Fundamental Tests of Physics with Optically Trapped Microspheres, 2013
Springer Theses Series

Fundamental Tests of Physics with Optically Trapped Microspheres details experiments on studying the Brownian motion of an optically trapped microsphere with ultrahigh resolution and the cooling of its motion towards the quantum ground state.

Glass microspheres were trapped in water, air, and vacuum with optical tweezers; and a detection system that can monitor the position of a trapped microsphere with Angstrom spatial resolution and microsecond temporal resolution was developed to study the Brownian motion of a trapped microsphere in air over a wide range of pressures. The instantaneous velocity of a Brownian particle, in particular, was studied for the very first time, and the results provide direct verification of the Maxwell-Boltzmann velocity distribution and the energy equipartition theorem for a Brownian particle. For short time scales, the ballistic regime of Brownian motion is observed, in contrast to the usual diffusive regime.

In vacuum, active feedback is used to cool the center-of-mass motion of an optically trapped microsphere from room temperature to a minimum temperature of about 1.5 mK. This is an important step toward studying the quantum behaviors of a macroscopic particle trapped in vacuum.

Introduction.- Physical Principle of Optical Tweezers.- Optical Trapping of Glass Microspheres in Air and Vacuum.- Measuring the Instantaneous Velocity of a Brownian Particle in Air.- Towards Measurement of the Instantaneous Velocity of a Brownian Particle in Water.- Millikelvin Cooling of an Optically Trapped Microsphere in Vacuum.- Towards Quantum Ground-State Cooling.- Appendix.

Tongcang Li received his Ph.D in Physics at the University of Texas at Austin in 2011, where he received the Outstanding Dissertation in Physics award by the Department of Physics. He is currently in the Nanoscale Science and Engineering Center at the University of California, Berkeley as a Postdoctoral Fellow. Previous positions include Graduate Research Assistant in the Center for Nonlinear Dynamics and Department of Physics at the University of Texas at Austin, as well as Postdoctoral Fellow at the University of Texas at Austin.