Fabrication and characterization of an infrared plasmonic metasurface

Vật lý<br /> <br /> FABRICATION AND CHARACTERIZATION OF AN INFRARED<br /> PLASMONIC METASURFACE<br /> Nguyen Thanh Tung*<br /> Abstract: Manipulating light by subwavelength resonators via plasmonic<br /> metasurface is currently studied intensively for various novel THz and optical<br /> applications. In this report, we present the fabrication of a simple plasmonic<br /> metasurface operating at infrared frequencies using the UV lithography and<br /> electron beam evaporation techniques. The strong plasmonic behavior of fabricated<br /> samples is then investigated by the micro Fourier transformed infrared<br /> spectroscopy. The parametric dependent plasmonics is discussed in light of<br /> measured results.<br /> Keywords: Metasurface, Plasmonic, Infrared frequency.<br /> <br /> 1. INTRODUCTION<br /> The interaction between electromagnetic waves and matters has been one of central<br /> problems in physics and materials science for many years. Exploring novel<br /> electromagnetic properties of matters and materials allows us to design and controll the<br /> wave propagation in a certain medium that lead to dozens of microwave, THz, and optical<br /> applications [1]. Unfortunately, the variability of the electromagnetic properties in<br /> naturally occuring materials is not always controllable since it rigorously depends on<br /> intrinsic nature of materials and their formation conditions as well. In order to brigde this<br /> gap, a new class of artificial metal-dielectric materials, the so-called metamaterials, was<br /> introduced to arbitrarily tailor electromagnetic properties of a medium without having<br /> material constitutes changed [2]. The main advantage of metamaterials is of its<br /> subwavelength resonators. These resonators are periodically arranged to effectively enact<br /> macroscopic electromagnetic behavior of the medium, in which the subwavelength<br /> resonators are embedded. For example, the refractive index in metamaterials can be tuned<br /> from positivity to negativity by independently controlling its permeability and permittivity<br /> [3]. Typical metamaterials can be designed to work as invisible cloaks, light absorbers,<br /> electromagnetic reflectors, light filters, etc ... [4-6].<br /> Recently, the use of single-sided metamaterials, namely metasurfaces, to represent<br /> electrically plasmonic antenna for sensing and energy storage purpose has been of an<br /> increasing interest [7-9]. A metasurface often contains of structured metallic patterns on<br /> top of a dielectric substrate and can be scaled to operate at various frequency ranges<br /> depending on the target application. For instance, the microwave metasurfaces need<br /> structures at millimeter sizes. The metasurfaces with micrometer-sized patterns exhibit<br /> plasmonic resonance at infrared frequencies. Eventually, the optical metasurfaces often<br /> have nanometer-sized structures. While the microwave metasurfaces can be fabricated<br /> conveniently using the classical photolithography and standard lift-off process, the<br /> fabrication of THz and optical ones is more challenging since it requires cutting-edge<br /> technologies and rigorious lift-off/developing conditions [10]. These practical<br /> difficulties, however, cannot slow down the research on micrometer- and nanometer-<br /> sized metasurfaces due to its very promising applicability, in particular, for infrared<br /> frequencies, where most of molecular vibrations can be probed and analyzed [8]. In this<br /> report, we fabricated and characterized infrared metasurfaces that show plasmonic<br /> behavior at THz frequencies. The influence of geometrical parameters on the plasmonic<br /> frequency is also discussed.<br /> <br /> <br /> 156 Nguyen Thanh Tung, “Fabrication and characterization… plasmonic metasurface.”<br /> Nghiên cứu khoa học công nghệ<br /> <br /> 2. EXPERIMENTAL AND COMPUTATIONAL SETUP<br /> <br /> <br /> <br /> <br /> Figure 1. (Left) Schematic drawing of a metasurface unit cell and (right) SEM images of<br /> four metasurface samples S1, S2, S3, and S4 corresponding to l = 1, 2, 3, and 4 µm,<br /> respectively. Other geometrical parameters of fabricated samples are unchanged as ax = 3<br /> µm, ay = 9 µm, and w = 1 µm.<br /> Although the results presented in this study obtained mainly by experimental<br /> techniques, the computational tool is also employed in the design and optmiztion. First, the<br /> geometrical structure of metasurfaces are simulated by finite-integration-simulation<br /> technique embedded in CST Microwave Studio Suite [11]. The structural dimensions of<br /> metasurface samples can be roughly estimated from our previous work [12-15] before<br /> optimized by CST simulations.<br /> A schematic drawing of the metasurface unit cell is illustrated in Fig. 1. It is composed<br /> of a cut-wire Au resonator on top of a commercial double-side polished Si substrate. The<br /> periodicities in the x and y directions are 3 and 9 µm, respectively. The width of the Au<br /> cut-wire is 1 µm while its length l is varied from 1 to 4 µm in this study. The thicknesses<br /> of Au resonators and Si substrate are fixed at 100 nm and 350 µm, respectively. The<br /> fabrication process is carried out with electron-beam evaporation of a 100-nm Au film<br /> onto a silicon substrate after a 5-nm Cr adhesion layer. The maskless UV-lithography<br /> technique (DL1000, Nano System Solutions), which might allow to obtain a metasurface<br /> with an operating area of up to few cm2 [16], is performed to pattern cut-wires on the Au<br /> film. The final structure is then obtained after lift-off process. The scanning electron<br /> microscope (SEM) images of fabricated samples with l =1- 4 µm are presented in Fig. 1.<br /> The transmission spectra of the metasurface is characterized using a micro Fourier-<br /> transformed infrared spectrometer (µ-FTIR 6300FV, Jasco). The incident wave is normal<br /> to the structure plane (z direction) while the electric and the magnetic directions can be<br /> applied in two modes TE and TM. To achieve a better signal-to-noise ratio of IR signals,<br /> the sample chamber is purged with dry nitrogen gas and a liquid nitrogen-cooled high-<br /> sensitive MCT (HgCdTe) detector is used with the frequency resolution of 2 cm-1. During<br /> the measurements, the metasurface is assumed to be embedded in the vacuum.<br /> <br /> <br /> Tạp chí Nghiên cứu KH&CN quân sự, Số 52, 12 - 2017 157<br />  Vật lý<br /> <br /> 3. RESULTS AND DISCUSSIONS<br /> It has been known that the cut-wire structure acts like an electric dipole and exhibit an<br /> electric resonance in the microwave frequencies [12,13]. Scaling down the cut-wire<br /> dimensions can theoretically provide higher-frequency resonances, for example, to<br /> infrared regime. However, in practical not only the dimensions but also the selection of<br /> suitable materials is also important when the frequency goes from microwave to higher<br /> ones. For instance, in this study, Si is used instead of FR4 as a dielectric substrate with the<br /> permittivity of 11 and the transmission window above 20 THz. Sapphire Al2O3 can also be<br /> used as dielectric substrate but the trasmission window starts at a higher frequency, about<br /> 60 THz. Two series of experiments have been performed to assign the plasmonic<br /> resonance of the fabricated Au cut-wires. First we measure the transmission spectra of S1,<br /> S2, S3, and S4 samples at the TE polarization (E field polarized along the y-axis). The<br /> results are displayed in Fig. 2. In particular, S1 exhibits a resonance at 54.6 GHz that is<br /> about two times higher than the resonance frequency of S2, about 27.7 GHz. The<br /> resonance of S3 likely appears at the lower edge of the frequency range, about 21 GHz<br /> while that of S4 is out of the measured range but surely lower than 20 GHz. According to<br /> the Drude model, the electric plasmon resonance can be exicited and its permittivity can be<br /> descirbed as<br />  = 1- (ep2 - eo2)/(2 – eo2), (1)<br /> in which, the length of cut-wires determines the electric plasmon frequency ωep, while the<br /> cut controls the cut-off resonance frequency ωeo [17,18]. Increasing the length l will<br /> linearly increase the electron density along the oscillation direction, resulting a higher<br /> plasmonic resonance frequency. This is consistent to the observed picture where the<br /> frequency ratio and length ratio of S1:S2:S3 samples are approximately as 1:2:3. From the<br /> length-depedent transmission behavior of Au cut-wire samples, we can identify the<br /> plasmonic resonance that is stimulated along the cut-wire in TE mode.<br /> <br /> <br /> <br /> <br /> Figure 2. Measured transmission spectra of S1, S2, S3, and S4 at TE polarization.<br /> The second series of experiments is carried out in TM mode to confirm the above<br /> explanation. In this configuration, the external electric field is polarized along the width of<br /> cut-wires, which is unchanged as varying from S1 to S4. Figure 3 plots the measured<br /> transmission spectra of S1, S2, S3, and S4 at TM polarization. One can see that samples<br /> S2, S3, and S4 exhibit a resonance at about 32.1 THz that is unchanged while increasing<br /> the length of cut-wires. It is well agreed with our previous discussion on the length-<br /> <br /> <br /> 158 Nguyen Thanh Tung, “Fabrication and characterization… plasmonic metasurface.”<br /> Nghiên cứu khoa học công nghệ<br /> <br /> dependent plasmonic resonance. Here the with of cut-wires defines “the length” along the<br /> electric field. Varying l does not affect the width and that is the reason why the plasmonic<br /> resonant frequency is unchanged for S4, S3, and S2. It is worth to mention that the<br /> resonant frequency of S1 sample (42.1 THz) is slightly higher and broader than that of S2,<br /> S3, and S4 due to the less uniform of S1 cut-wires over a large sample area (1 cm2).<br /> <br /> <br /> <br /> <br /> Figure 3. Measured transmission spectra of S1, S2, S3, and S4 at TM polarization.<br /> This is likely due to the fabrication limitation that appears for structures whose size are<br /> comparable to the highest resolution of the UV lithography technique (1 µm2). As a<br /> sequence, the plasmonic resonant frequency of S1 samples slightly shifts to higher<br /> frequency and to be significantly broader.<br /> 4. CONCLUSIONS<br /> In this report, we presented an experimental study on the infrared plasmonic<br /> metasurface. Two series of differently-geometrical samples corresponding to two<br /> polarizations are fabricated using UV lithography in combination with electron-beam<br /> evaporator technique and measured by FTIR spectroscopy. The operating plasmonic<br /> frequency is observed in the range of about 20-50 THz. The measured results indicate<br /> clearly that the longitudinal size of the cut-wires along the applied electric field is the key<br /> factor to control the plasmonic resonant frequency. The realization of the infrared<br /> metasurface can provide potential implementations for novel sensing applications in<br /> future.<br /> Acknowledgements: This work was supported by Japan Society for the Development of Science and<br /> Center for Advanced Photonics, RIKEN.<br /> REFERENCES<br /> [1]. Kotlarchyk, M. 2002. Electromagnetic Radiation and Interactions with Matter.<br /> Encyclopedia of Imaging Science and Technology.<br /> [2]. Y. Liu and X. Zhang, Chem. Soc. Rev. 40, 2494 (2011).<br /> [3]. D. R. Smith, W. J. Padilla, D. C. Vier, S. C. Nemat-Nasser, and S. Schultz, Phys.<br /> Rev. Lett. 84, 4184 (2000).<br /> [4]. D. Schurig, J. J. Mock, B. J. Justice, S. A. Cummer, J. B. Pendry, A. F. Starr, and D.<br /> R. Smith, Science 314, 977 (2006).<br /> <br /> <br /> Tạp chí Nghiên cứu KH&CN quân sự, Số 52, 12 - 2017 159<br />  Vật lý<br /> <br /> [5]. N. I. Landy, S. Sajuyigbe, J. J. Mock, D. R. Smith, and W. J. Padilla, Phys. Rev. Lett.<br /> 100, 207402 (2008).<br /> [6]. X. Shen, T. J. Cui, J. Zhao, H. F. Ma, W. X. Jiang, and H. Li, Opt. Express 19, 9401<br /> (2011).<br /> [7]. S. Wang et al., Nature Comm. 8, 187 (2017)<br /> [8]. N. Yu and F. Capasso, Nature Mater. 13, 139 (2014).<br /> [9]. A. Zhan, S. Colburn, C. M. Dodson, and A. Majumdar, Sci. Rep. 7, 1673 (2017).<br /> [10]. M. Tonouchi, Nature Photon. 1, 97 (2007).<br /> [11]. www.cst.com<br /> [12]. N. T. Tung, D. T. Viet, B. S. Tung, N. V. Hieu, P. Lievens, and V. D. Lam, Appl.<br /> Phys. Express 5, 112001 (2012).<br /> [13]. N. T. Tung, B. S. Tung, P. Lievens, E. Janssens, and V. D. Lam, J. Appl. Phys. 116,<br /> 083104 (2014).<br /> [14]. D. T. Viet, N. V. Hieu, V. D. Lam, and N. T. Tung, Appl. Phys. Express 8, 032001<br /> (2015).<br /> [15]. D. T. Anh, D. T. Viet, P. T. Trang, N. M. Thang, H. Q. Quy, N. V. Hieu, V. D. Lam,<br /> and N. T. Tung, AIP Advances 5, 077119 (2015).<br /> [16]. S. Prezioso et al., Langmuir 28, 5489 (2012).<br /> [17]. Z. G. Dong, M. X. Xu, S. Y. Lei, H. Liu, T. Li, F. M. Wang, and S. N. Zhu, Appl.<br /> Phys. Lett. 92, 064101 (2008).<br /> [18]. D. Schurig, J. J. Mock, and D. R. Smith, Appl. Phys. Lett. 88, 041109 (2006).<br /> TÓM TẮT<br /> NGHIÊN CỨU CHẾ TẠO VÀ ĐẶC TRƯNG CỦA CẤU TRÚC METASURFACE<br /> HOẠT ĐỘNG Ở TẦN SỐ HỒNG NGOẠI<br /> Điều khiển sóng điện từ bằng các cấu trúc cộng hưởng plasmon bề mặt<br /> (metasurface) hiện đang được quan tâm nghiên cứu cho các ứng dụng mới trong<br /> vùng hồng ngoại và quang học. Trong báo cáo này, chúng tôi trình bày kết quả<br /> nghiên cứu chế tạo một cấu trúc plasmon bề mặt hoạt động ở vùng hồng ngoại bằng<br /> phương pháp quang khắc bằng UV laser kết hợp với kỹ thuật bốc bay chùm điện tử.<br /> Phổ hồng ngoại của cấu trúc metasurface theo các kích thước và phân cực khác<br /> nhau được kiểm tra bằng kỹ thuật FTIR. Kết quả cho thấy chiều dài của cấu trúc<br /> metasurface theo phương điện trường có vai trò quan trọng trong việc điều khiển<br /> tần số cộng hưởng plasmon bề mặt.<br /> Từ khóa: Siêu vật liệu, Plasmon bề mặt, Tần số hồng ngoại.<br /> <br /> Nhận bài ngày 16 tháng 08 năm 2017<br /> Hoàn thiện ngày 06 tháng 11 năm 2017<br /> Chấp nhận đăng ngày 20 tháng 12 năm 2017<br /> <br /> Địa chỉ: Institute of Materials Science, Vietnam Academy of Science and Technology, Vietnam.<br /> *<br /> Email: tungnt@ims.vast.ac.vn.<br /> <br /> <br /> <br /> <br /> 160 Nguyen Thanh Tung, “Fabrication and characterization… plasmonic metasurface.”<br />