Page 99 - Optofluidics Fundamentals, Devices, and Applications

80 Cha pte r F i v e

arguably, is relatively weak, requiring often thousands of volts to
impart a noticeable velocity) but because the impulse can be applied
consistently over long distances (tens of centimeters).

5-1-4 Near-Field Optical Manipulation
One way to improve on the limitations imposed by the diffraction
limit is through the use of near-field methods [38] such as those based
on the use of surface plasmonic resonances [39,40] or other evanes-
cent field techniques [41]. The advantage of these approaches is that
the extremely high decay rate of the evanescent field leads to stronger
trapping forces than can be achieved with free-space systems. Exam-
ples include the work of Cizmar et al. [42], who demonstrated short-
range manipulation (on the order of 40 μm) of 350-nm polystyrene
beads, and Grigorenko et al. [43], who used plasmonic resonance in
surface bound metallic nanostructures to achieve high-quality trap-
ping of dielectric particles as small as 200 nm in diameter. While in
general these methods have in the past been successful at trapping
and even assembling [44] small particles, similar to free-space trap-
ping, they are limited by the distance through which they can trans-
port objects, since the optical manipulation region is limited by the
field of view of the focused laser, and the plasmon propagation dis-
tance is relatively short.

5-2 Optofluidic Transport
Though most readers of this book are likely to be at least somewhat
familiar with the topic, photonics is defined as interaction of light
with matter [45]. Photonic devices (e.g., waveguides, ring resona-
tors, and photonic crystals, see Saleh and Teich [46] or Pollock and
Lipson [47]) have found numerous applications in fields ranging
from telecommunications and computing to biochemical sensing
and detection.

5-2-1 Qualitative Description of Optofluidic Transport
For optofluidic transport, the photonic device we are primarily inter-
ested in is the dielectric waveguide. The reason for this is that they
can confine light by total internal reflection over very long distances
with very little lengthwise dilution of the optical energy. Though the
light is confined to propagate in a single direction in a waveguide, a
nonpropagating exponentially decaying component of this field
(referred to as the evanescent field) extends outside the waveguide.
The degree of this extension depends on the refractive index contrast
between the waveguide and the surrounding media [46] but is typi-
cally on the order of a 100 nm. In Fig. 5-2 we compare the forces on a
dielectric particle near an optically excited waveguide with those
imparted by a traditional optical tweezer. As can be seen in Fig. 5-2b,