Low-loss modified SU-8 waveguides by direct laser writing at 405 nm

In this work we present a fabrication process to obtain a low-loss waveguide in the photo-curable resin SU-8 using direct laser writing at 405 nm wavelength. Polymer-based devices offer low-cost prototype fabrication, fabrication flexibility, reliability, low power consumption and potential for mass production. These characteristics, coupled with its high optical performance and low propagation losses, make it an attractive material for applications related to optical biosensing. Initially, a method to reduce SU-8 viscosity is described to allow film thicknesses of a few hundred nanometers, thus guaranteeing single mode propagation at visible range. This is achieved while also introducing an H-nu 470 photoinitiator, providing the displacement of the absorption peak of the material from 365 nm to 470 nm, thus allowing H-line polymerization and the direct laser writing at wavelengths 405 nm and above. Key material and structure characteristics such as absorbance, transmittance, roughness and chemical composition on the surface are analyzed for both pure and modified SU-8. We observe lower RMS surface roughness in the latter one. In spite of the chemical modification of the material, optical parameters like absorption and refractive index in the wavelength of interest are not affected. Single-and multimode optical waveguides are demonstrated. The sidewall roughness is measured at 5 : 6 nm, and the propagation loss for the single mode waveguide is 4 : 4 dB/cm at 633 nm wavelength, providing a high quality and low-cost fabrication platform for optical nano-devices.