Backscattering-based detection scheme for dark-field optical tweezers

Laser nanomanipulation by metallic nanoparticles, which are smaller than the diffraction limit, has become a great interest, especially in the manipulation of unstained biological samples. However, they are hard to image in bright-field microscopy, because of the diffraction limit. Thus, incorporation of a dark-field microscopy technique with optical tweezers would be an inevitable choice, enforcing the use of the backward scattering detection scheme. Here in this paper, we have taken into account the reflected light from the coverslip along with backscattered light from the trapped particle to validate and reveal some precautions of using this detection scheme, based on Mie-Debye calculations. Also to hallmark the superior contrast enhancement capability of dark-field optical tweezers as well as rendering experimental proof of theoretical calculations, we have trapped and manipulated gold nanoparticle (d = 80 nm), polystyrene bead (d = 1.09 mu m), and red blood cells, in different accomplished experiments, in addition to the comparison of the obtained results in both forward and backward scattering detection schemes. (c) 2019 Optical Society of America