The present thesis investigates the dynamics of optically levitated particles in high vacuum. A levitated nanoparticle in high vacuum constitutes a nanomechanical resonator featuring a small mass and a ultra-high quality factor - significantly higher than that of fabricated nanomechanical resonators, which are approaching fundamental limits of dissipation.
The combination of small mass and high quality factor allows for sensing of feeble exotic forces, including gravitational forces, and quantum state preparation. Investigating the quantum behavior of a nanomechanical system and testing the predictions of quantum theory on meso- to macroscopic scales, addresses one of the outstanding challenges of modern physics and will provide answers to fundamental questions on our understanding of the world.
The scope of the thesis ranges from a detailed description of the experimental apparatus and proof-of-principle experiments (parametric feedback cooling) to the first observation of phenomena owing to the unique parameters of this novel optomechanical system (thermal nonlinearities). Aside from optomechanics and optical trapping, the topics covered include the dynamics of complex (nonlinear) systems and the experimental and theoretical study of fluctuation theorems, the latter playing a pivotal role in statistical physics.
Optically trapped nanoparticles are just beginning to emerge as a new class of optomechanical systems. Owing to their unique mechanical properties, there is clearly a vast and untapped potential for further research. Primary examples of how levitated particles in high vacuum can impact other fields and inspire new research avenues have been the first observation of thermal nonlinearities in a mechanical oscillator and the study of fluctuation relations with a high-Q nanomechanical resonator. Based on recent progress in the field, a plethora of fundamental research opportunities and novel applications are expected to emerge as this still young field matures.
Table of Contents
Introduction
0.1 Motivation
0.2 Overview and state of the art
0.2.1 Optomechanics and optical trapping
0.2.2 Complex systems
0.2.3 Statistical physics
1 Experimental Setup
1.1 Optical setup
1.1.1 Overview of the optical setup
1.1.2 Detector signal
1.1.3 Homodyne measurement
1.1.4 Heterodyne measurement
1.2 Feedback Electronics
1.2.1 Bandpass filter
1.2.2 Variable gain amplifier
1.2.3 Phase shifter
1.2.4 Frequency doubler
1.2.5 Adder
1.3 Particle Loading
1.3.1 Pulsed optical forces
1.3.2 Piezo approach
1.3.3 Nebuliser
1.4 Vacuum system
1.4.1 Towards ultra-high vacuum
1.5 Conclusion
2 Theory of Optical Tweezers
2.1 Introduction
2.2 Optical fields of a tightly focused beam
2.3 Forces in the Gaussian approximation
2.3.1 Derivation of optical forces
2.3.2 Discussion
2.4 Optical potential
2.5 Conclusions
3 Parametric Feedback Cooling
3.1 Introduction
3.2 Description of the experiment
3.2.1 Particle dynamics
3.2.2 Parametric feedback
3.3 Theory of parametric feedback cooling
3.3.1 Equations of motion
3.3.2 Stochastic differential equation for the energy
3.3.3 Energy distribution
3.3.4 Effective temperature
3.4 Experimental results
3.4.1 Power dependence of trap stiffness
3.4.2 Pressure dependence of damping coefficient
3.4.3 Effective temperature
3.5 Towards the ground state
3.5.1 The standard quantum limit
3.5.2 Recoil heating
3.5.3 Detector bandwidth
3.6 Conclusion
4 Dynamics of a parametrically driven levitated particle
4.1 Introduction
4.2 Theoretical background
4.2.1 Equation of motion
4.2.2 Overview of modulation parameter space
4.2.3 Secular perturbation theory
4.2.4 Steady state solution
4.3 Dynamics below threshold (linear regime)
4.3.1 Injection locking
4.3.2 Linear instability
4.3.3 Frequency pulling
4.3.4 Off-resonant modulation (low frequency)
4.4 Dynamics above threshold (nonlinear regime)
4.4.1 Nonlinear frequency shift
4.4.2 Nonlinear instability
4.4.3 Modulation frequency sweeps
4.4.4 Modulation depth sweeps
4.4.5 Relative phase between particle and external modulation
4.4.6 Nonlinear mode coupling
4.4.7 Sidebands
4.5 Conclusions
5 Thermal nonlinearities in a nanomechanical oscillator
5.1 Introduction
5.2 Description of the experiment
5.2.1 Origin of nonlinear frequency shift
5.2.2 Nonlinear spectra
5.3 Experimental results
5.3.1 Frequency and energy correlations
5.3.2 Pressure dependence of frequency fluctuations
5.3.3 Frequency stabilization by feedback cooling
5.4 Conclusion
6 Dynamic relaxation from an initial non-equilibrium steady state
6.1 Introduction
6.2 Description of the experiment
6.2.1 Average energy relaxation
6.3 Fluctuation theorem
6.3.1 General case
6.3.2 Relaxation from an initial equilibrium state
6.3.3 Relaxation from a steady state generated by parametric feedback
6.4 Experimental results
6.4.1 Relaxation from feedback cooling
6.4.2 Relaxation from excited state
6.5 Conclusion
Research Objectives and Topics
The primary objective of this dissertation is to investigate and control the dynamics of optically levitated nanoparticles in high vacuum. By achieving ultra-high quality (Q) factors through the suppression of clamping losses, this research explores the transition from classical to quantum regimes for nanomechanical oscillators, focusing on the characterization of nonlinear phenomena and the experimental validation of fluctuation theorems in non-equilibrium steady states.
- Development of an experimental setup for trapping and cooling nanoparticles in high vacuum.
- Implementation of a novel parametric feedback cooling mechanism for 3D motion control.
- Study of nonlinear dynamics in levitated particles, including Duffing non-linearity.
- Experimental analysis of thermal nonlinearities and frequency fluctuations in high-Q resonators.
- Verification of fluctuation theorems for non-equilibrium relaxation processes.
Excerpt from the Book
1.1.1 Overview of the optical setup
The optical setup for trapping and cooling is depicted schematically in figure 1.1. The light source is an ultra-stable low noise Nd:YAG laser1 with an optical wavelength of λ = 1064 nm (Fig. 1.1). The optical table2 has active vibration isolation to reduce mechanical noise.
A stable single beam optical trap is formed by focusing the laser (∼ 80 mW at focus) with a high NA objective3 (c.f. chapter 2), which is mounted inside a vacuum chamber. To parametrically actuate the particle, the beam passes through a Pockels cell (EOM)4 before entering the vacuum chamber. We use parametric actuation to either cool (c.f. chapter 3) or drive the particle (c.f. chapter 4).
For feedback cooling the particle position must be monitored with high precision and high temporal resolution. This is achieved with optical interferometry. The particle position is imprinted into the phase of light scattered by the particle. Through interference of the scattered light with a reference beam, the phase modulation induced by particle motion is converted into an intensity modulation. The intensity modulation is measured with fast balanced photodetectors. We have chosen to measure the forward scattered light. In this configuration, the non-scattered part of the incident beam serves as a reference. Since light scattered in the forward direction and the transmitted beam follow the same optical path, the relative phase between the two is fixed in the absence of particle motion. If the particle moves, the interference of scattered light and transmitted beam causes an intensity modulation of the light propagating in forward direction. We collimate the light propagating in forward direction with an aspheric lens5 which is mounted on a three dimensional piezo stage6 for alignment with respect to the objective (Fig. 1.2b).
Summary of Chapters
Introduction: Outlines the motivation for using nanomechanical oscillators and provides an overview of the field and current state-of-the-art.
1 Experimental Setup: Details the design and construction of the experimental apparatus, including optical trapping, detection, feedback electronics, and vacuum systems.
2 Theory of Optical Tweezers: Provides the analytical model for optical forces using Gaussian beam descriptions and the Rayleigh approximation.
3 Parametric Feedback Cooling: Demonstrates the experimental cooling of nanoparticles in high vacuum and discusses the theory of parametric feedback cooling.
4 Dynamics of a parametrically driven levitated particle: Investigates nonlinear dynamics and parametric driving regimes, including frequency pulling and injection locking.
5 Thermal nonlinearities in a nanomechanical oscillator: Characterizes frequency fluctuations caused by thermal motion and demonstrates mitigation via feedback cooling.
6 Dynamic relaxation from an initial non-equilibrium steady state: Explores the experimental and theoretical validation of fluctuation theorems during relaxation processes.
Keywords
Nanotechnology, Optomechanics, Optical Tweezers, Nanoparticles, Vacuum Trapping, Feedback Cooling, Nanomechanical Oscillators, Non-linear Dynamics, Fluctuation Theorem, Statistical Physics, Force Sensing, Q-factor, Langevin Equation, Duffing Oscillator, Injection Locking
Frequently Asked Questions
What is the core subject of this thesis?
This thesis focuses on the control and investigation of the dynamics of dielectric nanoparticles levitated by optical tweezers in a high vacuum environment.
What are the primary thematic areas covered?
The work spans experimental optomechanics, non-linear dynamics, statistical mechanics, and precision sensing using nanomechanical resonators.
What is the main goal or research question?
The main goal is to demonstrate the feasibility of cooling and controlling the center-of-mass motion of nanoparticles to achieve ultra-high quality factors, facilitating the study of quantum effects and non-equilibrium statistical mechanics.
Which scientific methods are utilized?
The research employs optical trapping, balanced detection, electronic parametric feedback, and stochastic analysis based on the Langevin equation.
What is the focus of the main body of the work?
The chapters cover the experimental design, the physical theory of optical tweezers, parametric cooling, non-linear dynamics under parametric driving, the analysis of thermal noise-induced frequency fluctuations, and the experimental verification of fluctuation theorems.
What key terms define this research?
Key terms include optical levitation, parametric feedback, nanomechanical oscillators, Duffing non-linearity, and fluctuation relations.
How does the experimental setup handle particles in vacuum?
The system uses a combination of an optical trap, specialized feedback electronics, and a load-lock vacuum chamber to isolate particles from thermal baths while maintaining precise control over their motion.
Why are nanoparticles better for high-Q sensing than traditional clamped systems?
Clamped systems suffer from dissipation losses at their anchors, whereas levitated nanoparticles in high vacuum are isolated from their environment, leading to significantly higher Q-factors.
What did the fluctuation theorem experiments demonstrate?
The experiments showed that the relaxation of a non-equilibrium steady state of a highly underdamped nanomechanical oscillator obeys the Crooks fluctuation theorem, providing insight into the transition between arbitrary steady states.
- Citation du texte
- Jan Gieseler (Auteur), 2014, Dynamics of optically levitated nanoparticles in high vacuum, Munich, GRIN Verlag, https://www.grin.com/document/300442
