Measurement of light scattering from trapped particles

We have developed an optical tweezers setup combined with a laser light scattering measurement system to measure the elastic light scattering from trapped particles. The setup consists of a near infrared laser (λ=1064nm), a water immersion objective for trapping, a single or double structure sample cuvette, and a HeNe-laser for illuminating the trapped particles. The light is detected with an amplified photomultiplier and a lock-in amplifier. Light scattering images from the trapped particles are also shown with a CCD camera. An optical trap keeps the particle stable during the measurement. The measured scattering patterns from 23.25 μm diameter polystyrene spheres were shown to have good comparability with theoretical modelling. 6.0 μm particles were also measured. The light scattering from trapped red blood cells was much weaker than that from 23.25 μm and 6.0 μm polystyrene spheres, almost at the detection limit of our current detection system configuration. The stability of the polystyrene sphere was much better during the measurements than that of the red blood cell.