NEAR RESONANCE TRAPPING

Research in the Kopelman laboratory involves the creation of nano-scale biochemical probes for sensing analyte concentrations within and around cells. A smaller probe offers the advantages of non-invasiveness, spatial resolution, response speed and lower absolute detection limit. In order to manipulate this smaller probe optically (i.e. with laser tweezers) for measurements in and around single cells, one must specifically trap the particle without perturbing the surrounding environment. To achieve this specificity, we proposed near-resonance trapping of absorptive particles.

The Classical Electron Oscillator(CEO) model provides a simple description of the frequency-dependent complex polarizability of a Rayleigh particle (d << lambda) in a focused laser beam. Real and imaginary parts of the polarizability are proportional to the gradient and scattering forces, respectively. Similar relations occur for components of the complex index of refraction in the geometrical optics regime (d >> lambda). By frequency tuning the trapping beam near the probe's absorption resonance, one can maximize the gradient force and minimize the scattering force, thus optimizing the single beam gradient trap.

For more details on the theoretical aspects please refer to the following publications:

Reference in Phys. Rev Focus

Applied Optics-LP, Volume 41, Issue 12, 2318-2327.