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    1700429 (1 of 10) 2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

    www.advopticalmat.de

    Batch Fabrication of Metasurface Holograms Enabled by Plasmonic Cavity Lithography

    Liqin Liu, Xiaohu Zhang, Zeyu Zhao, Mingbo Pu, Ping Gao, Yunfei Luo, Jinjin Jin, Changtao Wang, and Xiangang Luo*

    DOI: 10.1002/adom.201700429

    Therefore, it is of great significance to find a high yield nanofabrication method. Nanoimprint lithography seems to be a candidate, but it still needs further improvement in defect control, align-ment, etc.[19,20] The conventional lithog-raphy with ultraviolet light source has the advantages of low-cost and large area.[21,22] But, its resolution can only reach about one half of the wavelength due to the optical diffraction limit. The reason is the evanescent waves that carry the objects subwavelength information decay expo-nentially in a medium with positive per-mittivity and permeability, and thus could not contribute to the imaging. In order to improve the resolution, various technolo-gies have been exploited, such as reducing the light wavelength by employing deep or even extreme ultraviolet light sources,[23] improving numerical aperture by utilizing immersion lenses with high index mate-rials.[24] This inevitably requires complex projecting optics and control methods, not being affordable for common researchers.

    The limited refractive index also limits the further improve-ment of resolution.

    The near field lithography was proposed and was subse-quently demonstrated to address the diffraction limit of reso-lution.[2528] However, the great decaying feature of evanescent waves brings the imaging resist patterns with shallow depth and poor fidelity. In 2000, Pendry first proposed the concept of perfect lens capable of amplifying the evanescent waves by a negative index slab to overcome the diffraction limit.[29] This precursive research rapidly intrigues researchers interest on plasmonic lithography and super-resolution imaging. In 2004, silver grating with 300 nm period and 50 nm slit width is uti-lized to excite surface plasmonics (SPs), forming image of interference patterns with 100 nm period and 50 nm line width (about /9) in photoresist (Pr) layer.[30] In 2005, a superlens of Ag film was experimentally verified to achieve the 60 nm line width (about /6).[31] Subsequently, a series of theoretical and experimental investigations was developed to further improve resolution, depth, and fidelity, such as by utilizing the smooth superlens, reflective lens, and plasmonic cavity.[3238] The latest research results show that the plasmonic cavity associated with off-axis illumination could further improve the imaging resolu-tion and enlarge the working distance between the mask and

    Metasurface holograms consisting of nanostructures have shown great promise for various applications due to their unique capability of shaping light. Usually, they are fabricated by point-by-point scanning method, such as focused ion beam and electron beam lithography, which would greatly hamper their applications due to the high cost and low yield. In this work, plasmonic cavity lithography is proposed to fabricate metasurface holograms. The lithography system consists of Cr mask and plasmonic cavity that com-pose of 20 nm Ag/30 nm photoresist/50 nm Ag, where an air separation layer exists between them to avoid contamination and damage of mask patterns. The simulated results show that the cavity can effectively amplify the evanes-cent waves and modulate the electric field components on imaging plane, resulting in greatly improved resolution and fidelity compared to near field and superlens lithography. In experiments, the Au metaholograms are fabri-cated by the proposed lithography method and following etching processes. Furthermore, the designed holographic image of character E is successfully observed with the fabricated hologram. This approach is believed to open up a batch fabrication way for reproducing many copies of a metasurface hologram.

    L. Q. Liu, X. H. Zhang, Prof. Z. Y. Zhao, Prof. M. B. Pu, P. Gao, Y. F. Luo, J. J. Jin, Prof. C. T. Wang, Prof. X. G. LuoState Key Laboratory of Optical Technologies on Nano-Fabrication and Micro-EngineeringInstitute of Optics and ElectronicsChinese Academy of SciencesP.O. Box 350, Chengdu 610209, ChinaE-mail: lxg@ioe.ac.cnL. Q. Liu, X. H. Zhang, J. J. JinUniversity of Chinese Academy of SciencesBeijing 100049, China

    Holography

    1. Introduction

    Recently, metasurface holograms characterized with nano-structures have attracted a lot of attention due to their extraor-dinary ability of shaping light,[16] delivering various practical applications in holographic imaging, anticounterfeiting trade-mark, storage, and so on. However, almost all previously reported metasurfaces were fabricated by point-by-point scan-ning methods,[718] such as electronic beam lithography (EBL) and focused ion beam (FIB), and the poor efficiency imposes a serious barrier for large scale production and applications.

    Adv. Optical Mater. 2017, 1700429

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    2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700429 (2 of 10)

    www.advopticalmat.de

    imaging.[32] Compared to the point-by-point scanning type nanofabrication tools, plasmonic imaging lithography shows a great advantage in efficiency due to its one-step exposure. Also, it could realize deep subwavelength resolution far beyond the near field diffraction limit.[39] It is worth to note that most experimental investigations are focused on the resolution issue with results of dense nanolines, and few efforts are attributed to the plasmonic imaging lithography for fabricating complex arbitrary nanopatterns of functional devices, which impedes the extensive application of this technology.

    In this paper, we propose the plasmonic cavity lithography for the batch fabrication of metasurface holograms with ani-sotropic nanoapertures to solve the efficiency problem faced by current point-by-point approach. The principle is based on the fact that plasmonic cavity composed of Ag/Pr/Ag could amplify and modulate the electric field components of evanes-cent waves to improve imaging performance, and enlarge the working distance between mask patterns and cavity so as to relieve the contamination and damage of mask patterns. This point is demonstrated by the simulated transmission amplitude of electromagnetic field inside the cavity. In experiment, using the designed lithography associated with multilayer etching transfer technique, the Au holograms as a demo sample with nanoaperture size about 95 175 nm2 and patterns area 9 9 m2 are achieved, meanwhile the designed holographic image is successfully observed.

    2. Principle and Configuration for Plasmonic Cavity Lithography

    The schematic configuration of plasmonic cavity lithography in separated mode is shown in Figure 1a. The thickness param-eters of the cavity that composes of 20 nm Ag/30 nm Pr/50 nm Ag and 25 nm separated distance have been optimized, as depicted in Figure S1 (Supporting Information). A plane wave with 365 nm wavelength and in natural polarization normally illuminates on the Cr mask. To generate 25 nm air separation

    between mask patterns and cavity, a 25 nm thick Cr grating with 80 m period (line/space = 1) as a spacer is placed around the mask patterns region. The plasmonic cavity is physically contacted with Cr spacer by air pressure in experiment. In addi-tion, a 20 nm thick SiO2 hard film below the cavity is for pat-terns transfer, and a 50 nm thick Au film on the fused silica substrate is for fabricating metasurface hologram. Figure 1b displays the top view of the designed Cr mask patterns with nanoaperture size 60 170 nm2, period 300 nm, and patterns area 9 9 m2. Under 365 nm working wavelength, the relative permittivity of materials used for simulation are Cr = 8.55 + 8.96i, air = 1, Ag = 2.17 + 0.36i, Pr = 2.59.[40]

    Compared to lithography structures for near field and super-lens, the designed plasmonic cavity could greatly improve the imaging performance. To explain the principles of physics, Figure 2 presents the calculated transmission amplitudes of magnetic field Hy and electric field Ex and Ez under the three structures sketched in the insets of Figure 2bd, marked with I, II, and III, respectively. Here, only the incident light in TM (transverse magnetic) polarization is taken into account, since the light in TE (transverse electric) polarization could not excite the surface plasmon waves. In Figure 2a, it is clear that both the superlens and plasmonic cavity could enhance the transmission amplitude of evanescent waves, indicating higher resolution than that of near field. However, the electric field components decide the light field of imaging lithography. Figure 2b,c exhibits the transmissions of Ex and Ez electric field components. It is worth noting that Ex component makes the positive contribution for image, while the Ez component makes the negative contribution due to the /2 phase shift between Ex and Ez for evanescent waves.[41,42] In comparison with the Ex and Ez transmission amplitudes for near field and super-lens structures, the Ex of cavity is greatly enhanced and the Ez is depressed to kx < 6k0, which is believed to play a key role in resolution improvement. This point would be further veri-fied by the following simulated imaging results in Figure 3. In addition, one should note that the two sharp and strong peaks of dashed blue curves in Figure 2a,c represent the plasmonic

    Adv. Optical Mater. 2017, 1700429

    Figure 1. a) The schematic of plasmonic cavity lithography system in separated mode. b) The top view of the designed Cr mask patterns.

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    2017 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim1700429 (3 of 10)

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