IEEE PHOTONICS TECHNOLOGY LETTERS, VOL. 18, NO. 7, APRIL 1, 2006 865 High-Finesse Laterally Coupled Organic–Inorganic Hybrid Polymer Microring Resonators for VLSI Photonics Daniele Rezzonico, Andrea Guarino, Christian Herzog, Mojca Jazbinsek, and Peter Günter Abstract—We produced laterally coupled optical microring res- onators having high finesse ( at 1.5- m wavelength) using a two-step patterning technique based on optical photolithography. The technique used allows us to separately control the height of both ring and port waveguides and structure submicrometer gaps. The resonance spectrum of microrings with radii of 50 m made of an organic–inorganic hybrid polymer have an extinction ratio of about 12 dB and a filter bandwidth nm (full-width at half- maximum) at a wavelength nm. We show that the resonances can be thermooptically tuned by 0.2 nm C, thus al- lowing us to modulate the transmission of the through port signal. Index Terms—Integrated optics, lateral coupling, microring resonator, organic–inorganic hybrid polymers, Ormocer. I. INTRODUCTION P OLYMERIC optical microring resonators lately gained a considerable interest due to their suitability in photonic integrated circuits for filtering or modulation applications in the telecom environment [1]–[3], and more recently also in sensing technology [4], [5]. Today, integrated optical elements have to compete with bulk devices but the increasing demand on end user access network bandwidth will require low-cost mass production technologies and materials. Here polymers can play an important role. Besides the advantage to potentially reach any specification just by designing the appropriate functionality, e.g., using nonlinear or electrooptic active chromophores, poly- mers are relatively easily patterned and generally do not require any expensive technological feature. For the case of ring-like waveguides, however, the material’s low refractive index limits the miniaturization of the radii due to a reduced confinement of the electromagnetic mode fields if compared to semiconductors [6]. Most recent publications in this domain point out a presumed difficulty in patterning laterally coupled waveguides with optical fabrication technologies owing to a restriction in establishing submicrometer gaps of the asymmetric directional couplers, preferring, therefore, a vertically stacked arrangement [7], [8]. In this letter, we propose a simple and fast two-step patterning technique that allows us to clear the submicrometer gap between Manuscript received December 1, 2005; revised January 23, 2006. This work was supported by the Swiss National Science Foundation (NF200020-107541/1). The authors are with the Nonlinear Optics Laboratory, Swiss Federal Institute of Technology, ETH Zurich, CH-8093 Zürich, Switzerland (e-mail: rezzonico @phys.ethz.ch; guarino@phys.ethz.ch; herzog@phys.ethz.ch; mojca@phys. ethz.ch; nlo@phys.ethz.ch). Digital Object Identifier 10.1109/LPT.2006.871816 rings and port waveguides in lateral couplers, as well as inde- pendently adjust the height of the different waveguides. The pre- sented novel technique exploits the characteristics of the direct photopatternable hybrid polymer Ormocer, from Micro Resist Tech but originally developed by the Fraunhofer Gesellschaft. It enables us to produce higher finesse microrings with a radius of 50 m, while polymeric ring resonators with comparable prop- erties commonly have radii in the order of tenths of millimeters [1]–[4], [9], [10]. Finally, we demonstrate the tuning of the resonance spectrum via the thermooptic effect. II. MATERIAL CHARACTERISTICS AND FABRICATION PROCESS The material used for the waveguides is Ormocore, from the family of Ormocer, a commercially available inorganic-organic hybrid polymer that behaves as a negative photoresist, with re- fractive index 1.536 and typical losses of 0.6 dB/cm at 1.55- m wavelength, and glass transition temperature above 270 C [11]. A Silicon wafer, covered with a 2- m-thick thermally produced oxide as refractive index barrier , was used as substrate. After a careful cleaning and dehumidification of the substrate, and correct dilution of the Ormocore resin in propyl acetate, the first production step was to spin coat a droplet of the lacquer to get a wet 4- m-thick film. This was followed by the ultravi- olet (UV) exposure in oxygen poor atmosphere through a neg- ative chrome mask to pattern the port waveguides. The mask aligner we used is a standard Karl Süss MJB3 UV300 equipped for contact illumination only. Since a proximity mode exposure was required and the apparatus could not guarantee good par- allelism and precisely controlled distance from the wafer to the mask, 25- m overall separation was achieved by using a Kapton HN spacer from Goodfellow, which was placed around the coated surface of the sample. The exposed sample was then developed in methylpentanone. Directly after that we proceeded with the second production step. We spin coated a second film of about 5- m thickness from the same solution, which despite the already present waveguides appeared flat. After aligning the 50- m radius ring on the mask 0.5 m close to the waveguides produced before, we exposed and developed the second lacquer film. Final hard baking at 150 C in inert atmosphere was per- formed. The whole procedure lasts less than three hours. The resulting Ormocore waveguides had cross sections of 2.7- m width and 3.6- m height for the ports, and m for the ring, respectively (Fig. 1). 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