Counter-propagating optical trapping system for size and refractive index measurement of microparticles

Richard A. Flynn, Bing Shao, Mirianas Chachisvilis, Mihrimah Ozkan, Sadik C. Esener

Research output: Contribution to journalArticle

8 Scopus citations


We propose and demonstrate a novel approach to measure the size and refractive index of microparticles based on two beam optical trapping, where forward scattered light is detected to give information about the particle. The counter-propagating optical trap measurement (COTM) system exploits the capability of optical traps to measure pico-Newton forces for microparticles' refractive index and size characterization. Different from the current best technique for microparticles' refractive index measurement, refractometry, a bulk technique requiring changing the fluid composition of the sample, our optical trap technique works with any transparent fluid and enables single particle analysis without the use of biological markers. A ray-optics model is used to explore the physical operation of the COTM system, predict system performance and aid system design. Experiments demonstrate the accuracy of refractive index measurement of Δn = 0.013 and size measurement of 3% of diameter with 2% standard deviation. Present performance is instrumentation limited, and a potential improvement by more than two orders of magnitude can be expected in the future. With further development in parallelism and miniaturization, the system offers advantages for cell manipulation and bioanalysis compatible with lab-on-a-chip systems.

Original languageEnglish (US)
Pages (from-to)1029-1036
Number of pages8
JournalBiosensors and Bioelectronics
Issue number7
StatePublished - Jan 15 2006
Externally publishedYes


  • Mie theory
  • Optical trapping
  • Ray-optics
  • Refractive index
  • Scattering
  • Size

ASJC Scopus subject areas

  • Biotechnology
  • Biophysics
  • Biomedical Engineering
  • Electrochemistry

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