The hollow-core waveguide developed at UCSC can be used to guide light through liquid or gas samples on a silicon chip.
Image: D. Yin et al
October 25, 2004
Researchers guide light through liquids
and gases on a chip, a major step forward for optical sensing
By Tim Stephens
UCSC researchers 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
"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.
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
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
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
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
Schmidt's research on the liquid-core waveguides is supported
by the National Institutes of Health.
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