제목 [2018.6.5][colloquium] Tunable metasurfaces on stretchable substrates: Ultrathin flat zoom lens and switchable hologram
작성자 물리학과홈피 등록일 2018-05-31 오후 1:56:04 조회수 115
추천 0 비추천 0
날짜 : 6월 5일 화요일
시간 : 오후 4시 30분
장소 : R404
연사 : 이호석 교수님 ( 공주대 물리학과 )
제목 : Tunable metasurfaces on stretchable substrates: Ultrathin flat zoom lens and switchable hologram
내용 : Metasurfaces are ultrathin flat surfaces that produce abrupt changes in a light beam over a sub-wavelength length-scale. The 2D metasurface also enables the fabrication of tunable optical elements, making them attractive for the design of reconfigurable photonic devices. To demonstrate the concept of a tunable metasurface, we have developed several mechanically tunable Berry-phase metasurfaces operating in the visible frequency regime using a stretchable polydimethylsiloxane (PDMS) substrate. We first demonstrate that the anomalous refraction angle and therefore the optical wavefront can be continuously tuned by mechanical stretching of the substrate. We then demonstrate proof-of-concept prototypes of tunable metasurfaces with potential real-world applications: a 1.7× ultrathin flat optical zoom lens and a switchable metasurface hologram. The tunable metasurface structures composed of a array of Au nanoantennas with different orientation angles are designed and optimized using finite-difference time-domain simulations. The designed matasurfaces are first fabricated on a Si substrate using HSQ/PMMA bilayer e-beam lithography and e-beam evaporation of Au and then are transferred to a PDMS film. The fabricated metasurfaces are optically characterized for various stretch ratios. In a tunable metasurface with a constant phase gradient, measured anomalous refraction angles agreed well with calculations using the generalized law of refraction and successfully demonstrated that the refraction angle of the metasurface can be tuned from 11.4° to 14.9° by changing the stretch ratio of the substrate. We designed, fabricated, and optically characterized the first ultrathin flat stretchable metasurface zoom lens, proving that our tunable metasurfaces can control an optical wavefront for practical applications. By reconstructing captured light intensity profiles of the transmitted beam with the opposite circular polarization, longitudinal beam profiles are obtained. The results showed that the focal length of the lens increases from 150 to 250 μm as the stretch ratio changes from 100% to 130% successfully demonstrating the optical zoom function of the flat lens. We will also discuss our efforts to design a mechanically switchable hologram as another example of tunable metasurface operating in the visible region. To obtain the phase profiles required for the switchable hologram, a phase-only computer-generated hologram was designed with a multi-plane hologram generation technique and a metasurface that produced the obtained phase profile was fabricated and transferred to a PDMS film. Experimentally, the metasurface hologram switches the image when the PDMS film is stretched or relaxed by a certain amount. Tunable metasurfaces demonstrated in this work will be useful for a variety of applications in information technology, biomedical sciences, integrated optics, optical communications, imaging, flat displays, and wearable consumer electronics.

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