Kai Müller

Enhanced photoluminescence emission from two-dimensional silicon photonic crystal nanocavities

We present a temperature-dependent photoluminescence study of silicon optical nanocavities formed by introducing point defects into two-dimensional photonic crystals. In addition to the prominent TO-phonon-assisted transition from crystalline silicon at ~1.10u2009eV, we observe a broad defect band luminescence from ~1.05 to ~1.09u2009eV. Spatially resolved spectroscopy demonstrates that this defect band is present only in the region where air holes have been etched during the fabrication process. Detectable emission from the cavity mode persists up to room temperature; in strong contrast, the background emission vanishes for T≥150u2009K. An Arrhenius-type analysis of the temperature dependence of the luminescence signal recorded either in resonance with the cavity mode or weakly detuned suggests that the higher temperature stability may arise from an enhanced internal quantum efficiency due to the Purcell effect.

Electrical control of the exciton?biexciton splitting in self-assembled InGaAs quantum dots

The authors demonstrate how lateral electric fields can be used to precisely control the exciton-biexciton splitting in InGaAs quantum dots. By defining split-gate electrodes on the sample surface, optical studies show how the exciton transition can be tuned into resonance with the biexciton by exploiting the characteristically dissimilar DC Stark shifts. The results are compared to model calculations of the relative energies of the exciton and biexciton, demonstrating that the tuning can be traced to a dominance of hole-hole repulsion in the presence of a lateral field. Cascaded decay of the exciton-biexciton system enables the generation of entangled photon pairs without the need to suppress the fine structure splitting of the exciton. Our results demonstrate how the exciton-biexciton system can be electrically controlled.

Dynamical modeling of pulsed two-photon interference

Single-photon sources are at the heart of quantum-optical networks, with their uniquely quantum emission and phenomenon of two-photon interference allowing for the generation and transfer of nonclassical states. Although a few analytical methods have been briefly investigated for describing pulsed single-photon sources, these methods apply only to either perfectly ideal or at least extremely idealized sources. Here, we present the first complete picture of pulsed single-photon sources by elaborating how to numerically and fully characterize non-ideal single-photon sources operating in a pulsed regime. In order to achieve this result, we make the connection between quantum Monte--Carlo simulations, experimental characterizations, and an extended form of the quantum regression theorem. We elaborate on how an ideal pulsed single-photon source is connected to its photocount distribution and its measured degree of second- and first-order optical coherence. By doing so, we provide a description of the relationship between instantaneous source correlations and the typical experimental interferometers (Hanbury-Brown and Twiss, Hong-Ou-Mandel, and Mach-Zehnder) used to characterize such sources. Then, we use these techniques to explore several prototypical quantum systems and their non-ideal behaviors. As an example numerical result, we show that for the most popular single-photon source---a resonantly excited two-level system---its error probability is directly related to its excitation pulse length. We believe that the intuition gained from these representative systems and characters can be used to interpret future results with more complicated source Hamiltonians and behaviors. Finally, we have thoroughly documented our simulation methods with contributions to the Quantum Optics Toolbox in Python (QuTiP) in order to make our work easily accessible to other scientists and engineers.

Direct exciton emission from atomically thin transition metal dichalcogenide heterostructures near the lifetime limit

We demonstrate the reduction of the inhomogeneous linewidth of the free excitons in atomically thin transition metal dichalcogenides (TMDCs) MoSe2, WSe2 and MoS2 by encapsulation within few nanometre thick hBN. Encapsulation is shown to result in a significant reduction of the 10u2009K excitonic linewidths down to ∼3.5u2009meV for n-MoSe2, ∼5.0u2009u2009meV for p-WSe2 and ∼4.8u2009meV for n-MoS2. Evidence is obtained that the hBN environment effectively lowers the Fermi level since the relative spectral weight shifts towards the neutral exciton emission in n-doped TMDCs and towards charged exciton emission in p-doped TMDCs. Moreover, we find that fully encapsulated MoS2 shows resolvable exciton and trion emission even after high power density excitation in contrast to non-encapsulated materials. Our findings suggest that encapsulation of mechanically exfoliated few-monolayer TMDCs within nanometre thick hBN dramatically enhances optical quality, producing ultra-narrow linewidths that approach the homogeneous limit.

Imaging surface plasmon polaritons using proximal self-assembled InGaAs quantum dots

We present optical investigations of hybrid plasmonic nanosystems consisting of lithographically defined plasmonic Au-waveguides or beamsplitters on GaAs substrates coupled to proximal self-assembled InGaAs quantum dots. We designed a sample structure that enabled us to precisely tune the distance between quantum dots and the sample surface during nano-fabrication and demonstrated that non-radiative processes do not play a major role for separations down to ∼10u2009nm. A polarized laser beam focused on one end of the plasmonic nanostructure generates propagating surface plasmon polaritons that, in turn, create electron-hole pairs in the GaAs substrate during propagation. These free carriers are subsequently captured by the quantum dots ∼25u2009nm below the surface, giving rise to luminescence. The intensity of the spectrally integrated quantum dot luminescence is used to image the propagating plasmon modes. As the waveguide width reduces from 5u2009μm to 1u2009μm, we clearly observe different plasmonic modes at the remot...

Emission redistribution from a quantum dot-bowtie nanoantenna

Abstract. We present a combined experimental and simulation study of a single self-assembled InGaAs quantum dot coupled to a nearby (∼25u2009u2009nm) plasmonic antenna. Microphotoluminescence spectroscopy shows a ∼2.4× increase of intensity, which is attributed to spatial far-field redistribution of the emission from the quantum dot-antenna system. Power-dependent studies show similar saturation powers of 2.5u2009u2009μW for both coupled and uncoupled quantum dot emission in polarization-resolved measurements. Moreover, time-resolved spectroscopy reveals the absence of Purcell enhancement of the quantum dot coupled to the antenna as compared with an uncoupled dot, yielding comparable exciton lifetimes of τ∼0.5u2009u2009ns. This observation is supported by numerical simulations, suggesting only minor Purcell-effects of <2× for emitter–antenna separations >25u2009u2009nm. The observed increased emission from a coupled quantum dot–plasmonic antenna system is found to be in good qualitative agreement with numerical simulations and will lead to a better understanding of light–matter coupling in such semiconductor–plasmonic hybrid systems.

Pulsed Rabi oscillations in quantum two-level systems: beyond the area theorem

The area theorem states that when a short optical pulse drives a quantum two-level system, it undergoes Rabi oscillations in the probability of scattering a single photon. In this work, we investigate the breakdown of the area theorem as both the pulse length becomes non-negligible and for certain pulse areas. Using simple quantum trajectories, we provide an analytic approximation to the photon emission dynamics of a two-level system. Our model provides an intuitive way to understand re-excitation, which elucidates the mechanism behind the two-photon emission events that can spoil single-photon emission. We experimentally measure the emission statistics from a semiconductor quantum dot, acting as a two-level system, and show good agreement with our simple model for short pulses. Additionally, the model clearly explains our recent results (Fischer and Hanschke 2017 et al Nat. Phys.) showing dominant two-photon emission from a two-level system for pulses with interaction areas equal to an even multiple of π.

Surface acoustic wave controlled charge dynamics in a thin InGaAs quantum well

We experimentally study the optical emission of a thin quantum well and its dynamic modulation by a surface acoustic wave (SAW). We observe a characteristic transition of the modulation from one maximum to two maxima per SAW cycle as the acoustic power is increased which we find in good agreement with numerical calculations of the SAW controlled carrier dynamics. At low acoustic powers the carrier mobilities limit electron-hole pair dissociation, whereas at high power levels the induced electric fields give rise to efficient acousto-electric carrier transport. The direct comparison between the experimental data and the numerical simulations provide an absolute calibration of the local SAW phase.

Coupling of guided surface plasmon polaritons to proximal self-assembled InGaAs Quantum Dots

We present investigations of the propagation length of guided surface plasmon polaritons along Au waveguides on GaAs and their coupling to near surface InGaAs self-assembled quantum dots. Our results reveal surface plasmon propagation lengths ranging from 13.4 ± 1.7 μm to 27.5 ± 1.5 μm as the width of the waveguide increases from 2-5 μm. Experiments performed on active structures containing near surface quantum dots clearly show that the propagating plasmon mode excites the dot, providing a new method to spatially image the surface plasmon mode. We use low temperature confocal microscopy with polarization control in the excitation and detection channel. After excitation, plasmons propagate along the waveguide and are scattered into the far field at the end. By comparing length and width evolution of the waveguide losses we determine the plasmon propagation length to be 27.5 ± 1.5 μm at 830 nm (for a width of 5 μm), reducing to 13.4 ± 1.7 μm for a width of 2 μm. For active structures containing low density InGaAs quantum dots at a precisely controlled distance 7-120 nm from the Au-GaAs interface, we probed the mutual coupling between the quantum dot and plasmon mode. These investigations reveal a unidirectional energy transfer from the propagating surface plasmon to the quantum dot. The exquisite control of the position and shape afforded by lithography combined with near surface QDs promises efficient on-chip generation and guiding of single plasmons for future applications in nanoscale quantum optics operating below the diffraction limit.

Quantum dot single-photon sources with ultra-low multi-photon probability

High-quality sources of single photons are of paramount importance for quantum communication, sensing, and metrology. To these ends, resonantly excited two-level systems based on self-assembled quantum dots have recently generated widespread interest. Nevertheless, we have recently shown that for resonantly excited two-level systems, emission of a photon during the presence of the excitation laser pulse and subsequent re-excitation results in a degradation of the obtainable single-photon purity. Here, we demonstrate that generating single photons from self-assembled quantum dots with a scheme based on two-photon excitation of the biexciton strongly suppresses the re-excitation. Specifically, the pulse-length dependence of the multi-photon error rate reveals a quadratic dependence in contrast to the linear dependence of resonantly excited two-level systems, improving the obtainable multi-photon error rate by several orders of magnitude for short pulses. We support our experiments with a new theoretical framework and simulation methodology to understand few-photon sources.Quantum optics: Making lonelier photonsGoing beyond the simplest form of quantum emitter can improve the quality of single-photon emission. A common approach to making single-photon sources, which are an important base component for many quantum technologies, is to use quantum dots with a single excited state. An external light pulse excites the dot, which can then relax by emitting the desired single photon. However, sometimes the light pulse will cause re-excitation leading to an undesired second photon. Kai Müller and colleagues from Technische Universität München and Stanford University have shown that using additional quantum states can avoid these limitations. In their scheme emission of a photon leads to an intermediate state that is much less likely to be re-excited, improving performance by orders of magnitude. With additional engineering this proof-of-principle should become competitive with other approaches.