Master Thesis Defence by Clara Celeste Qvotrup

Deflecting waveguides for three-dimensional quantum photonic integrated circuits

Integrated photonic circuits provide a promising platform for quantum information processing. However, effective integrated photonics require an efficient method of chip-to-fiber coupling, and a method which is both scalable, effective and broadband have yet to be developed.

In this thesis, a novel three-dimensional cantilever waveguide working as a hybrid coupler is introduced. By exploiting the intrinsic stress mismatch between the GaAs wafer and Nickel strips placed on the chip by metal evaporation, bilayer cantilever waveguides with a circular curvatures are realized, containing a radius of curvature dependent on the stress mismatch. By tailoring the length of the cantilever, the orientation of the epoxy-clad cantilever tip can be calibrated to reach a 90 degree angle with the substrate. In this configuration the epoxy-clad waveguide tip acts as a coupler, allowing for vertical in and out-coupling of light from the top window of a cryostat.

The fabrication process of the device is described, as well as an introduction to the complex behavior of strained semi-conductors.  Using the finite element method software COMSOL, simulations of the piezo-electric response of the bent bilayer cantilever to an applied electric potential are carried out, and used to describe the potential for strain tuning of GaAs quantum dots embedded in the waveguide. Furthermore, simulations of a flat bilayer cantilever with intrinsic stress on Nickel layer are run, and the resulting strain in the curved waveguide extracted. Using this strain, theoretical models for the changes in optical properties due to respectively band gap change described by the Pikus-Bir Hamiltonian and sub-bandgap absorption due to the Franz-Keldysh effect from the electric fields induced by the piezo- and flexoelectric effect are described.

The fabricated chip is placed in a flow cryostat, and the transmission was characterized for the transverse electric (TE) and transverse magnetic (TM) modes at 10 K and 293 K. The bent cantilevers were found to be heat  While the efficiency for the bent cantilever waveguide coupler is low, it was found to be extremely broadband, with full-width half-maximum bandwidths over 150 nm being observed for both the TM and TE modes.

During characterization, an abrupt cutoff allowing no transmission for wavelengths below it was found. This cutoff greatly differed between the TE and TM modes (935 nm for TE vs 905 for TM at room temperature), and was similarly found to be greatly temperature dependent. By taking measurements for the TE transmission for a wide range of temperatures in the range of 293 - 10 K, the temperature dependence of the cutoff was found. Comparing this cutoff to the temperature-dependent GaAs bandgap, it was found that they followed exactly the same slope, but differed with an offset of approximately 0.1 eV. The possible configurations of strain causing this bandgap was investigated.