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'Tsunami' On A Silicon Chip: A World First For Light Waves

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Ezgi Sahin, a PhD student at Singapore University of Technology and Design.



A collaboration between the Sydney Nano Institute and
Singapore University of Technology and Design has for the first time
manipulated a light wave, or photonic information, on a silicon chip that
retains its overall 'shape'.

A tsunami holds its wave shape over very long distances
across the ocean, retaining its power and ‘information' far from its source.

In communications science, retaining information in an optic
fibre that spans continents is vital. Ideally, this requires the manipulation
of light in silicon chips at the source and reception end of the fibre without
altering the wave shape of the photonic packet of information. Doing so has
eluded scientists until now.

A collaboration between the University of Sydney Nano
Institute and Singapore University of Technology and Design has for the first
time manipulated a light wave, or photonic information, on a silicon chip that
retains its overall ‘shape'.

Such waves - whether a tsunami or a photonic packet of
information - are known as ‘solitons'. The Sydney-Singapore team has for the
first time observed ‘soliton' dynamics on an ultra-silicon-rich nitride (USRN)
device fabricated in Singapore using state-of-the-art optical characterisation
tools at Sydney Nano.

This foundational work, published in Laser & Photonics
Reviews, is important because most communications infrastructure still relies
on silicon-based devices for propagation and reception of information.
Manipulating solitons on-chip could potentially allow for the speed up of
photonic communications devices and infrastructure.

“The observation of complex soliton dynamics paves the way
to a wide range of applications, beyond pulse compression, for on-chip optical
signal processing,” Ms Sahin said. “I'm happy to be a part of this great
partnership between the two institutions with deep collaboration across theory,
device fabrication and measurement.”




Professor Ben Eggleton and Professor Dawn Tan.


Co-author of the study and Director of Sydney Nano,
Professor Ben Eggleton, said: “This represents a major breakthrough for the
field of soliton physics and is of fundamental technological importance.

“Solitons of this nature - so-called Bragg solitons - were
first observed about 20 years ago in optical fibres but have not been reported
on a chip because the standard silicon material upon which chips are based
constrains the propagation. This demonstration, which is based on a slightly
modified version of silicon that avoids these constraints, opens the field for
an entirely new paradigm for manipulating light on a chip.”

Professor Dawn Tan, a co-author of the paper at SUTD, said:
“We were able to convincingly demonstrate Bragg soliton formation and fission
because of the unique Bragg grating design and the ultra-silicon-rich nitride
material platform (USRN) we used. This platform prevents loss of information
which has compromised previous demonstrations.”




Artist's impression of the Bragg gate on a silicon
substrate.

Solitons are pulses that propagate without changing shape
and can survive collisions and interactions. They were first observed in a
Scottish canal 150 years ago and are familiar in the context of tsunami waves,
which propagate thousands of kilometers without changing shape.

Optical soliton waves have been studied since the 1980s in
optical fibres and offer enormous promise for optical communication systems
because they allow data to be sent over long distances without distortion.
Bragg solitons, which derive their properties from Bragg gratings (periodic
structures etched in to the silicon substrate), can be studied at the scale of
chip technology where they can be harnessed for advanced signal processing.

They are called Bragg solitons after Australian-born
Lawrence Bragg and his father William Henry Bragg, who first discussed the
concept of Bragg reflection in 1913 and went on to win the Nobel Prize in
Physics. They are the only father and son pair to have won Nobel Prizes.

Bragg solitons were first observed in 1996 in Bragg gratings
in optical fibres. This was demonstrated by Professor Eggleton while he was
working on his PhD at Bell Labs.

The silicon-based nature of the Bragg grating device also
ensures compatibility with complementary metal oxide semiconductor (CMOS)
processing. The ability to reliably initiate soliton compression and fission
allows ultrafast phenomena to be generated with longer pulses than previously
required. The chip-scale miniaturisation also advances the speed of optical
signal processes in applications necessitating compactness.

Funding declaration

Ezgi Sahin acknowledges scholarship funding Singapore
International Graduate Award (SINGA) from A*STAR and thanks the Institute for
Photonics and Optical Science (IPOS), the University of Sydney Nano Institute
and the School of Physics at the University of Sydney for hosting her to
conduct the experiments with Andrea Blanco Redondo. Dawn Tan acknowledges the
support of the National Research Foundation Competitive Research Grant, MOE
ACRF Tier 2 grant, SUTD - MIT International Design center, Digital
Manufacturing and DesignGrant and the National Research Foundation, Prime
Minister's Office, Singapore, under its Medium Sized Centre Program. Ben
Eggleton acknowledges the support of the Australian Research Council (ARC)
Laureate Fellowship (FL12010)








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