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Abstract
Active metasurfaces, which are tunable and reconfigurable nanophotonic structures with active materials, have been in spotlight as a versatile platform to control the profiles of scattered light. These nanoscale structures show surpassing functionalities compared to the conventional metasurfaces. They also play an important role in a wide range of applications for imaging, sensing, and data storage. Hence, the expansion of functionalities has been highly desired, in order to overcome the limited space constraints and realize the integration of several optical devices on a single compact platform. In this context, an electrically tunable metasurface that enables respective modulation of the phase and amplitude of reflected light, depending on the angle of incidence at the targeted wavelength, is proposed. This resonance-based device with hyperbolic metamaterial substrate excites different kinds of highly confined modes, according to the incident angle. Indium tin oxide is employed to offer electrically tunable optical properties in the near-infrared regime. At the wavelength of 1450 nm, the proposed device modulates the phase of reflected light with ~207 degrees of modulation depth for normal incidence, whereas it shows ~86% of relative reflectance change for oblique incidence of 60 degrees. In principle, the proposed scheme might provide a path to applications for the next-generation ultracompact integrated systems.
© 2018 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
In nanophotonics, thanks to the development of nanofabrication technologies, metasurfaces have been an attractive approach for implementation of ultracompact optical elements and systems [1–5]. This specific arrangement of resonant nanostructures with subwavelength period, has offered unprecedented flexibility to manipulate light in extraordinary manners at the nanoscale. Complex wavefront synthesis has been achieved by designing local nanoantennas of metasurfaces to control the phase, amplitude or polarization of the scattered light. A number of studies have reported a wide range of applications to replace the conventional bulk optic devices, including focusing lens [6–9], visible holograms [10–14], optical vortex generation [15–17], among others.
However, such passive metasurfaces have significant limitations in terms of device functionalities because the profiles of scattered wavefront are fixed once metasurfaces are designed and fabricated. For instance, meta-holograms can only generate a limited number of images under specific polarized incidences, and ultrathin beam deflectors with metasurfaces can only operate at a certain planned angle. To overcome these constraints of functionalities, new types of nanophotonic devices called active metasurfaces, the combination of passive metasurfaces and active optical materials, have been demonstrated in recent years [18–30]. The phase, amplitude, or polarization can be dynamically controlled by external physical stimuli to active materials. With enhanced device performance and functionalities, a wide range of applications of active metamaterials such as sensors [18–20], data storages [21,22], and tunable lenses [23,24] have been demonstrated. Among various physical mechanisms for control of active materials combined with metasurfaces such as phase-transition [25–28], mechanical reconfiguration [29,30], and chemical activation [18,31,32], electronic modulation using electro-optic phenomenon [33–40] has been in the spotlight due to its distinct advantages such as low power consumption, fast switching speed, and compatibility with semiconductor electronics. There have been many studies on the demonstration of dynamically tunable optical responses using active electro-optic materials such as transparent conducting oxides (TCOs) [33–38], graphene [39,40], and heavily doped semiconductors [41,42]. In particular, TCOs are well known as near-infrared (NIR) plasmonic materials due to their large bandgaps and quasi-parabolic conduction bands [43]. Thus, modulation of amplitude, phase or polarization in NIR regime deeply have been achieved by TCO-based metasurfaces [33–36].
Now, as a further step toward the ultimate goal of dynamic nanophotonics, realization of ultracompact all-optical devices and systems, the implementation of multiple electromagnetic functions in one design has been highly desired since such active metasurfaces that integrate diversified functionalities allow not only for efficient use of limited space but also for ease of optical system design. However, most of the reported studies about multifunctional devices were either amplitude- or phase-based applications including optical memory, beam deflecting, splitting, focusing and polarization switching [34,44–48]. It is still very challenging to simultaneously and independently modulate amplitude and phase of scattered light in one platform at the same wavelength. In this study, a dynamically tunable metasurface designed to implement two functions at one fixed target wavelength is proposed and theoretically investigated. In the NIR region, the proposed device operated by applied bias based on TCO-integrated structure can modulate the phase or amplitude of reflected light depending on the condition of incident light. In case of normal incidence, the phase modulation is achieved through the mode called gap plasmon polariton (GPP) modes designed strongly confined in TCO layer [Fig. 1(a)]. In addition, to perform the other function on the same structural basis, another type of highly confined modes is utilized by applying hyperbolic metamaterial (HMM) substrate [49]. In case of oblique incidence, the amplitude modulation is implemented by using these bulk plasmon polariton (BPP) modes since these kinds of guided modes are sensitive to oblique incidence [Fig. 1(b)]. The rest parts of manuscript are organized as follows. After briefly introducing the overall design of the proposed device, detailed analysis on the metal-oxide-semiconductor (MOS) capacitor and high-k modes from HMM structure is carried out in terms of design strategy and working principle. Then, modulation performances with numerical demonstration and relevant discussions are presented for both phase-type and amplitude-type modulation mechanisms. In our calculations, rigorous coupled wave analysis (RCWA), a kind of Fourier modal method for electromagnetic simulation, is used [50]. For two-dimensional (2D) simulation, the truncation order related to the size of Fourier coefficient matrix is set to 100 since the simulation results converge constantly when the truncation order is greater than 70.
Fig. 1 Device schematic of an electrically tunable multifunctional optical modulator constructed from a metal-oxide-semiconductor capacitor and hyperbolic metamaterial. The reflected light is controlled by electrically tuning the carrier density of ITO as an active semiconductor layer. (a) Phase of the reflected light is modulated with normal incidence and (b) amplitude of the reflected light is controlled in case of oblique incidence.
2. Design and working principle of proposed device
Schematic illustration of the unit cell of proposed device is depicted in Fig. 2(a). The device consists of a nanoscale MOS capacitor on the HMM substrate and a two-dimensional metallic nanogratings placed on the top. These three components of the proposed device are respectively utilized to demonstrate electrically-driven modulator, to support strongly confined modes [51], and to couple the incident light matching the momentum conditions inside the structure. The detailed physical mechanisms of each component and its working principles are explained as follows.
Fig. 2 (a) Schematic of unit cell of the proposed device. The Au nanograting is placed on the Au-HfO2-ITO capacitor, and the HMM that consists of the 3 sets of Au-HfO2 multilayer is under the capacitor as a substrate. Unit cell dimensions are chosen as follows: width of grating w = 230 nm, and periodicity of unit cell p = 530 nm. Each thickness of grating, ITO, Au, and HfO2 is tg = 50 nm, ts = 20 nm, tm = 20 nm, to = 10 nm. (b) Spatial distribution of carrier density and (c) the real value part of permittivity of ITO at wavelength of 1450 nm as a function of position from HfO2-ITO interface for different voltages. The charge accumulation region dependent on the voltage is divided into 13 layers in order to simulate the real condition accurately as much as possible (red rectangles). Gray area highlights the spatial region where the real value of permittivity acquires values between −1 and 1 representing the ENZ region.
2.1 MOS capacitor: electro-optic mechanism and quantitative analysis
In our device, the MOS capacitor consists of gold (Au), hafnium oxide (HfO2), and ITO [Inset of Fig. 2(b)]. As a key component of active control, the MOS capacitor serves charge accumulation or depletion in active layer according to the direction of applied electric field [33–36]. The amount of accumulated or depleted carriers is affected by applied voltage, thus complex refractive index of active region can be controlled by the application of voltage. To investigate the electrical properties of the MOS capacitor, the amount of charge accumulation or depletion is calculated with the commercial tool (COMSOL Multiphysics) by assuming a one-dimensional MOS capacitor. The material parameters are selected from several literature values as follows [33–36]. The carrier density of semiconductor is 3 × 1020 cm−3; the temperature is 293.15 K; the work function of Au is 5.1 eV; and the relative DC permittivity of HfO2 is 25 at the static condition. Figure 2(b) shows the calculated carrier densities at the HfO2-ITO interface under various applied voltages. Especially, the thickness of HfO2 is carefully designed to prevent dielectric breakdown. The range of voltage application is set from −2 to 3 V since the breakdown field of HfO2 is known to be approximately 3.1 MVcm−1 [35]. Electron depletion at the interface occurs when the zero voltage is applied. This phenomenon is due to charge redistribution by electron diffusion under equilibrium conditions. An accumulation layer is formed in ITO at the HfO2-ITO interface when the voltage application is larger than 1 V, and more charges accumulate with higher voltages. Importantly, substantial changes are concentrated within around 4 nm under various bias, whereas the carrier concentration over this active region maintains the background carrier density value regardless of an external electric field. Since the active region is very narrow, field effect using TCOs cannot inherently provide sufficiently large change to the device. For this reason, the device is designed to take advantage of the epsilon-near-zero (ENZ) conditions that the real value of permittivity is between −1 and 1, to achieve large variation of refractive index with a little change of permittivity according to the Eq. (1) [52] as follows:
For quantitative analysis of the variation of optical properties by such electrical stimuli, complex permittivity of ITO under applied voltages is calculated. Figure 2(c) shows the real value of permittivity at an operation wavelength of 1450 nm as a function of distance from the HfO2-ITO interface for different applied voltages. In our calculation, ITO is modeled as a Drude metal since it has sufficient amount of carrier density in the range of ~1020 cm−3. The permittivity of ITO (εITO) and the plasma frequency of ITO (ωp) are given by
Here, the parameters are selected from literature as follows: dielectric permittivity at infinite frequency ε∞ = 4.4, damping constant Γ = 1.8 × 1014 Hz, effective mass of electron m* = 0.22 × m0, where m0 is the electron rest mass, and the carrier density N = 3 × 1020 cm−3 [33–36]. It is worth noting that N can be controlled by applied voltages as shown in Fig. 2(b). Therefore, the permittivity of ITO in the accumulation or depletion region can also be adjusted by voltage application. Note that the value of real permittivity at the HfO2-ITO interface decreases as the applied bias increases. In particular, it reaches the ENZ region at an applied bias of approximately 1.9 V, and the sign of the real value of permittivity changes from positive to negative under higher bias of 1.9 V. It means that the region where charges accumulate becomes more metallic and lossy. In our simulations, the 5 nm region is divided into 13 layers to reflect the actual situation more accurately [53].
2.2 High-k modes: GPPs and BPPs
Now, we focus on the role of HMM substrate in the proposed device. Our HMM is constructed by alternatively stacking 3 sets of metal (Au) and dielectric (HfO2) layers. Since the metal-dielectric multilayer is stacked along the z-direction, the HMM exhibits anisotropic characteristics, which means the uniaxial dielectric tensor components are different from each other (ε∥ = εx = εy and ε⊥ = εz). Figure 3(a) shows the calculated relative permittivity tensor of HMM using the effective medium theory [54]. This is possible since the thicknesses of Au and HfO2 layer are sufficiently thin to be respectively 20 nm and 10 nm compared to the wavelength of incidence. It is confirmed that the HMM shows a hyperbolic dispersion at interesting wavelengths, where ε∥< 0 and ε⊥> 0.
Fig. 3 (a) Real and imaginary parts of effective permittivity of Au-HfO2 HMM calculated using effective medium theory. The dielectric permittivity tensor components in the parallel direction and the normal direction are depicted as solid and dashed lines, respectively. (b) Three kinds of high-k modes. The spatial distribution of y-component of magnetic field at gap plasmon (Left panel), bulk plasmon (Center panel), and surface plasmon (Right panel) resonance. (c) Dispersion relation of the proposed structure. Color map of the simulated reflectance versus wavevector and wavelength for the proposed structure (Left panel) and the structure without HMM in the proposed one (Right panel). The various types of resonance are named (Inset: structure diagrams).
Based on this unique characteristics, optical mode engineering is carried out. According to electronic characteristics of the MOS capacitor of the proposed device introduced in the previous section, there are limitations of modulation using ITO such as too small refractive index change and too thin thickness of accumulation layer. To enhance light-active layer interaction and expand functionality, the proposed device relies mainly on gap plasmon polaritons (GPPs) and bulk plasmon polaritons (BPPs), different kinds of strongly confined high-k modes [55,56]. Figure 3(b) represents mode profiles of three optical modes shown in the proposed structure. First of all, a metal-insulator-metal (MIM) waveguide consisting of Au gratings and the top Au layer of HMM supports GPP mode which has extremely high modal index [The left panel of Fig. 3(b)]. Note that black arrows indicating surface currents form a current loop that induces magnetic field flow along the parallel direction. In this case, incident wave feels HMM as metal (ε∥< 0), thus cannot penetrate inside HMM. Due to a merit to induce strong light-matter interaction, this kind of configuration has attracted considerable attention for use in nanophotonic applications such as a perfect absorber [33–36,57]. The thicknesses of HfO2 and ITO are respectively chosen to be 10 nm and 20 nm which are sufficiently thin since the modal index is approximately inversely proportional to the gap size between metal layers [50]. Most of the previously reported studies on TCO-based modulators have used GPP mode to control the amplitude or phase of reflected light due to its high modal index [33–36].
On the other hand, BPP mode, a different kind of highly confined mode that can be only observed with HMM substrate, is excited within HMM substrate unlike GPP mode [The center panel of Fig. 3(b)]. This mode is induced from surface plasmon polaritons (SPPs) excited at the interface between Au grating and HfO2. In general, SPP mode has a long tail to dielectric material and very short penetration depth to metal [The right panel of Fig. 3(b)]. However, the evanescent field of SPPs can be easily coupled to HMM owing to the thin thickness of metal and dielectric layers in HMM. Moreover, SPP wave oscillates along the parallel direction, thus this mode feels HMM as high refractive index dielectric material (ε⊥> 0).
These high-k modes including GPP and BPP supported by the proposed device can be intuitively explained by calculating the dispersion relation. The reflectance is calculated at different wavelengths and wavevectors for two cases: the proposed design [The left panel of Fig. 3(c)] and simple MOS capacitor without HMM substrate [The right panel of Fig. 3(c)]. The period, width and thickness of the nanogratings are respectively chosen as p = 530 nm, w = 230 nm, and tg = 50 nm to design the spectral position of the gap plasmon (GP) resonance at NIR frequencies. In both cases, surface plasmon (SP) resonance is observed between 1000 nm and 1100 nm. This mode is excited at the Au-SiO2 interface. The reflectance of SP resonance in the proposed structure is lower than that of the case without HMM due to the energy loss from coupling with the metal-dielectric multilayer. More importantly, there is a GP resonance around 1500 nm of wavelength, and the proposed device acts as a perfect absorber at these wavelengths. These results imply two important properties; GPP mode is not affected by additional changes in structure such as the set number of multilayer, and not sensitive to wavevector of the incidence. It is because this GPP is like localized resonance, not related to the structure of the lateral period but only to the geometric parameters of the grating and thickness of the dielectric layer. On the other hand, it is confirmed that BPP modes are supported by metal-dielectric multilayer (The left panel of Fig. 3b), and sensitive to the wavevector of the incidence. It means that the resonance characteristics of BPP modes can be tuned by changing the angle of incidence while those of GPP modes remain unchanged. Therefore, based on two different kinds of high-k modes, different kinds of functions can be implemented on a single platform depending on the angle of incidence.
3. Results and discussions
3.1 Phase modulation with GPP mode
Let us think about the situation where a wave is normally incident with transverse magnetic (TM) polarization along the z-direction. In this case, nearly perfect absorption phenomenon by GPP modes in the NIR range is utilized to modulate the phase of reflected light. Figure 4(a) shows the reflectance spectra as a function of wavelength for different applied voltages. The resonance dip shifts to shorter wavelengths as the applied bias increases due to reduction of the value of real permittivity. From the wavelength of 1450 to 1460 nm, the device absorbs almost all the incident light regardless of the changing applied voltage. As shown in Fig. 3(b), the electric field does not penetrate into HMM and light is highly confined between the metal grating and the topmost metal layer. The GPP mode is formed on ITO and HfO2 layers between Au nanogratings and HMM. It means that the incidence feels the HMM substrate as a metal layer. As discussed in the previous section, the resonance is not affected by external changes such as the wavevector of incidence or the number of multilayer since the momentum matching conditions to excite GPP mode in the MOS capacitor are already satisfied at the resonant wavelength, which leads to nearly constant reflectance value.
Fig. 4 For normal incidence, (a) Color map of the reflectance versus wavelength and applied voltages. (b) Calculated complex reflectance as a function of applied voltages for the wavelength of 1450 nm. Applied voltage increases along the clockwise direction. (c) Simulated phase shift and intensity as a function of applied voltages at target wavelength of 1450 nm. (d) The spatial distribution of y-component of magnetic field of reflected light when the applied voltage is −2 V (Left panel) and 3 V (Right panel).
To observe the phase shift of reflected light more intuitively, complex reflectance as a function of applied voltage from −2 to 3 V for target wavelength of 1450 nm is plotted in Fig. 4(b). In the complex map, the distance from the origin is related to the absolute value of reflectance, and the angle with respect to real axis means the phase of the reflected wave. In other words, phase modulation depth can be calculated using the shifted angle and the applied voltage around the origin. The complex reflectance shows a tendency to move along the trace of the curve in the clockwise direction regardless of the wavelength of incidence as the voltage increases. Basically, the variation of complex reflectance due to the voltage application is not large since the permittivity of the material changes only in a very thin region. In order to enable large phase modulation with such small changes, the variation of complex reflectance must occur near the origin, and the curve has to turn around the origin. Therefore, a wide range of phase modulation can only occur near the perfect absorption condition. Moreover, it has a wide bandwidth of approximately 20 nm under the condition of π-phase modulation due to broad bandwidth characteristics of GPP mode. The simulation results show that the proposed device can modulate the phase of reflected light with about 207 degrees of modulation depth at the operation wavelength of 1450 nm for normal incidence, whereas the amplitude is almost fixed around 1.7% [Fig. 4(c)]. For more intuitive comprehension, the phase shift is visualized in Fig. 4(d). These field distributions represent the case of applied voltages of −2 V (Left panel) and 3 V (Right panel), respectively.
3.2 Amplitude modulation via interactions of BPP and GPP modes
Now, let us consider the case of oblique incidence on the same structural condition. With TM polarization, the incident light can have a momentum of n0k0sinθ as the incidence angle is θ, and such momentum compensation by oblique incidence can excite several new types of modes that are not seen in normal incidence. To implement different types of modulation at the same operating wavelength, it is important to design the BPP modes to be as close to the wavelength as possible with the GPP mode. It can be engineered by adjusting the number of added metal-dielectric layers and the incident angle without affecting the GPP modes.
First, Fig. 5(a) represents the reflectance spectra as a function of wavelength for changing the number of sets in HMM. These BPP modes occur at shorter wavelengths as the number of sets in HMM increases. This is because as the number of layers increases, the number of nodes in the field increases which requires a higher energy level to induce the BPP modes. Then, the reflectance spectra as a function of wavelength for different incident angles are calculated [Fig. 5(b)]. In this calculation, it is assumed that the HMM consists of 3 sets of Au-HfO2 multilayers. Note that as the angle of incidence increases, the wavelength at which same order BPP resonance occurs becomes longer. It is reasonable that the wavenumber in vacuum k0 gets smaller as the sine value of angle increases because the amount of momentum needed to form this kind of mode is constant. This interpretation applies equally to SPP excited at the Au-SiO2 interface with a sharp resonance dip. As a result, the proposed modulator has another kind of resonance mode near the operating wavelength by introducing oblique incidence of 60 degrees. Unlike GPP modes, these kinds of modes are formed when the constructive interference between the waves guided from both lateral structures occurs, thus the x-component of the wavevector is key factor to determine the resonance characteristics. It is evident that the electric field penetrates into the metal-dielectric multilayer and confined on the dielectric layers in HMM [Fig. 3(b)]. The mode profile shown in the y-component of the magnetic field indicates that the mode is formed in the whole structure including HMM, and it is supported by a nanoscale multilayer as well as the MOS capacitor.
Fig. 5 The reflectance spectra (a) for the multiple Au-HfO2 sets in HMM with normal incidence and (b) as a function of the incident angles with the proposed structure. Several resonances are classified by types.
Figure 6(a) represents the reflectance spectra for different wavelengths and applied voltages. There are two resonance dips, BPP mode at around 1420 nm of wavelength and GPP mode at around 1470 nm of wavelength as discussed above. Note that the intensity varies in case of GPP mode depending on the applied voltages while BPP mode absorbs almost the entire incidence regardless of the applied voltages. This is because by bringing the GPP mode closer to the BPP mode, perfect absorption by GP resonance breaks, and GP resonance becomes unstable by interference between two high-k modes. Complex reflectance as a function of applied voltage from −2 to 3 V for target wavelength of 1450 nm is depicted in Fig. 6(b). Similar to the phase modulation case, each curve moves in the clockwise direction depending on the applied bias. Around the BP resonance wavelength, the complex reflection coefficient moves near the origin. On the other hand, in case of GP resonance at around 1450 nm, the values of complex reflection coefficients are away from the origin; thus, the curve no longer forms a shape to move around the origin. Therefore, as the applied voltage varies, the distance from the origin is changed greatly compared to the phase modulation case, while the phase shift is restricted. At the operation wavelength of 1450 nm, the proposed device can modulate the reflectance with ~6.8% of modulation depth (defined as ΔR(λ) = R(λ)V = 3 - R(λ) V = −2), and ~86% of relative reflectance change (defined as ΔR/RV = −2) while the phase shift is restricted below 16 degrees [Fig. 6(c)]. The intensities of magnetic fields for applied voltages of −2 V (Left panel) and 3 V (Right panel) confirm that the amplitude of the reflected light is modulated in Fig. 6(d).
Fig. 6 For oblique incidence of 60°, (a) Color map of the reflectance versus wavelength and applied voltages. (b) Calculated complex reflectance as a function of applied voltages for target wavelength of 1450 nm. Applied voltage increases along the clockwise direction. (c) Simulated phase shift and intensity as a function of applied voltages at operating wavelength of 1450 nm. (d) The intensity of magnetic field of reflected light when the applied voltage is −2 V (Left panel) and 3 V (Right panel).
The proposed device exhibits polarization-dependent properties since it is designed based on 1D grating. To change it to 2D rectangular or circular pattern, a similar performance can be expected [58].
4. Conclusion
In conclusion, a numerical demonstration of an electrically tunable multifunctional device in the NIR region is reported. The nanoscale MOS capacitor including ITO layer is introduced for implementation of electrical-driven device. Furthermore, by combining an HMM configuration, the proposed modulator was designed to have different kinds of high-k modes, BPP modes in HMM region, depending on the angle of incidence. The proposed device acts like a phase modulator with normal incidence using GPP mode formed in MOS capacitor, whereas it shows amplitude modulation with oblique incidence using the resonance coupling of GPP and BPP modes formed inside HMM substrate. It is expected that the proposed mechanism in optical mode engineering can open a new pathway to design practical multifunctional applications such as miniaturized multi-sensing systems and spatial light modulators capable of phase and amplitude modulation for the next generation ultracompact integrated optical systems.
Funding
Center for Advanced Meta-Materials (CAMM) funded by the Ministry of Science, ICT and Future Planning as Global Frontier Project (CAMM-2014M3A6B3063710).
References
1. N. Yu and F. Capasso, “Flat optics with designer metasurfaces,” Nat. Mater. 13(2), 139–150 (2014). [CrossRef] [PubMed]
2. A. E. Minovich, A. E. Miroshnichenko, A. Y. Bykov, T. V. Murzina, D. N. Neshev, and Y. S. Kivshar, “Functional and nonlinear optical metasurfaces,” Laser Photonics Rev. 9(2), 195–213 (2015). [CrossRef]
3. P. Genevet, F. Capasso, F. Aieta, M. Khorasaninejad, and R. Devlin, “Recent advances in planar optics: from plasmonic to dielectric metasurfaces,” Optica 4(1), 139–152 (2017). [CrossRef]
4. H. T. Chen, A. J. Taylor, and N. Yu, “A review of metasurfaces: physics and applications,” Rep. Prog. Phys. 79(7), 076401 (2016). [CrossRef] [PubMed]
5. P. Lalanne and P. Chavel, “Metalenses at visible wavelengths: past, present, perspectives,” Laser Photonics Rev. 11(3), 1600295 (2017). [CrossRef]
6. F. Aieta, P. Genevet, M. A. Kats, N. Yu, R. Blanchard, Z. Gaburro, and F. Capasso, “Aberration-free ultrathin flat lenses and axicons at telecom wavelengths based on plasmonic metasurfaces,” Nano Lett. 12(9), 4932–4936 (2012). [CrossRef] [PubMed]
7. S. Wang, P. C. Wu, V.-C. Su, Y.-C. Lai, M.-K. Chen, H. Y. Kuo, B. H. Chen, Y. H. Chen, T.-T. Huang, J.-H. Wang, R.-M. Lin, C.-H. Kuan, T. Li, Z. Wang, S. Zhu, and D. P. Tsai, “A broadband achromatic metalens in the visible,” Nat. Nanotechnol. 13(3), 227–232 (2018). [CrossRef] [PubMed]
8. A. Arbabi, E. Arbabi, S. M. Kamali, Y. Horie, S. Han, and A. Faraon, “Miniature optical planar camera based on a wide-angle metasurface doublet corrected for monochromatic aberrations,” Nat. Commun. 7, 13682 (2016). [CrossRef] [PubMed]
9. W. T. Chen, A. Y. Zhu, V. Sanjeev, M. Khorasaninejad, Z. Shi, E. Lee, and F. Capasso, “A broadband achromatic metalens for focusing and imaging in the visible,” Nat. Nanotechnol. 13(3), 220–226 (2018). [CrossRef] [PubMed]
10. L. Huang, X. Chen, H. Mühlenbernd, H. Zhang, S. Chen, B. Bai, Q. Tan, G. Jin, K.-W. Cheah, C.-W. Qiu, J. Li, T. Zentgraf, and S. Zhang, “Three-dimensional optical holography using a plasmonic metasurface,” Nat. Commun. 4(1), 2808 (2013). [CrossRef]
11. G. Zheng, H. Mühlenbernd, M. Kenney, G. Li, T. Zentgraf, and S. Zhang, “Metasurface holograms reaching 80% efficiency,” Nat. Nanotechnol. 10(4), 308–312 (2015). [CrossRef] [PubMed]
12. K. Huang, Z. Dong, S. Mei, L. Zhang, Y. Liu, H. Liu, H. Zhu, J. Teng, B. Luk’yanchuk, J. K. W. Yang, and C.-W. Qiu, “Silicon multi-meta-holograms for the broadband visible light,” Laser Photonics Rev. 10(3), 500–509 (2016). [CrossRef]
13. P. C. Wu, W.-Y. Tsai, W. T. Chen, Y.-W. Huang, T.-Y. Chen, J.-W. Chen, C. Y. Liao, C. H. Chu, G. Sun, and D. P. Tsai, “Versatile polarization generation with an aluminum plasmonic metasurface,” Nano Lett. 17(1), 445–452 (2017). [CrossRef] [PubMed]
14. G.-Y. Lee, G. Yoon, S.-Y. Lee, H. Yun, J. Cho, K. Lee, H. Kim, J. Rho, and B. Lee, “Complete amplitude and phase control of light using broadband holographic metasurfaces,” Nanoscale 10(9), 4237–4245 (2018). [CrossRef] [PubMed]
15. Y. Yang, W. Wang, P. Moitra, I. I. Kravchenko, D. P. Briggs, and J. Valentine, “Dielectric meta-reflectarray for broadband linear polarization conversion and optical vortex generation,” Nano Lett. 14(3), 1394–1399 (2014). [CrossRef] [PubMed]
16. X. Ma, M. Pu, X. Li, C. Huang, Y. Wang, W. Pan, B. Zhao, J. Cui, C. Wang, Z. Zhao, and X. Luo, “A planar chiral meta-surface for optical vortex generation and focusing,” Sci. Rep. 5(1), 10365 (2015). [CrossRef] [PubMed]
17. E. Karimi, S. A. Schulz, I. De Leon, H. Qassim, J. Upham, and R. W. Boyd, “Generation optical orbital angular momentum at visible wavelengths using a plasmonic metasurface,” Light Sci. Appl. 3(5), e167 (2014). [CrossRef]
18. F. Sterl, N. Strohfeldt, R. Walter, R. Griessen, A. Tittl, and H. Giessen, “Magnesium as novel material for active plasmonics in the visible wavelength range,” Nano Lett. 15(12), 7949–7955 (2015). [CrossRef] [PubMed]
19. H.-H. Jeong, A. G. Mark, and P. Fischer, “Magnesium plasmonics for UV applications and chiral sensing,” Chem. Commun. (Camb.) 52(82), 12179–12182 (2016). [CrossRef] [PubMed]
20. Y. Lee, S.-J. Kim, H. Park, and B. Lee, “Metamaterials and metasurfaces for sensor applications,” Sensors (Basel) 17(8), 1726 (2017). [CrossRef] [PubMed]
21. T. Driscoll, H. T. Kim, B. G. Chae, B. J. Kim, Y. W. Lee, N. M. Jokerst, S. Palit, D. R. Smith, M. Di Ventra, and D. N. Basov, “Memory metamaterials,” Science 325(5947), 1518–1521 (2009). [CrossRef] [PubMed]
22. R. E. Simpson, P. Fons, A. V. Kolobov, T. Fukaya, M. Krbal, T. Yagi, and J. Tominaga, “Interfacial phase-change memory,” Nat. Nanotechnol. 6(8), 501–505 (2011). [CrossRef] [PubMed]
23. A. She, S. Zhang, S. Shian, D. R. Clarke, and F. Capasso, “Adaptive metalenses with simultaneous electrical control of focal length, astigmatism, and shift,” Sci. Adv. 4(2), eaap9957 (2018).
24. H.-X. Xu, S. Ma, W. Luo, T. Cai, S. Sun, Q. He, and L. Zhou, “Aberration-free and functionality-switchable meta-lenses based on tunable metasurfaces,” Appl. Phys. Lett. 109(19), 193506 (2016). [CrossRef]
25. Z. Zhu, P. G. Evans, R. F. Haglund Jr., and J. G. Valentine, “Dynamically reconfigurable metadevice employing nanostructured phase-change materials,” Nano Lett. 17(8), 4881–4885 (2017). [CrossRef] [PubMed]
26. S.-J. Kim, H. Yun, K. Park, J. Hong, J.-G. Yun, K. Lee, J. Kim, S. J. Jeong, S.-E. Mun, J. Sung, Y. W. Lee, and B. Lee, “Active directional switching of surface plasmon polaritons using a phase transition material,” Sci. Rep. 7(1), 43723 (2017). [CrossRef] [PubMed]
27. Z. Tian, B. Xu, B. Hsu, L. Stan, Z. Yang, and Y. Mei, “Reconfigurable vanadium dioxide nanomembranes and microtubes with controllable phase transition temperatures,” Nano Lett. 18(5), 3017–3023 (2018). [CrossRef] [PubMed]
28. A. Tittl, A.-K. U. Michel, M. Schäferling, X. Yin, B. Gholipour, L. Cui, M. Wuttig, T. Taubner, F. Neubrech, and H. Giessen, “A switchable mid-infrared plasmonic perfect absorber with multispectral thermal imaging capability,” Adv. Mater. 27(31), 4597–4603 (2015). [CrossRef] [PubMed]
29. N. I. Zheludev and E. Plum, “Reconfigurable nanomechanical photonic metamaterials,” Nat. Nanotechnol. 11(1), 16–22 (2016). [CrossRef] [PubMed]
30. J. Valente, J.-Y. Ou, E. Plum, I. J. Youngs, and N. I. Zheludev, “A magneto-electro-optical effect in a plasmonic nanowire material,” Nat. Commun. 6(1), 7021 (2015). [CrossRef] [PubMed]
31. J. Li, S. Kamin, G. Zheng, F. Neubrech, S. Zhang, and N. Liu, “Addressable metasurfaces for dynamic holography and optical information encryption,” Sci. Adv. 4(6), eaar6768 (2018).
32. P. Yu, J. Li, S. Zhang, Z. Jin, G. Schütz, C.-W. Qiu, M. Hirscher, and N. Liu, “Dynamic Janus metasurfaces in the visible spectral region,” Nano Lett. 18(7), 4584–4589 (2018). [CrossRef] [PubMed]
33. Y.-W. Huang, H. W. H. Lee, R. Sokhoyan, R. A. Pala, K. Thyagarajan, S. Han, D. P. Tsai, and H. A. Atwater, “Gate-tunable conducting oxide metasurfaces,” Nano Lett. 16(9), 5319–5325 (2016). [CrossRef] [PubMed]
34. J. Park, J.-H. Kang, S. J. Kim, X. Liu, and M. L. Brongersma, “Dynamic reflection phase and polarization control in metasurfaces,” Nano Lett. 17(1), 407–413 (2017). [CrossRef] [PubMed]
35. J. Park, J.-H. Kang, X. Liu, and M. L. Brongersma, “Electrically tunable epsilon-near-zero (ENZ) metafilm absorbers,” Sci. Rep. 5(1), 15754 (2015). [CrossRef] [PubMed]
36. G. Kafaie Shirmanesh, R. Sokhoyan, R. A. Pala, and H. A. Atwater, “Dual-gated active metasurface at 1550 nm with wide (>300°) phase tunablity,” Nano Lett. 18(5), 2957–2963 (2018). [CrossRef] [PubMed]
37. G. V. Naik, J. Kim, and A. Boltasseva, “Oxides and nitrides as alternative plasmonic materials in the optical range,” Opt. Mater. Express 1(6), 1090–1099 (2011). [CrossRef]
38. A. Boltasseva and H. A. Atwater, “Low-loss plasmonic metamaterials,” Science 331(6015), 290–291 (2011). [CrossRef] [PubMed]
39. Y. Yao, M. A. Kats, P. Genevet, N. Yu, Y. Song, J. Kong, and F. Capasso, “Broad electrical tuning of graphene-loaded plasmonic antennas,” Nano Lett. 13(3), 1257–1264 (2013). [CrossRef] [PubMed]
40. M. C. Sherrott, P. W. C. Hon, K. T. Fountaine, J. C. Garcia, S. M. Ponti, V. W. Brar, L. A. Sweatlock, and H. A. Atwater, “Experimental demonstration of >230° phase modulation in gate-tunable graphene-gold reconfigurable mid-infrared metasurfaces,” Nano Lett. 17(5), 3027–3034 (2017). [CrossRef] [PubMed]
41. K. L. Koshelev and A. A. Bogdanov, “Temperature-tunable semiconductor metamaterial,” Phys. Rev. B Condens. Matter Mater. Phys. 92(8), 085305 (2015). [CrossRef]
42. J. Lee, S. Jung, P.-Y. Chen, F. Lu, F. Demmerle, G. Boehm, M.-C. Amann, A. Alù, and M. A. Belkin, “Ultrafast electrically tunable polaritonic metasurfaces,” Adv. Opt. Mater. 2(11), 1057–1063 (2014). [CrossRef]
43. A. Calzolari, A. Ruini, and A. Catellani, “Transparent conductive oxides as near-IR plasmonic materials: the case of Al-doped ZnO derivatives,” ACS Photonics 1(8), 703–709 (2014). [CrossRef]
44. H. Cai, S. Chen, C. Zou, Q. Huang, Y. Liu, X. Hu, Z. Fu, Y. Zhao, H. He, and Y. Lu, “Multifunctional hybrid metasurfaces for dynamic tuning of terahertz waves,” Adv. Opt. Mater. 6(14), 1800257 (2018). [CrossRef]
45. L. Cong, P. Pitchappa, Y. Wu, L. Ke, C. Lee, N. Singh, H. Yang, and R. Singh, “Active multifunctional microelectromechanical system metadevices: applications in polarization control, wavefront deflection, and holograms,” Adv. Opt. Mater. 5(2), 1600716 (2017). [CrossRef]
46. C. Huang, C. Zhang, J. Yang, B. Sun, B. Zhao, and X. Luo, “Reconfigurable metasurface for multifunctional control of electromagnetic waves,” Adv. Opt. Mater. 5(22), 1700485 (2017). [CrossRef]
47. L. Cong, Y. K. Srivastava, H. Zhang, X. Zhang, J. Han, and R. Singh, “All-optical active THz metasurfaces for ultrafast polarization switching and dynamic beam splitting,” Light Sci. Appl. 7(1), 28 (2018). [CrossRef]
48. H. Yang, X. Cao, F. Yang, J. Gao, S. Xu, M. Li, X. Chen, Y. Zhao, Y. Zheng, and S. Li, “A programmable metasurface with dynamic polarization, scattering and focusing control,” Sci. Rep. 6(1), 35692 (2016). [CrossRef] [PubMed]
49. G. T. Papadakis and H. A. Atwater, “Field-effect induced tunability in hyperbolic metamaterials,” Phys. Rev. B Condens. Matter Mater. Phys. 92(18), 184101 (2015). [CrossRef]
50. H. Kim, J. Park, and B. Lee, Fourier Modal Method and its Applications in Computational Nanophotonics (CRC, 2012).
51. I. Avrutsky, I. Salakhutdinov, J. Elser, and V. Podolskiy, “Highly confined optical modes in nanoscale metal-dielectric multilayers,” Phys. Rev. B Condens. Matter Mater. Phys. 75(24), 241402 (2007). [CrossRef]
52. M. Z. Alam, S. A. Schulz, J. Upham, I. De Leon, and R. W. Boyd, “Large optical nonlinearity of nanoantennas coupled to an epsilon-near-zero material,” Nat. Photonics 12(2), 79–83 (2018). [CrossRef]
53. X. Liu, J.-H. Kang, H. Yuan, J. Park, Y. Cui, H. Y. Hwang, and M. L. Brongersma, “Tuning of plasmons in transparent conductive oxides by carrier accumulation,” ACS Photonics 5(4), 1493–1498 (2018). [CrossRef]
54. G. A. Niklasson, C. G. Granqvist, and O. Hunderi, “Effective medium models for the optical properties of inhomogeneous materials,” Appl. Opt. 20(1), 26–30 (1981). [CrossRef] [PubMed]
55. S. V. Zhukovsky, O. Kidwai, and J. E. Sipe, “Physical nature of volume plasmon polaritons in hyperbolic metamaterials,” Opt. Express 21(12), 14982–14987 (2013). [CrossRef] [PubMed]
56. A. A. Bogdanov and R. A. Suris, “Effect of the anisotropy of a conducting layer on the dispersion law of electromagnetic waves in layered metal-dielectric structures,” JETP Lett. 96(1), 49–55 (2012). [CrossRef]
57. E. Feigenbaum, K. Diest, and H. A. Atwater, “Unity-order index change in transparent conducting oxides at visible frequencies,” Nano Lett. 10(6), 2111–2116 (2010). [CrossRef] [PubMed]
58. N. Liu, M. Mesch, T. Weiss, M. Hentschel, and H. Giessen, “Infrared perfect absorber and its application as plasmonic sensor,” Nano Lett. 10(7), 2342–2348 (2010). [CrossRef] [PubMed]
