Optical refractometry using lensless holography and autofocusing

Conventional optical refractometry methods are often limited by a narrow measurement range, complex hardware, or relatively high cost. Here, we present a novel refractometry method to measure the bulk refractive index (RI) of materials (including solids and liquids) using lensless holographic on-chip imaging and autofocusing, which is simple, cost-effective, and has a large RI measurement range. As a proof of concept, two compact prototypes were built to measure the RIs of solid materials and liquids, respectively, and they were tested by measuring the RIs of a ZnSe plate and a microscopy immersion oil. Experimental results show that our devices have an average accuracy of ~3 × 10−4 RI unit (RIU) with an estimated precision of ~3 × 10−3 RIU for solids; and an average accuracy of ~1 × 10−4 RIU with an estimated precision of ~3 × 10−4 RIU for liquids. We believe that this cost-effective and portable RI measurement platform holds promise to be used in laboratory and industrial settings.