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Ultrafast all optical switching in AlGaAs photonic crystal waveguides D.M. Szymanski1,2, B.D. Jones1, David O’Brien2, M.S. Skolnick1, A.M. Fox1, T.F.Krauss2 1
Department of Physics and Astronomy, University of Sheffield, UK S3 7RH
[email protected] 2 School of Physics and Astronomy, University of St Andrews, Fife, UK KY16 9SS
Abstract. We have demonstrated all optical switching using photonic crystals integrated into an AlGaAs Mach Zehnder interferometer (MZI). Enhanced phase modulation efficiency together with fast carrier recovery times of 4 ps and high nonlinearities of the AlGaAs material make this design suitable for ultra fast all optical switching applications.
Ultrafast all optical switching Nowadays telecommunication networks rely on electro – optic components that limit the network’s performance. Only full optical control of light by light can meet the demands of increased bandwidth. Optical components are not limited by the RC constants like their electronic counterparts and allow generation of much shorter pulses which allow higher network speeds. However still some work must be done to fulfil all the requirements for optical devices. The most important are low power consumption, high S/N ratio, and high repetition speed. It is very hard to meet all of these criteria at once so the device should be carefully chosen. All optical elements rely on the refractive index change. This optically induced change is limited by the material nonlinearity and the required ʌ phase change entails that the size of the device be in the range of millimetres. Proper device design, material choice and the optical nonlinearity used can significantly increase the device performance. High repetition rates can be achieved using nonresonant optical nonlinearities. The main disadvantage of that approach is the high power required to utilize the full material nonlinearity. Power consumption and the speed can be enhanced by utilizing resonant coherent effects. Coherent effects are not limited by real carrier relaxation. The pulse width of the laser should be shorter than the carriers’ relaxation times, if not, real carrier generation occurs and limits device speed. Even if the laser pulse width is shorter real carrier generation can still mask coherent effects. Carriers are generated then via multiple photon absorption. From a practical point of view it is easier to work with incoherent effects because the device can be operated at low powers. In this experiment we show a way to decrease carrier relaxation time utilizing TPA (two photon absorption).
Experimental methods We have investigated all optical switching using photonic crystals integrated into a GaAs/AlGaAs Mach Zehnder interferometer (MZI). The MZI employs multimode interference (MMI) optical power splitters and photonic crystal waveguides in the functional area of each arm. Optical switching can be achieved by optically pumping the photonic crystal sections of one of the arms causing a carrier induced refractive index change in this arm of the MZI. A compact 3dB 1x2 power splitter operating at 950nm centre wavelength was designed by selecting correct position along MMI’s and waveguides configurations.
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The imaging lenngth of the MMIs was optimised using u FDTD D simulatioon and 3/8L Lʌ was dex of 3.29 was w used duuring simulaation. foundd to get 1x22 power splitting. A reffractive ind This refractive index repreesents the effective e ind dex of the waveguide slab which h was calcuulated separrately. The optical banndwidth off the splitteer is inversely proporttional to thhe square of o the MMII’s width soo 6ȝm widtth was chossen as optim mal for the sake s of devvice perform mance and fabrication f p purposes [11]. As an ouutput waveg guides separration was ddesign to bee 3ȝm hence further deecreased in device d size could causee outputs ovverlapping. Optim mal geomeetry was fouund to be: 6ȝm MMIIs width, 64ȝm 6 MMIIs length, 1.5ȝm 1 widthh of the innput and ouutput waveeguides. S-b bend type waveguides w s were add ded to separrate MZ arm ms of 25ȝm m. Photonic crystaal waveguiddes are the most criticcal part of the all opttical MZ sw witch. Theirr function is i not only decreasing the function nal area of the device but, importtantly increeasing the operation o sppeed of devvice. The ho oles patternned in a sem miconductorr host systeem play thee role of reccombinationn centres an nd hence rellaxation tim me of the ex xcited carriers is reduuced. Samples were faabricated with w three different d tyypes of pho otonic f two tyypes rely on n engineereed defects iin the trian ngular crysttal waveguiides. The first latticce PhC. Row ws of the one and threee holes werre removedd to create W W1 and W3 3 type waveeguides. Thhe third typee utilizes self-collimatiion phenom mena in a squuare PhC laattice. The main advanntage of thiis type of structure s is that the guuiding doess not rely on o the uced into laattice. The fr fraction of etched e photoonic bandgaap, so no deefect need too be introdu holess and thus surface recombination is increaased in thatt type of lattice. The selfcollim mation regime of the sqquare latticee was found d using PWE E and FDTD D method. Lighht propagatioon in the phhotonic crystal is goveerned by its dispersion surface and d it is perpeendicular too it. Non patterned sem miconducto or slab exhiibits circulaar shape off EFC (equii-frequencyy contour) soo propagateed light is diiverged. Diffferent dispersion prop perties of thhe photonicc crystals at a different frequency give rise to differennt EFC con ntours shapes. One cann obtain a square s shappe of EFC contours annd hence coollimation of o the beam m by carefullly choosingg the geometry and freequency opeeration rangge for the square latticce. Square laattice has tw wo possible self-collim mation regionns [2].
Figu ure 1. Disperssion diagram for PhC compprising an AlG GaAs slab pattterned with airr holes in a sq quare latticee (top left). PW WE method was w used with effective refraactive index of o the slab 3.299. EFC contou urs for a first two bannds: band 0 (leeft) and band 1 (right) show w variety of shaape for differeent frequenciees.
Figurre 1 shows EFCs for first f two bannds. First band has a square s shapee of the EF FC for a gam mma – M diirection of the t lattice annd frequenccy in the rannge of 0.20996. In the seecond bandd self-collim mation takes place in gaamma – X direction. d Onne can obtaain the samee selfcollim mation propperties in thhe second baand by simp ple rescalinng and rotatiing the geom metry by 45 4 o. This haas one important conssequence in n fabricationn of self-coollimation based b
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devicce in the neaar infrared region. r Thee rescaling factor f of ¥2 makes the llattice largeer and easieer to fabricaate. FDTD calculation c w perform was med to conffirm proper frequency range for thhe device opperating in the t gamma – X directio on.
Sam mple fabriication an nd characcterisation n Deviices were patterned p o asymmeetric AlGaA on As/GaAs heeterostructuure slab sy ystem. The sample connsists of ann Al0.2Ga0.8As A core lay yer of thicknness d = 4000nm and lower l 500nm. A 200nm 2 thickk SiO2 layer was Al0.6Ga0.4As claadding layerr with thickkness dcl=15 depoosited on thee top of thee heterostruucture and acts a as a haard mask duuring dry etching stepss. Patterns were w writtenn into 400 nm PMMA A electron beam b resist then transfferred into silica maskk via fluorinne – based chemistry c CHF3. The resist r was thhen removed and p oxxide mask trransferred too the semicconductor laayers with C Cl2/Ar chem mistry. the patterned 2 2 This type of sysstem is charracterized by b low index contrast ∆ε=n ∆ n = 1.6. Tha at low ccl co m needs deeeply etchedd holes to decrease d scattering of the light at a the contrrast system bottoom of the hooles. The saample was etched e with an aspect raatio of 8:1 ((depth >1 um u for periood 260 nm, r/a=0.26). Figure 2 shhows integrrated MZ sw witch with photonic crrystal waveeguides in functional fu arrea of each MZ arms.
Figurre 2 Scanning electron micrrograph of thee fabricated MZ M switch (topp left). Cross section of the central c part of MZ switch s (top rigght) and the MMIs M splitter (bottom ( left).
The dynamics of the exccited carrieers were measured m byy pump prrobe techniiques. Expeeriments weere perform med using 130fs 1 pulsees from a frequency f m mode-locked d Ti– Sappphire laser. An A 800nm wavelengthh pump beaam of 1kHz repetition rrate was foccused onto the top surrface of onee of the PhC C regions. The T weakerr probe beaam is transm mitted throuugh the devvice and thee pump indduced transm mission chaange is regiistered by single s channnel Si detector. The main m advanttage of this low repetittion frequenncy is very y high peakk intensity meaning m thee material nonlinearity n can be fullly utilized. The pump beam was focused f to 8ȝm 8 in diam meter givingg a fluence of o 0.1 mJ/cm m2. The maximum change in the transmiission occu urs near zerro delay tim me IJ=0. A rapid decreease and reccovery of thhe transmisssion depend dent on geom metry used is observed d near zero delay (fig.33). This moodulation off the transm mission wass a result off a change in i the real part p of the refractive r inndex induceed by injectted carriers.. Free carrieers are geneerated by thhe two photoon absorptioon in AlGaA As core layeer.
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decay time[ps]
Transmission [a. u.]
7.3ps -60 -40 -20
0
20 40 32ps
60
80 100
0
20
40
delay time [ps]
60
5.5 5.0 4.5 4.0 3.5 0.27
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-60 -40 -20
6.0
selfcolimator w1-a280nm
4ps
w3-a330nm MZ
0.28
0.29
0.30
normalized freq
80 100
Figure 3 Time dependence of the transmission spectra (left).Clearly visible difference of the decay time for different type of waveguides used in active switch area. Right: Relationship between measured decay time and normalized frequency for self-collimator based MZ switch. Decay time was measured for different periods at 920nm wavelength and average pump power 2.25 uW.
The carrier decay time is highly dependent on the geometry of the photonic crystal waveguides used in the experiments. A non patterned MZ switch shows a decay time of 61 ps which is about a factor of 2 faster than in bulk AlGaAs material [3]. Introduction of a photonic crystal in the active area of the switch changes the mean distance of the carriers from the surface hence increasing surface recombination. The self-collimator based MZ switch has a larger fraction of etched holes in comparison to W1 and W3 waveguides, so the recombination of optically excited carriers is further enhanced. A time decay of 4 ps was measured for a self-collimator based MZ switch with a period of 280nm. A decay time has been measured for a range of periods close to the self collimation region. Figure 3 shows a decrease of the decay time with an increase of normalized frequency. This implies that the probe beam is more collimated for normalized frequency 0.304 and undergoes higher modulation by the pump beam. Modulation depth was calculated using: ݉ൌ
ሺିሻ
(1)
ሺାሻ
where Am is a peak-peak value of the modulated beam and A is peak-peak value of the unmodulated beam. The self-collimator based device showed a modulation depth of 34%. The estimated switching energy is 2.82nJ (for average absorbed power 2.25ȝW).
References [1] P.A. Besse, M. Bachmann, H. Melchior, L. B. Soldano, and M. K. Smit, "Optical Bandwidth and Fabrication Tolerances of Multimode Interference Couplers", Journal of Lightwave Technology, vol.12, no.6, June 1994 [2] J. Witzens, M. Loncar, and A. Scherer “Self-Collimation in Planar Photonic Crystals”. IEEE Journal of Selected Topics in Quantum Electronics, Vol. 8, no 6, November/December 2002 [3] M. J. LaGasse, K. K. Anderson, C.A. Wang, H.A Haus and J.G. Fujimoto, “Femtosecond measurements of the nonresonant nonlinear index in AlGaAs,” Appl. Phys. Lett. 56, 417 (1990).
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