Spectral shift response of optical whispering-gallery modes due to water vapor adsorption and desorption

The spectral shift response of optical whispering-gallery modes due to the adsorption and desorption of water molecules into a SiO₂ nano-coating is investigated. This coating is applied to a silica microsphere and optical-shift measurements are made at a very low humidity level (<10%). The micro-optical coupling system is incorporated into a sealed vacuum chamber for controlled humidity testing. The experimental observation of the whispering-gallery-mode spectral shift with refractive index is consistent with a theoretical analysis using Mie theory. A significant hysteresis effect is observed in the spectral shift of individual whispering-gallery modes during variable humidity tests. To understand the hysteresis mechanism, tests with step changes in humidity are performed, which reveal complex and dynamic water molecule transport phenomena between the nano-coating and surrounding environment.