We propose two types of structures to achieve the control of Fano and electromagnetically induced transparency (EIT) line shapes, in which dual one-dimensional (1D) photonic crystal nanobeam cavities (PCNCs) are side-coupled to a bus waveguide with different gaps. For the proposed type Ⅰ and type Ⅱ systems, the phase differences between the nanobeam periodic structures of the two cavities are π and 0, respectively. The whole structures are theoretically analyzed via the coupled mode theory and numerically demonstrated using the three-dimensional finite-difference time-domain (3D FDTD) method. The simulation results show that the proposed structure can achieve several kinds of spectra, including Fano, EIT and asymmetric EIT line shapes, which is dependent on the width of the bus waveguide. Compared to the previously proposed Fano resonator with 1D PCNCs, the proposed structures have the advantages of high transmission at the resonant peak, low insertion loss at non-resonant wavelengths, a wide free spectral range (FSR) and a high roll-off rate. Therefore, we believe the proposed structure can find broad applications in optical switches, modulators and sensors.

Photonic crystal nanobeam gratings (PCNGs) enable broad-band and flat-top reflection responses within short grating lengths due to their large coupling coefficients. In this Letter, we propose and experimentally demonstrate a channel-drop filter (CDF) based on a dual-waveguide apodized PCNG. The dual-waveguide structure realizes the decoupling of grating apodization from mode conversion. The reflected light is separated with low loss through a broadband asymmetric directional coupler (ADC). This design allows effective apodization using laterally shifted holes, which eases the fabrication requirements. The fabricated device on a silicon-on-insulator (SOI) platform exhibits a 3-dB bandwidth of 40.1 nm, an extinction ratio (ER) of 20 dB, a sidelobe suppression ratio (SLSR) of 19.5 dB, and an insertion loss (IL) of less than 1.5 dB, demonstrating a strong potential for integrated on-chip broadband filtering applications.

Fano-resonant photonic molecules hold great promise for ultra-compact optical filtering and sensing, yet their performance is usually limited by the free-spectral-range (FSR) of the resonators. In this letter, we demonstrate an FSR-free Fano resonance realized in a silicon-on-insulator photonic molecule comprising a bus waveguide side-coupled to two bent, $\pi $ -phase-shifted sidewall Bragg-grating (BG) cavities. Rigorous coupled-mode theory analysis reveals that the interference between the two gratings generates a single, sharp asymmetric line shape whose slope rate (SR) and extinction ratio (ER) are governed by the bus waveguide width. On the silicon-on-insulator (SOI) platform, we experimentally demonstrate a fabricated device exhibiting an extinction ratio (ER) of 37 dB, an average slope rate (SR) of 162 dB/nm, and an FSR-free response. The proposed structure is expected to find wide applications in optical filtering and sensing that demand single-resonance operation over a broad spectral window.