We aim to explore a novel optical resonator that diverges from the traditional symmetric Lorentzian line shape in optical cavities. Instead, an asymmetric Fano spectral line is produced to exhibit significant intensity variations with wavelength changes. This distinctive feature of Fano resonance with the sharp and asymmetric line profile has a high potential for applications in sensitive sensors, photodetection, and low-power optical switches. The principle behind its application in sensing is based on changes in the surrounding environment of the sensors, which alters the effective refractive index of the waveguide. This alteration causes a shift in the transmission spectral line, leading to substantial changes in the output light intensity at the working wavelength. The sensitivity of resonant sensors is characterized by the steepness of the transmission spectral line’s slope. A steeper slope indicates greater changes in light intensity for the same spectral line drift, thereby enhancing the sensor’s detection sensitivity. Therefore, the Fano resonance with the capacity for high sensitivity finds broad applications and catches research attention from various fields. In recent years, optical devices with Fano characteristics have been extensively studied. Examples include the metal-insulator-metal (MIM) waveguide structure with branched resonators and square ring open resonators. By varying the branch height, the geometric dimensions of the open rings, and the symmetry of the structure, the Fano resonance’s transmission characteristics are altered to yield high sensitivity up to 1500 nm/RIU and a quality factor exceeding 1800. Another example is the MIM waveguide structure with concentric double ring resonators, where a maximum sensitivity of 1400 nm/RIU and a quality factor of 1380 are obtained. We propose the research methodology in this paper involves a comprehensive approach combining theoretical analysis and experimental validation, and utilizes a dual-path interference structure within a microring cavity to create the Fano resonator.

We employ the transfer matrix method to analyze the phase conditions that lead to the generation of an asymmetric spectral line. This method is instrumental in understanding how various parameters influence the shape of the asymmetric spectral lines in the Fano resonator. Meanwhile, it allows for an in-depth examination of the phase conditions responsible for creating the distinctive asymmetric line profile of the Fano resonance. The design and analysis of the device modal patterns are conducted by adopting the finite difference-time domain (FDTD) method. This method is pivotal in determining the modal distribution and behavior of the device in different operational conditions and is helpful for device parameter fine-tuning to achieve the desired optical characteristics. The device is fabricated on a silicon-on-insulator (SOI) platform using electron beam lithography (EBL) etching technology. This technology is chosen for its precision and ability to create finely structured optical components, essential for the accurate realization of the Fano resonator. Following fabrication, the device’s features are characterized to validate the theoretical predictions. This involves testing the device in various conditions to observe its performance and confirm the theoretical models. The combination of these theoretical and experimental methods provides a robust framework for us. The proposed innovative Fano resonator structure opens new avenues for the design of high-performance devices in applications such as high-resolution optical sensing, low-power optical switches, and high-contrast optical detection.

The theoretical framework utilizing the transfer matrix method allows for an in-depth analysis of the phase conditions leading to the asymmetric line shape of the Fano resonance. The results show that for a coupling coefficient of 0.242, a loss coefficient of 0.995, and a phase difference of 94° between the two light paths, the Fano resonance spectrum can achieve an extinction ratio as high as 41.54 dB and a spectral slope as steep as 2372 dB/nm. These theoretical predictions are significant as they indicate the potential of the Fano resonator to yield high performance in optical applications. The research also provides formulas for calculating the wavelength shift at the spectral dip and conditions for complete extinction under ideal circumstances. These calculations are crucial for predicting and fine-tuning the resonator’s performance in practical applications. For device fabrication and validation, the experimental part involves fabricating the devices on an SOI platform using EBL. A multi-mode interference (MMI) structure is employed for combining the two light paths with varying phase differences to observe their effects on the asymmetric line shape of the Fano resonance. The experimental results are highly encouraging, demonstrating an extinction ratio of nearly -25 dB and a spectral slope of 1997 dB/nm in the described process conditions. Meanwhile, they nearly align with the theoretical predictions, revealing the practical viability of the proposed resonator design. The successful demonstration of the Fano resonator with such high-performance metrics underscores its potential in high-resolution optical sensing, low-power optical switches, and high-contrast optical detection. Additionally, we highlight the ability of this resonator-interferometer structure to manipulate the light phase and power distribution, opening new pathways for integrated optoelectronics. Finally, we conclude by emphasizing the Fano resonator’s superior performance in sensing capabilities, highlighting its applicability in nanobiological sensing and densely integrated nanophotonic devices.

We successfully propose, analyze, design, and validate a new type of Fano resonator assisted by a micro-ring cavity. This resonator exhibits a sharp, asymmetric Fano resonance, and a notable deviation from traditional resonator designs. A crucial finding is the ability to effectively control the spectral symmetry and slope by adjusting the phase difference between two light beams within the resonator. This capability to manipulate the spectral features is pivotal for various applications. Meanwhile, we observe that the spectral line shape of the Fano resonance is sensitive to phase noise, which plays a significant role in determining the resonator’s performance and potential applications. The experimental results show an impressive extinction ratio of up to -25 dB and a spectral slope of 1997 dB/nm, marking an improvement of nearly 20 dB in extinction ratio compared to traditional microring resonators in similar process and coupling conditions. The spectral line characteristic study reveals that the Fano resonator possesses excellent sensing capabilities. The resonator’s structure is highly suitable for applications in nanobiological sensing and densely integrated nanophotonic devices, highlighting its broad applicability in various fields of optical technology. Additionally, this shows its potential in advancing the design of high-performance devices in fields including high-resolution optical sensing, low-power optical switches, and high-contrast optical detection.