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Proceedings Paper

A new technique for high sensitive detection of rotational motion in optical tweezers by a differential measurement of backscattered intensity
Author(s): Basudev Roy; Sudipta K. Bera; Argha Mondal; Ayan Banerjee
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Paper Abstract

Asymmetric particles, such as biological cells, often experience torque under optical tweezers due to birefringence or unbalanced scattering forces, which makes precise determination of the torque crucial for calibration and control of the particles. The estimate of torque relies on the accurate measurement of rotational motion, which has been achieved by various techniques such as measuring the intensity fluctuations of the forward scattered light, or the polarization component orthogonal to the trapping light polarization in plasmonic nanoparticles and vaterite crystals. Here we present a simple yet high sensitive technique to measure rotation of such an asymmetric trapped particle by detecting the light backscattered onto a quadrant photodiode, and subtracting the signals along the two diagonals of the quadrants. This automatically suppresses the common mode translational signal obtained by taking the difference signal of the adjacent quadrants, while amplifying the rotational signal. Using this technique, we obtain a S/N of 200 for angular displacement of a trapped micro-rod by 5 degrees, which implies a sensitivity of 50 mdeg with S/N of 2. The technique is thus independent of birefringence and polarization properties of the asymmetric particle and depends only on the scattering cross-section.

Paper Details

Date Published: 16 September 2014
PDF: 6 pages
Proc. SPIE 9164, Optical Trapping and Optical Micromanipulation XI, 916414 (16 September 2014); doi: 10.1117/12.2061245
Show Author Affiliations
Basudev Roy, Indian Institute of Science Education and Research Kolkata (India)
Sudipta K. Bera, Indian Institute of Science Education and Research Kolkata (India)
Argha Mondal, Indian Institute of Science Education and Research Kolkata (India)
Ayan Banerjee, Indian Institute of Science Education and Research Kolkata (India)

Published in SPIE Proceedings Vol. 9164:
Optical Trapping and Optical Micromanipulation XI
Kishan Dholakia; Gabriel C. Spalding, Editor(s)

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