Low Loss AlN Optical Resonators for Sensing Applications

In this paper, we present a low loss optical resonator platform based on the aluminum nitride (AlN) material, which can be used to sense the temperature, chemical, and mechanical variations. The platform is built by use of the AlN rib-waveguide structures deposited on top of a thick oxide layer. Applications of temperature sensing, chemical sensing, and mechanical sensing of the AlN optical resonators are simulated.

AlN is a promising material for integrated opto-mechanics. It has not only excellent optical properties, such as ultra-low absorption at near infrared region, but also shows large piezoelectric coefficient, which allows efficient control of optomechanical systems by electrical means. It promises a high sensitive on-chip detection by optical method ^[1-3]. In this work, an AlN rib waveguide based opto-resonator is reported. The AlN rib waveguide is on top of a 3 µm thick oxide layer as shown in Fig. 1(a). Figure 1 (b) shows the designed AlN optical resonator structures.

The field distribution of the optical mode of the AlN optical resonator is simulated using the finite element method (FEM) tool by COMSOL. Figure 2 illustrates the optical field distribution of AlN rib waveguide. The dependence of n_eff of the rib waveguide on temperature, chemical and mechanical stress are also obtained using COMSOL, as shown in Fig. 3. The responses of the AlN rib waveguide to temperature, chemical, and mechanical stress are obtained via simulation.

Figure 4 show the scanning electron microscope (SEM) images of fabricated AlN rib waveguide. A rough surface of the sidewall and rib is achieved using an optimized etching recipe. The propagation loss of the AlN rib waveguide is greatly reduced with the merits of ultra-low optical scattering.