Ultra-fast nonlinear response around 1.5 μm in 2D AlGaAs/AlOx photonic crystal

We report the fast switching capabilities of a two-dimensional Al0.3Ga0.7As photonic crystal slab around 1.5 μm. The slab is supported by an AlOx low-index thick layer that plays the role of an efficient heat sink. By pumping at 0.8 μm in the

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  Appl. Phys. B 81, 333–336 (2005) AppliedPhysicsB DOI: 10.1007/s00340-005-1818-1  Lasers and Optics A. M. YACOMOTTIF. RAINERIG. VECCHII. SAGNESM. STRASSNERL. LE GRATIETR. RAJA. LEVENSON ✉ Ultra-fast nonlinear response around1.5 µ  m in 2D AlGaAs/AlOx photonic crystal Laboratoire de Photonique et de Nanostructures (CNRS UPR 20), Route de Nozay, 91460 Marcoussis, France Received: 7 February 2005Published online: 15 July 2005  •  © Springer-Verlag 2005 ABSTRACT  We report the fast switching capabilities of atwo-dimensional Al 0 . 3 Ga 0 . 7 As photonic crystal slab around1.5 µ  m. The slab is supported by an AlOx low-index thicklayer that plays the role of an efficient heat sink. By pump-ing at 0.8 µ  m in the absorption of the Al 0 . 3 Ga 0 . 7 As quantumwells, the optical response is modified in the transparency re-gion: a 200% change in the reflectivity is obtained with a totalresponse time of 8ps. PACS  42.65.Re; 42.70.Qs 1 Introduction In the last few years, two-dimensional photoniccrystal(2DPC)slabshaveshowntheircapabilitytocontrolthepropagationoftheelectromagneticfield[1].Mostapplications concentrate on in-plane or guided operation, though leakymodes have been the object of wide investigations due totheir role in losses [2]. Indeed, the coupling out of the guidein the out-of-plane direction is a major preoccupation in PCs.However,anumberofstudieshavealsoshownthatbyworkingat normal incidence, the slowing down of the light due to theflattening of the photonic bands around the    point (wavevector equal to zero) [3] can be profitably used to obtain very sharp resonances. These resonances result from the couplingof the radiative modes into these low-group-velocity modes.They attest to the enhancement of the optical mode densitythat can be used to obtain a laser effect [4] or to increase nonlinear interactions [5, 6]. For both laser and nonlinear  optical applications, it is of primary importance to fabricate2D PCs made of active or nonlinear materials that can bear high internal optical powers. In this context, the use of a heatdissipating low index layer as a support for the 2D PC ispreferred to the usual membrane suspended on air approach.On another front, the attainment of technological pattern-ingaccuracybyelectroniclithographyhasbroughtsignalhan-dling down to a few nanometers precision; but this still doesnot fulfil the requirement for certain applications in advancedoptical communication systems, which is less than 1nm inprecision in order to process signals efficiently. One way outwould be to actively manipulate the photonic modes of PCs ✉ Fax: + 33-1-69-63-6006, E-mail: ariel.levenson@lpn.cnrs.fr  through an external stimulus, in order to adapt their opticalresponses to specific needs. In general, the optical proper-ties of PCs are governed by the thickness, the filling factor,and the refractive indices of the constitutive materials. Theseparameters determine the spectral position, the width of thephotonic band gaps, and the transmission resonances. Thereare several methods which exist to tune the PC resonances[7], such as temperature [8], mechanical, and infiltration of  the PC hole pattern [9]. These methods are compatible with applications which do not require high-speed signal process-ing. Alternatively, by acting on the parameter which can bechanged by an external electromagnetic field, namely the re-fractive index, through photocarrier injection [10] in 2D PCs,the optical characteristics can be tailored for fast processing.Here we explore the possibility to perform ultra-fast con-trol of a photonic mode and possible switching operation byusing the interesting material system AlGaAs/AlOx. The useof AlOx as low-index layer was initially posed as a seriouscandidate in preference to air because of the mechanical sta-bility it offers; it has the additional advantage as an improvedheat dissipating cladding layer as compared to a classical air-bridged membrane. 2 Sample description We fabricated a 2D PC with a triangular latticeof   ∼ 1 . 5- µ  m-deep circular air holes drilled in a high indexcontrast Al 0 . 3 Ga 0 . 7 As/AlOx waveguide. The Al 0 . 3 Ga 0 . 7 Aslayer incorporates four 10-nm-thick GaAs quantum wells.This lattice was patterned through a nitride mask into thesemiconductor by electron-beam lithography followed byreactive ion etching (RIE). In the RIE of the semiconductor heterostructure,bybalancingtheratiooftheSiCl 4  andO 2 ,theconditions were optimized to obtain highly anisotropic etch-ing. The AlOx layer was then obtained by steam oxidation of Al 0 . 9 Ga 0 . 1 As. In order to render possible lateral oxidation, wechoose to place the structure on mesas (100 µ  m  ×  100 µ  m)situated in ‘swimming pools’ as shown in Fig. 1. The oxi- dation step is indeed a very delicate stage in the fabricationof the sample. When all conditions (temperature, humidity)are not fulfilled, the AlOx layer blows up and becomes dustand the structure may be completely destroyed as illustratedin Fig. 2. In Fig. 3 we show a scanning electron micrograph (SEM) picture of a structure that we studied in this paper obtained by optimizing all the fabrication parameters.  334 Applied Physics B – Lasers and Optics FIGURE 1  Schematic view of the ‘swimming pool’ used for the oxidationof the 2D PCs FIGURE 2  Photograph obtained by scanning electronic microscope of astructure obtained under non-optimized oxidation FIGURE 3  ScanningelectronmicrographofthestudiedAl0.3GaAs/AlOx2DPC.Anoverallviewisgivenin( a ). b Close-upoftheedgeofthestructure FIGURE 4  Photonic band structure of the triangular 2D PC. The  arrow indicates the low group velocity mode located around 1490nm The Al 0 . 3 Ga 0 . 7 As and AlOx layer thicknesses (respec-tively240nmand1 µ  m)werechosentoensureagoodconfine-ment in the third (out-of-plane) direction. The PC period andair-filling factor are respectively  a  = 810nm and  f   = 30%.The results of a plane-wave 2D calculation of the photonicband structure [11] taking into account the vertical confine- ment (effective index) predict a slow group velocity modearound 1490nm at the    point, as indicated by the arrow inFig. 4. 3 Experiments Initially, the sample is characterized at room tem-perature by linear reflectivity measurements using 150-fspulses from an optical parametric oscillator tunable around1550nm. The repetition rate of the source is 80MHz. Lightis focused down to a 10- µ  m-diameter spot via a microscopeobjective and is incident normal to the 2D PC surface. Thesample is mounted on a precision micropositioner that opti-mally couples the light into the structure. Indeed, high pre-cision is essential as it ensures an accurate characterizationof the patterned area, which is only 100 µ  m × 100 µ  m. Thereflectedsignalisthencollectedthroughthesamemicroscopeobjective and spectrally analyzed by focusing it onto a single-mode fiber (SMF) connected to an optical spectrum analyzer.A gold mirror is stuck next to the sample in order to obtaina reflection reference. The corrected reflectivity is then cal-culated by dividing the spectra of the pulses reflected by theslab by the spectra of the pulses reflected by the gold mirror.The nonlinear properties are explored by pump and probeexperiments performed at room temperature (Fig. 5). 120-fs FIGURE 5  Schematics of the experimental set-up. OPO: optical paramet-ric oscillator, PBS: polarizing beam splitter, SMF: single-mode fiber, OSA:optical spectrum analyzer   YACOMOTTI  et al. Ultra-fast nonlinear response around 1.5 µ  m in 2D AlGaAs/AlOx photonic crystal 335 FIGURE 6  Reflectivityspectrafornormalincidenceofthepumpandprobebeams onto the 2D periodicity.  Thin line : linear spectrum.  Dotted line : pumparriving after probe.  Thick line : pump arriving before probe pulses at 810nm from a Ti:sapphire laser are used to pumpvarious levels of carrier population and, as a consequence, tocreate different carrier densities in the AlGaAs slab. At thesame time, the spectral response of the PC is probed around1500nm using 150-fs pulses from an optical parametric os-cillator (OPO). Both the pump and probe beams are focuseddown to 5- µ  m spot diameter. The retro-reflected probe is col-lected and monitored using an optical spectrum analyzer. Thetemporally synchronized pump and probe are at normal in-cidence to the PC surface. The synchronization of the twopulses is done accurately by observing the up-converted sig-nal when the two beams are coincident at a nonlinear crystal.An optical delay on one of the beams (the probe) is used for varying the time delay between the two pulses. 4 Results and analysis of the nonlinear response In order to study the nonlinear response of the2D PC in detail, three pumping mean intensities were used:26.7kW/cm 2 , 16.6kW/cm 2 , and 5kW/cm 2 . These relativelylow mean intensities correspond to high peak intensities of  FIGURE 7  Dynamical response of the reflectivity for differentpumping intensities: 26.7kW/cm 2 ( squares ), 16.6kW/cm 2 ( circles ),and5kW/cm 2 .Theoriginofthetimeaxisissetatpump–probecoinci-dencebeforetheobjective;coincidenceofpulsesonthesampleoccursat  t   = 0 . 2.Thereflectivity changeis calculated at λ ref   = 1491 . 5nm,close to the minimum of the linear reflectivity. The  inset   is a close-upof the dynamics around   t   = 0 and includes the up-converted signalobtained before the objective, showing the pump–probe coincidence the order of 1GW/cm 2 , and low energy fluxes of the order of 0.1mJ/cm 2 , , for 100-fs-long pumping pulses.The linear resonance is represented in Fig. 6 as a thin line,whichwasobtainedintheabsenceofpumping.Asthepumpisturned on, a spectral shift of the sharp resonance is observed,analogous to that reported in [12, 13]. Before the arrival of  the pumping pulse the probed resonance is red shifted withrespect to the linear resonance (dashed line) while, after thepumping pulse arrival, the resonance shifts towards the blue(full line). The relative delay between pump and probe pulsesis defined as   t   = t  probe − t  pump . The coincidence of pumpand probe pulses was measured before the objective placed infrontof the sample bymeans of anup-conversion experiment.The up-converted intensity is shown in the inset of Fig. 7.Pump and probe pulses are coincident at   t   = 0 before theobjective. We observed that the objective introduces an ex-tra delay of    t  obj  = 0 . 2ps. As a result, the pump and probedelay on the sample is   t  samp  =  t   −  t  obj , i.e. the pulsesare coincident on the sample (  t  samp  = 0) for    t   = 0 . 2 inFig. 6. When the delay is   t   < − 0 . 85ps, or    t   >  20ps, thereflectivity spectrum is red shifted with respect to the linear resonance (seedotted linein Fig.6).Thisred shiftdepends on the mean pumping power. Since the femtosecond laser has ahigh repetition rate (80MHz) with respect to the typical ther-mal time scale (1MHz), and since the shift does not dependon the delay between pump and probe pulses for large |  t  | ,we can attribute the red shift to thermal srcin.A very different scenario occurs when   t   is positive andless than 15–20ps. In these conditions a blue shift (withrespect to the thermal ‘offset’) is observed. This blue shiftstrongly depends on   t   and pumping power. In particular, itattains a maximum around   t   ∼ 0 . 3ps, for which the reflec-tivity spectrum is blue shifted by about 1.5nm with respectto the linear resonance (see thick line in Fig. 6). This effect can be explained as an electronic blue shift due to the changeof refractive index induced by the pump (see Refs. [12, 13]). In order to study the temporal features of this effect we mea-sure the change of reflectivity at a given wavelength, namely   R  ≡ [  R ( λ ref  ) −  R 0 ( λ ref  )], where  λ ref   is the reference wave-length,  R  is the reflectivity in the presence of the pump (for   336 Applied Physics B – Lasers and Optics a given   t  ), and  R 0  is the linear reflectivity. We have chosen λ ref   = 1491 . 5nm, close to the minimum in  R 0 .    R /  R 0  as afunction of    t  , at the three pumping intensities, is shown inFig. 7. The electronic effect is as fast as 8ps for the complete recovery of the linear reflectivity. Note that negative    R  indi-cates that the electronic effect is not sufficient to compensatethe thermal effect. However, it is worth pointing out that theelectronic effect considered from the thermal offset, gives achange of reflectivity of the order of 200% for 26.7kW/cm 2 .Let us come back to the short response time we observe. Itisindeedshortconsideringthattheprocessresponsiblefortheshift is governed by carrier lifetime. In GaAs quantum wellselectronic lifetimes are of the order of a few nanoseconds.But, the fact of ‘drilling’ holes in the material has the ef-fect of creating centers of recombination, which considerablydiminishes the lifetime. Usually, this constitutes a drawbacksince it inhibits amplification and laser operation. But here itprovides us with the means of obtaining ultra-fast operation.The attempt to diminish the lifetime by creating centers of recombination is by no means new; for instance, bombardingGaAs with high-energy ions was used for just this end suc-cessfully, as well as low-temperature growth. In an AlGaAs2D PC this just happens to be a side benefit as the holes arethe very essence of the structure! 5 Conclusion In conclusion, we designed, fabricated, and oper-ated a defect-free 2D PC with the view to obtaining ultra-fastresponse. We demonstrated that a low group velocity groupmode is spectrally tuned around 1490nm by optical carrier injection at 810nm. The non-radiative carrier recombinationdue to the presence of holes in the AlGaAs material systemcontributes to significantly reducing the carrier lifetime. Theconsequence is an ultra-fast response for the wavelength tun-ing that can be advantageously used for optical switching. Wehave demonstrated experimentally total response times as fastas 8ps with high reflectivity contrast ratios. REFERENCES 1 C.M.Soukoulis(ed.),in  ProceedingsoftheNATOASIPhotonicCrystalsand Light Localization in the 21st Century  (Kluwer, Dordrecht, 2001),pp. 1812 H. Benisty, D. Labilloy, C. Weidbuch, C.J.M. Smith, T. F. Krauss,D. Cassagne, A. B´eraud, C. Jouanin, Appl. Phys. Lett.  76 , 532(2000)3 K. Sakoda, Opt. Express  4 , 167 (1999)4 M. Imada, S. Noda, A. Chutinan, T. Tokuda, Appl. Phys. Lett.  75 , 316(1999)5 M.G. Banaee, A.R. Cowan, J.F. Young, J. Opt. Soc. Am. 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