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Engineering Optical Antenna for Efficient Local Field Enhancement


Optical antennas have been widely used for variety of applications such as sensitive photodetection, efficient light emission, high-resolution imaging, heat-assisted magnetic recording, and surface-enhanced Raman spectroscopy (SERS) because they can capture and focus propagating electromagnetic energy into sub-diffraction-limited areas and vice versa. However, widespread application of optical antennas has been limited due to lack of appropriate methods for uniform and large area fabrication of antennas, as well as difficulty in achieving an efficient design with small mode volume (gap spacing < 10nm).

In this dissertation, we theoretically derived the local field enhancement of the optical antenna and found that optimized radiation and small gap spacing are required for maximum field enhancement of the antenna. To address these parameters, we experimentally demonstrated three different designs of optical antennas.

First, we report on applying a dipole antenna on a ground plane for radiation engineering. The dipole antenna design can achieve the optimum radiation condition by tuning the spacer thickness between the antenna array and the ground plane; however, reducing the gap spacing below 10 nm is challenging if using typical nanofabrication techniques such as e-beam lithography and focused ion beam milling.

Secondly, we report on using a patch antenna on ground plane for the uniform sub-5 nm gap spacing. The patch antenna design can be easily implemented with extremely small gaps; however, the poor radiation efficiency of the patch antenna with a nanometer-scale gap degrades the performance of the antenna.

Finally, we present a novel optical antenna design--the arch-dipole antenna--which has optimal radiation efficiency and small gap spacing (5 nm) fabricated by CMOS-compatible deep-UV spacer lithography. This antenna design achieves a strong SERS signal with an enhancement factor exceeding 108 compared to the arch-dipole antenna array; this is two orders of magnitude stronger than that obtained from the standard dipole antenna array fabricated by e-beam lithography. Because the antenna gap spacing--a critical dimension of the antenna--can be defined by deep-UV lithography, efficient optical antenna arrays with sub-10 nm gap can be mass-produced using current CMOS technology.

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