Micro bimorph cantilever switches for tuning integrated optical
systems
S.M. Chakkalakkal Abdulla, L.J. Kauppinen, M. Dijkstra, M.J. de Boer, R.M. de Ridder and Gijs J.M. Krijnen
MESA+ Institute for Nanotechnology, University of Twente, 7500 AE Enschede, The Netherlands. Phone: 053489-4373; fax: 053489-3343; email: s.m.chakkalakkalabdulla@ewi.utwente.nl.
We propose to use a self aligning technology (Figure 1) to integrate micro bimorph cantilevers with tips with respect to e.g. the holes in a photonic band gap (PBG) micro-resonator coupling section in order to perturb its evanescent field [1]. Using a simplified process, we first fabricated bimorph cantilevers on top of silicon, by surface micromachining techniques in which the upper electrode is a thin layer of Chromium on the top of a thick layer of dielectric material which is Silicon Rich Nitride (SiN). The resonance frequencies and pull in voltages of these electrostatically actuated bimorph cantilevers with off-state deflection [2] are analysed and it is found that the higher resonance frequencies come at the price of larger switching voltages (Figure 2-3). This allows for fabrication of relative stiff cantilevers with resonance frequencies in the MHz range to interact with the evanescent field of PBG crystals in which the mechanical elements start to play a role typically with a distance <400 nanometers.
We have also fabricated bimorph cantilevers without tips, integrated on top of various optical systems like ring resonators, photonic crystals and planar waveguides. Analytical and numerical models are developed to predict the resonance frequencies and the pull-in voltages of these switches, including the effect of undercut and validated it with experimental data.
We have observed selective wavelength on/off switching by perturbing the near band edge resonance of a waveguide grating with a 20 µm wide silicon nitride AFM cantilever, without using its tip area (Figure 4). The observed mechanical perturbation allows 15 dB on/off switching of a specific wavelength and a wavelength tuning of approximately 60 pm.
In conclusion, here we describe the technology for fabricating integrated bimorph switches, the optimization studies of the cantilever designs and measurements of mechano-optical interactions using an AFM based cantilever. These optical switches have potential application in the field of telecommunication networks.
Keywords: photonic band gap crystal, bimorph, optical switch, undercut
References
[1] Y. Kanamori, et al, ‘’Photonic crystal switch by inserting nano-crystal defects using MEMS actuator’’, Proceedings of the 2003 IEEE/LEOS International Conference on Optical MEMS Waikoloa, USA, 2003, pp. 107–108.
[2] Chakkalakkal Abdulla, S.M, et al, “Optimised Frequency Range of Active Joints for
Nanometre Range Stroke”, MicroMechanics Europe Workshop 2007, Guimarães, Portugal.
Figure 1. (A) Fabrication flow for self aligned bimorph on top of PBG . (B) Fabricated bimorph of length 80 µm on top of a ring resonator and (C) bimorph of length 60 µm on top of a PBG.
Figure 2. First and second (inset) mode resonance frequency versus the corrected length of the bimorphs (including the effect of undercut), for an electrode thickness of 204.85 nm.
Figure 3. Comparison between measured pull-in voltages and those calculated from the numerical analysis for three different electrode thicknesses. The solid line is presented as a guide line to the eye for values calculated from the numerical analysis and the starred values represents the measured data.
Figure 4. Measured transmission spectra of a grated silicon waveguide with different cantilever locations. 1533 1533.5 1534 1534.5 1535 1535.5 1536 1536.5 1537 -55 -50 -45 -40 -35 -30 -25 no cantilever Cantilever location A Cantilever location B Wavelength [nm] T ra n s m is s io n [ d B m ] 1533 1533.5 1534 1534.5 1535 1535.5 1536 1536.5 1537 -55 -50 -45 -40 -35 -30 -25 no cantilever Cantilever location A Cantilever location B Wavelength [nm] T ra n s m is s io n [ d B m ]
