Force detection in levitated optomechanics

Abstract

The use of levitated optomechanical systems as force sensors is a growing field with great potential. This thesis presents a system that achieves a sensitivity of ≈10^-22 N / √Hz by using on-resonance forces and an optically levitated nanoparticle in a gradient force trap. It is possible to reach pressures of 10^-6 mbar and trap particles with diameters of 50 nm to 300 nm. The particle's motion is detected with a homodyne-like detection system that measures the phase difference in the scattered and un-scattered, divergent, light. With this system it was possible to detect the changes in the particle's motion due to the application of an external AC and DC electric fields. DC electric fields showed a shift in the average position of up to 100 nm and also a shift of the relevant oscillator frequency of up to 1500 Hz. Applying an AC electric field resulted in the particle's motion being driven at the AC frequency. On resonance the detected signal increased by a factor of 200 which helps to measure smaller changes in the particle's motion compared to the undriven signal. Using the AC driving it was possible to detect a particle with a charge of just 4 ± 3 electrons. In addition to this, two vacuum sources were investigated, the first being an ablating source that generated particles directly in the chamber, and the second being a sonicating source that releases pre-made particles from a surface. The ablated source used a high power nano-second Neodymium-doped Yttrium aluminium garnet (Nd:YAG) laser that was able to remove material from a silicon wafer with a 200 nm layer of silicon dioxide. It was possible to trap a nanoparticle with a radius of 35 ± 3 nm at atmosphere but there was a large thermal distribution in the particle sizes. The sonicating source had the advantage that the particle's size range could be known before hand and also the source could be very close to the trap site. An acoustic horn was developed that focused the energy down to a 3 mm radius surface. It was possible to see a large release of 100 nm particles, however, none of them were trapped. It was assumed that the particles were still too large to trap so steps were taken towards a MHz source. This resulted in the first detection of a particle from an ultrasonic source at the trap site. The signal didn't last long but this still holds promise as a source once a transducer or even a horn have been designed.

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Identifiers

ORCID for Hendrik Ulbricht: orcid.org/0000-0003-0356-0065

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