Surface-immobilized whispering gallery mode resonator sphere for optical sensing

Whispering-gallery modes can be excited in micro-cavities, such as rings, spheres or toroids. These are sharp optical resonances at specific wavelengths. The electromagnetic wave is guided in the cavity by total internal reflection. In case the guided wave drives itself by returning in phase after one roundtrip, a travelling wave arises. Its resonance frequency depends on the radius and the refractive index of the sphere as well as the surrounding refractive index.

Microcavities that support whispering-gallery modes can be well applied as biological and optical sensors since changing one of the above parameters results in resonance wavelength shift, mode splitting or resonance line broadening. All physical properties varying the radius or the refractive indices of the sphere or the surrounding medium, respectively, can be recorded and quantified.

Ann Britt Petermann and colleagues from Leibniz University in Germany investigated surface-immobilized whispering gallery mode resonator sphere for optical sensing. In their work, they represented a simple and fast solution for fixation of whispering gallery mode resonator spheres in the form of a spin coated UV curing adhesive, maintaining an accuracy and reproducibility that can be comparable to the sensor without fixation. Their work is now published in peer-reviewed journal, Sensors and Actuators A: Physical.

In the experimental setup with a tunable narrow-band diode laser, a thin layer of a curing adhesive is spin coated. Due to total internal reflection, the plate acts as a light guide leading to an evanescent field at the surface of the plate. The field is seen to protrude in the spin coated layer where spheres are placed with random distribution before curing.

To investigate the influence of the resonator size, the authors used spheres with two different mean diameters. The diameters were chosen to compare the results of previous works. The evanescent field couples into the spheres and whispering gallery modes are generated. Some spheres are only in resonance at a particular wavelength, because they vary in size. Therefore, the intensity pattern produced by all spheres changes with the incident wavelength.

A suitable fixation layer must meet certain requirements. The refractive index must be smaller than the one of the spheres and the substrate. This is to ensure that light is still guided in the substrate by total internal reflection. The layer must be thin to permit coupling between plate and spheres via the evanescent field and to generate whispering gallery modes in these spheres.

Due to small variation in the mean diameter of the spheres, only a few of them are in resonance at a given wavelength. Using many spheres relieves the high demands on resonator quality. By recording the intensity profiles of several spheres simultaneously and unambiguous, determination of the present resonance condition is possible. The actual wavelength can be determined by comparing the intensity distribution of the spheres at the unknown wavelength.

This paper focused on demonstrating a method to attach polymer whispering gallery mode resonator spheres on a polymer substrate and realize an all-polymer sensor system. Using the low-cost polymer components made the sensor easy to manufacture, as a proof of the sensing capabilities, the researchers determined unknown optical wavelengths.