Construction of a scanning confocal microscope for Quantum Dot mapping at telecom wavelength

Detta är en Master-uppsats från KTH/Tillämpad fysik

Sammanfattning: Future commercial applications making use of non-classical light sources require bright quantum emitters. Such emitters can be achieved by embedding self-assembled, optically active semiconductor quantum dots (QDs) in centered photonic structures. Generation of nearly pure single-photon states as well as highly indistinguishable and strongly entangled photon pairs has been demonstrated with self-assembled QDs. However, these QDs form at random locations on the samples during the growth process. QD positioning therefore becomes a necessity in order to obtain a high yield of centered photonic structures. The spatial matching allows for a strong interaction between individual QDs and greatly confined optical modes. This thesis focuses on optical positioning methods both at 800 nm and in the telecom C-band (1530 nm - 1565 nm), which is the ideal wavelength range for long distance quantum communication. A modular optical setup is built and the data acquisition is fully automated on LabVIEW. In its primary application, the setup can be used for cryogenic micro-photoluminescence (PL) measurements at 800 and 1550 nm. A scanning mirror showing low hysteresis characteristics, whose advantages over piezo actuators are discussed, is introduced in the setup to perform PL mapping and QD positioning on the sample. A telecentric optical system is also added to minimize the distorsions due to the slight misalignments induced by the use of the scanning mirror. PL mapping is then performed on QD samples on which metallic alignment marks have been fabricated, where the markers act as a reference for the positioning of the QDs. In our scanning approach, a PL spectrum is acquired at each scanned position, resulting in a PL map of the scanned surface. Data visualization techniques are thus required, and convenient contrast and map wavelength control are implemented as part of this work. Furthermore, a study is conducted to reach an optimal illumination scheme enabling simultaneous detection of the QDs and the alignment marks. A two-colour illumination scheme is chosen, consisting in a HeNe laser for QD excitation and an incoherent LED for marker detection. Once the PL maps are acquired, an advanced image analysis method based on cross-correlation calculations is employed for detection and positioning of the alignment marks, along with two-dimensional Gaussian fits for the positioning of the imaged QDs. The scanning microscope shows an overall positioning uncertainty under 10 nm in the near infrared (NIR) range, providing a proof of concept for telecom QD mapping and positioning, since the design is fully transposable at telecom wavelength. Additionally, as self-assembled QDs have random emission wavelengths, the acquisition of the positioned QDs' PL spectra presents a great benefit for individual photonic structure tuning and optimal mode overlap. Finally, a supplementary optical path for direct camera imaging is modularly added, in order to assess the positioning performances of the scanning microscope. The analysis of the camera images gives a positioning uncertainty around 3 nm, exceeding the precision of current state of the art positioning setups and setting a new standard in the NIR range.

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