SANTA
CRUZ, CA--Researchers at the University of California,
Santa Cruz, have reported the first demonstration of
integrated optical waveguides with liquid cores, a technology
that enables light propagation through small volumes
of liquids on a chip. The new technology has a wide
range of potential applications, including chemical
and biological sensors with single-molecule sensitivity.
"It is an enabling technology that opens up a wide
range of fields to the use of optics on integrated semiconductors
to do experiments or build devices," said Holger
Schmidt, an assistant professor of electrical engineering
at UC Santa Cruz.
Schmidt and graduate student
Dongliang Yin designed the liquid-core waveguides
so they could be made using the standard silicon fabrication
technology used on an industrial scale to make computer
chips. The fabrication process yields a hollow-core
waveguide that works whether the core is filled with
liquid or gas. They described the novel waveguides
and the results of optical testing of the devices
in the October 18 issue of the journal Applied Physics
Letters.
Guiding light waves through
liquids and gases is a challenge because of their
relatively low refractive indexes. In an optical fiber,
light travels through a core with a high index of
refraction surrounded by cladding with a lower index
of refraction. The difference in refractive indexes
results in "total internal reflection" of
light waves, allowing transmission of optical signals
over long distances.
To build a waveguide with a
liquid or gas core, Schmidt relied on the principle
of antiresonant reflecting optical waveguides (ARROW).
ARROW waveguides with solid cores have been used for
semiconductor lasers and other applications. The technique
uses multiple layers of materials of precise thicknesses
as cladding to reflect light back into the core. Schmidt's
group has achieved low-loss propagation of light over
useful distances in hollow-core ARROW waveguides containing
air or liquids.
"Liquids and gases are
the natural environment for molecules in biology and
chemistry. If you can guide light through water and
air, all of the fields that rely on nonsolid materials
can take advantage of integrated optics technology,"
Schmidt said.
Schmidt is working toward chemical
sensing of single molecules using liquid-core waveguides.
He also sees potential applications for gas-core waveguides
in the areas of atomic physics and quantum optics.
As cladding materials for the
hollow-core waveguides, the researchers chose silicon
nitride and silicon dioxide because of their compatibility
with microfabrication techniques and the potential
for integration with silicon-based electronics. The
cladding layers are deposited over a sacrificial layer
that is later etched away to create the hollow core,
which has a rectangular shape. With a thickness of
3.5 microns and a width of 9 microns, it is the smallest
hollow light guide ever made. The fabrication was
done at a facility at Brigham Young University by
John Barber and Aaron Hawkins of BYU, both coauthors
on the paper.
"We can make many waveguides
in parallel on a chip, so you can imagine probing
20 to 30 channels at one time, with each channel containing
a different sample," Schmidt said. "And
because it is all silicon technology, we can integrate
it with electrical contacts and even put a silicon
photodetector right on the chip."
Schmidt's team has also made
two-dimensional arrays of waveguides that connect
with each other at 90 degree angles, another useful
feature made possible by silicon microfabrication
techniques.
The researchers have been able
to detect molecular fluorescence from a liquid sample
in the core of the waveguide, using light from a helium-neon
laser to stimulate a fluorescent dye. The experiment
detected fluorescence from 800 molecules of dye in
a sample volume of 200 picoliters (a picoliter is
one trillionth of a liter). Further refinements should
enable detection of single molecules, Schmidt said.
Fiberoptic connections can
channel light into the waveguides, which could also
be coupled with microfluidics systems--so-called "labs
on a chip"--to control the flow of samples into
and out of the waveguide cores.
Schmidt is also working with
David Deamer, professor and chair of biomolecular
engineering at UCSC, to combine liquid-core waveguides
with a nanopore device developed in Deamer's lab.
Deamer's nanopore device can feed linear molecules
such as single-stranded DNA through a 2.5-nanometer
channel one at a time.
"The idea is to use the
nanopore to feed single molecules one by one into
a very small volume in the core of the waveguide and
capture the photons released by each molecule. There
is really nothing like this--it's a totally novel
approach to single-molecule detection," said
Deamer, who is also a coauthor on the new paper.
Schmidt's research on the liquid-core waveguides is
supported by the National Institutes of Health.
Contact: Tim Stephens
stephens@ucsc.edu
831-459-4352
University of California - Santa Cruz
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