Silicon-On-Insulator ring resonators can be widely tuned with a cladding layer of Liquid Crystals. Reorientation of the director with an electric field results in a tuning range of more than 30nm

Baets, Roel

Beeckman, Jeroen

Claes, Tom

De Cort, Wout

Neyts, Kristiaan

English

Wout De Cort

Jeroen Beeckman

Tom Claes

Kristiaan Neyts

Roel Baets

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Widely tunable silicon-on-insulator ring resonators with a liquid crystal cladding

Ghent University Academic Bibliography

Widely Tunable Silicon-On-Insulator RingResonators with a Liquid Crystal CladdingWout De Cort∗†‡, Jeroen Beeckman†‡, Tom Claes∗‡, Kristiaan Neyts†‡ and Roel Baets∗‡∗Photonics Research Group, Dept. Information Technology, Ghent University - IMEC, Belgium†Liquid Crystals and Photonics Group, Dept. Electronics and Information Systems, Ghent University, Belgium‡Center for Nano- and Biophotonics (NB-Photonics), Ghent UniversityEmail: wout.decort@ugent.beAbstract—Silicon-On-Insulator ring resonators can be widelytuned with a cladding layer of Liquid Crystals. Reorientation ofthe director with an electric field results in a tuning range ofmore than 30nm.I. INTRODUCTIONRing resonators are efficient optical filters designed to filterout narrow wavelength bands from a broad spectrum. Thischaracteristic makes them very useful in larger telecommuni-cation components such as (de-)multiplexers. In some applica-tions, e.g. reconfigurable optical networks, it is necessary to beable to adjust the position of the resonances. Another reasonwhy tunablity is desirable is that current fabrication techniquescan not yet guarantee an exact resonance wavelength. Thistunability in ring resonators is often achieved by heating orcarrier injection. These methods provide a considerable butstill limited tuning range. They require a constant currentand thus a constant power consumption. Tuning with a liquidcrystal (LC) cladding layer is another method which is notwidely used but which has been investigated in the past.Nematic LC is a very attractive material for this purposedue to its high anisotropy (∆n ≈ 0.2). For ring resonatorsguiding TE-polarized (Transverse Electric field) light it hasbeen shown that with a top-bottom electrode configuration, theresonances can be shifted towards shorter wavelengths [1]. Thefield generated by electrodes in a finger pattern can shift theresonance towards longer wavelengths [2]. The mechanics oftuning with an LC cladding are becoming clear but the tuningranges achieved are still below 1nm [1], [2], [3] or requirecomplicated fabrication techniques [4]. In this work we showthat TM (Transverse Magnetic field) ring resonators in Silicon-On-Insulator (SOI) can be tuned over a wide range (>30nm)with a straightforward top-bottom electrode configuration.This is to the best of our knowledge the highest tuning rangereported for silicon-based resonators and opens opportunitiesfor widely tunable filters, lasers and detectors.II. OVERVIEW OF THE CELLA. Cell structureThe base of the cell consists of an SOI chip. This materialsystem is widely used in electronics and integrated optics andits fabrication processes are well-known. The waveguides areFig. 1. Schematic overview of a liquid crystal cell.defined in a 220nm thin Si layer which is separated from theSi substrate by an insulating SiO2 layer to prevent opticalleakage [5]. With glue and spacers (6µm diameter) we attacha glass plate to the chip. Near the gap between the glassplate and the chip we deposit a drop of E7 (a commerciallyavailable nematic LC often used in displays) which is suckedinto the cell by capillary forces. On the glass plate there aretwo additional layers. First, a thin Indium Tin Oxide (ITO)layer for electrical contacting. Second, a spincoated alignmentlayer (nylon) to give the LC director a fixed orientation nearthe surface of the glass plate. For easy contacting, electricalwires are soldered to the ITO layer and to the substrate whichacts as the bottom contact.B. Ring resonatorsThe waveguides measure 220nm high and 450nm wide. Thering resonators have a bend radius of 20µm and 4 straightsections of 10µm each. This adds up to a total length of166µm. The coupling section is 4µm long and the gap betweenthe bus waveguide and the resonator is 550nm wide. The buswaveguide is equipped with two grating couplers. These allowlight to be coupled vertically into and out of the waveguide [6].III. WORKING PRINCIPLES AND SIMULATIONSOn SOI surfaces, the LC aligns in a planar fashion. Thedirector is able to follow the structures on the chip downto the nanometer scale [1]. In case of ring resonators, thedirector will remain parallel with the ring waveguide in thearea near the waveguide due to geometrical alignment. TheTM mode travelling in the waveguide has evanescent tailsprotruding from the top and bottom of the waveguide into thecladding layers. As the electric field is mostly polarized alongthe normal to the surface, it will see the ordinary refractive15151525153515450 20 40 60 80 100Resonance Wavelength (nm)Voltage (V)SimulationExperiment3 FSR3 FSRFig. 2. The resonance wavelength as a function of voltage over thecell. Simulation (dashed), the actual measured curve (below) and the totalaccumulated shift (black) are shown.index (no) of the LC. This results in a low value of the effectiveindex of the mode. When an electric field is applied betweenthe silicon substrate and the ITO on the glass plate, the directorwill reorient vertically. Depending on the strength of the field,the evanescent tails of the TM mode will now see more of theextraordinary refractive index (ne) of the LC and the effectiveindex will rise. The change in effective index translates directlyto a change in resonance wavelength of the ring resonator.∆λλ=∆neffng, (1)where ng is the group index. We simulate the wavelengthshift by combining a calculation of the director with a fullyanisotropic mode solver. The results of the director calculationare imported in the mode solver to get a detailed view onthe behavior of the cell at different voltages. The simulatedtuning curve for the waveguides specified above shows atuning range of around 31nm for a voltage of 100V(Fig. 2).There is a threshold voltage around 5V where the field exactlycompensates for the elastic forces holding the LC in place.IV. CHARACTERISATIONLight from a tunable laser is coupled into the waveguidethrough the grating couplers and the output spectrum isanalyzed. To avoid ion drift which can cause shielding ofthe electrodes, we apply a 10kHz square wave to the LCcladding layer. For increasing voltage levels the resonanceshifts towards longer wavelengths (Fig. 3). Note that at certainvoltage levels we start tracking resonance dips located 3 freespectral ranges towards shorter wavelengths. The measuredcurve as well as the accumulated shift is shown in Fig. 2. Theagreement between simulation and experiment is excellent.At 10V rms the resonance wavelength has shifted about 4nmwhich covers a free spectral range in the rings used for thisexperiment. When we increase the voltage to 100V rms we canshift the resonance up to 31nm. At this point the resonancedoes not change significantly with increasing voltage as thedirector has reached its maximum tilt angle throughout thecell.00.20.40.60.810.40.60.810V 10V 15V20V 23V23V 30V 40V0.40.60.811515 1518 1521 1524 1527 15301 FSR =4.27 nm3 FSR3 FSRTransmission (a.u.)Wavelength (nm)60V60V100VFig. 3. Output spectrum for different values of the voltage.V. CONCLUSIONWe have shown that TM ring resonators on SOI can betuned widely with a cladding layer of LC. A tuning range ofmore than 30nm is achieved. We have explained the tuningmechanism and shown simulations that correspond well withthe experimental findings. These results have a large potentialfor widely tunable filters, lasers and detectors, with low tuningpower.ACKNOWLEDGMENTW. De Cort acknowledges the Institute for the Promotion ofInnovation through Science and Technology in Flanders (IWT-Vlaanderen) for a specialization grant. The work has beencarried out in the framework of the IAP project Photonics@beof the Belgian Science Policy.REFERENCES[1] W. De Cort, J. Beeckman, R. James, F. Fernandez, R. Baets, andK. Neyts, “Tuning of silicon-on-insulator ring resonators with liquidcrystal cladding using the longitudinal field component,” Optics Letters,vol. 34, no. 13, pp. 2054–2056, 2009.[2] ——, “Tuning silicon-on-insulator ring resonators with in-plane switchingliquid crystals,” Journal of the Optical Society of America B-OpticalPhysics, vol. 28, no. 1, pp. 79–85, 2011.[3] B. Maune, R. Lawson, C. Gunn, A. Scherer, and L. Dalton, “Electricallytunable ring resonators incorporating nematic liquid crystals as claddinglayers,” Applied Physics Letters, vol. 83, no. 23, pp. 4689–4691, 2003.[4] V. G. Chigrinov, L. Zhou, A. Muravsky, and A. W. O. Poon, “Electricallytunable microresonators using photoaligned liquid crystals,” U.S. Patent20 070 258 677, November 8, 2007.[5] S. Selvaraja, P. Jaenen, W. Bogaerts, D. Van Thourhout, P. Dumon, andR. Baets, “Fabrication of photonic wire and crystal circuits in silicon-on-insulator using 193 nm optical lithography,” Journal of LightwaveTechnology, vol. 27, no. 18, p. 4076 4083, 2009.[6] D. Taillaert, P. Bienstman, and R. Baets, “Compact efficient broad-band grating coupler for silicon-on-insulator waveguides,” Optics Letters,vol. 29, no. 23, pp. 2749–2751, 2004.

Silicon-On-Insulator ring resonators can be widely tuned with a cladding layer of Liquid Crystals. Reorientation of the director with an electric field results in a tuning range of more than 30nm. © 2011 IEEE

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Widely tunable silicon-on-insulator ring resonators with a liquid crystal cladding

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