Off-Axis Diffractive Focusing Reflectors for Refractive Index Sensing in Microfluidic Devices

Microfluidic devices with off-axis diffractive focusing reflectors (DFRs) were fabricated for refractive index sensing in biological and chemical fields. Such devices can detect refractive index changes of dielectric liquid samples just above the DFRs by measuring the changes of diffracted power from the fluidic devices. The focusing reflectors consist of curved SiO2 lines with periods which gradually varied from 1.23 to 3.73 µm. When a probe laser with 632.8 nm wavelength was coupled to the DFRs at the off-axis angle of 20°, a focal spot of 3.5 µm was observed at the distance of 1000 µm from the lens surface, which was close to theoretical value.