Ultraviolet Sensor Based on a Silica Optical Microresonator
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Ultraviolet Sensor Based on a Silica Optical Microresonator Simin Mehrabani1, Audrey Harker2, Andrea Armani1 1 Department of Chemical Engineering and Materials Science, University of Southern California, Los Angeles, CA, U.S.A. 2 Department of Chemical and Biomolecular Engineering, University of California, Berkeley, Berkeley, CA, U.S.A. ABSTRACT It is well known that exposure to ultraviolet (UV) light can result in various physical and psychological diseases. Therefore, there is a strong demand for a reliable sensor to monitor UV exposure levels in the physiologically relevant intensity ranges of mW/cm2. Here, we demonstrate a UV sensor based on a silica whispering gallery mode microresonator. This UV sensor works over physiologically relevant intensity ranges with linear performance both in the forward and backward operating directions, with very high signal-to-noise ratio that can be utilized in monitoring the UV exposure for various applications. INTRODUCTION Ultraviolet (UV) radiation is a part of the electromagnetic spectrum, which is naturally found in sunlight. Prolong exposure to UV radiation can cause serious health issues, the most prevalent of which is skin cancer. Our skin cells are designed to protect us from the harmful effects of UV radiation by producing a chemical named melanin. Melanin blocks UV radiation; however over-exposure can damage these cells. In addition, studies on working personnel in tanning bed facilities have shown negative effects of over-exposure to synthetic UV radiation. Therefore, a reliable UV sensor is required to monitor different levels of UV in the environment and workplace [1-3]. The so-far-developed UV sensors mainly work based on UV-responsive materials that are coated on optical or electrical transducers. As the sensor is exposed to UV radiation, the underlying sensor detects changes in the optical or electrical properties of the coating. Examples include dye-sensitized polymer coatings on optical fiber sensors [4-6] and zinc oxide (ZnO) based electrical sensors [7-9]. However, there are several challenges that are preventing their further development. First of all, the coating materials are not robust and experience degradation over time due to environmental changes. In addition, these sensors exhibit hysteretic response i.e. different behavior in forward and backward changes in the UV intensity. Also, there are parasitic effects that can mask the actual response of the material (for instance piezoelectricity of zinc oxide in addition to its photo responsiveness). In order to build a reliable UV sensor, a material must be chosen that is sensitive to UV, and at the same time does not have the above mentioned issues. Silica is a well-known, robust material that does not degrade in the ambient environment. In addition, it does not have parasitic effects. In this work, we demonstrate a UV sensor based on a silica whispering gallery mode microresonator. Upon exposure, UV radiation is absorbed by the silica and is converted into heat, which causes a slight increase in silica
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