Sunday May 17, 2009 WorkShop sw-f1: Control Issues in the Micro/Nano-World


- Jason J. Gorman,Thomas W. LeBrun and Arvind Balijepalli

Manufacturing Engineering Laboratory
National Institute of Standards and Technology,
Gaithersburg, MD 20899-8230
USA

Contact: gorman 'at' nist 'dot' gov

Title


Controlled Optical Trapping of Nanoparticles

Abstract


Optical trapping is a technique in which optical forces are used to manipulate particles ranging in diameter from 20 Ám to 20 nm. It has been used extensively in biophysics research, including the characterization of the mechanical properties of DNA, and more recently has been pursued as a research tool for nanotechnology, such as in the measurement of physical properties of carbon nanotubes. Controlling the position of a trapped particle when performing measurements such as these is critical to their success, where positioning resolution on the order of nanometers is typically required. However, Brownian motion caused by the thermal interaction between the surrounding medium and the trapped particle, along with other external disturbances, limits the open-loop positioning resolution, thereby necessitating the use of feedback control. This presentation will examine the closed-loop control of optically trapped nanoparticles for the suppression of Brownian motion. Each subsystem, including the trapping dynamics, actuation, and sensing are discussed, as well as the various tradeoffs involved in the selection of equipment for these subsystems. An overview of several control schemes will be presented for particles that are tethered to a surface (force and position clamping), as well as for free particles that have been optically trapped. In particular, a method which minimizes Brownian motion while limiting the maximum control activity will be presented. Experimental control results demonstrate a 73 % reduction in the maximum amplitude and a 44 % reduction in the standard deviation for the Brownian motion of an optically trapped silica sphere with a diameter of 0.97 Ám over a range between 10 Hz and 6.4 kHz.

Download the slide
here




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