Waveguide acousto-optical modulators for integrated quantum photonics

Résumé

The (quantum) optical waveguide structures presented in this thesis are novel and versatile devices for future integrated photonic circuits that need to be small, cost-effective, robust and that can be fabricated with high production tolerances in place. This work seeks to exploit the interaction between light and sound waves in dimensions down to the quantum emitter level in order to lead the way toward highly dynamical telecommunication systems. To this end, acoustically tuned optical switches on a monolithical (Al,Ga)As platform that enable robust, compact and fast responding systems improving on recently demonstrated technology are analysed. The presented work covers the full development process of modelling, designing, calculating and simulating the devices from the ground up. The detailed experimental sections show the fabrication steps as well as the complex measurements undertaken to characterise the samples. A comprehensive analysis and interpretation of the achieved experimental data brings the thorough study of the devices to a conclusive end. The devices’ fundamental functionality relies on multimode interference based Mach-Zehnder interferometer switches. The devices are tuned by surface acoustic wave (SAW) beams working in the low GHz range. In this way, the refractive index profile over a narrow modulation region can be modulated and thus real time control over the optical switching behaviour is gained via the applied SAW intensity and frequency. Using SAWs allows for a non-destructive manipulation of such a system’s optical responses. The here presented device category involves integrated quantum photonics wherein single photon sources are combined with complex and dynamic optical circuitry. Therefore, the acoustic modulation of the signal emitters and photons plays a key role for the realisation of state-of-the-art chip-based integrated photonic circuits. Novel acoustically tuned optical switches on an (Al,Ga)As platform that enable low-loss, small footprint and high signal fidelity systems using the on-chip generation of single photons are presented. As such a light source single quantum dots embedded in the waveguide structures are used. Modulation of the quantum dots by a surface acoustic wave grants control over the photon emission wavelength. The integrated optical circuit through which the photons travel consists of a Mach-Zehnder interferometer based on two multimode interference devices which are dynamically tuned by a second surface acoustic wave. The photons can be routed between multiple outputs by controlling the intensity and frequency of this surface acoustic wave. With the presented results it is shown how these complex modulation mechanisms can be combined and how they can be readily applied for wavelength modulation and fast signal routing in future quantum optical networking systems for secure quantum communication.