Feedback Control of Optically Trapped Nanoparticles and its Applications
thesisposted on 04.05.2020, 17:39 by Jaehoon Bang
In the 1970's, Arthur Ashkin developed a remarkable system called the ``optical tweezer'' which utilizes the radiation pressure of light to manipulate particles. Because of its non-invasive nature and controllability, optical tweezers have been widely adopted in biology, chemistry and physics. In this dissertation, two applications related to optical tweezers will be discussed. The first application is about the demonstration of multiple feedback controlled optical tweezers which let us conduct novel experiments which have not been performed before. For the second application, levitation of a silica nanodumbbell and cooling its motion in five degrees of freedom is executed.
To be more specific, the first chapter of the thesis focuses on an experiment using the feedback controlled optical tweezers in water. A well-known thought experiment called ``Feynman's ratchet and pawl'' is experimentally demonstrated. Feynman’s ratchet is a microscopic heat engine which can rectify the random thermal fluctuation of molecules to harness useful work. After Feynman proposed this system in the 1960’s, it has drawn a lot of interest. In this dissertation, we demonstrate a solvable model of Feynman’s ratchet using a silica nanoparticle inside a feedback controlled one dimensional optical trap. The idea and techniques to realize two separate thermal reservoirs and to keep them in contact with the ratchet is discussed in detail. Also, both experiment and simulation about the characteristics of our system as a heat engine are fully explored.
In the latter part of the dissertation, trapping silica nanodumbbell in vacuum and cooling its motion in five degrees of freedom is discussed. A levitated nanoparticle in vacuum is an extraordinary optomechanical system with an exceptionally high mechanical quality factor. Therefore, levitated particles are often utilized as a sensor in various research. Different from a levitated single nanosphere, which is only sensitive to force, a levitated nanodumbbell is sensitive to both force and torque. This is due to the asymmetry of the particle resulting it to have three rotational degrees of freedoms as well as three translational degrees of freedoms. In this dissertation, creating and levitating a silica nanodumbbell will be demonstrated. Active feedback cooling also known as cold damping will be employed to stabilize and cool the two torsional degrees of freedom of the particle along with the three center of mass DOF in vacuum. Additionally, both computational and experimental analysis is conducted on a levitated nanodumbbell which we call rotation-coupled torsional motion. The complex torsional motion can be fully explained with the effects of both thermal nonlinearity and rotational coupling. The new findings and knowledge of a levitated non-spherical particles leads us one step further towards levitated optomechanics with more complex particles.