Nano-Opto-Electro-Mechanical devices based on Silicon Slot ...

<strong>Nano</strong>-<strong>Opto</strong>-<strong>Electro</strong>-<strong>Mechanical</strong> <strong>devices</strong> <strong>based</strong> onSilicon Slot-Waveguides StructuresVilson R. Almeida 1,21 – Instituto de Estudos Avancados, IEAv2 – Instituto Tecnologico de Aeronautica, ITASao Jose dos Campos – SP, BRAZILe-mail address: vilson@ieav.cta.brRoberto R. PanepucciFlorida International University, FIUMiami, FL, USAe-mail address: roberto.panepucci@fiu.eduAbstract — We present device applications for <strong>Nano</strong>-<strong>Opto</strong>-<strong>Electro</strong>-<strong>Mechanical</strong> System (NOEMS) structures <strong>based</strong> on theevanescent-wave bonding acting on silicon slot-waveguidestructures. Useful all-optical and electrooptical functionalities areproposed, including: phase modulation, polarization modedispersion, near-field probing and reconfigurable optical delay.Keywords - silicon photonics; slot-waveguide; NOEMS;evanescent wave-bonding; integrated optics.I. INTRODUCTIONSlot-waveguides are channel-type high-index contrastoptical structures that present a slot filled with low-indexmaterial or even void [1-2], as shown in Fig 1; its uniqueoptical properties has arisen much recent interest from severalresearch groups that have been widening its range ofapplications, since its initial conception. Its optical propertiesstrongly depend on the slot width as well as on the refractiveindex contrast, which enables it for a wide variety ofapplications for slot-waveguide <strong>based</strong> <strong>devices</strong>.Povinelli et al. have shown that slot-waveguides experienceforces between its two half-sides, which are due to the dipolemoments induced by the guided light intensity [3]; such forcesmay be either attractive or repulsive, depending on the phase ofthe dipoles, dictated by the optical properties and thegeometrical parameters of the waveguide. A suspended sectionof a slot-waveguide deprived from cladding thus mayexperience significant mechanical deformation. The opticalforces have led researchers to pursue several novel and usefulmechanisms and applications [4-8], <strong>based</strong> on all-optical and/orelectrooptical control approaches; some applications may beenabled by turning the slot-waveguide into a <strong>Nano</strong>-<strong>Opto</strong>-<strong>Electro</strong>-<strong>Mechanical</strong> System (NOEMS) device, <strong>based</strong> on an allopticalpump-probe method. The evanescent-wave bondingbetween optical waveguides can be calculated either by meansof the Maxwell Stress Tensor or directly from the dispersiondiagram of the waveguide [3], and expressed as the opticalforce (F) given bydU U dωF = − = − , (1)dξω dξkkwhere U represents the optical energy traveling through theslot-waveguide, ξ represents the generalized transverse spatialvariable (usually associated with the transverse axis thatcrosses the two half-sides of the slot), ω is angular opticalfrequency, and k is the associated guided wavenumber.Unless otherwise stated, the cross-section parameters forthe slot-waveguide used in this work are as shown in Fig. 1(a).Ref. 3 shows that the resonance frequency for a 30-µm longcantilever stays around the MHz range. However, for a 1-µmlong (L) beam slot-waveguide, our theoretical predictionsindicate that mechanical resonance can reach up to 2.2 GHz fora fixed (doubly-clamped) beam seen in Fig. 1(b), and up to 340MHz for a cantilever (simply-clamped) beam seen in Fig. 1(c).(b)(c)(a)Lslot widthSiO 2L240 nmSiO 2SisubstrateSiO 2Sisubstrate280 nmFigure 1. (a) Typical slot-waveguide cross-section. (b) Slot-waveguidefixed-beam. (c) Slot-waveguide cantilever-beam.978-1-4244-5357-3/09/$26.00©2009IEEE 560

compensation ranging from negative to positive values, withexcursion of up 300 fs/(nm.cm), are achievable using typicalstructure dimensions.Figure 8. Schematic of a Mach-Zehnder interferometer for NOEMSamplitude modulation.3D-FDTD simulations were used in order to find thedependence of the optical phase (relative to a straight 80-nmwide slot waveguide) accumulated as light travels through thecontrolled slot arm, which is represented by Fig. 8. Notice thata phase modulation of π rad is needed for 100% modulationdepth on a MZI; however, much lower values are sufficient foran efficient modulation schemesSlot-waveguide NOEMS <strong>based</strong> on cantilever-beamstructures may also find useful applications as nano-tweezersor as NSOM tips; simulation results are shown in Fig. 9, where3D-FDTD simulations show strong (higher than 20%)modulation depth for both transmittive and reflective opticalcomponents from such structures (starting off an unperturbed80-nm slot width).Optical Property (%)10080604020d iniLd minTransmissionBack ReflectionLosses00 20 40 60 80 100 120Slot Width (nm)Figure 9. Optical properties of a deformed cantilever-beam slot-waveguide.Additionally, our calculations predict that PMDcompensation, on the order of 20 ps/cm, is possible; also, GVDIII.CONCLUSIONSeveral optical functionalities of Si/SiO 2 NOEMS <strong>based</strong> onslot-waveguides may be attained by means of the optomechanicallyinduced forces driven at relatively low opticalpowers. This effect is a promising area of research that maylead to novel applications in the field of silicon photonics.Therefore, several interesting applications for slot-waveguidesmay be attained by essentially controlling the slot width bymeans of all-optical modulation.[1] V. R. Almeida, Q. Xu, C. A. Barrios, and M. Lipson, “Guiding andconfining light in void nanostructure”, Opt. Lett., vol. 29, pp. 1209-1211, 2004.[2] Q. Xu, V. R. Almeida, R. R. Panepucci, and M. Lipson, “Experimentaldemonstration of guiding and confining light in nanometer-size lowrefractiveindex material”, Opt. Lett., vol. 29, pp. 1626-1628, 2004.[3] M. L. Povinelli et. al., "Evanescent-Wave Bonding between OpticalWaveguides", Opt. Lett., vol. 30, pp. 3042-3044, 2005.[4] V. R. Almeida and R. R. Panepucci, “Silicon Slot-Waveguide asNOEMS Photonic Platform”, in Proceedings of the Frontiers in Optics2006 Conference, FThK2, 2006.[5] M. Eichenfield, C. P. Michael, R. Perahia, and O. Painter, “Actuation ofmicrooptomechanical systems via cavity-enhanced optical dipoleforces”, Nature Photonics, v. 1, n. 7, pp. 416–422, 2007.[6] M. Li et al., “Harnessing optical forces in integrated photonic circuits”,Nature, v. 456, n. 7221, pp. 480-484, 2008.[7] W. H. P. Pernice, M. Li, and H. X. Tang, “Theoretical investigation ofthe transverse optical force between a silicon nanowire waveguide and asubstrate”, Optics Express, v. 17, n. 3, pp. 1806-1816, 2009.[8] M. Li, W. H. P. Pernice and H. X. Tang, “Tunable bipolar opticalinteractions between guided lightwaves”, Nature Photonics, v. 3, n. 8,pp. 464-468, 2009.[9] N. Lobontiu and E. Garcia, Mechanics of MicroelectromechanicalSystems, Kluwer Academic Publishers, Ch. 5, 2005.[10] X. Li, T. Ono, Y. Wang and M. Esashi, “Ultrathin single-crystallinesiliconcantilever resonators: Fabrication technology and significantspecimen size effect on Young’s modulus”, Appl. Phys. Lett., vol. 83,pp. 3081-3083, 2003.[11] R. R. Panepucci, V. R. Almeida, X. Wang, A. Lavrenov, and A. M. P.Fievre, “Sensing using <strong>Nano</strong>-<strong>Opto</strong>-<strong>Electro</strong>mechanical Systems(NOEMS)”, in 2006 Emerging Technology Workshop on<strong>Nano</strong>technologies for Microelectronics, 2006.[12] V. R. Almeida, Q. Xu, and M. Lipson, “Ultrafast integratedsemiconductor optical modulator <strong>based</strong> on the plasma-dispersion effect”,Optics Letters, vol. 30, pp. 2403-2405, 2005.2009 SBMO/IEEE MTT-S International Microwave & <strong>Opto</strong>electronics Conference (IMOC 2009) 563