# The Achievements and Challenges of Silicon Photonics

Authors: Richard Soref
Journal: Advances in Optical Technologies
 (2008)

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## Abstract

A brief overview of silicon photonics is given here in order to provide a context for invited and contributed papers in this special issue. Recent progress on silicon-based photonic components, photonic integrated circuits, and optoelectronic integrated circuits is surveyed.  Present and potential applications are identified along with the scientific and engineering challenges that must be met in order to actualize applications. Some on-going government-sponsored projects in silicon optoelectronics are also described.


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## Body

## 1. Introduction

We are fortunate
in this special issue to have authoritative invited papers
written by key contributors to the field silicon-based photonics. It is a
pleasure for me to be in the company of these leaders as I write this introductory
article. The forward-looking papers here
highlight the “technical momentum” that has built up in silicon photonics. They
describe discoveries in science and technology that are being made worldwide at
an increased rate thanks to the ramped-up investment by industry, universities,
and governments [1, 2]. Transitions from
“lab-to-fab” are taking place for commercial and military uses. Products are
being developed (Luxtera, Kotura, SiLight). This is the beginning of
optoelectronic (OE) manufacturing on a much wider scale [3]. My mission here is
to give a context or background for the papers.
I will review recent achievements in Si-based photonic integrated
circuits (PICs) and optoelectronic integrated circuits (OEICs). I will pinpoint
important challenges for the emerging OE industry, and I will identify
applications that will be actualized if and when those challenges are met.


## 2. Recent Projects

Several new and
on-going programs related to PICs and OEICs are described in this section. PICs
are important in their own right, but the main driver today of the Si photonics
field is the quest for low-cost large-scale-integrated OEICs manufactured in
state-of-the-art high-volume silicon CMOS foundries (silicon fabs). Potentially, these chips have vast and highly
significant applications which, if implemented, would make those circuits
pervasive in our planet.The
essential role that silicon nanophotonics will play in the future is
highlighted in a new 2007 initiative on ultraperformance nanophotonic intrachip
communication (UNIC) sponsored by the US Defense Advanced Research Projects
Agency. The goal is to demonstrate low-power, high-bandwidth, low-latency
intrachip photonic communication networks designed to enable chip
multiprocessors with hundreds or thousands of compute cores to realize
extremely high computational efficiency; a goal that embodies the convergence
of computation and communication envisioned by Lionel C. Kimerling. The
optoelectronic UNIC project poses a major challenge to the technical community
because it will require revolutionary rather than evolutionary advances in
science, devices, circuits, and computing systems.An
important investigation of nanoscale devices was made at the March 19-20, 2007
workshop on “very large-scale photonic integration” sponsored by the US
National Science Foundation and chaired by Ronging Hui, Usha Varney, and Thomas
Koch. Nanophotonic strategies for VLSI were explored, but the path to mass
production was not clear. Corporations and universities now engaged in the
fledgling OE industry need a cost-effective way to demonstrate and optimize
their individual OE prototype chips. For that reason, I and other workshop
participants recommended that a government-sponsored national CMOS-photonics
User Facility should be set up in the United States
to establish a
cost-shared “photonics-ready” silicon foundry that will provide
application-specific OEIC prototypes for users throughout the technical
community. The proposed user fab would
rely upon the SOI photonic-component manufacturing “libraries” that are being
developed at BAE Systems, Luxtera, and elsewhere. Both BAE and Luxtera are
currently “on track” with their Phase II milestones for DARPA EPIC [1] (www.darpa.mil/MTO/Programs/epic/index.html).
I am aware that “serious” OE programs are starting up in France, South
Korea, and China, but I don’t have much
information on those efforts.The basic motivation for OE is
to attain electronic drivers and controllers that are intimately integrated
with their laser diodes, modulators, amplifiers and photodetectors. The larger
OE goal is to create greatly improved devices, subsystems, and systems. Silicon
of course is not the only OE medium, and silicon OE is currently struggling
with the issue of on-chip light sources, a problem solved years ago by the
III-V semiconductor laser industry. That
is why OE based upon GaAs and InP is likely in the near term to give Si OE
strong competition in “small-scale” integration situations, for example, in
InP-based 1.55μm transceivers (Infinera Corporation, Sunnyvale, Calif., USA) where III-V lasers and photodetectors can
be integrated monolithically. However, two key advantages of silicon appear in
the bigger picture: the very low chip costs that high-volume Si OE production
will ultimately give, and the very high level of functionality that Si OE will
eventually provide sophistication derived from hundreds or thousands of
photonic components integrated on-chip with perhaps a million transistors.Some of the cutting-edge
research topics in Si photonics are nanophotonics, plasmonics [4], photonic
crystals, nanomembranes, SiGeSn alloys, commercial manufacturing methods,
nonlinear optics, nanoelectrooptical mechanics, and microfluidics. The papers
presented at the IEEE LEOS 4th International Conference on Group IV Photonics (Tokyo, September 19-21
2007) give a good indication of the present R&D thrusts of silicon
photonics (http://www.ieee.org/organizations/society/leos/LEOS-CONF/GFP2007/index.html).
The sessions there deal with the Japanese MARAI optical interconnection
project, waveguides and filters, OE and III-V hybrid integration, MOEMs and 3D
structures, modulators and switches, disruptive materials and process
technologies, nonlinear optics and active functions, slow-light devices and
passive photonic crystals, light-source materials, light-source devices and
structures, and detectors.The new thrust in group IV
nanomembranes, catalyzed by a 2008 AFOSR multiuniversity research initiative (ONR
BAA Announcement Number 07-036, Research Opportunity number 10) can yield
single-crystal membranes of Si or Ge or layered Group IV heterostructures whose
thickness is, for example, in the 5 to 500 nm range. Such membranes could be
deployed in flexible intelligent photonics and more generally in all of the
applications areas listed below in Section5.


## 3. The Light-Source Challenge

An optical
network, whether fiber-optic or on-chip, requires light sources, and an Si OEIC
or PIC is incomplete without sources. Because bulk crystalline silicon has
inefficient electroluminescence, light sources have been critical issues for
silicon photonics since its inception. For these reasons, the creation of
practical silicon light sources is a major, ongoing R&D focus.Off-chip
light sources can be thought of as a “photon supply” or “optical power supply”
for the chip. In a particular subsystem or application, the decision about
whether to locate the light source on- or off-chip is “situational,” depending upon
factors such as heat sinking, average power consumption, energy dissipated per
bit per second, cost, and size. Both on- and off-chip solutions have been
proposed for intrachip interconnects. The preferred light
source is usually (but not always) a laser. The case of a spectroscopic
laboratory on a chip [5, 6] presents an exception to the “rule” of laser. There,
a spectrally broad source like an on-chip light-emitting diode would be
optimum, although a WDM laser array could serve also.Silicon
Raman lasers and amplifiers are certainly useful in silicon photonics, but
intense optical pumping is required for these on-chip devices (it takes a laser
to make a Raman laser) so the pump is likely to be off-chip and capable of
pumping several on-chip gain devices.Significant
Si-based sources are beginning to emerge due to hard work by the technical
community. We can divide these sources
into those that will be available in five years and those emerging in a year or
two. Regarding the integration of on-chip sources, the categories are
monolithic and hybrid. These terms are a bit vague: I will interpret the word
“monolithic” to mean “entirely within group IV” and “hybrid” to mean “silicon
plus III-V” or silicon plus “other” (in other words, heterogeneous integration).The
III-V/Si hybrid source technique of John Bower’s group at UCSB is a very potent
short-term solution that provides on-chip evanescently coupled lasers-on-SOI,
amplifiers, photodetectors, and modulators created by low-temperature bonding
of the PIN III-V components to the top surfaces of the waveguided silicon
network. The remaining questions about
these hybrids are in the areas of manufacturing and economics. Will the hybrids be viable in a CMOS Fab and
will the yields be high? The answers are not known yet.The
US Air Force Office of Scientific Research has a multiuniversity research
initiative on electrically pumped silicon-based lasers, an effort in its second
year (seehttp://www.mphotonics.mit.edu/about_mphc/MURI/sili-conlasers.php and
http://www.asu.edu/news/stories/200603/20060310_MURI.htm). The lasers of the MIT team are either
extrinsic or intrinsic. A quantum dot or nanocrystal gain medium within Si is
extrinsic, while a tensile-Ge gain medium within an Si/Ge/Si heterostructure is
intrinsic. The Arizona State University
team utilizes an intrinsic “all in group IV” approach employing intersubband
emission in Ge/SiGeSn multiquantum-well structures for the far infrared, and
band-to-band emission in GeSn/SiGeSn or Ge/SiGeSn heterostructures for the near/mid
infrared. The MIT and ASU intrinsic lasers appear to be CMOS manufacturable.
The five-year MURI results will be very important to the SiOE industry if the
resulting lasers have adequate efficiency and intensity.


## 4. The High-Volume Challenge

Job creation and an expanded economic infrastructure
will be triggered by sustained growth of the “nacent” silicon optoelectronics
industry, growth that would be supported by long-term high-volume markets. A strong industry would provide
optoelectronic “hardware for the information age,” thereby benefiting a global
society that values “bits more than things” [7]. We examine high-volume SiOE markets in this
section.Which devices will likely be
purchased at the rate of ten million units per year? The answer according to
the MIT CTR consortium [7] is the transceiver, a Si-based send-and-receive
optoelectonic chip. Two fast chips interconnected by a fiber create a duplex
fiber-optic communication link that, with near-future technology, is capable of
transmitting data at rate anywhere from 1 Gbps up to 100 Gbps, depending upon
the sophistication of the chip consitutents.
If the chip can be manufactured to sell at a price of one dollar per
Gbps or less, and if the chip will draw less than 1 mW/Gbps of power, then the transceiver
will probably meet the needs of several major markets including: broadband core-,
metro- and access-networks (fiber to the home, internet box, Ethernet LAN, FTTX),
supercomputers, high-performance computers, enterprise networks, data centers (active
cables as computer patch cords; optical interconnects for cabinet to cabinet, board
to board, chip to board, and chip to chip), avionics-automotive-shipboard
(communication and control links for aircraft, cars and ships), and microwave
photonics (optical control of a phased-array antennas). Potentially, there are
also huge, pervasive markets for “nanotransceivers” found inside new computer
chips where waveguides link CPU cores.Optics will replace copper if
and only if the optics has compelling advantages. If SiOE research and
development succeed in making on-chip optical linkages sufficiently fast,
small, efficient, cheap, and reliable, then “long” copper paths would be
replaced and many millions of such OE chips would appear in next-generation
personal computers, notebooks, and gaming boxes (Playstation, Nintendo).In addition to the DARPA effort
mentioned above, the optical interconnect MIRAI project sponsored by Japan
's
MITI is making a major R&D investment in the intrachip area [8]. The main objective is ultrafast on-chip
global interconnects having a total path length of 5 to 30 mm. A second goal is optical clock distribution.
Their optical interconnect approach is quite pragmatic and “wavelength
agnostic”; whatever wavelength and technology are cost-effective and work
well are appropriate. They will use an
off-chip light source (or an off-chip WDM laser array) emitting in the 800 nm
range, together with SiON waveguides and Si photodiodes. Modulation will be
accomplished by poled PLZT films.I will conclude this section
with some speculations about chip-scale 1550-nm global interconnects and the
“not-yet-born” nanolasers that will be necessary to produce them. There are many feasible architectures for an
SOI waveguided optical network that gives communication among multiple processor
cores on a computer chip. I will illustrate here the example of a
reconfigurable broadcast network for eight-core interconnection. The four-ring layout
of Figure1 is similar to the WDM- and switched-network geometries that I presented in slides at the
high-speed interconnect workshop [9]. In Figure 1, the green octagons are made
from Si strip waveguides. Each bus-coupled circular ring (red, orange, green,
blue) in Figure 1 is a reconfigurable optical add-drop multiplexer (ROADM) that
can be electrically tuned over eight laser wavelengths around 1550-nm. Each PIN laser diode and photodiode in Figure 1
(integrated in a silicon strip waveguide shown as a green line) would use
tensile Ge as the gain medium [10]. In
[1, Figure 5], I suggested that a carrier-injected lateral superlattice formed
within a germanium PIN-diode microring could be the heart of a 1550-nm laser. To
attain a smaller-than-ring mode volume in a laser resonator, the Ge nanolasers
and the nanoscale Ge photodetectors presented here in Figure 1 would each have an
inline Fabry-Perot cavity created by a pair of 1D photonic crystal (PC)
“mirrors” in a silicon strip (photonic wire) waveguide as illustrated in the
enlarged top view of Figure 2. The
resonator, consisting of one or two PC point defects, would have a mode volume
of approximately (λ/2n)3.Figure 1
Top view of proposed SOI waveguided
intrachip photonic interconnect system for global communication among eight cores.Figure 2
Structure of the strip-waveguided group IV
nanolasers and nanophotodetectors used in Figure1.The tapered hole-diameter
mirrors in Figure2 minimize out-of-plane loss and maximize Q as discussed in a
“nanolink” patent application [11]. The reflectors in Figure 2 consist of a row
of air columns and are equivalent to a set of deeply etched rectangular slots
in the SOI strip (a tapered Bragg mirror).As shown in the detailed views
of Figure3, the tensile-Ge or GeSn parallelepiped (an ultrasmall bulk crystal) is
grown selectively within an Si strip trench to form a P-Si/I-Ge/N-Si
heterodiode, forward- or reverse-biased, as needed. The lateral P-type and N-type silicon “wings” bracketing the Ge create a
localized rib waveguide within the strip waveguide, a rib that does not disturb
the fundamental guided mode. Addressing the ROADMs in Figure 1 will be discussed
elsewhere. Thermal stability is a challenge
for any of the resonant devices discussed in this paper.Figure 3
Closeup views of the Figure2 resonant lateral PIN Ge-in-Si (or
GeSn-in-Si) heterodiode laser (and photodetector) used in the Figure 1 nanophotonic system.


## 5. The Dream and Challenge of High-Impact Applications

I believe that “well-developed” Si-OEICs will have
major impact on global society and commerce.
Generally speaking, the Si OE application areas are optical
interconnects, sensor technologies for the visible and near-, mid-, and
far-infrared [5, 12, 13], signal processing functions, imaging, displays, energy
conversion, illumination, optical storage, and gaming. If I look inside of these
eight categories, I can identify specific high-impact cases. These are the
challenges and opportunities for those of us involved in Si OE:(a)
interconnects: all of the transceiver applications listed above
in the high-volume section, electrooptically
switched (reconfigured) optical networks;(b)
sensors: infrared spectrometer-on-a-chip, photonic laboratory-on-a-chip for sensing chemical
and biological agents, lab-on-a-chip for environmental monitoring or process
control or medical diagnosis;(c)
signal processing: wireless mobile multifunction “phone-like”
device, optical time-delay beam-steerer for a phased-array microwave antenna, RF-optical
receivers for RF spectrum analysis, ultrafast analog-to-digital converters, reconfigurable
wavelength-division multiplexers and demultiplexers, reconfigurable optical
filters, electronic warfare processors, photonically enhanced microwave and
millimeter-wave circuits, optical buffer memories, electrooptical logic that
operates on phase-coherent light beams, quantum communication-cryptography-metrology-computing,
photonic testing of electronic ICs, bionic signal processors; neural network
processors; data-fusion chips using inputs from several sensors;(d)
imaging: focal-plane-array imager with integral readout: infrared-to-visible
image converter chip;(e)
displays: chip-scale electrooptical display with integral scanning;(f)
energy: highly efficient group IV photovoltaic solar cells with
integral signal processing—perhaps with thin-film supercapacitors for energy
storage;(g)
illumination: efficient group IV solid-state lighting devices;(h)
optical storage: read/write chips for ultradense CDs and atom-scale memories;(i)
gaming: ultrafast graphic computation chips for Nintendo and Playstation.
I feel that nonlinear optical effects in group IV PCs
and photonic-wire devices will have applications in signal processing. These NLO effects encompass all-optical
modulation, all-optical wavelength conversion, electrooptically tuned resonator
wavelength conversion, four wave mixing, broadband parametric gain, all-optical
signal regeneration, electromagnetically induced transparency, Pockels-effect polymer/Si
self-phase modulation, cross-phase modulation, stimulated-Raman slow-light
delay lines, temporal pulse compression, and soliton generation. Many of these apply
to optical networks.


## 6. Recent Achievements

This section contains a brief sketch of results achieved
during the past six months. Most of results
streaming out of Europe, North America, and Asia pertain to 1550 nm, but we are beginning to witness progress at longer
wavelengths, highlighted here in Sections6.2 and 6.3. In 2005, I proposed that silicon-integrated
optelectronics could be migrated into the wide, longwave infrared region that
stretches from 1.6 to 200μm. That migration will require
innovative components whose dimensions are scaled-up from 1550 nm [12]. In a recent 2008 talk [5], I suggest that the
time is ripe for chip-scale LWIR integration.


### 6.1. Results at 1550 nm

Modeling of an n+-doped layer of tensile Ge [10] predicts strong 1550-nm
luminescence because the As doping
populates the lower-energy L conduction valley, thereby transferring
electrons from “excited L” into the Γ valley that sits at higher energy. Thus strong
radiative recombination is obtained. I do not know whether the amount of
electron-hole injection in a PNN+ diode will be adequate compared to
that of a PIN diode. With regard to
Sn-alloy emission experiments, the SiGeSn photoluminescence results of ASU [15, 16]
are precursors of their electroluminescence studies. Their new SiSn alloy [16]
is predicted to have a direct bandgap at 1460 nm when Si0.65Sn0.35 is grown lattice-matched upon Ge0.88Sn0.12-buffered silicon.On the topic of PCs, the
first photonic crystal in a film of crystal Ge has been reported [17]. Strong luminescence from a point-defect
resonant cavity in a 2D PC slab of GeOI was attained. On the topic of microring resonators,
considerable progress has been made as evidenced by an experimental SOI
photonic-wire fifth-order filter [14]. An
alternative to the SOI strip ring has now surfaced in the form of the SOI 2D photonic-crystal
ring resonator (PCRR) presented in [18, 19]. As shown in Figure 4, the dual-ring
PCRR has a property not found in photonic-wire rings. It can drop
a resonant wavelength in either the forward or the backward direction [18].
Also, the PCRR diameter can be made smaller than that of the microwire without
suffering size-dependent losses like those present in the microwire resonators.Two designs for an SOI
photonic-crystal dual-ring-resonator wavelength-division add-drop
multiplexer optimized for
(a) backward dropping, (b) forward dropping. (Illustrations taken from [18]).
(a)(b)


### 6.2. Results at the Near and Mid Infrared

The 3.39μm midinfrared silicon Raman amplifier
reported by Bahram Jalali’s group at UCLA represents an important migration of
silicon photonics into the 3 to 5μm atmospheric-transmission window [20]. The
room temperature electrically pumped germanium MIS-diode laser developed in the
group of Chee-Wee Liu appears to be a world first [21]. This novel
tunneling-injection device utilizes an unstrained bulk Ge crystal with a
strip-shaped metal-insulator gate on the Ge top surface, plus a contact on the
bottom Ge surface. This multimode laser has output lines from 1600 nm to 2140 nm, and there are temporal instabilities in the laser output associated with
local heating under the gate.I propose improvements to this MIS laser diode laser that
would reduce indirect-bandgap effects and would increase infrared mode
confinement under the gate without relying upon heating. Since the direct-gap wavelength of a Ge1-xSnx crystal film moves from 1550 nm in pure Ge to 1900 nm as 5% Sn is added to Ge,
and since GeSn can be grown directly upon Si, the improved MIS laser would
employ a ~1μm-high nearly direct GeSn stripe mesa
waveguide upon a thin Si substrate. The emission wavelength would be in the
1900 to 2100 nm range. A
multiquantum-well GeSn/Ge active region would lase closer to 1600 nm.As GeSn technology becomes
mature, I feel that several practical GeSn/Si PIN laser diodes and photo diodes
will be invented whose wavelengths-of-operation are from 1.8 to 2.5μm. At these near/mid-IR wavelengths, the
attenuation of a glass optical fiber is too large for meter-scale transmission,
but the high-speed GeSn-transceiver fiber-optic links would work well over a
few cm and fluoride fiber could serve for multimeter links.


### 6.3. Results at the Far Infrared

ASU researchers have demonstrated a relaxed-crystal SiGeSn buffer on
silicon, offering a template for subsequent MQW epitaxy. This provides a real-world basis for design
studies. A quantum-cascade laser design of
a strain-free Ge/SiGeSn MQW lattice-matched to the ternary buffer was recently
presented by Sun et al. [22]. This conduction intersubband device, a unique
advance in Group IV technology, was predicted to have 120 cm-1 of
gain at the 49μm wavelength.Because the deposited material Ge0.23Sb0.07S0.70 has mid IR and far IR transparency, as well as CMOS compatibility, the low-loss
GeSbS waveguides announced by the MIT group [23] appear to be good candidates
for silicon-based LWIR on-chip networks.


## 6.1. Results at 1550 nm

Modeling of an n+-doped layer of tensile Ge [10] predicts strong 1550-nm
luminescence because the As doping
populates the lower-energy L conduction valley, thereby transferring
electrons from “excited L” into the Γ valley that sits at higher energy. Thus strong
radiative recombination is obtained. I do not know whether the amount of
electron-hole injection in a PNN+ diode will be adequate compared to
that of a PIN diode. With regard to
Sn-alloy emission experiments, the SiGeSn photoluminescence results of ASU [15, 16]
are precursors of their electroluminescence studies. Their new SiSn alloy [16]
is predicted to have a direct bandgap at 1460 nm when Si0.65Sn0.35 is grown lattice-matched upon Ge0.88Sn0.12-buffered silicon.On the topic of PCs, the
first photonic crystal in a film of crystal Ge has been reported [17]. Strong luminescence from a point-defect
resonant cavity in a 2D PC slab of GeOI was attained. On the topic of microring resonators,
considerable progress has been made as evidenced by an experimental SOI
photonic-wire fifth-order filter [14]. An
alternative to the SOI strip ring has now surfaced in the form of the SOI 2D photonic-crystal
ring resonator (PCRR) presented in [18, 19]. As shown in Figure 4, the dual-ring
PCRR has a property not found in photonic-wire rings. It can drop
a resonant wavelength in either the forward or the backward direction [18].
Also, the PCRR diameter can be made smaller than that of the microwire without
suffering size-dependent losses like those present in the microwire resonators.Two designs for an SOI
photonic-crystal dual-ring-resonator wavelength-division add-drop
multiplexer optimized for
(a) backward dropping, (b) forward dropping. (Illustrations taken from [18]).
(a)(b)


## 6.2. Results at the Near and Mid Infrared

The 3.39μm midinfrared silicon Raman amplifier
reported by Bahram Jalali’s group at UCLA represents an important migration of
silicon photonics into the 3 to 5μm atmospheric-transmission window [20]. The
room temperature electrically pumped germanium MIS-diode laser developed in the
group of Chee-Wee Liu appears to be a world first [21]. This novel
tunneling-injection device utilizes an unstrained bulk Ge crystal with a
strip-shaped metal-insulator gate on the Ge top surface, plus a contact on the
bottom Ge surface. This multimode laser has output lines from 1600 nm to 2140 nm, and there are temporal instabilities in the laser output associated with
local heating under the gate.I propose improvements to this MIS laser diode laser that
would reduce indirect-bandgap effects and would increase infrared mode
confinement under the gate without relying upon heating. Since the direct-gap wavelength of a Ge1-xSnx crystal film moves from 1550 nm in pure Ge to 1900 nm as 5% Sn is added to Ge,
and since GeSn can be grown directly upon Si, the improved MIS laser would
employ a ~1μm-high nearly direct GeSn stripe mesa
waveguide upon a thin Si substrate. The emission wavelength would be in the
1900 to 2100 nm range. A
multiquantum-well GeSn/Ge active region would lase closer to 1600 nm.As GeSn technology becomes
mature, I feel that several practical GeSn/Si PIN laser diodes and photo diodes
will be invented whose wavelengths-of-operation are from 1.8 to 2.5μm. At these near/mid-IR wavelengths, the
attenuation of a glass optical fiber is too large for meter-scale transmission,
but the high-speed GeSn-transceiver fiber-optic links would work well over a
few cm and fluoride fiber could serve for multimeter links.


## 6.3. Results at the Far Infrared

ASU researchers have demonstrated a relaxed-crystal SiGeSn buffer on
silicon, offering a template for subsequent MQW epitaxy. This provides a real-world basis for design
studies. A quantum-cascade laser design of
a strain-free Ge/SiGeSn MQW lattice-matched to the ternary buffer was recently
presented by Sun et al. [22]. This conduction intersubband device, a unique
advance in Group IV technology, was predicted to have 120 cm-1 of
gain at the 49μm wavelength.Because the deposited material Ge0.23Sb0.07S0.70 has mid IR and far IR transparency, as well as CMOS compatibility, the low-loss
GeSbS waveguides announced by the MIT group [23] appear to be good candidates
for silicon-based LWIR on-chip networks.


## 7. Overview of this Special Issue

As I read through the invited papers,
I was impressed by their depth, diversity, timeliness, and innovation. The main themes of this issue are
efficient silicon light sources (Gaburro et
al.), optical switching and memory in photonic crystal nanocavities (Notomi et
al.), macroporous silicon photonic
crystals (Kitzerow et
al.), hybrid evanescent
III-V integration (Park et
al.), light
emitters in 3D Si/SiGe nanostructures (Lockwood et
al.), chip-scale silicon photonic bio-sensors (Carracosa et al.),
reflective gratings for photonic interconnects (Bidnyk et
al.), high-speed waveguided Ge integrated photodetectors (Masini
et al.), rare-earth doped silicon nano-emitters (Li et
al.), foundry tools for photonic integration on CMOS (Fedeli et
al.), gigascale silicon optical
modulators (Basak et
al.), sub-wavelength
grating structures in SOI (Schmid et
al.),
low-cost silicon photonic links (Luff et
al.), ion-implanted optical Bragg filters in SOI (Knights et al.), polarization
control via stress induction (Xu et
al.),
electromagnetic modeling of silicon lasers (Prather et
al.), engineering of material-photon interaction (Wada), and active
photonic crystal devices for interconnects (Fauchet). The advances reported
here lift silicon-integrated photonics to a higher level of capability and lay a
foundation for further progress. These advances
will help silicon reach preeminence in microphotonics.



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