WP 3: Applications of Cavity Solitons WP managers: T. Ackemann, R. Jäger happy to be here SUPA and Department of Physics, University of Strathclyde Glasgow, UK ULM Photonics, Lise-Meitner-Str. 13, 89081 Ulm, Germany 31/05/2006 FunFACS review meeting, Brussels

Outline Task 1 of WP 3: All-optical delay line Motivation and principal scheme Results on optical delay line Fabrication of device with improved homogeneity Experimental demonstration of delay line Theoretical analysis of drift speed Comparison to other “slow light” systems Supporting activities and future directions Fabrication of top-emitters with tailored current injection Incoherent switching Summary

What is special about CS? Properties of CS: motion „plasticity“, „motility“ novel ingredient compared to micro-machined pixels translational symmetry continuous bistable all-optical processing all-optical network parallelism optical interconnects best „bang“ you can get from a cavity soliton should involve motility enabling conceptionally new approaches impact ! self-luminous 2D displays; memory or pattern recognition systems; massively parallel and reconfigurable all-optical packet switching and routing schemes; optical delay lines; shift registers and beam fanning. To all these schemes one can add the possibility of high-frequency self-pulsing in the case of a pulsed CSL. Task 1 of WP 3: All-optical delay line discrete

„Slow light“ and all-optical delay line Boyd et al., OPN 17(4) 18 (2006) Hau et al., Nature 397, 594 (1999)

All-optical buffers and delay lines buffers can enhance performance of networks future high-performance photonic networks should be all-optical need for all-optical buffers with controllable delay Boyd et al., OPN 17(4) 18 (2006); Gauthier, Physics World, Dec. 2005, p. 30

All-optical delay line based on CS parameter gradient read out at other side inject train of solitons here time delayed version of input train all-optical delay line buffer register for free: serial to parallel conversion and beam fanning note: won‘t work for non-solitons / diffractive beams Harkness et al., USTRAT (1998)

Prerequisite: High homogeneity state-of the art at the end of PIANOS: gradient of cavity resonance 0.27 GHz / µm quite good for university growth reactor (U Ulm) but: limits the existence range of CS in transverse plane  for drifting of CS over large distances higher homogeneity required ! Barland et al., Nature 419, 699 (2002)

Homogeneity before optimization substrate holder in production reactor: immediate improvement by a factor of about 3 and more origin of the inhomogeneity: beam shape of the source material temperature inhomogeneity of the substrate R. Jäger et al., UP, unpublished

Homogeneity after optimization tangential improve beam shape optimize temperature difference between bottom and top heating filament of the effusions cells improve homogeneity of substrate temperature reduce temperature level of growth enhance uniformity of substrate holder rings result: < 0.012 GHz / µm < 2.5 GHz / 200 µm radial R. Jäger et al., UP, unpublished

Set-up for all-optical delay line spatio-temporal detection system: 6 local detectors + synchronized digital oscilloscopes BW about 300 MHz VCSEL (UP) 200 µm diameter pumped above transparency but below threshold  amplifier suitable preparation of holding beam F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

Preparation of holding beam Mach-Zehnder interferometer with 6 detectors you cannot investigate two-dimensional spatio-temporal structures create quasi-1D situation channeling of motion of CS there is also a phase gradient along the stripes  drift stripes with modulation depth of  1

Results: Noise-driven events intentional (bang on table) or intrinsic perturbations trigger release of pulse anti-phase oscillation possible interpretation: structure oscillating back and forth in a potential well F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

Reproducibility superposition of 50 events  deterministic propagation noise triggered events appear at fairly random time intervals compatible with interpretation of a noise triggered drifting CS F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

optically addressed drifting structure Optical addressing gate addressing beam with an electro-optical modulator rise/fall times < 1 ns 100 ns optically addressed drifting structure delay  12 ns distance  25 µm velocity  2.1 µm/ns delay / width  2-4 this is an embryonic all – optical delay line ! F. Pedaci, S. Barland, M. Giudici, J. Tredicce, INLN, Nice, unpublished

Velocity experiment suggests velocity of about 2 µm / ns = 2000 m / s = 7200 km / h theoretical expectation here amplifier model (‚standard‘ parameters) perturbative regime saturation speed limit  1.5 µm/ns semi-quantitative agreement fortuitous (at present stage) Tissoni et al., INFM, unpublished

Bandwidth and bit rate velocity: 2 µm / ns CS diameter typically 10 µm  a local detector would see a signal of length 10 µm/(2 µm/ns) = 5 ns  bit rate 100 Mbit/s not great, but certainly a start limit: time constant of medium (carriers) typically assumed to be about 1 ns  d-response some ns 10 µm / 3 ns = 3.3 µm /ns  probably origin of numerically observed saturation behaviour even this makes sense with experiment (HWHM  3ns)

„Slow media“: Non-instantaneous Kerr cavity g  0.01  semiconductor velocity determined by response time saturation for instantaneous medium  faster medium will speed up response ! log (velocity / gradient) slope 1 response time can be engineered by growers: low-temperature growth, ion implantation, QW close to surface, quantum dots need to pay for it by increase of power log (g) A. Scroggie, USTRAT, unpublished (1D, perturbation analysis)

„Conventional“ approaches to slow light modification of group velocity in vicinity of a resonance two-level atom electro-magnetically induced transparency cavity resonance .... large effect needs steep slope, narrow resonance bandwidth limited by absorption high-order dispersion Hau et al., Nature 397, 594 (1999)

Intermediate résumé: CS-based delay line it works ! drifting CS are a quite different approach to slow light  pros and cons should be assessed potentially very large delays potentially simple combination with processing/routing (bistability!) in a cavity soliton laser there are (at least) two other twists relaxation oscillations are faster than carrier decay time and modulation frequency of modern SC lasers is certainly faster (at least 10 Gbit/s)  drift velocity might not be limited simply by material response time possibility of fast spontaneous motion N. N. Rosanov, e.g. Spatial hysteresis and optical patterns, Springer, 2002

Future and supporting activities lot‘s of things to be done theory: saturation behaviour of velocity material response t N = - A N – B N2 – C N3 +... new features in lasing regime fabrication: homogeneity, purpose made devices experiment: control gradients, improve ignition, larger distances ... ongoing at INLN: set-up and characterization of liquid-crystal two-dimensional spatial light modulator other activities in first year to enhance delay-line activities speed-up of laser simulations by improved adiabatic elimination technique optimization of current injection incoherent switching of CS

Optimization of current injection standard broad-area top-emitting VCSEL (below threshold) IL max 100µm A IL min B strongly inhomogeneous (1:2) carrier density due to current crowding at device perimeter (oxide aperture) corroborated by device simulations Fontaine et al., LAAS, unpublished

Results: VCSEL with ITO layer ITO layer (250 nm) Anode current & carrier density profiles in MQW P-DBR Oxide Cavity + MQW N-DBR Cathode reduction of inhomogeneity to about a factor of 1.6 taken alone, not sufficient Fontaine et al., LAAS, unpublished

Results: Patterned injection Cavity + MQW Anode Cathode P-DBR N-DBR Localized oxides reduction of inhomogeneity to about a factor of 1.06

Design for patterned injection patterning with oxide apertures and/or metal mesh Dimensions of the mesh: d = 4µm e = 2µm d = 5µm e = 3µm d = 20µm e=5µm soliton should average over mesh coupled solitons „discrete solitons“ ?! d e 100µm Camps et al., LAAS, unpublished

Design for delay line create quasi-1D situation for all-optical delay line: rectangular VCSEL channeling minimize electrical and optical power requirements dimensions: d = 5µm, 10µm, 20µm L = 320µm L d explore possibility of electrically controlled gradient Camps et al., LAAS, unpublished

Towards incoherent manipulation of CS previous CS manipulation in SC amplifier system relied on phase coherence switch-on: constructive interference between holding beam and writing beam switch-off: destructive interference between holding beam and writing beam phase-sensitive interactions do not tend to be robust motivation to study feasibility of incoherent control! especially for a cavity soliton laser ! feasibility demonstrated by simulations in laser with saturable absorber first experiments on VEGSEL experiments on optically pumped amplifier

Setup: Optically pumped amplifier Barbay et al., Opt. Lett. 31, 1504 (2006)

demonstration of incoherent switch-on quite fast switch but long delay thermal effects ? on demonstration of incoherent switch-on Barbay et al., Opt. Lett. 31, 1504 (2006)

demonstration of incoherent switch-off same pulse can switch CS off at slightly different bias level quite fast switch demonstration of incoherent switch-off full control would give robust optical flip-flop Barbay et al., Opt. Lett. 31, 1504 (2006)

Summary experimental demonstration of embryonic all-optical delay line speed about 2 µm/ns in accordance with expectation milestone fully reached novel approach to slow light well-placed among other slow light systems (large delays) further achievements: incoherent switching, improved device design future work directed on better understanding and optimization on good track for robust, phase-insensitive soliton-based delay-line