Unidirectional Coupling Of Light From Nanowire Lasers Into Silicon Waveguides

In the beginning of 2017, I joined the Walter Schottky Institut, TUM [1, 2], where research efforts focus on challenges ranging from solid-state physics, photonics and quantum science to nanotechnology and materials science. I was working within the Nanowire Group, a subgroup of Prof. Jonathan J. Finley’s Chair for Semiconductor Nanostructures and Quantum Systems (E24) [3-5].

One of the main challenges to overcome on the road towards integrated photonics is to achieve a monolithic integration of efficient, reliable and cheap nanoscale sources on silicon in the desired wavelength range, which remains the holy grail for chip-level optical interconnects to date. In this context, (III-V) semiconductor nanowires (NWs) are a promising candidate.

In my thesis, I focused on numerical simulations of NW lasers on silicon waveguides with the emphasis on chiral optics effects, i.e., exploring how to achieve a (uni)directional coupling of light from a nanowire laser into a silicon waveguide.By means of numerical simulations, I have shown – for the first time – that via an effect called spin-orbit coupling of light, circularly polarized light generated in a nanowire laser can be coupled unidirectionally into a silicon waveguide.

I defended my thesis in September 2017 and secured the best possible grade (1,0 at TUM and 10 at University of Ljubljana). A peer-reviewed publication is in preparation.

The full Thesis is available HERE.

Following below is a short abstract.


Before: light couples symmetrically, no preferred direction.


Result: unidirectional coupling of light into the silicon waveguide. The preferred direction can be changed by reversing the circular polarization (left-handed <-> right-handed).

Abstract

The thesis focus is on numerical simulations of GaAs-AlGaAs core-shell nanowire lasers on silicon waveguides, which are promising in the context of chip-level optical interconnects. We employ several numerical simulation tools to investigate different properties of the chosen structures. We start by studying the propagating modes defined by the nanowire waveguide. The nanowire end facet reflectivity, an important parameter for low-threshold lasing, can be enhanced from 40% to 90% with a metal mirror. By asymmetrically placing a nanowire which supports a circularly polarized propagating wave on top of a Si waveguide, unidirectional coupling of light can be achieved via the spin-orbit coupling. The ratio between the amount of light coupled in opposite directions of the waveguide can reach values of nearly 104 but is extremely sensitive to the position of the nanowire. The coupling is influenced by many parameters, such as nanowire and waveguide dimensions and the thickness of the oxide, resulting in a huge parameter space which makes the task of finding an optimal structure more challenging.

References

[1] Walter Schottky Institute, TUM. http://www.wsi.tum.de/

[2] Technical University Munich. https://www.tum.de/en/homepage/

[3] Chair for Semiconductor Nanostructures and Quantum Systems (E24). http://www.wsi.tum.de/Research/FinleygroupE24/tabid/143/Default.aspx

[4] Prof. Jonathan J. Finley’s profile on TUM Professors’ website. http://www.professoren.tum.de/en/finley-jonathan/

[5] Nanowire Group, E24, WSI.  http://www.wsi.tum.de/Research/FinleygroupE24/ResearchAreas/QuantumNanomaterials/tabid/309/Default.aspx