Control of field enhancement in plasmonic nanoantennas by use of VO2 and interference technique : 바나듐 이산화물과 간섭을 이용한 플라즈모닉 나노안테나의 빛의 세기 능동 조정에 관한 연구

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공과대학 전기·컴퓨터공학부
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서울대학교 대학원
plamsonicsnanoantennasurface plasmons
학위논문 (박사)-- 서울대학교 대학원 : 전기·컴퓨터공학부, 2014. 8. 이병호.
The interaction between a light and a metallic nanostructure is well known to provide an excitation of the electromagnetic surface mode, surface plasmon polaritons (SPPs), coupled with collective motion of conduction electrons. If the geometry of a nanostructure is properly designed to support the plasmonic resonances, an enhanced field with several orders of magnitude to the incident field, a hot spot, can be locally formed at nanoscale. The plasmonic hot spots provide novel means of overcoming the optical diffraction limit and have triggered a considerable number of investigations due to their various fascinating properties such as sub-wavelength confinement characteristics and strong electromagnetic field enhancement near metal surface.
One of the necessary components of optical interfacing device is a plasmonic nanoantenna, like dipole or bowtie nanoantenna, which is the promising application as compact solutions to the coupling between far-field and nanoscale devices. Interesting features of the plasmonic nanoantenna can be obtained by modification of the geometry of the nanostructures under plasmonic resonances. However, less interest has been devoted to active plasmonic nanoantenna. The works done in this dissertation are focused on the active characteristics and the field enhancement of the nanoantennas designed with dipole, cross-shape and bow-tie shape and tapered slot nanoantenna.
After brief introduction to the SPPs, the various experiment tools applied to the fabrication and measurement of proposed plasmonic structure in this thesis are introduced with e-beam lithography, direct laser writing, template-stripping method, and fluorescence imaging measurement. These subjects are worthy of notice, due to their particular purpose in the realization of the plasomonic nanoantennas compared with other conventional methods.
For the purpose of the active control of the plasmonic nanoantenna, the phase transition materials, vanadium dioxide, are considered due to their change of the optical properties between semiconductor and metallic phase across the critical temperature. By adopting the vanadium dioxide as a nanoantenna materials, the active dipole nanoantenna is proposed and analyzed via numerical simulation focusing on the enhancement of the intensity and the tunability. The principles on resonances at both phases are helpful to extend the basic nanoantenna design (dipole) to a complicated and more functioned design. On this wise, another active plasmonic nanoantenna, active cross-shaped nanoantenna, is proposed based on the results from the active dipole nanoantenna. The simulation results confirm the similar spectral responses of active dipole nanoantenna. Especially, more distinct characteristics are the generation of the multiple hot-spots and thier selective generation of the hot-spots from the polarization state of the incident light. This characteristics can be recognized as the spatial multiplexing of the nanoantenna by polarization state of the incident light.
Another important characteristics is the modulation of the nanoantenna emission. This investigation starts from the quantum dots located at the feed gap of the nanoantenna as a nanoantenna source. The main principle of the modulation is the interference at the feed gap of the dipole nanoantenna by two SPPs propagating beyond the plasmonic nanoantenna. Thus, by inducing the phase difference of two SPPs, excitation of the quantum dots located at the feed gap can be modulated, resulting in modulated emission of the nanoantenna coupled with quantum dots. Based on this principles, optimized geometry to provide the efficient modulation depth is investigated by numerical simulation. And the experiment results confirm the effective modulation in proposed naanoantenna.
An approach to improve the characteristics of a hot-spot from a bow-tie shaped and tapered slot nanoantenna is proposed without degradation such as increment of the subwaelength spot size. Two advantageous features of the proposed tapered structure are investigated: At first, by enlarging the entrance area of the aperture, it could collect more light with respect to the regular one. Thus the funneled light contributes to the field enhancement. Furthermore, the tapered edges of the exit surface of the aperture provide confined field, a wedge mode, which is bounded strongly and enhances the local electric field around the edge of the aperture. The enhanced characteristics of subwavelength hot-spot in bow-tie shaped and tapered slot nanoantenna including peak intensity, power throughput, and full width half maximum were obtained numerically. The relative tapered angle-dependent enhancements are discussed with experimental and quantitative demonstrations of the proposed structure.
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College of Engineering/Engineering Practice School (공과대학/대학원)Dept. of Electrical and Computer Engineering (전기·정보공학부)Theses (Ph.D. / Sc.D._전기·정보공학부)
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