In order to achieve electromagnetically induced transparency (EIT) in the quantum field, it is necessary to meet harsh experimental conditions such as extremely low experimental temperatures, high-intensity light sources, and huge experimental equipment. Therefore, the development of EIT is greatly limited. With the development of photonics, the realization of EIT in the field of photonics will avoid harsh experimental conditions and accelerate the research on EIT. The electromagnetically induced absorption (EIA) effect is contrary to EIT. The physical mechanism of EIA is radiation and sub-radiation resonator, which realizes EIA through the near-field coupling between them. The EIA effect can be used in the fields of optical switches and slow light devices. In addition, compared with Lorentz linetype, Fano linetype has the characteristics of asymmetry, and the transmission efficiency of the Fano effect is higher. In view of the difficulty in realizing the EIT effect in the quantum field, the application scope of the EIA effect, and the transmission advantages of the Fano effect, the feasibility of these physical effects in a simple and compact device is worthy of studying.
Two main research methods are used in this study, namely the transmission matrix method and the finite difference time domain (FDTD) method. The transmission matrix method is used to analyze the transmission characteristics of devices. Specifically, the transmission matrixes of the air hole, Fabry-Perot (FP) cavity, and coupling between the microring and the FP cavity are established using the parameters such as the reflection coefficient of the air hole, the length of the FP cavity, the wavelength, the effective refractive index, the circumference of the microring, and the transmission loss coefficient. Through the relationship between these matrices, the input mode and the output mode in the waveguide are connected. By analyzing the input and output modes, the expression of normalized transmittance is obtained. The device is simulated by FDTD. It mainly simulates the mode field, transmission spectrum, and performance parameters [quality factor and extinction ratio (ER)] of a microring resonator (MRR). The mode field is simulated at the wavelength of 1550 nm. According to Eq. (9) and the group refractive index in the simulation results, the bending losses of microgroove microring and single waveguide microring are compared. The simulation of the transmission spectrum is mainly shown in the device structure simulation and optimization module. By changing the structural parameters, such as the coupling distance, the length of the FP cavity, the radius of the air hole, and the width of the microgroove, the optimal output linetype can be ensured.
In this study, the coupling structure of the FP cavity and microgroove microring is adopted (Fig. 1), which makes the light mode of continuous state in the FP cavity and that of discrete state in the microring couple interfere with each other. In addition, the waveform is distorted by the high refractive index difference of the straight waveguide and the FP cavity, and Lorentz linetype, Fano linetype, EIT-like linetype, and EIA-like linetype appear between two adjacent resonance peaks of the FP cavity. In order to improve the utilization of light, reduce the loss, and improve the quality factor of the device, two air holes are introduced outside the FP cavity, and the microgroove structure is adopted. The microgroove structure restricts light. When the distance between the external air hole and the FP cavity increases, the cavity length (L) of the FP cavity of the reflector composed of the air hole increases. In this process, it can be seen from Eq. (8) that the transmissivity of the structure mentioned in this study increases. This phenomenon can be clearly seen in Fig. 7. With the increase in L, the resonance intensity in the EIT transparent window gradually increases. When the radius (Rhole) of the air hole increases, L will decrease relatively. In this process, it can be seen from Eq. (8) that the transmissivity of the device will gradually decrease, and this process is consistent with the results shown in [Fig. 8(a)]. Fig. 8(b) also shows that when Rhole increases, the quality factor slowly decreases.
In this study, a microgroove MRR based on the FP cavity is proposed. Two air holes are introduced outside the FP cavity. By adding air holes, the utilization of light is improved, the coupling ability between the FP cavity and the microring is enhanced, and the transmissivity of the device is improved. FDTD is used to simulate the effects of coupling distance, FP cavity length, air hole radius, and micro slot width on the output linetype of the device. The results show that: the coupling distance can directly control the EIT-like linetype, and the EIT transparent window can be opened and closed by changing the coupling distance; the length of the FP cavity and the radius of the air hole determine the utilization of light; the width of the microgroove can realize the regulation of EIA. In addition, this study also compares the advantages and disadvantages of single waveguide microring and microgroove microring. The fabrication of single waveguide microring is simple, but the bending loss of microgroove microring is small. In order to simulate the realizability of the device, the fabrication tolerance of the device is simulated on the premise of ensuring the optimal output linetype. The results show that the proposed device has favorable fabrication tolerance and strong realizability. Through the simulation analysis, the multiline output is realized; the Q value of the structure reaches 90112, and the ER is about 15 dB. The proposed structure can be used in the field of optical switches.
张江峰, 梁龙学, 吴小所, 吴朝阳, 王嘉伟, 孙成龙. 输出多线型微环谐振器的研究[J]. 光学学报, 2023, 43(9): 0923002. Jiangfeng Zhang, Longxue Liang, Xiaosuo Wu, Chaoyang Wu, Jiawei Wang, Chenglong Sun. Research on Output Multiline Microring Resonator[J]. Acta Optica Sinica, 2023, 43(9): 0923002.