Engineering the Optical Properties of Nanowire Quantum Dots

Engineering the Optical Properties of Nanowire Quantum Dots

Bouwes Bavinck, M.H.

Kouwenhoven, L.P. (promotor)

Applied Sciences

QN/Quantum Nanoscience

2016-01-07

In the future world of quantum communication and computing, infomation cannot be hacked and computers solve difficult problems, such as factorizing large number or searching in a large databases, much faster than possible with ordinary computers. However, to realize such systems still much research is needed. One of the systems that has gained a lot of attention for applications in this field are optically active quantum dots, because of their ability to emit single and entangled photons. Also, they have the properties to form a bridge between stationary qubits and flying qubits. However, many challenges remain for their application in quantum information. In this thesis, we study optically active quantum dots in nanowires to answer the questions: how we can improve extraction efficiencies and how can we gain more control over the optical properties. The first part of this thesis discusses our experiments on InAsP quantumdots that are embedded in bottom-up grown InP nanowires. We investigate the effect of the nanowire diameter on the extraction efficiency. We find that a nanowire waveguide on a gold mirror can improve extraction efficiencies up to at least 42%. The photons in such waveguides are emitted in the HE11 optical mode, that has a Gaussian profile. This allows the photons to be coupled to a fiber with an efficiency of 93%. We show that by growing a simple dielectric envelope by plasma enhanced chemical vapor deposition (PECVD), it is possible to shift the emission energy of the nanowire and quantum dot over 200 meV. We can tune the shift by changing the growth conditions, resulting in red and blue shifts. We explain that the observed shifts are induced by strain from the dielectric envelope and support this with tight-binding calculations. The quantum dots that are subject of the first part of this thesis are defined by a change in material (composition) and we refer to them as hetero quantum dots. One of the great challenges of such quantum dots is to control them in growth, allowing the control over the optical properties and the ability to design complex systems containing multiple quantum dots. In the second part of this thesis, we study a different type of quantum dots - crystal phase quantumdots - that are defined by a change in the crystallographic structure. Due to the high surface to volume ratio in nanowires, it is possible to grow crystal structures that are not found in bulk and a local change in crystallographic structure can form a quantum dot, such as a small zinc blende section in a wurtzite nanowire. This type of quantumdots would in principle allow for an unprecedented degree of control as the interface between the quantum dot and matrix is defined by the crystal structure and therefore within one monolayer. In the second part, we optically study single crystal phase quantum dots in InP and GaP nanowires. We study InP nanowires containingmultiple crystal phase quantumdots andwe find that these are excellent single photon emitters, that can be source of cascaded single photon emission. We observe that the lines in our spectra our often split and we explain this with a comprehensive theory. In magnetic field, crystal phase quantumdots show a clear diamagnetic shift (10-20 mueV/T2), but the observed behavior differs from what hasbeen observed for hetero quantum dots in magnetic field. In electric field, we find clear evidence of charging of the crystal phase quantum dots. However, the growth control of InP crystal phase quantum dots is not yet achieved. In GaP, the growth of crystal phase quantum dots can be well controlled, however, the understanding the optical properties is still subject of debate. By measuring the magnetic field, we try to identify the origin of some of the observed lines that could origin from crystal phase quantumdots.

nanowire
optically active quantumdots
single photons

https://doi.org/10.4233/uuid:9228b5e1-a80c-4524-9eec-18e4c94c95a8

978-90-8593-242-0

Institutional Repository

doctoral thesis

(c) 2016 Bouwes Bavinck, M.H.