Whispering gallery mode microcavities for photonic sensing applications

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Light interacts with everything it touches. Whenever light passes through a substance or reflects from a surface, its properties are affected in ways that can give important information about the environment. Photonic sensors use light to “sense” the surroundings on a length scale that is truly tiny. Being small is important; it can reduce cost, increase portability for operation “in the field”, yield fast results, and minimize the need for large and expensive laboratories. Photonic sensors can be used in a considerable range of applications and are being developed by research groups and companies worldwide. Applications range from hand-held toxic gas sensors to the detection of disease in at-risk populations. Making a photonic sensor for a particular application can be difficult, however. Many competing effects can “overlap”, making it hard to detect the presence of the specific substance of interest. Many analytes are chemically complex (e.g., blood), and it is hard to be selective for only one specific target. Also, one must typically detect very weak signals. We developed a sensor based on light emitted inside specially prepared microcapillaries. The channel can be as small as 10 micrometers wide; when a fluid is pumped in, the device emission wavelength is controlled by the chemical composition of the analyte. We aim to push this technology to ultra-low detection limits, such that changes of less than one part in a million can be reliably and quickly measured. The project further aims toward the application of specialized methods to detect the presence of single target molecules that are made to bind onto the device, achieving extraordinary chemical sensitivity and selectivity. We will search for ways to make biocompatible lasers in which the optical “gain medium” is the target analyte itself. Since laser signals are bright, this method can turn small changes into large signals, ideal for sensing applications. This work aims to overcome the key issues of selectivity and signal strength, and could help open a whole range of applications for photonic sensor technologies. Specific targets include measurement of vitamin D3 and explosives detection.