Benjamin Wohlfeil

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.

Optimization of fiber grating couplers on SOI using advanced search algorithms

A one-dimensional fiber grating coupler is derived from a waveguide with random etches using implementations of particle swarm and genetic algorithms. The resulting gratings yield a theoretical coupling efficiency of up to 1.1 dB and prompt clear design rules for the layout of highly efficient fiber grating couplers.

Integrated optical fiber grating coupler on SOI for the excitation of several higher order fiber modes

Experimental and numerical results of an integrated optical fiber grating coupler on SOI capable of exciting LP₀₁, LP_{11, a} and LP_{11, b} modes in a standard few mode fiber in both TE and TM polarization are presented.

A Two-Dimensional Fiber Grating Coupler on SOI for Mode Division Multiplexing

A grating-based approach to exciting higher order fiber modes in a few mode fibers is presented. By exploitation of higher order modes of silicon on insulator nanowaveguides and by the illumination of the grating from opposing ends, the LP01, LP11,a, LP11,b, and LP21,a fiber modes could be excited in two orthogonal polarizations. Simulative and experimental results as a proof of principle are presented.

Numerical simulation of grating couplers for mode multiplexed systems

A numerical investigation of a two dimensional integrated fiber grating coupler capable of exciting several LP fiber modes in both TE and TM polarization is presented. Simulation results and an assessment of the numerical complexity of the 3D, fully vectorial finite element model of the device are shown.

Cross-talk in mode-division-multiplex optical fiber transmission systems

Multi-core and multi-mode fibers are compared for the use in future mode-division-multiplex transmission systems. While trench assisted multi-core fibers with 37 cores and a cross-talk of less than −63 dB may be designed; a principal mode based mode-division-multiplex transmission with multi-mode fibers will significantly suffer under high cross-talk.