Colloidal Quantum Dots Enabling Coherent Light Sources for Integrated Silicon-Nitride Photonics

Abstract

Integrated photonic circuits, increasingly based on silicon (-nitride), are at the core of the next generation of low-cost, energy efficient optical devices ranging from on-chip interconnects to biosensors. One of the main bottlenecks in developing such components is that of implementing sufficient functionalities on the often passive backbone, such as light emission and amplification. A possible route is that of hybridization where a new material is combined with the existing framework to provide a desired functionality. Here, we present a detailed design flow for the hybridization of silicon nitride-based integrated photonic circuits with so-called colloidal quantum dots (QDs). QDs are nanometer sized pieces of semiconductor crystals obtained in a colloidal dispersion which are able to absorb, emit and amplify light in a wide spectral region. Moreover, they combine costeffective solution based deposition methods, ambient stability and low fabrication cost. Starting from the linear and non-linear material properties obtained on the starting colloidal dispersions, we can predict and evaluate thin film and-device performance, which we demonstrate through characterization of the first onchip QD based laser.

A bright future for colloidal quantum dot lasers

“…This combination was attributed to attractive exciton-exciton interactions, opposite from the common perception that blue-shifted, repulsive, type II bi-excitons are key to efficient gain with QDs. Such high material-gain values, in turn, enabled the fabrication of the previously mentioned microlasers since the modal gain that follows can quite readily overcome typical cavity losses 9 .…”

“…It contrasts with the numerous ad-hoc cavities fabricated by dropcasting ill-defined QD films onto low-quality resonators, such as microstains, beads, and so on, which have little application or upscaling potential. A case example is the work of Xie et al and Zhu et al 9,17 in which a patterning and embedding process was developed to incorporate QDs into silicon-nitride-based microcavities operating in multi-or single-mode operation under quasi-CW conditions, see Fig. 2c.…”

“…Here, quantitative femtosecond pump-probe spectroscopy, which measures the material gain g i (λ, t) in cm −1 as a function of wavelength and pump-probe delay, offers a useful alternative 7 . In particular, the material gain is a measure to compare the vastly different samples and material systems available today 22 , and it can be translated to a device context where it yields a good estimate of the modal gain 9 . Moreover, its spectral and transient behaviors provide insights into the gain mechanism and the role of non-radiative Auger processes.…”

“…The material gain g i , which is the gain, measured in cm −1 , provided by a fictitious ensemble of QDs packed with a QD volume fraction of unity. It can be translated to device scenarios using the volume fraction and optical mode confinement of the QD layer in a laser cavity 9 . Since the discovery of light amplification by QDs 4 , it was realized that net stimulated emission requires biexciton states, i.e., QDs containing two electron-hole pairs.…”

“…As CdSe/CdS QDs can have a material gain up to several 1000 cm −1 7 , these core/shell QDs make an excellent model system to explore and improve the performance of QD-based lasers. This point has become evident in the past few years through the demonstration of applicationrelevant excitation schemes, such as electrical 12 and continuous-wave operation 13 and the demonstration of integrated QD lasers 9 . These milestones set the scene for a reflection on the future of QD lasers.…”