All-polymer whispering gallery mode sensor in the low-Q regime

Microcavities, such as spheres, rings or toroids, show sharp optical resonances at specific wavelengths, called whispering gallery modes (WGMs). They correspond to light waves circling around the cavity, relying on continuous total internal reflection at the cavity surface. After one roundtrip, the light waves return to the same point with the same phase, hence interfere constructively with themselves and travelling waves arise. So the resonance wavelengths of such micro-resonators depend on the radius and the refractive index of the sphere as well as the surrounding refractive index [1]. All physical quantities which change one of these parameters can be well detected and quantified with such resonators, because they cause a resonance wavelength shift [2]. Besides the possibility to determine quantities like temperature, pressure or the use of the WGMs as biosensors [3], they can also be employed for high resolution wavelength measurement [4]. The most frequently applied techniques to determine the wavelength use interferometric principles, for example Michelsonor Fabry-Perotinterferometers, which are often large and very expensive. Instead of using interferometers, the wavelength could also be determined by recording the shift of the resonance frequency of a single resonator. The WGM resonator can be described by the free spectral range (FSR) c/(2πnR) between two neighboring modes, the wavelength shift ∆λ of the resonance wavelength λ as well as the quality factor (Q-factor) [2]. R is the radius and n the refractive index of the resonator. A reduction of the size simplifies the detection of the resonance wavelength shift, because the FSR and the wavelength shift scale with 1/R. Unfortunately reducing the size of the resonator causes a bandwidth broadening of the resonance due to the higher radius curvature of smaller resonators and this leads to a lower Q-factor. Using an array of microspheres with slightly different diameters takes advantage of the fact that every single microsphere has a different resonance behavior. Therefore, using many spheres instead of one relieves the high demands on the resonance quality and thus allows using inexpensive polymer spheres instead of high quality resonators [4, 5]. This opens the perspective of simple and low cost WGM sensing.