PbS/CdS quantum dots as an active medium in ultrafast nanophotonics

Ulrike Woggon^a, Nina Owschimikow^a, Yücel Kaptan^a, Bastian Herzog^a, Mirco Kolarczik^a, Christian Ulbrich^a, Pieter Geiregat^b, Zeger Hens^b, Yunpeng Zhu^c, Dries van Thourhout^c, Akshay Singh^d, Xiaoqin Li^d

^aGent University - BE, Krijgslaan 281 - S3, Gent, Belgium

^bGent University - BE, Krijgslaan 281 - S3, Gent, Belgium

^cPhysics Department, University of Texas, 2515 Speedway, C1600 Austin, TX 78712-1192, United States

Materials for Sustainable Development Conference (MATSUS)
Proceedings of September Meeting 2016 (NFM16)

Berlin, Germany, 2016 September 5th - 13th

Organizers: Marin Alexe, Enrique Cánovas, Celso de Mello Donega, Ivan Infante, Thomas Kirchartz, Maksym Kovalenko, Federico Rosei, Lukas Schmidt-Mende, Laurens Siebbeles, Peter Strasser, Teodor K Todorov, Roel van de Krol and Ulrike Woggon

Poster, Christian Ulbrich, 157
Publication date: 14th June 2016

Colloidal quantum dots constitute an attractive active medium for nanophotonics due to widely tunable optical properties and scalable production. In particular PbS/CdS quantum dots are promising, as their emission wavelength can be tuned through the telecommunications windows. Although the long radiative lifetime of the exciton ground state of these nanocrystals hinders their application for rapid data processing, ultrafast dynamics can be achieved by nonlinear processes such as multiexciton formation or intraband transitions.

We investigate the response of an optical modulator consisting of PbS/CdS quantum dots spin-coated onto a silicon nitride waveguide to an optical excitation with high repetition rate (74.5 MHz). Silicon nitride is a CMOS-compatible platform widely used in nanophotonics, which does not display large intrinsic nonlinearities. Its low refractive index, however, causes an only weak evanescent coupling between the guided mode and the quantum dots. To resolve the response of the quantum dots, we use sideband pump-probe spectroscopy, a highly sensitive technique allowing us to extract amplitude and phase changes induced by the quantum dots in an optical pulse propagating through the waveguide.

We find a modulation decaying on a timescale typical for biexcitons in this systems. The phase response reveals a second resonance, which extends into the quantum dot band gap, and is linked to interband absorption processes. The feasibility of exploiting nonlinear optical processes in colloidal quantum dots as a modulation source in nanophotonics is thus demonstrated. Integrating the quantum dots into the mode maximum of the waveguide and/or using highly refractive waveguide media such as silicon will enhance the modulation depth.

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