Page 100 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Trapping and Transport Using Planar Photonic Devices 81

F stokes

F grad
~100 μm

F scat + F abs

(a) Free-space optical tweezing
Light in
evanescent
field F stokes F scat + F abs

Waveguide F grad

Substrate

>1 m
(b) Nanoscale optofluidic transport

FIGURE 5-2 Comparison between (a) traditional optical tweezing and
(b) optofl uidic transport on a dielectric waveguide.

the evanescent mode extends outside the waveguide decaying
exponentially into the surrounding medium with a portion of it inter-
acting with the particle. This optical gradient partially polarizes the
particle, resulting in a strong Lorenz force. This serves to attract the
particle to the waveguide (F ). When this particle is trapped within
grad
the evanescent field, a certain percentage of the photons that flow
through the waveguide are either scattered (radiated in a random
direction) or absorbed when they contact the particle. Each of these
photons has a momentum given by Planck’s constant divided by the
wavelength, h/λ. These scattering (F ) and absorption (F ) events
scatt abs
result in momentum transfer to the particle and a net forward veloc-
ity that is proportional to intensity and impeded by viscous drag
(F ). In a sentence, what optofluidic transport allows us to do is simulta-
stokes
neously exploit the extremely high trapping strength available in the near
field with the ability to apply a radiation pressure like transport force over
indefinitely long distances.