On-chip Spectroscopy of PbS/CdS Colloidal QDs Integrated into Plasmonic Gap Antennas using superconducting Nanowire Detectors on a Silicon Nitride Photonic Platform

Lukas Elsinger^a, Ronan Gourgues^b, Iman E. Zadeh^b, Jorick Maes^c, Vigneshwaran Chandrasekaran^c, Pieter Geiregat^c, Gabriele Bulgarini^b, Val Zwiller^d, Zeger Hens^c, Dries Van Thourhout^a

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

^bSingle Quantum B.V., The Nehterlands, Lausbergstraat 17, Delft, 2628LA, Netherlands

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

^dDepartment of Applied Physics, KTH, Stockholm Sweden, Sweden

Materials for Sustainable Development Conference (MATSUS)
Proceedings of nanoGe Fall Meeting 2018 (NFM18)

S4 Nanophotonics by Nanocrystals

Torremolinos, Spain, 2018 October 22nd - 26th

Organizers: Daniel Vanmaekelbergh and Zeger Hens

Poster, Lukas Elsinger, 296
Publication date: 6th July 2018

There is potential for a fast-emitting and reliable single-photon source with emission in a telecom band to replace attenuated lasers and decoy state protocols for the application of quantum cryptography. Colloidal PbS/CdS quantum dots (QDs) can be synthesized to emit in the telecom O-band, but are a challenging material to work with because of temperature sensitivity during processing, microsecond exciton lifetimes and a broad emission spectrum.

Using a room-temperature lift-off process, we integrated PbS/CdS QDs into the gap of plasmonic antennas on top of silicon nitride waveguides in order to speed up their emission. After passing a pump filter implemented as a corrugated sidewall grating, the emitted light was guided to a Planar Concave Grating (PCG) spectrometer with four output channels. The waveguide mode of each channel was evanescently coupled to a Superconducting Nanowire Single-Photon Detector (SNSPD) located on the same photonic chip, enabling an efficient detection of the QD emission. The whole experiment was performed in a helium cryostat at temperature of 4K.

With our measurements we are able to spectrally resolve the increase of the radiative rate, while not exceeding the thermal budget of the QDs. At the same time, the combination of pump filter and spectrometer made it possible to reject the 700 nm excitation laser vertically incident on the QDs with an extinction ratio >50dB. Additional stray light suppression was achieved by having an absorbing metal layer enclosing the trenches etched to define the waveguides.

While it should be possible to scale our method of patterning the QDs down to single emitters, the long intrinsic radiative lifetime and broad emission spectrum of PbS/CdS QDs remain a hindrance for any application in Quantum Information Technologies. However, with our approach we are also able to incorporate faster emitters e.g. recently developed InAs-based QDs in the future, making it a generic platform that can be used with different kinds of quantum emitters in the visible and IR.

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