Sensor properties and surface characterization of aluminum-deposited SPR optical fibers

The response curves and sensor properties of Al-deposited optical fibers with Al film thicknesses of 7–70 nm based on surface plasmon resonance (SPR) were investigated. The response of the Al-deposited optical fiber sensor strongly depends on the thickness of the Al film. Though the sensors with Al show lower sensitivities than those deposited with Au or Ag, the Al-deposited optical fiber sensors have wider refractivity response ranges. The reflection properties of thin (11–31 nm) Al films due to the SPR phenomenon were also measured and considered. The surface characterization of the Al films (15–45 nm) was made by atomic force microscopy (AFM) and X-ray photoelectron spectroscopy (XPS). The surfaces of these Al films consist of small grains with diameters of 40–80 nm and the surface height distribution is almost random, having a value of more than 8 nm. The XPS analyses showed the presence of native oxide layers with thicknesses of 2–3 nm on the Al films. The Al films having oxide layers with thicknesses of 2–4 nm exhibited no significant change in the sensor properties. The Al-deposited optical fiber sensor also exhibited no change in the sensor properties following prolonged use for 5 months. The response curves of the Al-deposited optical fiber sensors calculated from SPR theoretical equations in consideration of the surface oxide layers, agreed well with those obtained by experimentation.