Final Report
Project number:
IST-033651
Project acronym:
SPLASH
Project title:
Slow Photon Light Activated Switch
Period covered:
from: 01-01-2007
Date of preparation:
June 2010
Start date of project:
1st January 2007
Project duration:
3.5 years
Project coordinator name:
Thomas F. Krauss
Project coordinator organisation:
University of St Andrews (USTAN)
to: 30-06-2010
Table of Contents
1
2
3
4
5
6
7
Introduction
Project objectives and their realisation
2.1
Objective 1 - The nature of slow Bloch modes in photonic crystals
2.1.1
Result (USTAN, CNRS, POLIMI, AMOLF)
2.1.2
Result (CNRS, USTAN)
2.2
Objective 2 - Realisation of dispersion-engineered photonic crystal
waveguides
2.2.1
Result (USTAN)
2.3
Objective 3 - Efficient injection of light into a slow mode
2.3.1
Result (CNRS, USTAN)
2.4
Objective 4 - Coupled-resonator delay lines
2.4.1
Result (GU, POLIMI)
2.4.2
Result (POLIMI, USTAN, GU)
2.5
Objective 5 - Slow light tuneable switch
2.5.1
Result (USTAN, AMOLF)
2.6
Objective 6 - Electronic tuning of a silicon-based photonic crystal
2.6.1
Result (USTAN)
2.7
Objective 7 - Increase bandwidth and storage capacity
2.7.1
Result (POLIMI)
2.8
Objective 8 - Tuning of slow light structures and optical storage
2.8.1
Result (GU, POLIMI, USTAN)
2.8.2
Result (POLIMI)
2.8.3
Result (AMOLF, USTAN)
CUDOS Interaction
State-of-the-Art table
Conclusion
Papers in progress
References
3
5
5
5
6
7
7
8
8
9
9
10
11
11
13
13
13
13
14
14
15
15
16
18
19
20
20
1
Introduction
When SPLASH was conceived 2005 and 2006, slow light was still a new topic in
guided-wave photonics. Until then, the thrust of slow light work had been conducted
by the atomic physics/laser physics community who studied electromagnetically
induced transparency (EIT), population oscillations and similar effects whereby an
optical state excites a material state, is stored for a time and then converted back into
an optical state, thus creating an effective delay.
The guided-wave equivalent is to excite a photonic resonance, store the signal in the
resonance and release it back into a regular waveguide. This work was pioneered by
some of the founding members of the Splash consortium, e.g. slow light in coupled
ring resonators by Melloni (OQE 2003) and in photonic crystals by Krauss, whose
invited talk at the photonic crystal meeting PECS in 2005 explored the opportunities
for slow light in photonic crystal waveguides [1]. Some theoretical papers also
existed, most notably by Khurgin on the comparison between EIT-type and photonic
resonance-type slow light [2] and by Fan on stopping light in chains of photonic
crystal resonators. [3], but very little experimental work had been done, especially in
the high refractive index contrast regime; Khurgin’s paper had made the point,
however, that the performance of a slow light buffer scaled with the refractive index
contrast in terms of the delay-bandwidth product. Motivated by this insight and the
emergence of Silicon Photonics, SPLASH was designed to focus on silicon, which
was also the favourite material of the photonic crystal community.
The key questions to address were as follows:
1. What limits the performance of a slow light delay line? Is it dispersion or
propagation loss? How many bits of information can be stored realistically?
2. Can the losses in slow light waveguides be reduced, and what is the maximum
achievable group index and total delay?
3. How do photonic resonance devices compare to EIT devices, and are photonic
crystals better or worse than rings?
4. Some papers had predicted the enhancement of linear electro-optic effects by the
slowdown factor, and nonlinear effects by the slowdown factor squared [4, 5].
Could these enhancements be realised in practice?
In order to answer these questions, the project was divided into 4 technical
workpackages, 2 focussing on rings and 2 on photonic crystal waveguides; the first
two were more fundamental in nature while the second two were more applied,
yielding the following matrix:
SPLASH
Fundamental
Applied
Nonlinear
Photonic crystals
WP 1 Bloch modes
WP 3 Slow light switch
CUDOS
Microrings
WP 2 Coupled microrings
WP 4 Tuneable delay
Figure 1. SPLASH was set up to establish the best geometry for slow light based on
photonic resonances, to find out whether arrays of coupled rings or photonic crystal
waveguides would be better for a given application and to understand the limitations of
each approach.
Shortly after approval of the project, the interaction with the Australian CUDOS
collaboration intensified to the extent that TF Krauss and L Kuipers became partner
investigators of CUDOS; as a result, CUDOS also became an associate partner of
SPLASH. The CUDOS interaction focussed on enhanced nonlinear interactions in the
slow light regime, an area that the original proposal had shied away from, as well as
the development of chalcogenide waveguides that offered a promising new platform
for slow light enhanced nonlinear optics. Several very impressive results were
achieved and the CUDOS expertise in optical communications systems proved very
valuable. For example, we observed slow light enhanced self-phase modulation with a
view towards all-optical regenerators; enhanced two-photon absorption, which is
useful for optical limiting applications, and, most importantly, we observed green
light from a silicon waveguide by third harmonic generation, which was also shown
to be capable of monitoring optical signals up to 640 Gbit/s.
A number of objectives were formulated in order to address the key questions posed
above. Given their ambitious nature and the vagaries associated with a high risk FET
project, it is impressive how many of these were met in full, and how many at least in
part. This attests to the high quality of the consortium and the close collaboration
between partners. These partners were the University of St Andrews, coordinator
(USTAN, UK, led by TF Krauss), the University of Glasgow (GU, UK, led by M
Sorel and R De La Rue), Politecnico di Milano (POLIMI, Italy, led by A Melloni), the
FOM Institute, Amsterdam, (AMOLF, NL, led by L Kuipers) and the Institut
d’Optique, Paris (CNRS, France led by P Lalanne).
2
Project objectives and their realisation
2.1 Objective 1 - The nature of slow Bloch modes in
photonic crystals
We aim to understand the nature of losses in photonic crystal waveguides and develop
devices with a propagation loss per unit time equal to or lower than that of an
equivalent fast light structure and a bandwidth of 500 GHz or more.
2.1.1 Result (USTAN, CNRS, POLIMI, AMOLF)
Following an intensive effort over the full three years of the project, we have
understood the nature of Bloch modes and their associated losses better than any other
group in the world. This is manifested by the fact that we were able to model the
rather complicated dependence of loss on group index for our dispersion-engineered
waveguides (cf objective 2, figure 2), which required including the out-of plane loss,
accounting for the variation of mode shape with group index, and, most importantly,
realising the importance of the interaction between multiple scatterers in the unit cell
and how to capture this in the calculation method. In fact, the latter was already
captured in the method developed early on in the project [6], but we had not realised
its impact. Fully understanding the method led us to realise that interference effects
can be exploited for the reduction of losses, in fact, one can think of this as an analogy
between EIT-based slow light and photonic resonance-based slow light: The
phenomenon of EIT is based on the destructive interference between two decay-paths
of an excited state; equally, one can think of minimising the losses in a slow light
photonic crystal waveguide by exploiting the destructive interference between
scatterers.
Figure 2. Comparison between experimental (blue dots) and modelled (red, green line)
data for the loss vs. group index curve. Using standard methods that consider scattering
loss in isolation, e.g. a single scatterer per unit cell, the experimental curve can not be
fitted (dashed green curve). Only by considering the possibility of multiple scatterers per
unit cell and taking interference effects into account (“with phase”, red curve) is it possible
to match the experimental data.
This insight is rather profound and will lead us to designing slow light waveguides
with lower propagation losses than their fast light counterparts in due course.
2.1.2 Result (CNRS, USTAN)
The second major insight is more of the “what-not-to-do” type, but also related to a
clear understanding of the nature of Bloch mode propagation and based on the
interplay between device length and the balance between out-of-plane losses and
backscattering losses. If the propagation losses are dominated by backscattering and
accumulate to 3dB or larger, we found that the transmission becomes
in-deterministic [7]. This is most dramatically demonstrated by figure 3 b) below,
where a set of 18 nominally identical devices was analysed experimentally and
numerically; for an average transmission of 50%, i.e. 3dB loss, it is almost impossible
to predict the transmission of the waveguide, as it ranged between 10% and 80% in
the experiment [8]. The 3dB point is the point where the device length equals the
localisation length and it is statistical whether light is transmitted ballistically,
whether it is scattered once or scattered multiple times.
The two insights arising from addressing this objective highlight some important
design rules for slow light photonic crystal waveguides:
a) Try to minimise backscattering losses by interference effects.
b) Avoid operating in the regime where the loss due to backscattering exceeds 2-3
dB. In practice, this restricts the application of slow light waveguides of current
technology and design to lengths of a few 100 µm (ng 30-40) or correspondingly
longer if less slowdown is applied.
Figure 3. Histogram for 18 nominally identical devices (black bars) for an average
transmission of <T>=0.9 (a), <T>=0.5 (b) and <T>=0.1 (c). The red curves correspond to
the numerical average over 5x105 cases. The waveguides were 80µm long, of regular W1
type and operated around a group index of n g=55.
2.2 Objective 2 - Realisation of dispersion-engineered
photonic crystal waveguides
We will develop dispersion-engineered waveguides that are fault-tolerant and aim to
demonstrate waveguides with a slow down factor of 20 or greater over a bandwidth of
500 GHz with negligible higher order dispersion.
2.2.1 Result (USTAN)
This objective was met early on in the project. Based on the idea of creating a slow
light region in the anti-crossing regime between the fundamental index-guided mode
and the corresponding bandgap-guided mode of the W1 waveguide, we conducted a
systematic study of modifying this anti-crossing regime. We established that moving
rows of holes was more fault-tolerant than changing hole radii and yielded a very
versatile design tool that allowed us to adjust the group index between ng=20 to
ng=110 with corresponding bandwidths of =18 nm to 3 nm, respectively [9, 10].
Figure 4 provides an overview of the corresponding wavelength vs. group index
curves. These curves are obtained by inserting the waveguides into an interferometric
setup [11] and obtaining the group index from the resulting interference pattern.
These dispersion engineered waveguides have been instrumental for almost all of the
photonic crystal work in the project, as they have been used to achieve the nonlinear
results, they were key to understanding the losses and they were used extensively in
the phase shifter sections of the MZI modulators.
Figure 4. Example for the dispersion engineering available by shifting the holes of a
photonic crystal waveguide. The region of constant group index highlighted in yellow is
defined as the ±10% interval around the central value.
2.3
Objective 3 - Efficient injection of light into a slow mode
We aim to design and demonstrate compact mode converters that inject light with
target characteristics of efficiency >90% for S=20 over a 500 GHz bandwidth.
2.3.1 Result (CNRS, USTAN)
The mode conversion problem was initially believed to be a rather difficult problem
and had been addressed by designing tapers [12] that were quite lengthy; the idea was
an adiabatic conversion of modes from the slow light to the fast light regime. We
conducted a number of studies taking into account the fact that Bloch modes differed
fundamentally from TIR waveguide modes [13] and concluded that adding a simple
interface region as sketched in figure 5 would address the problem. The mode first
couples from the ridge waveguide into a fast photonic crystal waveguide and then on
into a slow guide, with efficient mode conversion taking place in the intermediate
section.
A complementary study led by CUDOS and based on evanescent modes [14] yielded
a similar result and concluded that the mode conversion length scaled with the
difference in group velocity.
The entire slow-light PhC activity, including both fundamental (i.e. loss
measurements, etc.) and applied (i.e. switches, etc.) work has benefitted from the
availability of such efficient injectors since the end of year 1.
Figure 5. A mode conversion interface. Light is coupled from the access ridge waveguide
(left) into a 4-10 period PhC section where the hole spacing along the waveguide is
increased, thus operating in the fast mode regime. This is coupled directly into a slow light
waveguide without further transition layers. This design can be applied to standard W1 or
dispersion engineered waveguides. The increase in hole spacing is typically 30nm (<10%
of the lattice spacing, typ. 420 nm) and is exaggerated here for clarity.
2.4
Objective 4 - Coupled-resonator delay lines
We will demonstrate a chain of at least 20 ring-resonators that are able to induce a
maximum delay equal to 8 bits at 10Gbit/s. Chains of coupled resonators based on
microrings and photonic crystals will be compared. The goal is to achieve an insertion
loss per stored bit of 5-10 dB and a footprint smaller then 1 mm2/bit.
2.4.1 Result (GU, POLIMI)
This objective was met in a number of configurations. Our initial result was realised
in glass and achieved a tunable delay of 8bit at 10 Gbit/s [15]; it was later extended to
the same 8-bit storage at 100 Gbit/s. Figure 6 illustrates the chosen architecture and
the results obtained by tuning 8 rings. By using 12 rings, we achieved delays of up to
89 ps, corresponding to a tunable capacity of more than 1 byte and a fractional delay
of 0.82 bit/RR, with a fractional loss is 0.6 dB/bit, an acceptable pulse broadening
(around 20%) and a moderate intersymbol interference. Please note that these results
far outstrip the goals set in the objectives, e.g. the insertion loss per stored bit is <1dB
vs. 5-10 dB assumed, and the footprint is much smaller with <0.1 mm2 for the entire
device (including heaters) able to store 8 bit, vs. 1 mm2/bit as stated in the objective
[16].
Figure 6. (a) Sketch of the reflective tunable CROW delay line, which effectively uses
each ring twice. Each heater can be addressed individually. (b) Measured spectral intensity
and (c) group delay characteristic of the CROW. The number of rings contributing to the
delay is shown as “open” rings [16].
2.4.2 Result (POLIMI, USTAN, GU)
The comparison between ring resonator CROWs and photonic crystal waveguides
yielded the result that the two are surprisingly similar, especially if realised in the
same technology. In fact, the photonic crystal waveguide can be seen as the limiting
case of a CROW, with each lattice constant representing a microring in CROW taken
to its absolute minimum in size, i.e. /2. The similarity is also show in the
performance, which is compared in figure 7, where the delay of 9 ps pulses by ≈100
ps in either a ring resonator CROW or a photonic crystal waveguide is shown.
Apart from the similarity, the most striking result of this comparison is that the
propagation loss in the photonic crystal case (c) is lower than in the microring case
(a). In fact, the loss per unit time in the photonic crystal case is around 35 dB/ns while
it is close to 100 dB/ns for the rings. The 35dB/ns loss value is the lowest reported in
the literature for silicon-based slow light structure, to our knowledge. The
counterintuitive result of the microring structure exhibiting higher losses is explained
by the fact that, while the waveguide loss is low, the microrings suffer from additional
losses at the couplers. The additional loss comes from the excess loss of the
directional couplers, estimated at about 0.06 dB/coupler, increasing the round trip
attenuation from 0.04 to 0.16 dB; this is sufficient to increase the losses above those
of the photonic crystals.
Figure 7. Delay of a single 9 ps Gaussian pulse in a microring CROW (a,b) and in a
photonic crystal waveguide (c,d). (a) Delay after propagation through 4, 8 and 12
microrings. (b) Same result, but with pulses normalised and overlapped. (c) Delay
achieved by wavelength tuning in a W1 photonic crystal waveguide. The total delay is
more than 1 byte or 83ps. (d) Same result, but with pulses normalised and overlapped.
2.5
Objective 5 - Slow light tuneable switch
We will demonstrate the scaling of light-matter interaction with the slowdown factor
S via a switch that will operate with S-fold reduced size. We will aim for S=10-20 and
a bandwidth of 500 GHz. This corresponds to a switch of 50-100 µm length for a
refractive index change of ≈ 10-3 that would otherwise be ≈1 mm long.
2.5.1 Result (USTAN, AMOLF)
This objective was met in two parts. Firstly, we were able to demonstrate the world’s
smallest switch based on a dispersion engineered photonic crystal directional coupler
[17], shown in figure 8. The device had an active length of only 5 µm (12 unit cells,
see figure 8) and actuated on a refractive index contrast of n≈4x10-3. This is 40-fold
smaller than a comparable device based on photonic wires that actuates on the same
refractive index change, thus clearly demonstrating the size advantage of the photonic
crystal slow light approach. Due to the small size of the switch, we were also able to
demonstrate ultrafast actuation (3 ps) [18], which was only limited by the length of
the pump pulse and the bandwidth of the device ( ≈ 1nm) rather than by the
transient delay of the pulse passing through the structure.
Figure 8. Directional coupler switch based on dispersion-engineered photonic crystal
waveguides. The central switching area consisting of 12 unit cells can be easily discerned
via the larger holes; there are 4 unit cells each at the input and output to facilitate coupling
into the slow light regime.
Secondly, we designed Mach-Zehnder interferometer switches with slow light phase
shifters. These devices were typically 100 µm long and were thermally actuated, with
a refractive index change of n≈1x10-3 and an operating bandwidth of >10 nm, or
>1 THz, which is as good as or better than targeted in the objective. The concept
sketch of such a device is shown in figure 9 and an optical micrograph in
figure 10 b).
Figure 9. Concept sketch of photonic crystal Mach-Zehnder interferometer. The phase
shifter sections are realised by slow light waveguides.
2.6 Objective 6 - Electronic tuning of a silicon-based
photonic crystal
We will demonstrate tuning via carrier injection and depletion width modulation in a
photonic crystal waveguide. We will aim for a refractive index modulation of order
n=10-3.
2.6.1 Result (USTAN)
This objective has only been met in part. Due to problems with achieving reliable
doping and contacts, we were not able to demonstrate reliable pin junction operation
until late in the project and did then not complete the planned programme in terms of
depletion layer modulation. Both thermal and electro-injection modulation was
demonstrated, however, as shown in figure 11.
Figure 10. Photonic crystal Mach-Zehnder modulator actuated by current injection. (a)
The device only requires 6 mA to switch from ON to OFF and achieves a refractive index
change of n≈10-2, which we assume is due to thermal, rather than carrier-induced
refractive index change. The device does not operate in the slow light regime (the doped
regions are thinned by the HF stripping process, thus detuning the photonic crystal). (b)
Top view (optical micrograph) of the device. The outer two contacts are n-type doped and
the inner contact is doped p-type; the figure in (a) was achieved by driving a single p-i-n
junction.
2.7
Objective 7 - Increase bandwidth and storage capacity
Higher-order chromatic dispersion limits the number of bits that can be stored in a
coupled resonator system. By side-coupling additional cavities, the bandwidth of the
structure can be increased. We will explore the limits of this method and determine
how close to the bandwidth limit we can get in reality.
2.7.1 Result (POLIMI)
This objective was formulated in the belief that slow light delay lines were limited by
chromatic dispersion. As shown in figure 7, this is actually not the case, and both
coupled rings and photonic crystals are limited by loss rather than by dispersion.
Figure 11. Schematic of the “bouncing buffer” architecture. Initially, all rings are on
resonance and the pulse is loaded form the top left (“In”). Once the pulse is contained in
the structure, the front two rings that form a “gate” are detuned (left, +f) so the pulse
cannot escape back into the bus waveguide. The two rings on the right determine the
length of the buffer; the longer the buffer (central section of “blue” rings), the slower the
switching time can be; the shorter the buffer, the finer the granularity of possible delay
times, as the delay time is a multiple of round trips until the gate is opened again.
2.8 Objective 8 - Tuning of slow light structures and optical
storage
We will study both slow and fast tuning of coupled resonator systems. Slow, i.e.
thermal, tuning will allow us to demonstrate a coupled resonator system with variable
time delay between 0 and 8 bits. Fast (i.e. electronic, sub-100 ps) tuning will allow us
to realize a coupled resonator system able to store 1 bit (100 ps @ 10 Gbit/s) for at
least 1 ns.
2.8.1 Result (GU, POLIMI, USTAN)
The achievement of tunable delay by up to 1 byte (8 bits) in both photonic crystals
and coupled microrings was already discussed in objective 4, where thermal tuning
was successfully demonstrated. The work on ultrafast (ps-level) electronic tuning is in
progress and the first complete batch of devices has been fabricated, comprising
implanted pn junctions, contacts, and large arrays of ring resonators. An example for
such a device is shown in figure 12.
Figure 12. Optical micrograph of a fabricated CROW 9 in the “bouncing buffer”
configuration with 10 RRs, each with a P-i-N junction and heater for tuning. All the p-i-n
junctions and heaters are wired to gold pad contacts at the edge of the sample in order to
allow wire-bonding for external control.
2.8.2 Result (POLIMI)
Firstly, it was necessary to determine the optimum architecture for the “stopping
light” experiment. Three different architectures had been proposed in the literature,
namely the periodically activated CROW [19], the CROW with side-coupled
resonators [3] and our own “bouncing buffer” geometry [20]. In the ideal case, all
approaches require similar numbers of rings (15-30) and can be actuated on a similar
timescale (≈100ps). All of these proposals, especially the theoretical ones [3, 19]
assume ideal conditions. Once the tolerances of the fabrication and the actuation
mechanism are taken into account, however, the “bouncing buffer” emerged as a clear
winner and was therefore chosen. A schematic of this architecture is shown in
figure 11.
2.8.3 Result (AMOLF, USTAN)
An alternative architecture that relies on wavelength tuning and a dispersive delay
line emerged during the project [21]. Here, an optical signal is frequency-shifted,
typically using four-wave mixing, and then injected into a dispersive delay line; by its
very nature, the dispersive delay line delays signals according to wavelength. This
architecture, in fact, is the same as we already used for the PhC waveguides shown in
figure 7 c), 7 d). The problem is that of wavelength change, which, if done by four
wave mixing, typically suffers a loss of -20dB [21]. In order to address this low
efficiency, we considered the alternative approach of adiabatic wavelength
conversion. This process was first proposed by Notomi [22] and relies on tuning the
properties of a cavity while the optical pulse is contained within the cavity.
The results were rather striking: By containing an optical pulse within a slow light
waveguide and tuning the entire length of the waveguide while the pulse is caught
inside, we also observed wavelength tuning. Surprisingly, the process does not require
a cavity at all (contrary to the “guitar string” analogy) and can be done with very high
efficiency. We observed a conversion efficiency of 80%, the loss being caused by the
free carriers generated in the tuning process. Figure 13 summarises the experiment.
Figure 13. Ultrafast adiabatic frequency shifting. (a) Experimentally determined and (b)
calculated intensity spectra of the photonic crystal waveguide response to a 1.3-ps long
input pulse. (c) Measured and calculated output spectra at τ = −4 ps and τ = -1ps. For (a-c),
the pulse is enrtirely contained in the waveguide while it is being tuned. (d-f) Same as (a-c)
but for a 2.3-ps input pulse. The pulse being longer means that it is not entirely contained
within the waveguide, so only a part of the pulse is adiabatically shifted.
3
CUDOS Interaction
The CUDOS interaction yielded impressive results that had not been foreseen at the
onset of the project; in total, >10 joint (journal) papers resulted from the
collaboration.
The key results, green light generation and ultrafast signal processing, are highlighted
in figures 14 and 15. Figure 14 sketches the phenomenon of third harmonic
generation (THG) in silicon [23], highlighting the emission angle of -10 deg that
indicates phase matching by the photonic crystal.
The same process can be used for optical signal monitoring at bandwidths up to 640
Gbit/s [24] (figure 15). This was a major breakthrough, as it showed the ability of
slow light waveguides to provide substantial enhancements of nonlinear effects over
significant bandwidth in a systems context. In fact, it emphasizes the importance of
deploying photonic technology in areas inaccessible to electronics. In particular, we
demonstrated that the entire bandwidth of the 500 fs pulses used in the experiment
fitted within the dispersion-engineered slow light bandwidth of the waveguide, which
is essential for obtaining the green light generation used in the monitoring process.
Figure 14. Sketch of the third harmonic conversion process generating green light in slow
light photonic crystal waveguides [23].
Figure 15. Optical performance monitoring in slow light waveguides up to 640 Gbit/s. The
entire bandwidth of the 500fs pulses used in this experiment was supported by the slow
light waveguide, hence allowing for the efficient green light generation that is used for
performance monitoring.
4
State-of-the-Art table
WP1
Slow light interface
Losses
Transmission
WP2
Photonic wire propagation
losses
Number of microrings
(bandwidth as a measure
of disorder)
Microrings vs. Photonic
crystals
Resonator-based static
delay performance
WP3
Switch
Length x refr. index
product for modulator
WP4
Tunable delay
Frequency shift for
dispersive delay line
SPLASH
Rest of the World
Understanding slow light
injection based on Bloch
mode excitation [13].
Comprehensive
understanding of
interference effects –
phase matters [32]!
Highlighting of randomness of transmission events
due to the randomness of
scattering [7, 8].
Mainly tapers [12, 25].
Analysis of the role of
evanescent modes [14].
Concentration on
backscatter losses.
Interference effects not
taken into account [27].
n/a
0.9 dB/cm [28]
1.7 dB/cm [29]
20, individually tunable
0.6 nm bandwidth on 64
rings
The first comprehensive
comparison between
microrings, CROW and
PhC delay lines [10, 16].
25 dB/ns (10 Gbit/s)
35 dB/ns (100 Gbit/s)
Photonic crystal [10]
100, fixed [29]
0.4 nm bandwidth on 56
rings
Nothing similar was done
before.
Smallest all-optical switch
in the world (5µm x 8µm),
photonic crystal
directional coupler) [17]
0.12 µm (80µm x 1.5e-3)
Optically pumped CROW
(40 x 12µm2) [31]
Continuously tunable
delay line in SOI operating
up to 100 Gbit/s and up to
1 byte / 8bits tunable
delay [20, 16].
First adiabatic frequency
shift in a slow light
waveguide. 80% efficiency
[18].
Fractional group delay >10
bits for bit rates as high as
20 Gbit/s. Not tunable.
[29]
44 dB/ns [29]
microrings
0.8µm [Liu, Green]
Typically, Four Wave
Mixing is used. Efficiency
of order -20 dB [21].
5
Conclusion
Having reviewed the objectives and how well they have been addressed, we can
return to the questions posed in the beginning.
1. What limits the performance of a slow light delay line and how many bits of
information can be stored realistically?
2. Can the losses in slow light waveguides be reduced, and what is the maximum
achievable group index and total delay?
3. How do photonic resonance devices compare to EIT devices, and are photonic
crystals better or worse than rings?
By comparing microring CROWs and photonic crystal waveguides in the same silicon
technology, we obtained the result that backscattering losses ultimately limit the
performance of a slow light delay line and that dispersive broadening is a much
smaller issue than initially anticipated. Both rings and photonic crystals exhibit the
characteristic ng2 - scaling of the backscatter losses, but, especially in photonic
crystals, design options exist to reduce the backscatter component in favour of the
out-of-plane loss component, which is much less detrimental to device performance.
The “dispersion engineered” waveguides we developed are a particular highlight in
this respect as they display lower losses and a broader useful bandwidth than the
typical “W1” type waveguides widely used by the community. Typical group indices
where backscatter losses start to dominate are in the ng≈30-60 range (higher for
microring CROWs), so we do not believe that much higher group index values are
practical with present design and technology; in particular, we showed that the
transmission through photonic crystal waveguides exhibits strong statistical
fluctuations at the onset of multiple scattering.
Regarding total delay, we demonstrated an 8 bit tunable delay at 10-100 Gbit/s in both
systems and it is clear that microring CROWs perform better for longer delays and
lower data rates, while photonic crystals are more suitable for larger bandwidth and
shorter delays; in fact, there is a clear trend from EIT-type systems (narrow
bandwidth (<10GHz), long delays) to microring CROWS (medium bandwidth (10100 GHZ)) to photonic crystals (large bandwith (100GHz-1THz), short delays). An
additional surprise was the fact that microring CROWs suffer higher losses than slow
light photonic crystal waveguides due to the additional losses imposed by the
directional couplers; even though this loss is small (0.06 dB/coupler/roundtrip (typ.)),
the large number of roundtrips causes the losses to build up.
4. Can the predicted nonlinear enhancements be realised in practice?
A somewhat unexpected highlight was the success of the nonlinear work, mainly
conducted with the Australian CUDOS consortium. We were able to demonstrate a
number of slow light enhanced nonlinear effects and their potential application in
communication systems, such as self-phase modulation and third harmonic
generation, as well as four wave mixing in microrings. These effects scaled with slow
light as predicted, i.e. with the second, third or even fourth power of the slowdown
factor, which was a very exciting outcome.
Nonlinear optics in silicon is always limited by two photon absorption and free carrier
effects. We therefore applied the slow light toolkit to chalcogenide waveguides that
have an intrinsically much higher figure of merit, given by their lower nonlinear
absorption. We were able to demonstrate slow light and dispersion engineered
waveguides in chalcogenide glass as well, but the material proved to be a lot more
sensitive (aging, fabrication) than silicon, so the full potential of the slow light
approach in this more exotic material system remains to be explored.
Overall, SPLASH has set new benchmarks for the realisation of slow light structures
based on photonic resonances, it has highlighted key limitations but also provided
design rules for further improving these structures into the future.
6
Publications Summary
Nature
Photonics
Nature
Physics
PRL
Optics
Express
Optics
Letters
APL
Other
(impact factor
< 3.5)
Total
Papers
Total
Invited
Talks
Y1
1
1
1
5
2
1
8
18
29
Y2
2
2
5
4
10
23
20
2
4
7
5
16
39
58
Total
80
107
Science
Y3
1
3
Papers of note
Probing the magnetic field of light at optical frequencies
M. Burresi, D. van Oosten, T. Kampfrath, H. Schoenmaker, R. Heideman,
A. Leinse and L. Kuipers, Science 326, 550-553 (2009) (featured in perspectives
H. Giessen and R. Volgelgesang, Science 326, 529-530 (2009))
Slow guided surface plasmons at telecom frequencies
M. Sandtke, L. Kuipers, Nature Photonics 1 573-576 (2007)
Why do we need slow light?
T.F. Krauss, Nature Photonics 2 (8) 448-450 (2008)
Slower for longer
R.M. De La Rue, Nature Photonics 2 (12) 715*-716 (2008)
Green light emission in silicon through slow-light enhanced third-harmonic
generation in photonic-crystal waveguides
B. Corcoran, C. Monat, C. Grillet, D. J. Moss, B. J. Eggleton, T. P. White,
L. O'Faolain, T. F. Krauss, Nature Photonics 3 (4) 206-210 (2009)
The long march of slow photonics
A. Melloni and F. Morichetti, Nature Photonics, vol. 3, no. 3, p. 119, March 2009.
Ultrafast evolution of photonic eigenstates in k-space
R.J.P. Engelen, Y. Sugimoto, H. Gersen, N. Ikeda, K. Asakawa, L. Kuipers,
Nature Physics 3, 401-405 (2007)(featured on the cover of Nature Physics)
7
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Nature Photonics, 3, 206-210 (2009)
24. Corcoran, C. Monat, M. Pelusi, C. Grillet, T. P. White, L. O’faolain, T. F. Krauss,
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