Sven Rodt

Size-dependent fine-structure splitting in self-organized InAs/GaAs quantum dots

A systematic variation of the exciton fine-structure splitting with quantum dot size in single quantum dots grown by metal-organic chemical vapor deposition is observed. The splitting increases from to as much as with quantum dot size. A change of sign is reported for small quantum dots. Model calculations within the framework of eight-band theory and the configuration interaction method were performed. Different sources for the fine-structure splitting are discussed, and piezoelectricity is pinpointed as the only effect reproducing the observed trend.

Highly indistinguishable photons from deterministic quantum-dot microlenses utilizing three-dimensional in situ electron-beam lithography

The success of advanced quantum communication relies crucially on non-classical light sources emitting single indistinguishable photons at high flux rates and purity. We report on deterministically fabricated microlenses with single quantum dots inside which fulfil these requirements in a flexible and robust quantum device approach. In our concept we combine cathodoluminescence spectroscopy with advanced in situ three-dimensional electron-beam lithography at cryogenic temperatures to pattern monolithic microlenses precisely aligned to pre-selected single quantum dots above a distributed Bragg reflector. We demonstrate that the resulting deterministic quantum-dot microlenses enhance the photon-extraction efficiency to (23±3)%. Furthermore we prove that such microlenses assure close to pure emission of triggered single photons with a high degree of photon indistinguishability up to (80±7)% at saturation. As a unique feature, both single-photon purity and photon indistinguishability are preserved at high excitation power and pulsed excitation, even above saturation of the quantum emitter.The prospect of realizing building blocks for long-distance quantum communication is a major driving force for the development of advanced nanophotonic devices. Significant progress has been achieved in this field with respect to the fabrication of efficient quantum-dot-based single-photon sources. More recently, even spin-photon entanglement and quantum teleportation have been demonstrated in semiconductor systems. These results are considered as crucial steps towards the realization of a quantum repeater. The related work has almost exclusively been performed on self-assembled quantum dots (QDs) and random device technology. At this point it is clear that further progress in this field towards real applications will rely crucially on deterministic device technologies which will, for instance, enable the processing of bright quantum light sources with pre-defined emission energy. Here we report on enhanced photon-extraction efficiency from monolithically integrated microlenses which are coupled deterministically to single QDs. The microlenses with diameters down to 800 nm were aligned to single QDs by in-situ electron-beam lithography using a low-temperature cathodoluminescence setup. This deterministic device technology allowed us to obtain an enhancement of photon extraction efficiency for QDs integrated into microlenses as compared to QDs in unstructured surfaces. The excellent optical quality of the structures is demonstrated by cathodoluminescence and micro-photoluminescence spectroscopy. A Hong-Ou-Mandel experiment states the emission of single indistinguishable photons.

Multi-excitonic complexes in single InGaN quantum dots

Cathodoluminescence spectra employing a shadow mask technique of InGaN layers grown by metalorganic chemical vapor deposition on Si(111) substrates are reported. Sharp lines originating from InGaN quantum dots are observed. Temperature dependent measurements reveal thermally induced carrier redistribution between the quantum dots. Spectral diffusion is observed and was used as a tool to correlate up to three lines that originate from the same quantum dot. Variation of excitation density leads to identification of exciton and biexciton. Binding and anti-binding complexes are discovered.

Single-photon emission from InGaAs quantum dots grown on (111) GaAs

In this letter, we demonstrate that self-organized InGaAs quantum dots (QDs) grown on GaAs (111) substrate using droplet epitaxy have great potential for the generation of entangled photon pairs. The QDs show spectrally sharp luminescence lines and low spatial density. A second order correlation value of g(2)(0)<0.3 proves single-photon emission. By comparing the power dependence of the luminescence from a number of QDs we identify a typical luminescence fingerprint. In polarization dependent microphotoluminescence studies a fine-structure splitting ranging ≤40 μeV down to the determination limit of our setup (10 μeV) was observed.

In situ electron-beam lithography of deterministic single-quantum-dot mesa-structures using low-temperature cathodoluminescence spectroscopy

We report on the deterministic fabrication of sub-μm mesa-structures containing single quantum dots (QDs) by in situ electron-beam lithography. The fabrication method is based on a two-step lithography process: After detecting the position and spectral features of single InGaAs QDs by cathodoluminescence (CL) spectroscopy, circular sub-μm mesa-structures are defined by high-resolution electron-beam lithography and subsequent etching. Micro-photoluminescence spectroscopy demonstrates the high optical quality of the single-QD mesa-structures with emission linewidths below 15 μeV and g(2)(0) = 0.04. Our lithography method has an alignment precision better than 100 nm which paves the way for a fully deterministic device technology using in situ CL lithography.

Polarized emission lines from A- and B-type excitonic complexes in single InGaN/GaN quantum dots

Cathodoluminescence measurements on single InGaN/GaN quantum dots (QDs) are reported. Complex spectra with up to five emission lines per QD are observed. The lines are polarized along the orthogonal crystal directions [112¯0] and [1¯100]. Realistic eight-band k⋅p electronic structure calculations show that the polarization of the lines can be explained by excitonic recombinations involving hole states which are formed either by the A or the B valence band.

Large internal dipole moment in InGaN/GaN quantum dots

Direct observation of large permanent dipole moments of excitonic complexes in InGaN/GaN quantum dots is reported. Characteristic traces of spectral diffusion, observed in cathodoluminescence of InGaN/GaN quantum dots, allow deducing the magnitude of the intrinsic dipole moment. Our experimental results are in good agreement with realistic calculations of quantum dot transition energies for position-dependent external electric fields.

Quantum Dots for Single- and Entangled-Photon Emitters

The efficient generation of polarized single or entangled photons is a crucial requirement for the implementation of quantum key distribution (QKD) systems. Self-organized semiconductor quantum dots (QDs) are capable of emitting one polarized photon or an entangled photon pair at a time using appropriate electrical current injection. We realized a highly efficient single-photon source (SPS) based on well-established semiconductor technology: In a pin structure, a single electron and a single hole are funneled into a single InAs QD using a submicron AlOx current aperture. Efficient radiative recombination leads to emission of single polarized photons with an all-time record purity of the spectrum. Non-classicality of the emitted light without using additional spectral filtering is demonstrated. The out-coupling efficiency and the emission rate are increased by embedding the SPS into a micro-cavity. The design of the micro-cavity is based on detailed modeling to optimize its performance. The resulting resonant single-QD diode is driven at a repetition rate of 1 GHz, exhibiting a second-order correlation function of g(2)(0) = 0. Eventually, QDs grown on (111)-oriented substrates are proposed as a source of entangled photon pairs. Intrinsic symmetry-lowering effects leading to the splitting of the exciton bright states are shown to be absent for this substrate orientation. As a result, the XX rarr X rarr 0 recombination cascade of a QD can be used for the generation of entangled photons without further tuning of the fine-structure splitting via QD size and/or shape.

GaN/AlN quantum dots for single qubit emitters

We study theoretically the electronic properties of c-plane GaN/AlN quantum dots (QDs) with the focus on their potential as sources of single polarized photons for future quantum communication systems. Within the framework of eight-band theory we calculate the optical interband transitions of the QDs and their polarization properties. We show that an anisotropy of the QD confinement potential in the basal plane (e.g. QD elongation or strain anisotropy) leads to a pronounced linear polarization of the ground-state and excited-state transitions. An externally applied uniaxial stress can be used to either induce a linear polarization of the ground-state transition for emission of single polarized photons or even to compensate the polarization induced by the structural elongation.

Single-photon emission at a rate of 143 MHz from a deterministic quantum-dot microlens triggered by a mode-locked vertical-external-cavity surface-emitting laser

We report on the realization of a quantum dot (QD) based single-photon source with a record-high single-photon emission rate. The quantum light source consists of an InGaAs QD which is deterministically integrated within a monolithic microlens with a distributed Bragg reflector as back-side mirror, which is triggered using the frequency-doubled emission of a mode-locked vertical-external-cavity surface-emitting laser (ML-VECSEL). The utilized compact and stable laser system allows us to excite the single-QD microlens at a wavelength of 508 nm with a pulse repetition rate close to 500 MHz at a pulse width of 4.2 ps. Probing the photon statistics of the emission from a single QD state at saturation, we demonstrate single-photon emission of the QD-microlens chip with g(2)(0) < 0.03 at a record-high single-photon flux of (143 ± 16) MHz collected by the first lens of the detection system. Our approach is fully compatible with resonant excitation schemes using wavelength tunable ML-VECSELs, which will optimize ...