solid-state emitters in photonic nanowires or microcavities. The cavity approach presents the attractive possibility to greatly increase the source operation frequency. However, it is perceived as technologically demanding because the emitter resonance must match the cav...

Quantum dots in cavities have been shown to be very bright sources of indistinguishable single photons. Yet the quantum interference between two bright quantum dot sources, a critical step for photon based quantum computation, has never been investigated. Here we report on such a measurement, taking advantage of a deterministic fabrication of the d...

We investigate theoretically the generation of indistinguishable single photons from a strongly dissipative quantum system placed inside an optical cavity. The degree of indistinguishability of photons emitted by the cavity is calculated as a function of the emitter-cavity coupling strength and the cavity linewidth. For a quantum emitter subject to...

Quantum cascade lasers made of nanowire superlattices are proposed. The dissipative quantum dynamics of their carriers is theoretically investigated using non-equilibrium Green functions. Their transport and gain properties are calculated for varying nanowire thickness, evidencing the transition from the classical to the quantum wire regime. Our ca...

Electronic transport is theoretically investigated in laterally confined semiconductor superlattices using the formalism of nonequilibrium Green's functions. Velocity-field characteristics are calculated for nanowire superlattices of varying diameters, from the quantum dot superlattice regime to the quantum well superlattice regime. Scattering proc...

The potential for scale-up coupled with minimised system size is likely to be a major determining factor in the realisation of applicable quantum information systems. Nano-fabrication technology utilising the III-V semiconductor system provides a path to scalable quantum bit (qubit) integration and a materials platform with combined electronic/phot...

The origin of the dephasing of the S-P intersublevel transitions in semiconductor quantum dots is theoretically investigated. The coherence time of this transition is shown to be lifetime-limited at low temperature, while at higher temperature pure dephasing induced by the coupling to acoustic phonons dominates the coherence decay. Population relax...

We present calculations of intersublevel population and phase relaxation in semiconductor quantum dots. Anharmonic polaron decay is shown to be responsible for population relaxation over a large spectral range. Additional pure dephasing processes are caused by acoustic phonons, which cause the appearance of peaked sidebands and induce broadening th...

Carrier relaxation is a key issue in determining the efficiency of semiconductor optoelectronic device operation. Devices incorporating semiconductor quantum dots have the potential to overcome many of the limitations of quantum-well-based devices because of the predicted long quantum-dot excited-state lifetimes. For example, the population inversi...

We investigate theoretically acoustic phonon induced decoherence in quantum dots. We calculate the dephasing of fundamental (interband or intraband) optical transitions due to real and virtual transitions with higher energy levels. Up to two acoustic phonon processes (absorption and/or emission) are taken into account simultaneously in a non-pertur...

We present a review of coherence properties of interband and intraband optical transitions in self assembled InAs/GaAs quantun dots. Indeed, recent experimental and theoretical investigations of the optical transitions in both spectral domains have allowed a better understanding of the different phenomena that affects the interaction of confined ca...

Resonant photoluminescence experiments performed on self-assembled InAs/GaAs quantum dots under strong magnetic field up to 28T give rise to an accurate determination of the interband magneto-optical transitions. As this technique minimizes the effect of the homogeneous broadening of the transitions due to the size and composition fluctuations of t...

This thesis presents a theoretical study of electron-phonon interaction in InAs/GaAs quantum dots, where the strong coupling regime between confined carriers and optical phonons leads to entangled states known as polarons. Firstly, we take into account the strong coupling between exciton and optical phonons in order to calculate interband magneto-a...

We calculate the lifetime of conduction band excited states in self-assembled quantum dots by taking into account LO-phonon-electron interaction and various anharmonic phonon couplings. We show that polaron relaxation cannot be accurately described by a semiclassical model. The contributions of different anharmonic decay channels are shown to depen...

Polaron dephasing processes are investigated in InAs/GaAs dots using far-infrared transient four wave mixing (FWM) spectroscopy. We observe an oscillatory behaviour in the FWM signal shortly (< 5 ps) after resonant excitation of the lowest energy conduction band transition due to coherent acoustic phonon generation. The subsequent single exponentia...

Polaron relaxation in self-assembled InAs/GaAs quantum dot samples containing 2 electrons per dot is studied using far-infrared, time-resolved pump–probe measurements for transitions between the s-like ground and p-like first excited conduction band states. Spin–flip transitions between singlet and triplet states are observed experimentally in the...

Electron capture and relaxation processes in n-type InAs/GaAs quantum dots were investigated using mid-infrared degenerated pump-probe spectroscopy. Fast (~4¿10ps) intraband relaxation/capture times were measured even in the absence of electron-hole scattering.

We investigate the interband transitions of self‐assembled InAs/GaAs quantum dots by using resonant photoluminescence spectroscopy under strong magnetic field. Well defined resonances are observed in the spectra. A strong anti crossing between two transitions is observed, which cannot be accounted for by a purely excitonic model. The coupling betw...

We consider the role of one‐electron spin‐dependent couplings on the energy relaxation triggered by polaron disintegration in quantum dots charged with two electrons.

We investigate the interband transitions in several ensembles of self-assembled InAs∕GaAs quantum dots by using photoluminescence excitation spectroscopy under strong magnetic fields up to 28 T. Well-defined resonances are observed in the spectra. The magnetic field dependence of the resonance energies allows an unambiguous assignment of the interb...

Cette thèse présente une étude théorique des interactions électron-phonon dans les boîtes quantiques InAs/GaAs, où le régime de couplage fort entre les porteurs confinés dans les boîtes et les phonons optiques a pour conséquence la formation d'états intriqués appelés polarons. Nous prenons tout d'abord en compte le couplage fort entre excitons et p...