NORIAKI HORIUCHI

news & views

In-depth discussions were carried out among the attendees during the interactive poster session
constituted by about 800 posters.

minimal loss of energy is important for thedevelopment of efficient environmental
sensors and biosensors. To achieve this,Susumu Noda from Kyoto University
developed a method for concentrating
broadband sunlight into a narrowband
spectrum by accumulating thermal energy
in a device made of photonic crystals. The

narrowband output spectrum was tailoredsuch that it coincided with the absorption
spectrum of solar cells. As a proof-ofprinciple experiment, the researchers
injected currents into the photonic crystal
to demonstrate the spectral narrowing ofthermal radiation. The spectral width of thethermal radiation from the photonic crystal

was a thirtieth of that from the reference
sample. The conversion of broadband
sunlight into narrowband light near awavelength of 10 μm attracted significant
interest from the audience.
“The reported conversion efficiencyof our solar cells is so far around 10–20%
due to the large amount of unabsorbed
sunlight. The percentage of renewableenergy in sunlight can be increased if thebroad spectrum of sunlight can successfully
be converted into narrowband lightwithout energy loss. We expect to achieve
conversion efficiencies of more than 40%
using our method,” said Noda. He also
added that the theoretical maximum
conversion efficiency of 70% is achievable
if the thermal energy loss in the photonic
crystal can be significantly reduced.
Although the thermal loss is large, theconversion efficiency of a solar-cell systemusing the photonic crystal would be atleast 40%.
The next JSAP meeting will be held at theKanagawa Institute of Technology on 27–30
March 2013.
❒Noriaki Horiuchi is at Nature Photonics, Chiyoda
Building, 2-37 Ichigayatamachi, Shinjuku-ku, Tokyo
162-0843, Japan.
e-mail: n.horiuchi@natureasia.com

VIEW FROM… GROUP IV PHOTONICS

Industry meets academia

Silicon photonics and devices based on group iv elements are overcoming the tough economic downturns that haverocked industry over the past 12 years. Cross fertilization between academia and industry may lead to new devices
that are both innovative and profitable.

David PileIn 2004, Hong Kong hosted the first IEEE
International Conference on Group iv
Photonics, an annual meeting dedicatedto silicon photonics and photonic materials
and devices based on group iv elements.
Since then, the conference has been hosted
across Asia, Europe and the USA. This year’s
meeting — co-chaired by Mehdi Asgharifrom Kotura and Jurgen Michel from theMassachusetts Institute of Technology —was held in San Diego, California, on 29–31
August 2012.
Aside from the exciting meeting scheduleand competitive post-deadline session,
one aspect of the conference in particularstood out as being refreshing, practical andinnovative: the fact that around half thecontributors were from industry. This ratio

also holds true for the co-chairs — one beingfrom academia and the other from industry.
Those involved in the conference came from awide range of backgrounds, including venture
capitalists, software designers, electronic
chip manufacturers, wafer manufactures andsub-system and system companies. On theindustrial side, talks focused on topics suchas the future of silicon photonics, the need foron-chip photonic integration for large datawarehouse switches, photonics for scaling theinternet, developments in complementarymetal–oxide–semiconductor (CMOS)
photonics, photonics for access network
technologies, platforms for sub-picojoule-perbit macrochip interconnects, and the latest
developments in InP and other materials
technology. From academia came themes

such as photonic nanostructure transceiverdevices for advanced optical networks, GeSn
light-emitting p-i-n diodes on Si and Ge-onSi to form electrically pumped CMOS lasers.
One particular highlight of the meeting
occurred on the sunset boat cruise banquet
on the San Diego bay, during which
Andrew Rickman told his inspiring life
story. After gaining a PhD in photonics,
Richman founded one of the first companies
to commercialize silicon photonics, andfinally went on to chair a venture capitalist
company (Rockley Group). He commented
on the economic downturns that have hurt somany companies in the field, but was positive
about recognizing the mistakes of the pastmade by companies in the silicon photonics
industry. He also noted that new types ofNATURE PHOTONICS | VOL 6 | DECEMBER 2012 | www.nature.com/naturephotonics© 2012 Macmillan Publishers Limited. All rights reserved.

807

devices will hopefully realize a stable industry
in the future.
Interesting talks from industry included
an invited paper by Glenn Li (Oracle) on
the technical progress towards a CMOS
photonics toolkit for Oracle’s Macrochip
vision. In a post-deadline paper, Li and coworkers group also reported a >40 Gbit s–1
silicon CMOS microring modulator witha low voltage of 2 V (peak-to-peak at thedevice) and a high extinction ratio of 7 dB.
Joint Kotura–Oracle papers also gave
impressive results. Aaron Zilkie and coworkers presented a group iii–v/silicon laser
with an uncooled external DBR laser cavity
that can reach a record wall plug efficiencyof 9.5%. Dazeng Feng and co-workers,
also from Kotura–Oracle, presented a GeSi
electroabsorption modulator operating over30 nm near 1,550 nm with a 3 dB bandwidthof 40.7 Ghz at a bias of 2.8 V.
Gary Bulman presented research
performed at Research Triangle Institute(RTI) International with Jayesh Bharathan
on low-dark-current Ge-on-Si photodiodes
grown by chemical vapour deposition. He
explained that germanium offers a distinct
advantage over silicon not only because of
its superior electron and hole mobility, butalso because of its direct bandgap of 0.8 eV
at 300 K, which makes it possible to design
efficient photodetectors operating in the lowloss optical fibre range of 1,300–1,600 nm.
Their structures, grown on p‑type <100>silicon substrates, consisted of a 0.65-μmthick undoped germanium film grownthrough a two-step process, a 10-Å-thickgermanium/70-Å-thick silicon superlattice
structure with a variable number of periods,
and a 1,200-Å-thick n+ silicon contact layer.
This structure confines the high field region
to the wide-bandgap superlattice and siliconlayers, while lowering the electric field in thephoton-absorbing germanium layer, which
has a smaller bandgap. The researchers
annealed the thick germanium films in situ at
900 °C prior to superlattice deposition, which
reduced the threading dislocation density.
X‑ray diffraction analysis of the structure
confirmed the presence of the superlattice
structure. The researchers studied how thenumber of superlattice periods affects thedark current and responsivity, and achieved
a 50-fold reduction in dark current down
to 1.3 mA cm–2 at –1 V for a 100-periodsuperlattice, compared with a similar
structure not containing the superlattice.
Each device also exhibited a photoresponse
beyond 1,600 nm. This work demonstrates
that a silicon/germanium superlatticebased device structure placed between thephotoactive germanium-film and the top
electrical contact layer can dramaticallyreduce dark currents, thereby providing a key808

DAVID PILE

news & views

Researchers enjoying a harbour cruise at Group iv Photonics, held in San Diego, California, on 29–31
August 2012. The meeting, which brought together researchers from industry and academia, covered
topics from fundamental material problems to device-level and systems issues.

approach for designing photodetectors withlow leakage currents.
Tetsuya Mizumoto from the Tokyo
Institute of Technology in Japan presented an
optical isolator based on a silicon nanowire
Mach–Zehnder interferometer that involves
bonding Ce:YIG directly on top of thesilicon waveguide. His group obtained alarge magneto-optic phase shift and an
optical isolation of 18 dB at a wavelength of
1,322 nm. They also presented a transverse
electric (TE)–transverse magnetic (TM)mode converter constructed from a singletrench SiON waveguide. The trench isfabricated in a rectangular waveguide corein an asymmetric manner. By choosing anappropriate geometry, two orthogonallypolarized eigenmodes propagate along axes
rotated by 45° with respect to the horizontal
and vertical axes. When the lightwave
polarized along either the horizontal (TE) orvertical (TM) axis is launched into the trench
waveguide, two eigenmodes are excitedwith equal amplitude and propagate withdifferent velocities. After propagating halfa beat length, there is a π phase difference
between the two eigenmodes. This results
in a 90° rotation of launched polarization;that is, the launched TE (or TM) mode
is converted into a TM (or TE) mode.
The device is compatible with a silicon
nanowire waveguide and is fabricatedthrough a single mask and etching process.
The researchers achieved a TE–TM mode
conversion efficiency of 97% at a wavelengthof 1,270 nm. The device also exhibits a wide
operation wavelength range of ~100 nm
with a conversion efficiency more than94%. The conference wrapped up with apost-deadline session sponsored by Nature
Photonics, in which authors of the session’s‘best postdeadline paper’ award werepresented with personal subscriptions toNature Photonics. In the winning talk, Hong
Nguyen from Yokohama National University
in Japan presented a paper on a silicon
Mach–Zehnder optical modulator with a
sub-100-μm phase-shifter that operates bycarrier-depletion at a rate of 40 Gbit s–1. They
achieved this by incorporating photonic
crystal waveguides to generate slow lightwith a group index of more than 30, resulting
in a slow-light enhancement that is almost10 times larger than that in rib-waveguidedevices and hence enabling a corresponding
reduction in the device length to 90 μm.
Nguyen explained that although suchresonant structures are usually associatedwith a narrow spectral bandwidth, using amodified photonic crystal waveguide andgenerating low-dispersion slow light allows
bandwidths of 12.5 nm to be achieved in a
50-μm-long device at 10 Gbit s–1.
The 10th International Conference onGroup iv Photonics will take place in Seoul,
Korea, on 28–30 August, 2013.
❒David Pile is at Nature Photonics, 225 Bush Street,Suite 1453, San Francisco, California 94104, USA.
e-mail: d.pile@us.nature.com

NATURE PHOTONICS | VOL 6 | DECEMBER 2012 | www.nature.com/naturephotonics© 2012 Macmillan Publishers Limited. All rights reserved.