Optical Tweezers
To explore biology at the single cell level, we have built optical tweezers with integrated microfluidics, and fluorescence, brightfield and back focal plane interferometry detection methods.
Optical trapping is based on an equilibrium of forces in the vicinity of the beam waist of a laser beam tightly focused by a high numerical aperture (NA) objective. A strong electric field gradient at the beam waist attracts dielectric particles (including biological cells) towards the maxima of the electric field at the centre of the beam, while a scattering force acts on the particle in the beam direction. The net effect is that the particle is trapped in a region displaced just downstream of the narrowest part of the beam waist.
We have developed Optical Tweezers optimised for observations of single bacterial cells stained with fluorescent dyes. The configuration consists of a holographic optical trap operating at near infra-red (NIR) wavelength integrated with microfluidics and combined with epifluorescence and bright field microscopy, and back focal plane (BFP) interferometry.
Example of a microfluidic chip
a) E. coli bacteria stained with SYTO 9 trapped in optical tweezers and imaged with epifluorescence.
b) 1.8 µm green fluorescent beads trapped in optical tweezers and imaged using epifluorescence. Position of the trap is indicated by the yellow circle.