Page 107 - Optofluidics Fundamentals, Devices, and Applications

88 Cha pte r F i v e

Both the waveguides and microfluidic devices were fabricated
using common photolithographic and soft-lithography techniques.
More details are available in the Schmidt et al. [49], however briefly;
the waveguides were fabricated using SU-8 resist with MicroChem
formulation 2000.5 by exposing the film with desired waveguide
pattern using a standard photolithography arrangement, perform-
ing the recommended postexposure bake and developing proce-
dure. Figure 5-5c shows an SEM image of two of these waveguides
in close proximity. The input and output facets of the waveguides
were diced from the backside with a dicing saw to a distance of
50 μm from the top surface and then cleaved by applying simple
pressure to the substrate by hand. The microfluidics were made using
a standard procedure for creating PDMS microfluidics by solution
casting using a lithographically patterned mold [57,58]. The channels
were designed to dimensions of 5 μm in height and 100 μm in width.
We used relatively shallow channels to confine the flowing particles
as close as possible to the waveguides in order to increase the capture
rate.
To assemble the structure, the PDMS channels and the wave-
guide sample were both plasma-cleaned in air for several seconds
and then bonded by placing them in conformal contact. As shown
in Fig. 5-5a the arrangement was such that the channel ran perpen-
dicular to the waveguides, though this is by no means a necessity. In
the absence of a plasma cleaner (oxidizer), placing the two halves
together will still form a temporary seal, sufficient to carry out most
experiments that do not involve very high fluid pressures. The con-
formable nature of the PDMS greatly facilitated sealing of the micro-
fluidics over the waveguides without greatly disturbing the optical
mode. The use of other nonconforming materials complicates this
process. In this case the channels were aligned perpendicularly to
the waveguide inputs, leaving between 500 μm and 1 mm of space
between the edge of the chip and the start of the PDMS. Leaving an
air clad region at the edge of the chip facilitates coupling the light
into the waveguides.
In the experiment shown in Fig. 5-5d through 5-5f, we flow fluo-
rescently tagged dielectric particles in the main microfluidic channel
toward the optically excited waveguide using pressure-driven flow.
The particles used in our experiment were polystyrene spheres with
refractive index n = 1.574 in a 100-mM phosphate buffer solution
(PBS) with a regulated pH of 7.0. The light source used for testing was
a fiber-coupled laser diode module with a wavelength of λ= 975 nm.
To excite the waveguides we used a micrometer-controlled fiber posi-
tioning stage to position a lensed fiber near the end of the waveguide
of interest. The light was considered optimally coupled into the wave-
guide when we received a maximum output power reading on a
detector placed near the output end of the waveguide or by directly
observing the scattered light (on a CCD camera sensitive to 1-μm