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 References 1. TF Krauss, “Slow light- opportunity for photonic crystals?” PECS VI, Crete, Greece, June (2005) (invited) 2. J. Khurgin, “Optical buffers based on slow light in electromagnetically induced transparent media and coupled resonator structures: comparative analysis,” J. Opt. Soc. Am. B 22, 1062-1074 (2005) 3. M. F. Yanik, and S. 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