Innovative Light-Delivery Technique Improves Biosensors

New approach improves practicality of high-sensitivity sensors for
cancer, other diseases

WASHINGTON–(BUSINESS WIRE)–#Cancer–There is a continuing need for practical chip-based sensors that can be
used at the point of care to detect cancer and other diseases. An
innovative way to inject light into tiny silicon microdisks could help
meet this need by bringing down the cost and improving the performance
of chip-based biosensors. The advance could eventually lead to a
portable and low cost optical sensor for early-stage cancer diagnostics.

Microdisks are a type of microscale resonator that use the
whispering-gallery optical effect to confine and enhance light that
enters the disk. Just as the curved walls of a whispering gallery carry
sound waves to allow whispers to be clearly heard across a room, the
curved inner surface of a microdisk carries light waves across the disk,
enhancing the light. This allows the microdisk to boost a light-based
signal coming from a cell, protein or virus of interest, allowing more
sensitive detection of subtle changes associated with diseases such as
lupus, fibromyalgia and certain heart problems.

“Although there are whispering gallery mode micro-resonators that can
already be used to resolve single molecules, their application is
limited by problems in device repeatability, stability and wavelength
range,” said research team leader Qinghai Song from Harbin
Institute of Technology
, China. “Our new design enables excellent
device performance that works with a variety of wavelengths with low
cost, higher stability and better device repeatability.”

In Optica,
The Optical Society’s journal for high impact research, the researchers
detail their new end-fire injection configuration, which offers a
simple, cost-effective and efficient way to get light into the microdisk
resonator. They also show that devices using microdisks and end-fire
injection can be used to detect temperature changes and the presence of

The researchers’ ultimate goal is to use their new end-fire injection
technique to create a portable and low-cost sensor that can detect
changes in cells that are early indicators of cancer. However, they
point out that the new light-coupling configuration could also be useful
for integrated photonic circuits for communication applications and a
variety of sensors such as those used in homeland security or
environmental monitoring.

Using time reversal

Most microdisks are designed so that light is indirectly injected into
the microdisk using an optical phenomenon known as evanescent light
coupling. However, this method requires very precise alignment between
the waveguide and the microdisk, which increases manufacturing costs and
makes devices susceptible to stability problems.

The researchers’ end-fire injection technique uses a waveguide that is
directly connected to the edge of the microdisk. Although light that is
exactly perpendicular to the disk’s side will bounce off the interface,
using light angled just slightly less than perpendicular induces a
counterintuitive phenomenon known as laser time-reversal. This creates a
laser that absorbs light rather than emits it, allowing the light to
efficiently enter the microdisk.

“Because this configuration doesn’t require any parts that are smaller
than 500 nanometers, it can be fabricated with low-cost techniques,”
said Song.

To test their design, the researchers fabricated a device that included
a microdisk with a 5-micron radius connected to a waveguide. To measure
the end-fire injection, they incorporated a Y-splitter that allowed
light passing through the splitter to be injected into the microdisk and
then be transmitted out of the microdisk along the same waveguide.
Recording the spectrum coming from the Y-junction showed that light
could be coupled into the microdisk with an efficiency as high as 57

They also showed that the device exhibited a high Q-factor, a measure of
how well the microdisk confines and amplifies the light. In addition,
the device maintained good performance parameters even with fabrication
deviations such as increasing the waveguide width from 400 nanometers to
700 nanometers.

“We show that the performance of the end-fire injection technique is
comparable to that of conventional microdisks but with improved
robustness and reduced cost,” said Song. “Overall, our findings show
that microdisks are now ready for commercial applications.”

The researchers also demonstrated that sensors incorporating microdisks
and end-fire injection could detect the presence of multiple large
nanoparticles as well as single nanoparticles as small as 30 nanometers.
They are interested in using cell-derived vesicles that are around
40-to-100 nanometers to detect cancer, which should be possible based on
these results.

The researchers are now working on other parts of the device that would
be needed to use the end-fire injection technique to create a portable
and low-cost sensor that can detect early indicators of cancer.

Paper: S. Liu, W. Sun, Y. Wang, X. Yu, K. Xu, Y. Huang, S. Xiao,
Q. Song, “End-fire
Injection of Light into High-Q Silicon Microdisks
,” Optica,
Volume 5, Issue 5, 612-616 (2018).
DOI: 10.1364/OPTICA.5.000612

About Optica

Optica is an open-access, online-only journal dedicated to the
rapid dissemination of high-impact peer-reviewed research across the
entire spectrum of optics and photonics. Published monthly by The
Optical Society (OSA), Optica provides a forum for pioneering
research to be swiftly accessed by the international community, whether
that research is theoretical or experimental, fundamental or applied. Optica
maintains a distinguished editorial board of more than 50 associate
editors from around the world and is overseen by Editor-in-Chief Alex
Gaeta, Columbia University, USA. For more information, visit Optica.

About The Optical Society

Founded in 1916, The Optical Society (OSA) is the leading professional
organization for scientists, engineers, students and business leaders
who fuel discoveries, shape real-life applications and accelerate
achievements in the science of light. Through world-renowned
publications, meetings and membership initiatives, OSA provides quality
research, inspired interactions and dedicated resources for its
extensive global network of optics and photonics experts. For more
information, visit


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