(627e) Relative Humidity Optical Microsensor Based On Polymer Coated Microtoroids

Monitoring the relative humidity (RH) in the environment plays a major role in various applications ranging from fundamental science and industry to biomedical applications. For instance, detection, control and elimination of traces amounts of water vapor in moisture-sensitive environments such as semiconductor fabrication plants can increase the production yield significantly.  Thus far, different optical and electronic techniques have been implemented for the relative humidity detection.  However, electronic methods are sensitive to the ambient electromagnetic sources of noise therefore optical based techniques are more robust.

One of the optical detection techniques that has shown promising results in relative humidity detection is based on whispering gallery mode optical microresonators. In whispering gallery mode microresonators, light propagates inside a circular path confined at a specific resonant wavelength at the periphery of the device.  Because the resonant wavelength is dependent on both the refractive index and the geometrical properties of the device, any changes to either of these characteristics can be detected by measuring the resonant wavelength shift.  However, conventionally, the detection mechanism is based solely on a refractive index change. 

In this study, we have developed an optical humidity sensing technique based on the combination of a whispering gallery mode silica microtoroidal cavity sensor and a novel nanometer thick humidity-responsive polymer coating, poly N-isopropylacrylamide (pNIPAAm).  Once pNIPAAM is exposed to humidity, it induces a change in both the refractive index and the diameter of the hybrid microresonator, and therefore it is capable of improving the detection signal significantly. As a result of the conformational change by the polymer in response to humidity, the detection response of the mircoresonator was increased by nearly two orders of magnitude.  Additional measurements exploring the hysteresis behavior of the sensing device, the response rate, the recovery time, and the effect of the pNIPAAm film thickness were also studied.