Two-beam optical traps: Refractive index and size measurements of microscale objects

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

Research output: Contribution to journalArticlepeer-review

20 Scopus citations


A counter-propagating optical trap measurement (COTM) system is proposed and analyzed based on the ray-optics model. In this system, refractive index and size of trapped objects can be estimated by using forward scattered light from the two-beam laser trap with resolution Δn = 0.013 for the refractive index measurements and 3.3% for the size measurements, which is comparable with current bulk techniques, such as refractometry, and flow cytometry. The unique advantage of the COTM system over conventional approaches lies in its capability of marker-free single-particle characterization in whatever transparent buffer required by living cell, eliminating the necessity of changing the fluid composition of the sample in refractometry, and of tagging target with toxic fluorescence dyes in flow cytometry. Noise analysis predicts a potential improvement in the system resolution by more than two orders of magnitude. This non-invasive and sterile tool complements lab-on-a-chips with single cell manipulation and analysis in living friendly ambient.

Original languageEnglish (US)
Pages (from-to)93-97
Number of pages5
JournalBiomedical Microdevices
Issue number2
StatePublished - Jun 2005
Externally publishedYes


  • Optical trapping
  • Ray optics
  • Refractive index measurement
  • Scattering
  • Size measurement

ASJC Scopus subject areas

  • Biomedical Engineering
  • Molecular Biology


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