Planar near-infrared surface plasmon resonance sensor with Si prism and grating coupler

We have developed a silicon-microfabrication-based surface plasmon resonance (SPR) sensor with Si prisms and an optical grating. Since this sensor is compatible with microelectromechanical systems technology, it can be further miniaturized and integrated with optical parts or fluidic components for the realization of a one-chip SPR sensor. The Si prism structure, transparent in the near-infrared range, was used to lead the 1550-nm measuring light to the sensing surface where the evanescent light was generated. Smooth prism slopes were obtained by using anisotropic etching to utilize the (1 1 1) planes on the (1 0 0) surface plane of single-crystalline Si wafer. The result was a 200-μm-pitch array of prisms with 54.74° slope angles. In this configuration the range of incident angles at which near-infrared light produces an SPR dip is from 38.26° to 71.24°. Because the SPR angles for major specimens (16.60° for air, 22.06° for water, and 22.79° for ethanol) are out of range, we used an optical grating with a roughly 400-nm-pitch to make an SPR dip position move to fit within the available angle range by the diffraction effect. SPR measurements with several specimens confirmed that the combination of Si prisms and the optical grating excited surface plasmons. The measured SPR dip positions and their sensitivity to refractive index were compared with the theoretical values, and their consistency confirmed the utility of this device as an SPR sensor.