Optical modulators containing high Q-factor microring resonators are gaining popularity due to their robust functionality and compact size. The resonators enable exponential growth of electro-optical (EO) coefficients on the order of Moore’s Law. Microring resonators may also allow lossless power conversion due to their ultrafast response time.
Researchers at the University of Washington in Seattle used electron beam bleaching to fabricate microring resonators. The used the chromophore YL124, historically used for optical rectification and EO modulation, and doped it at a 20% loading density into the PMMA host polymer. The PMMA host had a molecular weight of 950,000, making it an attractive candidate for electron beam resist. The solid mixture was then dissolved in liquid chlorobenzene, and the resultant solution was then spin coated onto a silicon substrate. UV16 was precoated and cured on the substrate’s surface, acting as a cladding material to ensure sufficient adhesion of the doped polymer to the substrate. Electron beam bleaching formed the desired microring structure. A vacuum oven heated the polymer film at 65°C.
Key Parameters and Requirements
The objective of the microring is to dissipate resonance; so, the target metric is the maximum-achievable resonance extinction ratio, which defines optimal resonator characteristics for the application. The researchers used two applications, transverse electric (TE) and transverse magnetic (TM) polarization, to determine the resonator specifications. The highest resonance mode present in both spectra defined the target extinction ratio.
The UV16 polymer enables electron beam writing of the microring by maintaining a robust and high-integrity bond between the substrate and solution. Without it, the high resonance extinction ratios may not be achieved.
Electron beam irradiation decomposes chromophore molecules to reduce the refractive index of the polymer. The electron beam bleaching process enables the usage of microring resonators, which require a polymer adhesive that avoids organic solvents that can attack the chromophore. UV16 provided the thin adhesive layer and desired bond characteristics that enable the fabrication of these resonators.