Bilayer Metasurfaces for Dual- and Broadband Optical Antireflection

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Letter

Bi-layer metasurfaces for dual and broadband optical antireflection Li Huang, CHUN-CHIEH CHANG, Beibei Zeng, John Nogan, ShengNian Luo, Antoinette J. Taylor, Abul K. Azad, and Hou-Tong Chen ACS Photonics, Just Accepted Manuscript • DOI: 10.1021/acsphotonics.7b00471 • Publication Date (Web): 09 Aug 2017 Downloaded from http://pubs.acs.org on August 10, 2017

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Bi-layer metasurfaces for dual and broadband optical antireflection Li Huang,∗,†,⊥ Chun-Chieh Chang,‡,⊥ Beibei Zeng,‡,⊥ John Nogan,¶ Sheng-Nian Luo,§ Antoinette J. Taylor,k Abul K. Azad,‡ and Hou-Tong Chen∗,‡ †Physics Department, Harbin Institute of Technology, Harbin, Heilongjiang 150001, China ‡Center for Integrated Nanotechnologies, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA ¶Center for Integrated Nanotechnologies, Sandia National Laboratories, Albuquerque, New Mexico 87185, USA §The Peac Institute of Multiscale Sciences, Chengdu, Sichuan 610031, China kChemistry, Life, and Earth Sciences Directorate, Los Alamos National Laboratory, Los Alamos, New Mexico 87545, USA ⊥These authors contribute equally to this work E-mail: [email protected]; [email protected] Phone: +1-505-665-7365

Keywords metasurfaces; metamaterials; optical antireflection; terahertz spectroscopy; mid-infrared; metal-dielectric-metal structure

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Abstract Optical antireflection has long been pursued for a wide range of applications, but existing approaches encounter issues in the performance, bandwidth, and structure complexity, particularly in the long wavelength infrared regime. Here we present the demonstration of bi-layer metasurfaces that accomplish dual and broadband optical antireflection in the terahertz and mid-infrared spectral ranges. By simply tailoring the structural geometry and dimensions, we show that subwavelength metal/dielectric structures enable dramatic reduction of Fresnel reflection and significant enhancement of transmission at a substrate surface, operating either at two discrete narrow bands or over a broad bandwidth up to 28%. We also use a semi-analytical interference model to interpret the obtained results, in which we find that the dispersion of the constituent structures play a critical role in achieving the observed broadband optical antireflection.

Optical antireflection has been critical for a wide range of applications from reducing losses to avoiding adverse effects in optical systems and optoelectronic devices. Based on the principle of interference, quarter-wave antireflection coating is the oldest, 1 yet still simplest and widely used, approach to realize narrowband antireflection by introducing a dielectric film with a matched refractive index and a quarter-wave thickness. 2 Broadband antireflection coatings can be accomplished using multilayer dielectric thin films with appropriately arranged refractive index and thickness profiles. 3 However, in the mid- and far-infrared wavelength regions, it becomes a challenge to identify low-loss dielectric materials with the required refractive indices and, at the same time, suitable for cost-effective, high-quality film coating with increasing thicknesses. Alternative approaches have been intensively investigated, particularly using surface relief structures that create a gradient effective refractive index profile. 4,5 However, they often involve very complex fabrication processes and may be only applicable to particular kinds of materials. High-refractive-index dielectric nanoparticle arrays based on Mie resonances have been exploited to enable significant reduction of optical reflection, 6,7 although they typically do not enhance the transmission. 8 In the terahertz (THz) frequency range, ultrathin metallic films 9–11 and, more recently metal grids 12,13 and 2


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graphene 14 with appropriate surface impedance, have been used to realize broadband reduction of reflection from an optically dense medium to an optically less dense one. However, the one-directional operation and high insertion loss (only ∼50% transmission) have prevented them from applications where transmission enhancement is desired. The development of metasurfaces – two-dimensional equivalent of metamaterials – has enabled the unprecedented control of the amplitude, phase, and polarization states of reflection and transmission using an array of ultrathin subwavelength metal/dielectric resonators. 15–18 It has also been shown that few-layer metasurfaces – a stack of metasurfaces separated by dielectric spacers of subwavelength thicknesses – can not only dramatically enhance the performance, but also create new functionalities beyond the constituent metasurface layers. 19–23 Here we demonstrate dual and broadband optical antireflection using simple bi-layer metallic metasurfaces in the THz and mid-infrared (mid-IR) spectral ranges. In contrast to the previously demonstrated antireflection metasurfaces operating only within a narrow frequency range, 23–26 in this work we exploit specifically tailored dispersion of the individual metasurface layers to greatly expand the antireflection operational bandwidth. The high-performance dual and broadband antireflection metasurfaces are validated by numerical simulations and experimental measurements with excellent agreement, and analyzed using a semi-analytical model to elucidate the operational principle and provide design guidance for future metasurface devices. We find that in general broadband metasurface antireflection requires two metasurface layers with similar reflection amplitude but opposite phase dispersion, so to simultaneously satisfy the amplitude and phase conditions in realizing destructive interference over an extended frequency range. Figure 1 shows the antireflection metasurface structures consisting of a square array of silicon cross-pillars directly created on top of a high-resistivity silicon substrate by photolithography / ebeam-lithography and reactive ion etching (RIE), where the top and bottom surfaces, but not the sidewall of the silicon cross-pillars, are then coated with thin Ti/Au films (5 nm/200 nm thick for THz and 1 nm/30 nm thick for mid-IR), illustrated by the unit

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cell schematic in Figure 1a and false colored scanning electron microscopy (SEM) image in Figure 1b. Essentially, the structure can be considered as two cascade metallic metasurfaces separated by the silicon cross-pillars (and the surrounding air). Full-wave numerical simulations were first carried out using CST Microwave Studio, where the thin titanium layer was ignored and we used Drude conductivity for the gold film 27 and a frequency independent refractive index of 3.4 for the high-resistivity silicon substrate with negligible loss. 28 In the simulations, we tuned the geometric parameters to reach satisfying antireflection performance, including the length L, width w, and height h (i.e., etching depth) of the silicon cross-pillars with the period p = 1.1 × L (for THz) or p = 1.2 µm (for mid-IR). The optimized structure geometry and dimensions were then used to fabricate samples with example SEM images shown in Figure 1c and 1d to operate in the THz and mid-IR spectral ranges, respectively. In Figure 2a,b we plot the numerically simulated reflection and transmission spectra for a set of close-to-optimized dual-band THz metasurfaces, where we vary the cross-pillar (a) Gold

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Figure 1: Antireflection metasurface structures. (a) Unit cell schematic of the bi-layer metasurface structure consisting of top gold cross-resonator, bottom gold cross-slot, and silicon cross-pillar, with materials and geometric dimensions specified. (b) False colored SEM image of the unit cell (scale bar: 50 µm) and (c) large scale view (scale bar: 200 µm) of a fabricated THz metasurface. (d) Large scale view of a mid-IR metasurface (scale bar: 3 µm).

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length L and the corresponding period p, while other geometric parameters have been nearly optimized and fixed to w = 20 µm and h = 20 µm. The dual-band antireflection is evident from the observed two reflection dips and the correspondingly enhanced transmission peaks. When L = 76 µm, the two antireflection bands occur at ν1 = 0.550 THz with R1 = 1.09%, T1 = 96.4%, and at ν2 = 0.911 THz with R2 = 0.01%, T2 = 96.4%. As a comparison, for a bare silicon surface the relatively high refractive index results in Fresnel reflectance R = 30% and transmittance T = 70%. In general, by varying one of the geometric parameters, here L, we can realize near-zero reflection at one reflection dip, but not both, accompanied with a shift in antireflection frequencies. This can be clearly seen in the plots where reducing L to 70 µm and 64 µm improves the antireflection performance of the first (lower frequency) band but degrades the second (higher frequency) band, and shifts the antireflection bands to overall higher frequencies. For L = 70 µm, we obtain ν1 = 0.600 THz with R1 = 0.21%, T1 = 97.0%, and ν2 = 0.979 THz with R2 = 2.19%, T2 = 94.4%; for L = 64 µm, we have ν1 = 0.654 THz with R1 = 0.03%, T1 = 97.5%, and ν2 = 1.055 THz with R2 = 6.92%, T2 = 90.3%. Simultaneous zero reflection at both dips is possible in principle, but requires further optimization of geometric parameters also including cross-pillar width w and etching depth h. The designed antireflection THz metasurfaces were fabricated on top of 1 mm thick

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Figure 2: Dual-band THz antireflection metasurfaces. (a) Reflection and (b) transmission obtained by full-wave numerical simulations for three different values of cross length L. (c) Reflection and (d) transmission accordingly measured in experiments. Dotted lines: reflection and transmission for a bare silicon surface. 5


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double-side polished high-resistivity (ρ > 25, 000 Ω-cm) (100) silicon wafers, with an SEM image shown in Figure 1c. The fabrication involves standard photolithography methods, an RIE process, and directional e-beam metal evaporation (see Methods). The THz metasurfaces were characterized by measuring the reflection and transmission spectra under normal incidence using THz time-domain spectroscopy (THz-TDS, see Supporting Information for the schematic setup), 28–30 where time-windowing allows us to eliminate the effects arising from the reflection at the substrate back surface. Note that all transmission data presented in this paper have been processed to represent the absolute power transmittance through the metasurface-coated silicon surface. That is, they were normalized to the incident THz spectrum (without the sample) and the reflection loss at the sample back surface has been taken into account. The measured reflection and transmission spectra are shown in Figure 2c,d. The experimental results reveal excellent agreement with numerical simulations and reproduce all of the important characteristics of the dual-band antireflection metasurfaces. For L = 76 µm, we achieve low reflection and high transmission at two frequencies, ν1 = 0.587 THz with R1 = 2.43%, T1 = 91.3%, and ν2 = 0.932 THz with R2 = 0.56%, T2 = 89.3%. With reduced L, similar trends are observed as compared to simulations: for L = 70 µm, we realize ν1 = 0.631 THz with R1 = 1.71%, T1 = 92.0%, and ν2 = 0.998 THz with R2 = 1.75%, T2 = 87.6%; for L = 64 µm, we obtain ν1 = 0.675 THz with R1 = 0.45%, T1 = 95.0%, and ν2 = 1.071 THz with R2 = 8.08%, T2 = 83.6%. Broadband antireflection THz metasurfaces become intuitively possible through tuning the structural geometry to bring the two antireflection bands closer, where the crossresonators provide us the increasing degrees of freedom to tailor the resonant dispersion. This strategy contrasts remarkably to the one used to demonstrate broadband metamaterial absorbers, where resonators with different sizes were typically used. 31,32 Here we simply increase the etching depth of the silicon cross-pillars to h = 37 µm and reduce the crosspillar width to w = 15 µm. The simulated reflection and transmission spectra are shown in Figure 3a,b, validating such a strategy and revealing excellent broadband antireflection

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performance. When L = 70 µm, we accomplish R < 3% in a frequency range between 0.646 and 0.835 THz, where the corresponding transmittance is T > 94%. Increasing or decreasing the cross-pillar length L will shift the frequency of the antireflection band and alter the band flatness, as shown by the plots with L = 78 and 62 µm, similar to the case of dual-band metasurfaces. Transmittance Reflectance

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Figure 3: Broadband THz antireflection metasurfaces. (a) Reflection and (b) transmission obtained by full-wave numerical simulations for three different values of cross length L. (c) Reflection and (d) transmission accordingly measured in experiments. Dotted lines: reflection and transmission for a bare silicon surface. In experiments using metasurface structures similar to the one shown in Figure 1c, the best broadband antireflection performance occurs when L = 62 µm, as shown in Figure 3c,d, mainly due to the fabrication imperfection (see Supporting Information). The measured reflectance is R < 3% in a frequency range between 0.719 and 0.953 THz, achieving a bandwidth of 234 GHz or a relative bandwidth of 28%. The corresponding experimental transmittance is T > 87% in this frequency range, with the highest transmittance Tmax = 92% at near 0.9 THz. It is worthwhile pointing out that, in the frequency range between 0.748 and 0.932 THz (a relative bandwidth of 22%), the experimentally measured reflectance is R < 1%, attractive for many applications where eliminating the undesirable reflection over a broad bandwidth is critical. Consistent with the numerical simulations, increasing the cross-pillar length L shifts the antireflection band to low frequencies, and degrades the band flatness, as shown by the tilted spectra when L = 70 and 78 µm. The dual and broadband antireflection metasurfaces are expected to operate at other 7


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spectral regions by structural scaling. Here we plot the simulations in Figure 4a for the reflection and transmission spectra of a mid-IR broadband antireflection metasurface structure where the nano cross-pillars have period p = 1.2 µm, length L = 900 nm, width w = 400 nm, and height h = 600 nm. The simulated reflectance is less than 5% at wavelengths from 5.36 to 6.66 µm, where the transmittance is higher than 94%. The mid-IR metasurface structure was also fabricated (see Supporting Information) with an SEM image shown in Figure 1d and then characterized using a Fourier transform infrared spectroscopy (FTIR) microscope. A single-side polished silicon substrate was used to fabricate the metasurface structure for reflection measurements to avoid the undesirable reflection arising from the substrate back surface. The experimental results are shown in Figure 4b. The reflectance is less than 5% at wavelengths from 4.76 to 6.57 µm, and the transmittance is higher than 85% between 5.34 and 6.83 µm, which is slightly red-shifted as compared to the reflection reduction, and may be an artifact of the FTIR microscope measurements and from the use of different samples (polished and unpolished back surface for transmission and reflection measurements, respec1.0 0.8 Reflectance / Transmittance

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Figure 4: Broadband antireflection metasurface at mid-infrared wavelengths. (a) Simulated and (b) measured reflection and transmission spectra. Dotted lines: reflection and transmission for a bare silicon surface. 8


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tively). Note that when comparing to the mid-IR antireflection previously realized using single-layer cross-resonator arrays on top of a thin dielectric film, 26 the present antireflection metasurface structure allows for a flatter performance in addition to achieving a broader bandwidth. Despite the overall agreement between simulations and experiments, we find some obvious deviations arising from the fabrication imperfection, particularly the shift of the antireflection frequency and the slightly less amount of transmission enhancement. The fabrication tolerance of the THz metasurfaces is typically about ±1 µm for the horizontal dimensions. When performing RIE, it is expected that the sidewall of the silicon pillars would not be completely vertical and the bottom surface not be completely flat (see Supporting Information). These factors contribute to the observed shift of the antireflection frequency as well as the performance. For instance, in experiments the optimal THz broadband antireflection performance is obtained in the sample with L = 62 µm rather than the designed L = 70 µm in numerical simulations. Additionally, there is photoresist residual remained on the top surface (see Figure 1a) and significant roughness at the bottom surface caused by RIE (see Supporting Information), both of which contribute to the increasing losses in the subsequently deposited gold films, and result in slightly lower transmission enhancement obtained in experiments. However, the photoresist residual can be removed by optimizing the fabrication process, and the silicon surface roughness due to RIE can be reduced by hydrogen annealing, 33 thereby further improving the antireflection performance. In order to achieve an even broader antireflection bandwidth, it is important to understand the underlying mechanism. In the present antireflection metasurface structures the two complementary metallic layers are separated by a spacing equal to the thickness of the silicon cross-pillars in the wave propagation direction (see Figure 1a). The metal cross-resonator array on the top of the silicon cross-pillars and the metal cross-slot array at the bottom surface can be considered as two effective interfaces for the propagating waves, while the silicon cross-pillars form a subwavelength waveguide array and can be treated as

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an effective dielectric medium with an effective index that can be computed numerically. The feasibility of this consideration has been discussed in detail in literature, 34 showing that only the fundamental mode can propagate and travel between the two boundaries of the subwavelength waveguides in the same way as multireflection and interference in a dielectric thin film. Such a treatment has also been widely used recently in dielectric metasurfaces consisting of a subwavelength dielectric pillar array. 35,36 Differing from usual stratified media, there are possible near-field interactions between the resonant metallic structures at the two interfaces. 37,38 That is, the resonant response at one interface will influence the resonance at the other interface, particularly when the spacing between the two layers (i.e., cross-pillar height h) is sufficiently small. However, this near-field coupling decays rapidly with increasing spacer thickness, transitioning from the reactive near-field region to radiative near-field region, which is roughly the case in our antireflection metasurface structures. Therefore, as a first-order approximation, it is reasonable to treat the individual metasurface layers as independent interfaces with complex and dispersive reflection and transmission coefficients, which can be obtained by performing numerical simulations. Then the overall response of the antireflection metasurface can be analytically calculated based on the interference between the two effective interfaces forming a Fabry-Pérot-like subwavelength cavity 39 (see Supporting Information), with the effective dielectric spacer given by the subwavelength cross-pillars. Our previous works have shown that such an analysis can successfully model few-layer metasurfaces with subwavelength spacer thicknesses (though continuous dielectric films were used instead of two-dimensional dielectric grids employed here) for functionalities including perfect absorption, 37,39 antireflection, 23,26 and polarization rotation and phase gradient. 22 The simulated reflection amplitude and phase spectra at the two effective interfaces are plotted in Figure 5a,b for the dual-band and Figure 5d,e for the broadband THz antireflection metasurfaces, where the subscript ‘ij ’ represents incidence from medium ‘i’ to medium ‘j ’, with ‘1’, ‘2’ and ‘3’ indicating air, spacer, and substrate, respectively. The effective

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dielectric spacer provides a round-trip propagation phase delay 2β, which is also obtained through numerical simulations and shown in Figure 5b,e. With details provided in Supporting Information, the semi-analytically calculated reflectance and transmittance spectra are shown in Figure 5c,f for the dual and broadband antireflection metasurfaces with L = 70 µm, respectively. Our calculations reproduce all important characteristics revealed in both fullwave simulations and experimental measurements presented in Figures 2 and 3, despite our neglect of the near-field interactions between the two layers of metallic structures, which

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Figure 5: Amplitude and phase conditions required for realizing dual and broadband antireflection metasurfaces. (a,d) Reflection amplitude r and (b,e) phase φ spectra of the two constituent metasurfaces, and (c,f) semi-analytically calculated reflectance and transmittance spectra for the dual-band (a-c) and broadband (d-f) antireflection metasurfaces. Also plotted in (b,e) is the propagation phase delay 2β within the effective spacer and the total round-trip phase φ21 + φ23 + 2β within the metasurface cavity. The vertical dotted lines in (a-c) and the shaded regions in (d-f) indicate the antireflection frequencies where equal (or similar) reflection amplitude and zero round-trip phase conditions are satisfied (or approximately satisfied). Indicated by the vertical dotted lines in Figure 5a-c for the dual-band antireflection metasurface, the semi-analytical calculations reveal that both amplitude (r12 = r21 = r23 ) and phase (φ21 +φ23 +2β = 0) conditions are satisfied at two discrete frequencies located at below 11


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and above the resonance frequency of the bottom metal cross-slot array. This is contrast to the previous demonstration of single narrowband antireflection metasurface where the antireflection conditions were satisfied only at one off-resonance frequency. 23 For the broadband antireflection metasurface, the reflection coefficients of the two constituent metasurface layers cross each other at frequencies slightly below and above the resonance frequency of the bottom metal cross-slot array, resulting in similar reflection amplitude over a broad frequency range between 0.62 and 0.80 THz. At the same time, their phase dispersion, particularly in the cross-slot array, results in φ21 + φ23 + 2β ≈ 0 in this frequency range as indicated by the shaded regions in Figure 5d-f. It becomes clear that the phase dispersion of the cross-slot array plays a critical role in achieving destructive interference to minimize the reflection over a relatively broad bandwidth. These calculations also suggest that, in order to further broaden the bandwidth of antireflection metasurfaces, it is beneficial to use structures with weaker resonances at both metasurface layers so to maximize the frequency range of the equal reflection coefficients and satisfy the near-zero phase condition. It is also possible to design the metasurface layers operating at off-resonance frequencies where the reflection amplitude is approximately equal at the two layers and remains nearly unchanged with frequency, but the phase has opposite dispersion, for instance, resulting from respectively the capacitive and inductive response of the two metasurface layers. Such an understanding of the underlying mechanism is of particular importance and provides practical guidance in initially designing metasurface structures with desirable response, which can be further optimized using full-wave numerical simulations in order to ultimately accomplish high-performance few-layer functional metasurfaces with increasing operational bandwidths.

Conclusion In summary, we have demonstrated bi-layer antireflection metasurfaces to enable dual and broadband operation in the THz and mid-IR spectral ranges. By tailoring the geometry

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of the subwavelength metal/dielectric structures, we numerically and experimentally show dramatical reduction of Fresnel reflection and significant enhancement of transmission at a substrate surface, either at two discrete narrow bands or over a broad bandwidth exceeding 20%. By exploiting the self-aligned bi-layer metallic metasurfaces, this antireflection scheme avoids the deposition of thick dielectric films, and is applicable to substrates with arbitrary refractive indices. Using a semi-analytical interference model, our calculations are in excellent agreement with full-wave simulations and experimental measurements. We found that the dispersion of the constituent structures play a critical role in achieving broadband destructive interference to minimize the reflection and enhance the transmission. Our antireflection metasurfaces address many issues encountered in optical antireflection for expanded bandwidths, and the identified operational principle will provide a practical guidance in designing future broadband metasurface functional devices for a variety of optical and optoelectronic applications.

Acknowledgement The authors acknowledge partial financial support from Los Alamos National Laboratory LDRD Program and Fundamental Research Funds for the Central Universities and Program for Innovation Research of Science in Harbin Institute of Technology (PIRS OF HIT) under Grant No. T201408. This work was performed, in part, at the Center for Integrated Nanotechnologies, a U.S. Department of Energy, Office of Basic Energy Sciences Nanoscale Science Research Center operated jointly by Los Alamos and Sandia National Laboratories. Los Alamos National Laboratory, an affirmative action/equal opportunity employer, is operated by Los Alamos National Security, LLC, for the National Nuclear Security Administration of the U.S. Department of Energy under Contract No. DE-AC52-06NA25396.

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Methods Fabrication.

For the fabrication of THz metasurfaces, 4′′ double-side polished, high-

resistivity (ρ > 25,000 Ω-cm) (100) silicon wafers were used. A layer of negative photoresist (AZ nLOF2035) was first spin-coated on a silicon wafer, and metasurface patterns were defined by conventional contact photolithography. The photoresist patterns then served as a mask to etch the underlying silicon by a standard deep reactive ion etching (DRIE) process with SF6 /C4 F8 chemistry, forming a square array of silicon pillars (array area = 1 cm × 1 cm). After silicon DRIE, the remaining photoresist was removed by O2 plasma. Lastly, 10 nm/200 nm Ti/Au films were deposited by electron-beam evaporation, where the samples are perpendicular to the metal sources. The mid-IR metasurfaces were fabricated similarly using standard electron-beam lithography methods (positive resist PMMA950 A2). After developing the e-beam lithography resist, Ti/Au (1 nm/30 nm) were deposited on the sample. After the lift-off process, the Ti/Au patterns served as a hard etching mask for RIE. Next, the Ti/Au hard mask was removed by potassium iodide (KI) solution (KI:I2 :H2 O = 4:1:40). The final step was the e-beam deposition of a thin Au (30 nm) layer on the sample with etched nano-pillars to form bi-layer metasurfaces. In order to achieve accurate normalized spectra in the FTIR measurements, samples on double-side and single-side polished silicon substrates were used for the transmission and reflection measurements, respectively.

Supporting Information Available The Supporting Information provides the interference model for semi-analytical calculations of bi-layer antireflection metasurfaces, schematic of the THz time-domain spectrometer for THz reflection and transmission measurements, and additional SEM images of the metasurface samples revealing fabrication imperfection. This material is available free of charge via the Internet at http://pubs.acs.org/.

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Graphical TOC Entry Transmittance Reflectance

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1.0 0.8 0.6 0.4 0.2 0.0 0.8 0.6 0.4 0.2 0.0 0.4


1.2

For Table of Contents Only Manuscript title: Bi-layer metasurfaces for dual and broadband optical antireflection Authors: Li Huang, Chun-Chieh Chang, Beibei Zeng, John Nogan, ShengNian Luo, Antoinette J. Taylor, Abul K. Azad, and Hou-Tong Chen Description: False colored SEM image of the metasurface unit cell (left panel) that enables broadband antireflection performance (right panel).

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Bilayer Metasurfaces for Dual- and Broadband Optical Antireflection

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