Ultrasensitive optical detection of nanoparticles is highly desirable for applications in early-stage diagnosis of human diseases, environmental monitoring and homeland security, but remains extremely difficult due to ultralow polarizabilities of small-sized, low-index particles. Optical whispering-gallery-mode (WGM) microcavities, with high Q factors up to 108, provide a promising platforms for label-free detection of nano-scaled objects, due to significantly enhanced light-matter interaction. The mechanisms of the conventional WGM sensors, based on the reactive (or dispersive) interaction, measure the mode shift induced by the environmental variations of refractive index, which may fail to detect low-index nanoparticles. In this work, we propose a different dissipative sensing scheme, reacting as linewidth change of WGMs, to detect single nanoparticle using a silica toroidal microcavity fabricated on a silicon substrate. In experiment, detection of single gold nanorods in aqueous environment is realized by monitoring simultaneously the linewidth change and shift of cavity mode. Besides a good consistent with the theoretical predictions, the experimental result shows that the dissipative sensing achieves a better signal-to-noise-ratio compared to the dispersive mechanism. Remarkably, by setting the probe wavelength on and off the surface plasmon resonance of the gold nanoparticles, the great potential of the dissipative sensing method to detect single lossy nanoparticles is demonstrated. This dissipative sensing method holds great potential in detecting lossy nanoparticles, and may become a promising lab-on-a-chip platform for detecting small-sized, low-index particles with ultralow polarizabilities.
Yanyan Zhi, Bo-Qiang Shen, Xiao-Chong Yu, Li Wang, Donghyun Kim, Qihuang Gong, and Yun-Feng Xiao, "Optical microcavity sensing: from reactive to dissipative interactions (Conference Presentation)," Proc. SPIE 10090, Laser Resonators, Microresonators, and Beam Control XIX, 1009015 (Presented at SPIE LASE: February 01, 2017; Published: 21 April 2017); https://doi.org/10.1117/12.2249557.5387835789001.
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