Suzanne Bisschop

Nanoscale and Single-Dot Patterning of Colloidal Quantum Dots.

Using an optimized lift-off process we develop a technique for both nanoscale and single-dot patterning of colloidal quantum dot films, demonstrating feature sizes down to ~30 nm for uniform films and a yield of 40% for single-dot positioning, which is in good agreement with a newly developed theoretical model. While first of all presenting a unique tool for studying physics of single quantum dots, the process also provides a pathway toward practical quantum dot-based optoelectronic devices.

On‐Chip Integrated Quantum‐Dot–Silicon‐Nitride Microdisk Lasers

Hybrid silicon nitride (SiN)-quantum-dot (QD) microlasers coupled to a passive SiN output waveguide with a 7 µm diameter and a record-low threshold density of 27 µJ cm-2 are demonstrated. A new design and processing scheme offers long-term stability and facilitates in-depth QD material and device characterization, thereby opening new paths for optical communication, sensing, and on-chip cavity quantum optics based on colloidal QDs.

Broadband enhancement of single photon emission and polarization dependent coupling in silicon nitride waveguides

Single-photon (SP) sources are important for a number of optical quantum information processing applications. We study the possibility to integrate triggered solid-state SP emitters directly on a photonic chip. A major challenge consists in efficiently extracting their emission into a single guided mode. Using 3D finite-difference time-domain simulations, we investigate the SP emission from dipole-like nanometer-sized inclusions embedded into different silicon nitride (SiNx) photonic nanowire waveguide designs. We elucidate the effect of the geometry on the emission lifetime and the polarization of the emitted SP. The results show that highly efficient and polarized SP sources can be realized using suspended SiNx slot-waveguides. Combining this with the well-established CMOS-compatible processing technology, fully integrated and complex optical circuits for quantum optics experiments can be developed.

Fabrication and characterization of SiNx/Au cavities with colloidal nanocrystals

We demonstrate the fabrication and characterization of on-chip vertically-emitting SiNx/Au nanopatch cavities containing a monolayer of colloidal quantum dots. The fabrication process is based on electron-beam lithography and deterministically positions both the cavity and the emitters within the cavity with an accuracy of 10 nm. The Purcell enhancement of the spontaneous emission of the quantum dots is studied theoretically and experimentally. The fabrication technique makes it possible to pattern the quantum dot monolayer such that the quantum dots only occupy the center of the nanopatch cavity where a Purcell factor up to 7 can be reached. The work paves the way towards scalable fabrication of bright and directive single-photon sources.

The Impact of Core/Shell Sizes on the Optical Gain Characteristics of CdSe/CdS Quantum Dots

Colloidal quantum dots (QDs) are highly attractive as the active material for optical amplifiers and lasers. Here, we address the relation between the structure of CdSe/CdS core/shell QDs, the material gain they can deliver, and the threshold needed to attain net stimulated emission by optical pumping. On the basis of an initial gain model, we predict that reducing the thickness of the CdS shell grown around a given CdSe core will increase the maximal material gain, while increasing the shell thickness will lower the gain threshold. We assess this trade-off by means of transient absorption spectroscopy. Our results confirm that thin-shell QDs exhibit the highest material gain. In quantitative agreement with the model, core and shell sizes hugely impact on the material gain, which ranges from 2800 cm-1 for large core/thin shell QDs to less than 250 cm-1 for small core/thick shell QDs. On the other hand, the significant threshold reduction expected for thick-shell QDs is absent. We relate this discrepancy between model and experiment to a transition from attractive to repulsive exciton-exciton interactions with increasing shell thickness. The spectral blue-shift that comes with exciton-exciton repulsion leads to competition between stimulated emission and higher energy absorbing transitions, which raises the gain threshold. As a result, small-core/thick-shell QDs need up to 3.7 excitations per QD to reach transparency, whereas large-core/thin shell QDs only need 1.0, a number often seen as a hard limit for biexciton-mediated optical gain. This makes large-core/thin-shell QDs that feature attractive exciton-exciton interactions the overall champion core/shell configuration in view of highest material gain, lowest threshold exciton occupation, and longest gain lifetime.

Progress towards on-chip single photon sources based on colloidal quantum dots in silicon nitride devices

New results on integration of colloidal quantum dots (QDs) into SiN microstructures are reported, including QD positioning with nanometric accuracy and the efficient coupling of their emission to waveguides and cavities. The results are relevant to on-chip quantum optics and information processing.