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Nanoscale Optical Devices: Force, Torque and Modulator


Manipulating and utilizing light in nanoscale are becoming tasks of not only scientific interest, but also industrial importance. My research includes two major topics in nanoscale optics: 1. Nanoscale optical motor. 2. Optical modulator based on novel materials.

Light carries both linear and angular momentum, and therefore generating force or torque with light is feasible. The ability to provide torque in nano-meter scale opens up a new realm of applications in physics, biology and chemistry, ranging from DNA unfolding and sequencing, to active Nano-Electro-Mechanical Systems (NEMS). In the first part of this dissertation, I demonstrate a nano-scale plasmonic structure generating a significantly large rotational force when illuminated with linearly-polarized light. I show that a metallic particle with size of 1/10 of the wavelength is capable of rotating a silica microdisk, 4,000 times larger in volume. Furthermore, the rotation velocity and direction can be controlled by merely varying the wavelength of the incident light, thereby inducing different plasmonic modes which possess different torque directions. The tiny dimensions along with the tremendous torque may have a profound impact over a broad range of applications such as energy conversion, and in-vivo biological manipulation and detection.

Compared with the interaction between particles and photons, the optical force between particles is of fundamentally important as well. In the end of the first part, I propose a new technique to measure the optical binding forces between two plasmonic particles. By using localization technique on a build-in cantilever, I prove the potential to measure the force with accuracy up to sub-pico-Newton.

The second part of this dissertation is about the modulation of light, an optical modulator. Integrated optical modulator with high modulation speed, small footprint and large optical bandwidth is poised to be the enabling device for on-chip optical interconnects. However, present devices suffer from intrinsic narrow-bandwidth aside from their sophisticated optical design, stringent fabrication, temperature tolerances and large foot print. By using graphene, a monolayer of carbon atoms, I experimentally demonstrated a broadband, high-speed, waveguide-integrated electroabsorption modulator. The extremely strong interaction between light and relativistic electrons in graphene allows us to integrate an optical modulator within an ultra-small footprint while operating at a high speed with broad bandwidth under ambient conditions. Even monolayer of being less than 1 nm in thickness, its modulation effectiveness is comparable with the best materials like Ge and SiGe with tens nanometers. In addition, the athermal optoelectronic properties of graphene make the device immune to harsh operation environments, in sharp contrast to all existing semiconductor approaches.

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