Alexander, Koen

Asselberghs, I

Brems, S

Gorza, SP

Hu, Yingtao

Huyghebaert, C

Kuyken, Bart

Pantouvaki, M

Van Campenhout, J

Van Thourhout, Dries

English

Ghent University Academic Bibliography

Electrically Controllable Saturable Absorption inHybrid Graphene-Silicon WaveguidesKoen Alexander1,2, Yingtao Hu1,2, Marianna Pantouvaki3, Steven Brems3, IngeAsselberghs3, Simon-Pierre Gorza4, Cedric Huyghebaert3, Joris Van Campenhout3,Bart Kuyken1,2, Dries Van Thourhout1,21Photonics Research Group, INTEC, Ghent University-IMEC, Ghent B-9000, Belgium2Center for Nano-and Biophotonics (NB-Photonics), Ghent University, Gent B-9000, Belgium3IMEC, Kapeldreef 75, B-3001 Leuven, Belgium4Service OPERA-photonique, Universite´ libre de Bruxelles (U.L.B.), B-1050 Bruxelles, BelgiumKoen.Alexander@intec.ugent.beAbstract: Electrical tunability of saturable absorption is demonstrated in a graphene/SOIhybrid waveguide. The saturation modulation depth is tunable between 0 and 2.2 dB (∼0-40%decrease of absorption at saturation), with saturation powers between ∼1.25-2.5 W.© 2014 Optical Society of AmericaOCIS codes: (130.0130) Integrated Optics, (190.0190) Nonlinear Optics, (320.5390) Picosecond phenomena1. IntroductionGraphene has extraordinary electrical and optical properties, originating from its two-dimensional crystal structureand its linear and gapless band structure [1, 2]. One of these well known optical properties is saturable absorption:band filling at high optical powers causes the absorption of graphene to decrease [3]. This phenomenon is exploitedin spontaneously mode-locked lasers, taking advantage of the ultra-fast carrier dynamics and broadband absorptionof graphene. Early studies have demonstrated mode-locked fiber lasers using an exfoliated monolayer of graphene ona fiber end [3], or a saturable absorber made of graphene flakes dispersed in a polymer [4]. More recently, in-planesaturable absorption of graphene on a silicon waveguide has been demonstrated [5]. This way, the light-grapheneinteraction can be tailored by changing the waveguide length, and the strong optical confinement and high integrationdensity of silicon-on-insulator (SOI) platforms can lead to compact and cost-efficient integrated devices.In this work, a graphene-oxide-silicon capacitor structure on top of a silicon rib waveguide is created, so that theFermi level of graphene can be electrostatically tuned. The voltage dependence of the nonlinear absorption of thestructure is investigated and tunable saturable absorption is demonstrated. Since the saturable absorption modulationdepth can strongly influence the pulse duration of mode-locked lasers [6], the concepts demonstrated in this workcould eventually lead to integrated spontaneously mode-locked lasers with an electrically tunable pulse duration.Input9peak9power9LdBmc20 25 30 35 40Transmission9LdBc-18-17-16-15-14-13-12-11+99V+39V+09V-39V-69V-99VVoltage9LVc-10 -5 0 5 10Sat.9abs.9modulation9depth9LdBc012Low-power9transmission9LdBc-15-10Lac LbcLccV- +Au/Ti Au/CrGrapheneThermal OxideSiSiO22 µm 2 µmn++ nn+0.3 µm0.659µmFig. 1. (a) Transmission of the hybrid graphene/Si waveguide as a function of input peak power(pulse duration: ∼8 ps, wavelength: 1550 nm). (b) Schematic cross section of the hybrid monolayergraphene-Si waveguide structure (doping levels n: 1018 cm−3, n+: 1019 cm−3, n++: 1020 cm−3). (c)Low-power transmission and saturable absorption modulation depth for different voltages.2. Fabrication and Experimental resultsThe cross section of the structure is shown in figure 1(b). A planarized Si rib waveguide (650 nm wide) confinesthe optical mode. Phosphorous ion implantation was used to lower the electrical resistance of the Si (while keepingthe optical loss acceptable, hence the different doping levels) and a 5 nm thick gate oxide was grown on top of thewaveguide. The graphene was grown by chemical vapor deposition (CVD) on a Si/SiO2/Cu substrate and transferredusing an elastomer stamp. Both the graphene and the Si waveguide are contacted and by changing the voltage overthe graphene/oxide/Si capacitor structure the Fermi level of the graphene can be tuned. These structures can also beused as electro-absorption modulators and more details about this and their fabrication can be found in ref. [7]. Thedevice under study consists of a 150 µm long graphene/Si hybrid waveguide. The light is coupled in and out throughgrating couplers. The couplers and waveguides are optimized for TM light, as the evanescent fields that interact withthe graphene are stronger for the TM mode. The coupling loss of the gratings is about 9-10 dB. A PriTel, Inc. FFL-1550-10 laser is used to generate ∼8 ps pulses with a repetition rate of 10 MHz with a wavelength of 1550 nm. Thesepulses are amplified using a Pritel, Inc. HPP-PMFA-20 high peak power Er-doped fiber amplifier and an additionalattenuator (JDS FiTel, HA9 Optical attenuator) is used to change the pulse power without significantly changing thepulse shape. Using picosecond pulses and a relatively low repetition rate, the influence of free carriers generated bytwo-photon absorption in Si is minimized.The measured transmission as a function of the input peak power is plotted in figure 1(a), for different voltages. Figure1(c) shows the low-power transmission and the saturable absorption modulation depth, which is the difference betweenthe peak transmission and the the low-power transmission. At low voltages, the low-power absorption (transmission)is relatively high (low) but shows strong saturation when the optical power increases. This corresponds to a Fermilevel close to zero: Pauli blocking is unlikely at low power due to the low electron and hole occupation probabilities atthe relevant electronic states (at energies ±h¯ω/2). At higher input powers these occupation probabilities can signifi-cantly increase, leading to absorption saturation. At high voltages, the Fermi level becomes increasingly larger and theoccupation probability at h¯ω/2 gradually increases, effectively reducing the number of states available for absorptionand thus reducing (increasing) the low-power absorption (transmission). At higher optical powers the changes dueto further band-filling will be less dramatic, and the saturable absorption modulation depth will drop. Note that theFermi level only reaches zero if the voltage is about -9 V, this is due to spontaneous atmospheric doping of the sample.Furthermore a relatively steep drop in transmission can be observed in the limit of high power. This is caused by strongtwo-photon absorption in Si. The saturation power Ps (optical power at which the absorption change has reached halfof its final value) can be estimated to be in the range 31-34 dBm (∼ 1.25-2.5 W) for all measurements.3. ConclusionIn this paper, electrically tunable saturable absorption in a hybrid graphene/silicon waveguide is to our knowledgefor the first time experimentally demonstrated. By tuning the voltage, the saturable absorption modulation depth wastuned between 0 and 2.2 dB (corresponding to a 40% decrease in absorption at saturation). Saturation powers rangedbetween ∼1.25-2.5 W. This principle can potentially be used for integrated spontaneously mode-locked lasers with anelectrically tunable pulse duration.References1. A. K. Geim, K. S. Novoselov, “The rise of graphene,” Nature materials, vol. 6, no. 3, pp. 183–191, 2007.2. F. Bonaccorso, Z. Sun, T. Hasan, and A. Ferrari, “Graphene photonics and optoelectronics,” Nature Photonics,vol. 4, no. 9, pp. 611–622, 2010.3. Q. Bao, H. Zhang, Y. Wang, Z. Ni, Y. Yan, Z. X. Shen, K. P. Loh, D. Y. Tang, “Atomic-layer graphene as asaturable absorber for ultrafast pulsed lasers,” Adv. Funct. Mater., vol. 19, no. 19, 3077–3083 (2009).4. Z. Sun, T. Hasan, F. Torrisi, D. Popa, G. Privitera, F. Wang, F. Bonaccorso, D. M. Basko, A. C. Ferrari,“Graphene mode-locked ultrafast laser,” ACS Nano, vol. 4, no. 2, 803–810, (2010).5. Z. Shi, C. Y. Wong, Z. Cheng, K. Xu, H. K. Tsang, “In-plane saturable absorption of graphene on siliconwaveguides,” in Lasers and Electro-Optics Pacific Rim (CLEO-PR), 1–2 (2013).6. X. He, D. Wang, Z.-B. Liu, “Pulse-width tuning in a passively mode-locked fiber laser with graphene saturableabsorber,” Photonics Technology Letters, IEEE, vol. 26, 360–363 (2014).7. Y. T. Hu, M. Pantouvaki, S. Brems, I. Asselberghs, C. Huyghebaert, M. Geisler, C. Alessandri, R. Baets, P. Ab-sil, D. Van Thourhout, and J. Van Campenhout, “Broadband 10gb/s graphene electro-absorption modulator onsilicon for chip-level optical interconnects,” Electron Devices Meeting (IEDM), 2014 IEEE International.

Electrically controllable saturable absorption in hybrid graphene-silicon waveguides

Koen Alexander

Yingtao Hu

Marianna Pantouvaki

Steven Brems

Inge Asselberghs

Simon-Pierre Gorza

Cedric Huyghebaert

Joris Van Campenhout

Bart Kuyken

Dries Van Thourhout

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Electrically Controllable Saturable Absorption in Hybrid Graphene-Silicon Waveguides

https://biblio.ugent.be/publication/6955973/file/6956038.pdf

Electrically controllable saturable absorption in hybrid graphene-silicon waveguides

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