Ph.D. Student, Durham University, from 2006
Optical Trapping in Evanescent Waves
Colloidal particles can be trapped within the evanescent fields created by total internal reflection of two counterpropagating laser beams. The particles can form lines, regular arrays or large open clusters depending on the sizes and refractive indices of the particles. Particle spacings within lines and arrays are also influenced by the polarisation, intensity and interference fringe spacing of the evanescent field. My work aims to elucidate the forces acting on multiple particles in an evanescent field.
OPTICAL TRAPPING of individual particles occurs due to scattering of the evanescent field, the periodic interference fringe potential, and the gradient force arising from the Gaussian shaped laser beam. This is illustrated in the diagram below.
When several particles are present, lines and subsequently arrays form with spacings that are sometimes incommensurate with the fringe spacing. Incommensurate spacings can be attributed to a competition between the optical trapping of the individual particles and the optical binding of many particles. Optical binding refers to the multiple scattering of the optical field by the particles resulting in a scattered field that can be incommensurate with the incident field. The still below demonstrates this effect, with 800 nm polystyrene particles trapped in the evanescent field of two interfering, counterpropagating laser beams. Adjacent particles are not separated by multiples of the fringe spacing. However, each particle is separated from its 2nd nearest neighbour by an integer number of fringe spacings. This modulated structure is due to the competing forces of optical trapping and optical binding.
In the following video, an array of 460nm polystyrene particles are similarly trapped. The modulated column spacings are also a result of competing optical trapping and binding forces.
The particle arrays are imaged in 2D using optical microscopy, limiting our particle sizes to diameters larger than 300nm.
J. M. Taylor, L. Y. Wong, C. D. Bain, G. D. Love "Emergent Properties in Optically Bound Matter" Optics Express 2008, 16, 6921-6929 (DOI).
I spend an unhealthy amount of time on cooking, eating and planning what to cook next. To the annoyance of my housemates, I have taken up the tin whistle and will soon start learning the harmonica. In quieter moments, I read fantasy novels and practice sign language.