近日，美国亚利桑那大学的Judith Su及其研究团队取得一项新进展。经过不懈努力，他们实现微环面谐振器的超高Q自由空间耦合。相关研究成果已于2024年3月15日在国际知名学术期刊《光：科学与应用》上发表。

该研究团队通过自由空间耦合和监测散射共振光来耦合光进出环面，从而消除了对光纤的需求。研究人员采用单一长工作距离物镜结合数字微镜装置(DMD)进行光注入、散射光收集和成像。通过这种方法，研究人员获得了高达1.6×10⁸的Q因子。由于自由空间的间接耦合，在单个腔内观察到类似电磁感应透明(EIT)和范诺(Fano)共振。这可以提高传感灵敏度。

此外，其大有效耦合面积（数值孔径为0.14时，直径约为10μm）有效消除了对精确定位的需求。研究人员结合锁频回音倏逝谐振器(FLOWER)方法进行温度传感实验，验证了系统的传感性能。在调节输入功率的同时，通过FLOWER跟踪谐振，检测了热非线性光学效应。研究人员相信这项工作将为将WGM微环形谐振器扩展到实际应用奠定基础。

据悉，回音壁模式微环谐振器是目前最灵敏的生化传感器之一，能够检测单个分子。然而，这些设备在实际应用中却面临着一个关键的挑战：光需要通过锥形光纤进行短暂耦合才能进入这些设备。这种耦合方式不仅限制了设备的移动性，更因锥形光纤的脆弱性而容易受到机械振动的影响，同时还要求极高的定位精度。

附：英文原文

Title: Ultra-high-Q free-space coupling to microtoroid resonators

Author: Suebka, Sartanee, McLeod, Euan, Su, Judith

Issue&Volume: 2024-03-15

Abstract: Whispering gallery mode (WGM) microtoroid resonators are one of the most sensitive biochemical sensors in existence, capable of detecting single molecules. The main barrier for translating these devices out of the laboratory is that light is evanescently coupled into these devices though a tapered optical fiber. This hinders translation of these devices as the taper is fragile, suffers from mechanical vibration, and requires precise positioning. Here, we eliminate the need for an optical fiber by coupling light into and out from a toroid via free-space coupling and monitoring the scattered resonant light. A single long working distance objective lens combined with a digital micromirror device (DMD) was used for light injection, scattered light collection, and imaging. We obtain Q-factors as high as 1.6×10⁸ with this approach. Electromagnetically induced transparency (EIT)-like and Fano resonances were observed in a single cavity due to indirect coupling in free space. This enables improved sensing sensitivity. The large effective coupling area (~10μm in diameter for numerical aperture = 0.14) removes the need for precise positioning. Sensing performance was verified by combining the system with the frequency locked whispering evanescent resonator (FLOWER) approach to perform temperature sensing experiments. A thermal nonlinear optical effect was examined by tracking the resonance through FLOWER while adjusting the input power. We believe that this work will be a foundation for expanding the implementation of WGM microtoroid resonators to real-world applications.

DOI: 10.1038/s41377-024-01418-0

Source: https://www.nature.com/articles/s41377-024-01418-0