Final PhD Oral Examination (Thesis Title: “A Silicon Photonic Circuit for Optical Trapping and Characterization of Single Nanoparticles”)

Event Date and Time: 
Thu, 2017-03-23 16:00 - 18:00
Room 318, Hennings Building
Local Contact: 
Physics and Astronomy, UBC
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In this thesis, two slightly different silicon-on-insulator (SOI) planar photonic integrated circuits for optically trapping and characterizing single nanoparticles are designed, fabricated, and fully characterized. These symmetric (input/output) structures are formed by etching two dimensional patterns through a 220 nm thick silicon slab atop a micrometer thick layer of silicon dioxide, and are operated in a fluidic cell at wavelengths of ≈ 1.55 μm. Each consist of two grating couplers, two parabolic tapered waveguides, two single mode ridge waveguides, two photonic crystal waveguides and a single photonic crystal slot (PCS) microcavity, designed using a Finite Difference Time Domain (FDTD) electromagnetic simulation tool. The circuits are designed to concentrate continuous wave laser light incident on the input grating coupler to a small volume within the fluidic channel of the microcavity in order to achieve a high electric field intensity gradient capable of attracting and trapping nanoparticles from the solution via optical gradient forces.

The fabricated PCS cavities exhibit Q factors > 7500 and resonant transmissions as high as T = 6%, when operated in hexane and without undercutting the cavities. Due to fabrication imperfections, the cavity Q and peak transmission values were not as high as simulation predicted, nevertheless, these robust, devices were successfully used to optically trap single sub-50 nm Au nanospheres and nanorods (this is to our knowledge the first experimental demonstration of trapping sub-100 nm anisotropic nanoparticles using photonic crystal optical tweezers) with < 0.5 mW of laser power. Furthermore, it was found that while the particles were trapped, the transmitted laser intensity varied randomly in time, providing a simple means of characterizing the Brownian motion of the particle in the trap. The intensity variation is caused by the backaction of the dielectric object on the cavity resonance, the magnitude of which depends on the real part of the trapped particle’s polarizability tensor, and its position in the cavity. By exploiting this cavity-nanoparticle interaction, we developed a self-consistent analysis of the transmission signal of circuits that enabled us to determine the size and anisotropy of the trapped nanoparticles without any direct imaging, with nanometer sensitivity.

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