Low-Cost Fabrication of Functional Plasmonic Fiber-Optic Based Sensors Using Microsphere Photolithography

This paper studies the use of Microsphere Photolithography (MPL) as an alternative to Focused Ion Beam milling or e-beam lithography to pattern plasmonic fiber-optic based sensors. In the MPL approach, silica microspheres are self-assembled to form a Hexagonal Close-Packed (HCP) array on top of a layer of photoresist. The microspheres serve as an optical element and focus collimated UV radiation to an array of photonic jets inside the photoresist layer. The exposed region is dependent on the angle of incidence of the UV radiation which facilitates hierarchical patterning. Pattern transfer can be accomplished using either etching or lift-off with the size of the features dependent on the exposure dose. While low-cost and very scalable, the use of MPL influences the design and performance of the fiber probe in several ways. Specifically, the entire cleaved face is patterned without alignment to the fiber core and any defects in the self-assembled microsphere lattice are transferred to the surface. This paper presents the low-cost fabrication of Extraordinary Optical Transmission (EOT) type plasmonic fiber-optic based sensors. The sensors consist of a thin aluminum film on the cleaved face of single mode optical fiber, perforated with a HCP hole-array. At resonance, Extraordinary Optical Transmission (EOT), decreases the reflection from the fiber tip. The conditions for resonance are dependent on the local environment surrounding the fiber tip and the resonant wavelength can be used to measure the index of refraction of a liquid. Experiments show that viable sensors can be created with MPL. The reflection spectra of the sensors was measured in various concentrations of sugar water with a measured sensitivity of 8.33 mg/mL/nm. These results are compared to simulation results which provides a sensitivity analysis of the sensors.