Tunable Fano filtering based on silicon one-dimensional photonic crystal cavities

Abstract

In a conventional optical resonator, the resonance transmission spectrum is always a Lorentz line shape, which is symmetrical with respect to the resonant wavelength. Recently, different configurations based on ring resonators [1], surface plasmons [2] and photonic crystal cavities [3] have been demonstrated to have asymmetric line shape, i.e., Fano resonance. Asymmetric profile in Fano resonance has sharper features than normal Lorentz line shape, which leads to a smaller shift of the resonant wavelength required for the same variation of optical power. Compared to other structures possessing Fano resonance, PhC cavities are superior with the advantages of ultra-high Q factors, small effective mode volumes and high integrabilities with optical circuits. In this work, we design and demonstrate a high Q 1D PhC cavity which can obtain tunable Fano line shape filtering. It is designed for fabrication on SilicononInsulator (SOI) wafer. The cavity is formed by two periodic arrays of circular air holes etched on a silicon strip waveguide. By utilizing a deterministic design method, we obtain a high calculated Q-factor of 1.03 × 106 for the fundamental resonant mode. Fig. 1(a) shows the SEM image of the fabricated device, the inset is the grating coupler integrated with the cavity. Experimental results show that, the cavity not only possesses transmission spectrum of Lorentz line shape, it can also obtain asymmetric sharp Fano line shape. By shifting the input fiber from the center relative to the grating coupler in the lateral direction, we can get tunable and asymmetric Fano line shapes. The two typical Fano spectrums are shown in Fig. 1(b), we can see that the Fano spectrums are asymmetric and very sharp around the resonant wavelength. Besides, our designed 1D PhC cavity has a small footprint of 14 × 0.56 µm2. It has great potential in the applications of efficient and compact optical modulators and sensors.