E-Thesis 52 views 37 downloads
Spectral Analysis and Parameter Estimation in Fibre Levitated Optomechanics / CHRISTOPHER DAWSON
Swansea University Author: CHRISTOPHER, DAWSON
PDF | E-Thesis – open access
Copyright: The author, Christopher Dawson, 2020.Download (40.07MB)
DOI (Published version): 10.23889/SUthesis.56947
In levitated optomechanics, nano-scale objects are optically trapped so that their motion can be studied. These trapped nanoparticles are held in a 3D quadratic potential and act as damped harmonic oscillators; they are thermally and mechanically decoupled from the apparatus and their position is me...
|Supervisor:||Bateman, James E.|
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In levitated optomechanics, nano-scale objects are optically trapped so that their motion can be studied. These trapped nanoparticles are held in a 3D quadratic potential and act as damped harmonic oscillators; they are thermally and mechanically decoupled from the apparatus and their position is measured interfer-ometrically to picometre accuracy. These systems are well suited to sensing and metrology applications, as any external disturbance of the particle can be observed using the scattered trapping light.When examining the motion of a levitated nanoparticle, it’s position is recorded and used to estimate a power spectral density (PSD), from which state parameters can be estimated. In this thesis an experi-mental setup is presented, optimised for maximum collection of particle position information in 1D, using a ﬁbre-based parabolic mirror trap and heterodyne measurement system in order to produce spectra with minimal noise and unwanted artefacts.A novel application of the Middleton expansion from RF engineering is used to generate a complete power spectrum that depends on the physical parameters of the system. This method treats the particle as a stochastic harmonic oscillator, phase modulated by a Gaussian random process with known PSD. We reproduce the PSD of intensity at a detector, a quantity that is sinusoidally dependent on particle posi-tion. This technique generates a single, full PSD using modiﬁed Bessel functions, and does not depend on assumptions about the relative phases of the interfered ﬁelds, highlighting the non-linear dependence of measured signal on position. Theoretical spectra are ﬁtted to a measured PSD and the phase modulation depth is extracted; this is used to calculate the particle oscillation amplitude and, by an equipartition ar-gument, the centre of mass temperature to mass ratio. State parameters are tracked as environmental conditions change and an increase in centre of mass temperature as a function of decreasing background gas pressure is observed.
Optomechanics, Levitated Optomechanics, Fibre optics, Signal Processing, Optics, Photonics, Motion control, Spectral Analysis
College of Science