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Optofluidic Trapping and Transport Using Planar Photonic Devices 79

on the particles of interest, resulting in the radiation pressure force
that propels them forward. Because the net impulse imparted to a
larger particle is greater than that imparted to a smaller particle, they
will travel at different speeds and can thus be separated (this will be
expanded on at several points in this chapter). Recent demonstrations
of optical separations include those by Hart et al., who have demon-
strated refractive index separation of colloids [30] and other biopar-
ticles [31]. They have also recently integrated this into a microfluidic
device format for pathogen detection [32], demonstrating very pre-
cise separation between very closely related bacteria Bacillus anthracis
and Bacillus thuringiensis and millimeter scale separation [33].

5-1-3 Some Limitations of Traditional Optical
Manipulation Systems
Despite these successes, the above optically based microfluidic
transport systems are fundamentally limited in two ways. The first
is by the diffraction limit. It is well known [34] that the diffraction
limit places a lower bound on size to which light can be focused and
is given by d = 1.2λ/NA, where NA is the numerical aperture and
min
λ is the wavelength. In an aqueous environment and for an 850-nm
wavelength (consistent with that used by others for optical chroma-
tography [33]) and with a high numerical aperture, the minimum
spot size is 550 nm. Since light intensity is given by the input power
divided by the illuminated area, this places a fundamental limita-
tion on the trapping and propulsive forces that can be applied to a
particle. In practice this limits the size of targets we can trap to tar-
gets on the order of a few 100 nm in diameter and the speed with
which we can transport them. The second (and ultimately more
important here) is the light/species interaction length. From the dif-
fraction limit equation given earlier, it is apparent that the simplest
ways to decrease the area over which the optical energy is spread
involve either reducing the wavelength of the laser (e.g., into the
blue) or increasing the effective numerical aperture [(e.g., via solid
immersion lenses (SIL)]. Decreasing the wavelength to 488 nm
would reduce the spot size by slightly less than half. The SIL tech-
nique has been developed in a number of different flavors [35–37]
with the general principle being that increasing the refractive index
of the optical head gives one a nominal improvement in ultimate
resolution (1/n ). In either of these techniques the decrease in the
i
spot size is necessarily offset by an equivalent decrease in the depth
of focus. As such the light/species interaction length (i.e., the dis-
tance over which the optical impulse can be applied) becomes small,
making it impossible to perform optical transport over long dis-
tances. The reason why the traditional channel-based transport
techniques like pressure or electrokinetics are useful is not because
they are particularly well suited to microfluidics (electrokinetics,