Manipulating self-assembly of microcavities through lateral Casimir forces

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Examensarbete för masterexamen
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2021
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Tornéus, Emelie
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Gold nanoflakes in a liquid solution have been found to form stable optical microcavities through self-assembly in room temperature. By applying the nanoflake solution on a gold mirror, the nanoflakesorder themselves at a stable distance of∼100 - 200 nm from the mirror. The size of the cavity allowsfor a fundamental optical resonance modes in the visible spectral regime. By depositing a thin film ofSiO2on the gold mirror it is possible to increase the cavity size and enable the optical modes to reachthe infrared regime. The visible and infrared regime are highly interesting in technological applicationssuch as controlling chemical reaction rates and nanomachines. The configuration of the system makesit possible to actively tune the resonance modes of the cavities using laser light to exert a pressureupon the nanoflake, which is interesting for applications in light-matter coupling. The main forcesacting in this system are attractive Casimir forces and repulsive electrostatic forces. The forces aredescribed by the Lifshitz formalism and diffusive double layer, respectively within the frames of theDLVO theory. An introduction of nanoholes in the SiO2thin film (called spacer) generate areas onthe mirror that have a lower energy potential, which in turn attracts the nanoflakes. The nanoholesexerts a mean of control over where the microcavities form on the mirror.To deduce the impact nanoholes have on the formation of the microcavities, a nanofabrication processwas developed to create a gold mirror with a spacer containing nanoholes. A combination methods suchas thermal evaporation, reactive sputtering, electron beam lithography and reactive ion sputtering wasused to create 80 nm deep nanoholes with vertical walls in the spacer. An optical setup was constructedcontaining an inverted microscope, a fibre coupled spectrometer and a continuous wave laser. The setupallowed for real-time observation of the nanoflakes, reflectivity measurements of the microcavities andthe laser is utilized as optical tweezers to gain real-time control of the nanoflakes. The nanoflakes wasfound to form microcavities inside the nanoholes rather than outside, which corresponds with resultsfrom calculations made with the Lifshitz formalism. The quality of the nanofabrication process turnedout to have a significant impact on the reflectivity measurements and on how the nanoflakes react tothe nanoholes.
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self-assembly, Casimir force, nanofabrication, reflectivity, optical cavity, Lifshitz theory, laser tweezers, nanoflake
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