The hybrid plasmon polariton (HPP) nanoscale waveguide consists of a semiconductor strip separated from a metallic surface by a low dielectric gap. Schematic shows HPP waveguide responding when a metal slit at the guide’s input end is illuminated. (courtesy of Zhang group) |
The
creation of a new quasiparticle called the “hybrid plasmon polariton”
may throw open the doors to integrated photonic circuits and optical
computing for the 21st century. Researchers with the U.S. Department of
Energy (DOE)’s Lawrence Berkeley National Laboratory (Berkeley Lab) have
demonstrated the first true nanoscale waveguides for next generation
on-chip optical communication systems.
“We
have directly demonstrated the nanoscale waveguiding of light at
visible and near infrared frequencies in a metal-insulator-semiconductor
device featuring low loss and broadband operation,” says Xiang Zhang,
the leader of this research. “The novel mode design of our nanoscale
waveguide holds great potential for nanoscale photonic applications,
such as intra-chip optical communication, signal modulation, nanoscale
lasers and bio-medical sensing.”
Zhang,
a principal investigator with Berkeley Lab’s Materials Sciences
Division and director of the University of California at Berkeley’s
Nano-scale Science and Engineering Center (SINAM), is the corresponding
author of a paper published by Nature Communications that describes this
work titled “Experimental Demonstration of Low-Loss Optical Waveguiding
at Deep Sub-wavelength Scales.” Co-authoring the paper with Zhang were
Volker Sorger, Ziliang Ye, Rupert Oulton, Yuan Wang, Guy Bartal and
Xiaobo Yin.
In
this paper, Zhang and his co-authors describe the use of the hybrid
plasmon polariton, a quasi-particle they conceptualized and created, in a
nanoscale waveguide system that is capable of shepherding light waves
along a metal-dielectric nanostructure interface over sufficient
distances for the routing of optical communication signals in photonic
devices. The key is the insertion of a thin low-dielectric layer between
the metal and a semiconductor strip.
“We
reveal mode sizes down to 50-by-60 square nanometers using Near-field
scanning optical microscopy (NSOM) at optical wavelengths,” says Volker
Sorger a graduate student in Zhang’s research group and one of the two
lead authors on the Nature Communications paper. “The propagation
lengths were 10 times the vacuum wavelength of visible light and 20
times that of near infrared.”
The
high-technology world is eagerly anticipating the replacement of
today’s electronic circuits in microprocessors and other devices with
circuits based on the transmission of light and other forms of
electromagnetic waves. Photonic technology, or “photonics,” promises to
be superfast and ultrasensitive in comparison to electronic technology.
From left, Berkeley Lab’s Xiang Zhang, Ziliang Ye and Volker Sorger have demonstrated the first true nanoscale waveguides for next generation on-chip optical communication systems. (Photo by Roy Kaltschmidt, Berkeley Lab Public Affairs) |
“To
meet the ever-growing demand for higher data bandwidth and lower power
consumption, we need to reduce the energy required to create, transmit
and detect each bit of information,” says Sorger. “This requires
reducing physical photonic component sizes down beyond the diffraction
limit of light while still providing integrated functionality.”
Until
recently, the size and performance of photonic devices was constrained
by the interference that arises between closely spaced light waves. This
diffraction limit results in weak photonic-electronic interactions that
can only be avoided through the use of devices much larger in size than
today’s electronic circuits. A breakthrough came with the discovery
that it is possible to couple photons with electrons by squeezing light
waves through the interface between a metal/dielectric nanostructure
whose dimensions are smaller than half the wavelengths of the incident
photons in free space.
Directing
waves of light across the surface of a metal nanostructure generates
electronic surface waves – called plasmons – that roll through the
metal’s conduction electrons (those loosely attached to molecules and
atoms). The resulting interaction between plasmons and photons creates a
quasi-particle called a surface plasmon polariton(SPP) that can serve
as a carrier of information. Hopes were high for SPPs in nanoscale
photonic devices because their wavelengths can be scaled down below the
diffraction limit, but problems arose because any light signal loses
strength as it passes through the metal portion of a metal-dielectric
interface.
“Until
now, the direct experimental demonstration of low-loss propagation of
deep sub-wavelength optical modes was not realized due to the huge
propagation loss in the optical mode that resulted from the
electromagnetic field being pushed into the metal,” Zhang says. “With
this trade-off between optical confinement and metallic losses, the use
of plasmonics for integrated photonics, in particular for optical
interconnects, has remained uncertain.”
3-D image overlap of the deep sub-wavelength HPP mode signal (red spot) that indicates the waveguide’s potential to create strong light-matter-interaction for compact and highly functional photonic components. (courtesy of Zhang group) |
To
solve the problem of optical signal loss, Zhang and his group proposed
the hybrid plasmon polariton (HPP) concept. A semiconductor
(high-dielectric) strip is placed on a metal interface, just barely
separated by a thin oxide (low-dielectric) layer. This new
metal-oxide-semiconductor design results in a redistribution of an
incoming light wave’s energy. Instead of being concentrated in the
metal, where optical losses are high, some of the light wave’s energy is
squeezed into the low dielectric gap where optical losses are
substantially less compared to the plasmonic metal.
“With
this design, we create an HPP mode, a hybrid of the photonic and
plasmonic modes that takes the best from both systems and gives us high
confinement with low signal loss,” says Ziliang Ye, the other lead
authors of the Nature Communications paper who is also a graduate
student in Zhang’s research group. “The HPP mode is not only
advantageous for down-scaling physical device sizes, but also for
delivering novel physical effects at the device level that pave the way
for nanolasers, as well as for quantum photonics and single-photon
all-optical switches.”
The
HPP waveguide system is fully compatible with current
semiconductor/CMOS processing techniques, as well as with the
Silicon-on-Insulator (SOI) platform used today for photonic integration.
This should make it easier to incorporate the technology into low-cost,
large-scale integration and manufacturing schemes. Sorger believes that
prototypes based on this technology could be ready within the next two
years and the first actual products could be on the market within five
years.
“We
are already working on demonstrating an all-optical transistor and
electro-optical modulator based on the HPP waveguide system,” Sorger
says. “We’re also now looking into bio-medical applications, such as
using the HPP waveguide to make a molecular sensor.”
This research was supported by the National Science Foundation’s Nano-Scale Science and Engineering Center.
Berkeley Nano-scale Science and Engineering Center