Optical Micro-Resonators as Refractive Index Sensors in Microfluidics
M. Grad, S. Kocaman, C-C Tsai, C.W. Wong, D. Attinger
Columbia University, US
Keywords: microfluidics, sensors, optical ring resonators, refractive index, label-free
Abstract:We describe the use of photonic micro-resonators as label free refractive index sensors for microfluidics. Common fluids used in this field can be discerned by their refractive index. Also, subtle changes in temperature or concentration can be characterized by measuring the index of refraction. Therefore precise index of refraction sensors can serve multiple purposes on a microfluidic chip. Two micro-ring resonators are made from silicon waveguides with a thickness of 250 nm, nanofabricated on top of a silica cladding layer. The waveguides are lithographically patterned with a 248-nm lithography scanner, and the Si is plasma-etched. The resonators’ diameters are 8 um and 35 um, and their respective Q-factors were measured as 9000 and 11 000 while immersed in water, and up to 25000 for the dry 35 um resonator. The were tested in microfluidic channels with different concentrations of NaCl. The sensitivity of each resonator, defined as d(lambda)/dn (with units nm/Refractive Index Unit, or RIU), is 97 nm/RIU and 131 nm/RIU for the respective 8 mm and 35mm resonators. These values are better than the previously published micro-ring sensors.