Page 104 - Optofluidics Fundamentals, Devices, and Applications

Optofluidic Trapping and Transport Using Planar Photonic Devices 85

wavelength, the light in the waveguide is more strongly confined,
leading to stronger trapping forces and higher propulsion veloci-
ties. Gaugiran et al. were also the first to propose an analysis of the
optical propulsion and trapping forces for a particle near a wave-
guide using finite-element methods, in particular showing a devia-
tion from the analytical Rayleigh particle assumption at larger par-
ticle sizes. Furthermore, the authors provided some of the first
experimental quantification of numerical predictions of propulsion
and trapping forces. Along similar lines, Ng et al. [53] demonstrated
the propulsion of high-absorption gold nanoparticles, seeking to
now exploit high-absorption materials to generate higher propul-
sion velocities and trapping forces. In combination, these two papers
provided experimental evidence of the diverse materials that could
be transported on waveguiding structures.
One of the advantages of using optically driven transport is that
there are many types of devices that can be used to divert and alter
the behavior of optical fields. Evanescent coupling can be used to
cause light to effectively tunnel through a lower refractive index
medium into an adjacent waveguide. Resonator devices allow for the
attenuating properties of constructive and destructive interference to
enable switching and/or creating highly focused hot spots in the
guiding structure. Recently, there have been a few demonstrations of
methods to create more complex optical fields for particle sorting/
manipulation. Of particular note, Grujic et al. [54] was the first dem-
onstration using Y-branch waveguides as a sorting mechanism for
transported particles, shown in Fig. 5-4. The system consisted of CS +
ion-exchange waveguides on class. Polystyrene microparticles were
guided down the “upper” or “lower” waveguides at the Y-split by
altering the physical position of the input fiber, creating preferential
pathways for particles to follow. An improvement over this type of
device would be one that accomplishes the sorting based on the
intrinsic properties of the particle in question, as opposed to the arbi-
trary position of the input fiber.
Before moving on to a detailed example of optofluidic transport,
it is important to at least briefly describe a slightly different architec-
ture for optofluidic transport. The essential flaw with all the previ-
ously mentioned devices is that the majority of the guided optical
energy is confined within the solid core of the waveguide and the
particles only interact with the 10% to 20% of the energy that is acces-
sible in the evanescent field. As such a number of recent works have
investigated the possibility of using “liquid-core” waveguiding struc-
tures for optical transport. Since the overlap between the guided
mode and the transported optical energy is stronger in these systems,
the potential exists for greater transport speeds. As an example, Mandal
and Erickson [55] recently demonstrated the use of a specially tailored
hollow-core photonic crystal fiber (HCPCF) to propagate light within
a liquid-core environment and levitate/transport dielectric particles.