Ultra-fast and ultra-stiff optical tweezers

Abstract

This thesis describes our efforts to build an ultra-fast and ultra-stiff optical tweezers to be used for many fundamental science, engineering, and biological applications. This thesis is mainly composed of two sections: the first section describes the fundamental Brownian motion theories of an optically trapped particle, the experimental challenges and new techniques to measure it at very short times scales. The second part illustrates the structured illumination of the incident light field to enhance the optical trap stiffness by several orders of magnitude. In the first part of this thesis, we describe the design procedures to construct a stable and robust customized microscope employing high numerical aperture (1.3) trapping and condenser objectives to trap and track micro/nano-particles with unprecedented high bandwidth. We have developed a customized mechanical design of the microscope which possess heavy structure and provides 6-axis degrees of freedom for relative alignment of objectives having very small 160 µm working distance. The structure is stable enough to measure the motion of the particle as low as close to 1 Hz where mechanical vibrations are of significant concern. Finally, we demonstrate a new split-flipped waveplate based particle tracking scheme which is capable of measuring very high 160 mW of scattered light from the trapped particle resulting in unprecedented ultra-high position measurement bandwidth close to 10 MHz. It allows accessing Brownian motion at very short timescales leading to instantaneous velocity measurement of the particle. These measurements are further used to measure the unknown viscosity of the different fluids at fast timescales. The second part of the thesis demonstrates the spatial structuring of incident trapping light to create interference in the particle which makes it behave like a beam-splitter. On the contrary to the regular particle scattering phenomenon in which particle changes the direction of incident light, this new type of interferometric technique changes the relative intensity around the optical axis due to the Brownian motion of the particle. This phenomenon enhances the optical trap stiffness for several orders of magnitude especially for the particles larger than incident beam wavelength. These techniques are implemented in a holographic optical tweezers for the particle sizes ranging from 1.5 ∼ 10 µm particle and achieved trap stiffens enhancement by a factor of 27.5 for 10 µm particle. This ultra-fast and ultrastiff optical tweezers could play a vital role in testing the fundamental physics laws such MaxwellBoltzmann distribution, quantum ground state cooling, micro-robotics and correct viscosity of the biological fluids, especially in the cell.