Holographic microrefractometer

In-line holographic microscopy of micrometer-scale colloidal spheres yields heterodyne scattering patterns that may be interpreted with Lorenz-Mie theory to obtain precise time-resolved information on the refractive index of the suspending medium. We demonstrate this approach to spatially resolved refractometry with measurements on calibrated refractive index standards, and use it to monitor chemical concentration in a microfluidic channel. Using commercially available colloidal spheres as probe particles and a standard video camera for detection yields values for the fluid’s refractive index at the position of each probe particle in each holographic snapshot with a demonstrated resolution of 2×10−3 refractive index units (RIU) and a potential resolution surpassing 10−4 RIU. The combination of spatial resolution, temporal resolution, multipoint in situ access and technical simplicity recommend this technique for cost-effective lab-on-a-chip applications.