Optical characterization of colloidal quantum dots at cryogenic temperatures

In this project, we will use established techniques for integrating colloidal quantum dots,
that are attractive candidates for realizing a single-photon source, with nanophotonic
waveguides and characterize the spectral properties of their optical emission. As excitonic
signatures are difficult to observe at room temperatures we will translate adequately pre-
characterized chips into cryogenic measurement stations and record optical spectra of such
quantum dots at 1 Kelvin temperatures.

Integration of colloidal quantum dots in photonic crystal cavities

We have recently developed a new generation of highly performant photonic crystal
resonators for our photonic integrated circuit platforms. In this project, we will investigate
the quality factors of these devices and integrate colloidal quantum dots into optimal
resonators in order to observe the Purcell effect, which should force the emission of single
photons from quantum dots into the cavity mode and control the coherence properties of
such quantum emitters. We will further be interested in the statistical properties of the
quantum dot emission, which should be analyzed by performing a so-called Hanbury-Brown-
Twiss experiment to reveal quantum optical properties in a highly integrated photonic
system.

Lifetime measurements of SWCNTs

Defects in single-walled carbon nanotubes (SWCNTs) can be used as single-photon sources.
In collaboration with colleagues at the Ruhr-University Bochum we will investigate custom
engineered SWCNTs, analyze their optical properties first in confocal microscopy and then
attempt their integration with nanophotonic waveguides. We will particularly be interested
in their photostability, perform lifetime measurements and investigate the possibility of
generating emission at technologically relevant telecommunication wavelengths. In a second
step we plan on performing measurements with custom-designed superconducting
nanowire single-photon detectors that should allow to resolve extremely fast optical
processes due to superior timing accuracy.

Realization of a nanophotonic integrated optical phase shifter based on photo- switchable molecules

In this project, we will investigate molecules for realizing an optical switch that switches light
with light. Molecules from the azobenzol-family can exist in two isomer-states, exhibiting
different refractive indices if embedded in suitable host systems (such as polymers). Light at
one wavelength can then be used to switch between the isomer states inducing refractive
index changes for light at another wavelength. If such molecules are integrated with
nanophotonic waveguides in a Mach-Zehnder interferometer configuration the refractive
index changes will translate to effective phase shifts allowing to switch between the
interferometer outputs. We will here try out the integration of such molecules, and investigate
their switching behaviour as a function of concentration, wavelengths, etc. for finding optimal
configurations.

Optimization of fabrication of subwavelength-grating metamaterial waveguides

Photonic integrated waveguides are typically made of only one core material (e.g., silicon,
silicon nitride, or tantalum pentoxide) on top of an insulator layer (silicon dioxide). This
material choice imposes a refractive-index contrast that limits the design space of
nanophotonic devices. Subwavelength grating (SWG) metamaterials are periodic structures
with such a small period that diffraction or Bragg reflection effects are prevented, thereby
enabling diffractionless light propagation. Thus, SWG waveguides can be modelled as
equivalent homogeneous waveguides with tailorable refractive indices. Owing to the small
features that are required, the fabrication of these structures is challenging. While some
SWG waveguides have already been developed in the Münster Nanofabrication Facility
(MNF), the fabrication process is not optimized, and their performance has not yet been
sufficiently tested. In this Bachelor project, the student will have to write the mask layouts
for test chips, refine the fabrication recipes for silicon nitride SWG structures, and assess the
performance of the devices experimentally.

Only with prior clean room experience or an additional 2-3 months for training at the
MNF!

Design of adiabatic transitions for strong integrated Bragg reflectors

Integrated Bragg reflectors are periodic photonic waveguides that reflect the propagating
light. These structures are pivotal to creating on-chip spectral filters and Fabry-Pérot cavities.
Many implementations involve small corrugations in the sidewalls of the waveguides, which
leads to narrow bandwidths. However, when wider bandgaps are required, strong periodic
variations need to be applied to the optical waveguides. These variations induce high
scattering due to the mismatch between the unperturbed waveguide and the Bragg
waveguide. As a consequence, to reduce optical loss, transitions must be carefully designed.
In this Bachelor project, firstly, the student will have to simulate different types of
transitions and evaluate their performance; then, experimental characterization in an optical
measurement setup will be performed.

Implementation of new electromagnetic simulation capabilities for nanophotonic inverse design

In addition to designing nanophotonic circuit components using conventional methods, we
work on fully automated inverse design algorithms to find abstract structures enabling novel
functionalities. With computational efficiency being a critical aspect, we employ our own
Python/C++-based backend to simulate the electromagnetic field distribution needed in
specific stages of the iterative design process. The simulation backend is not yet feature-
complete and certain capabilities (such as adaptive mesh refinement in specific regions of
the simulation cell, a time-domain simulation routine, and advanced sources for interfacing
with quantum emitters) would broaden the range of devices that can be optimized using our
inverse-design tool. Please note that good programming skills are essential for this project.

Optimization of Total Internal Reflection Couplers for Photonic Integrated Circuits

Efficient in- and out-coupling of light onto a chip is crucial for the functionality of photonic
integrated circuits. This project focuses on the utilization of 3D-printed total internal
reflection couplers to achieve this objective. The primary aim is to enhance the performance
of the coupler, specifically tailored for the 1550 nm telecom wavelength on a Silicon-on-
insulator (SOI) platform.
The project includes the simulation of the coupling through the structure onto the chip to
achieve enhanced performance, with a particular emphasis on minimizing losses.
Furthermore, the investigation includes the characterization of the coupler's properties post-
fabrication, enabling a dynamic development process and the formulation of conclusions.
The ultimate goal is to gain valuable insights towards the optimization of total reflection
couplers for photonic integrated circuits.

Developing a characterization stage for integrated photonic circuits

Optical characterization is a critical aspect in the analysis of nanophotonic circuits. By
measuring wavelength-dependent transmission, the optical properties can be evaluated to
determine if the circuit is functional and meets its intended properties as per the design.
The objective of this project is to expand a currently developed setup with several new
features. Existing setups provide efficient in- and out-coupling of signals to the chip for a
fixed distance of 127 μm between the couplers on the chip. An additional, independent fiber
for in- and out-coupling will provide more flexibility for nanophotonic circuit design, opening
the door for the analysis of more complex chip designs. Electrical circuits can be employed to
characterize resistances and I-V curves of devices on the same stage. The properties of
devices may be temperature dependent, and to study this influence, Peltier elements can be
utilized to control the temperature.