Core/shell nanoparticle ratio changes microdisk-resonator sensing properties

Dec. 4, 2014
Microring and microdisk optical resonators are sensitive detectors of nanoparticles, including individual cells or viruses.

Microring and microdisk optical resonators are sensitive detectors of nanoparticles, including individual cells or viruses. When a particle touches such a resonator, it causes changes in the evanescent wave surrounding the resonator; as a result, light coupled out of the resonator changes intensity. Researchers at the University of Michigan (Dearborn) and Massachusetts Institute of Technology (MIT; Cambridge) have done numerical simulations of gold-core/polymer-shell nanoparticles with varying core/shell ratios being sensed by an on-chip four-port integrated microdisk resonator and then determined the sensitivity in terms of resonance-wavelength shift as a function of core/shell ratio. They found that, at a certain core size/shell thickness ratio, the resonance-wavelength shift virtually disappears due to coupling between the core, shell, and resonator, that is, the particle becomes invisible to the resonator.

The resonator itself is 8 μm in diameter; light at a 1550 nm wavelength is coupled into and out of the disk via 600-nm-wide waveguides. The resonator and waveguides are made of silicon nitride (Si3N4) clad with silicon dioxide (SiO2). The nanoparticles have a diameter of 200 nm, with the gold core varied from 0 to 200 nm. The study is being done to investigate optimum geometries of nanoparticles for applications such as sensing biomolecules via the use of dyes absorbed by the particles. Future research will include modeling on nonspherical nanoparticles and a wider variety of resonance modes. Reference: Y. Xiong et al., IEEE J. Quantum Electron. 50, 8, 677 (2014).

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