Low Index Cladding For Polymer Microring Resonator Waveguides

Abstract

Polymericmaterials are one option to realize photonic integrated circuits (PICs). Pureand hybrid polymer materials have garnered significant interest due to theirdistinctive properties, such as a simple, efficient, and scalable fabricationprocess as well as refractive index tuning [1-3]. Microring resonators (MRRs)fabricated in polymer waveguides have demonstrated groundbreaking applicationsin biosensing, medical diagnostics, food analysis, and environmental monitoring[1-4]. Additionally, they are emerging as miniaturized ultrasonic detectors forphotoacoustic imaging [5], [6]. However, polymers face challenges in achievingboth low and high refractive indices, which are crucial for versatileapplications. This limitation highlights the need to improve the refractive indexcontrast between the core and cladding in various applications. Recentadvancements in virtual - and augmented reality (VR/AR) devices have expanded the range of available polymersolutions. For example, low- refractive-index polymers have been used for lowercladding layers [7], while UV-curable hybrid organic-inorganic materials havebeen employed as waveguide materials.In this work, the MRRs are developed for photoacoustic imaging where a smallradius of the ring is beneficial. Our target is to achieve R<25 mm indicating a need to have sufficientrefractive index contrast between core and claddings. We have prepared theseMRRs using the established nanoimprint lithography (NIL) technique [1], [2], [5],[6] which is known for its high resolution and cost-effectiveness [1], [3]. Wereport on using using MgF2 (n=1.375) as well as a low refractive index (n=1.25)polymer (IOC-501TM from Inkron) as bottom claddings of the waveguides whosecore is imprinted on established ORMOCORETM resist. For the top claddingencapsulating the waveguide also a low refractive index (n=1.33) solution(MY-133-MCTM from MY Polymers) is used. Figure 1a shows a MRR fabricated on a MgF2 substrate. One of the resonance dips is shownin Fig. 1b. This work addresses key factors for improving the MRR performanceincluding a low residual layer thickness of <100 nm as depicted in Fig. 1c,low scattering and bending losses and a small ring radius.