Microresonators for compact optical sensors (uRCOS) for gas detection

This paper discusses ongoing research at Lawrence Livermore National Laboratory (LLNL) that investigates the effectiveness of a whispering gallery mode micro-resonator as a biosensor. Whispering Gallery mode resonators have properties such as ultrahigh quality factors (Q factors up to 1011), very high power density, and small mode volume that make them suitable for sensing applications1. In this work, silica microspheres (spheres on the order of 250?m) are used as resonators. These resonators are coupled to a tapered optical fiber connected to an infrared laser. Using critical coupling techniques, resonant wavelengths (wavelengths of zero power transmission) are produced. The resonant wavelengths of the coupled system are dependent upon properties of the microsphere such as diameter and index of refraction. Conjugation of biological organisms to the sphere causes a small change in these properties and thus creates a shift in resonant wavelengths (free spectral range) which can be characterized and used as a sensor. This paper will discuss microsphere and taper fabrication, the tested functionalization process, and the effect conjugation has on the microsphere Q factors. Future work includes real time analysis of biological organism conjugation and bringing the sensor down to the chip sized scale.