High‐Speed Transmission Control in Gate‐Tunable Metasurfaces Using Hybrid Plasmonic Waveguide Mode

Dynamic control of light based on gate‐tunable metasurfaces has revolutionized traditional optoelectronic devices due to its unprecedented compactness and versatile functionalities. However, these devices are typically based on metal‐insulator‐metal geometries that enable field‐effect modulation of only reflected light. Transmittance modulation techniques based on dielectric metasurfaces, despite their large modulation depth, have a disadvantage of low modulation speed due to high resistance of dielectric materials. Here, a high‐efficiency transmittance modulator that enables high switching speed, as well as large modulation depth, is demonstrated using indium‐tin‐oxide‐based metasurfaces. To realize these devices, the hybrid plasmonic waveguide mode is used which allows electromagnetic energy storage within the nanoscale permittivity‐tunable region between metal and high‐refractive dielectric layers. Experimental measurements reveal a change in the transmittance (≈33%) by applying 6 V gate bias, and a fast modulation speed (≈826 kHz of 3 dB cut‐off frequency). This work provides a promising avenue for developing ultracompact optical components such as dynamic holograms, lenses with active focal lengths, or spatial light modulators.


Introduction
Dynamic control of constitutive properties of light such as phase, intensity, polarization, or spectrum using tunable metasurfaces has introduced a new route towards miniaturization of optical devices far beyond the limit of current optoelectronic technologies. [1][2][3][4][5][6][7][8][9][10] The integration of passive metasurfaces with active materials enables the construction of tunable, highly compact devices with additional functionality. [11][12][13][14][15] Based on the field-effect modulation mechanism, the formation of charge accumulation or depletion region by gate bias, metal-oxide-semiconductor (MOS) combined metasurfaces have been used for controlling the complex refractive index of materials, resulting in modulation of the optical output. Significant efforts have been made to create field-effect-based optical modulators that operate from THz to visible frequencies.
Among various electro-optical materials including transparent conducting oxides (TCOs), [4][5][6][7][8] graphene, [16][17][18][19][20][21] transition metal dichalcogenide, [22][23][24] and highly doped semiconductors, [25][26][27] TCO-based modulation is particularly useful for application in telecommunication systems because of its operation feasibility at near-infrared frequencies with high speed modulation and low power dissipation. This is possible due to their relatively large plasma frequency (≈6 eV) that places the epsilon-nearzero (ENZ), which is the wavelength in which their real part of dielectric permittivity is between −1 and 1, in the near-infrared range. To achieve large modulation depth, most TCO-based metasurfaces have been designed to operate in ENZ region, therefore attaining strong electric-field enhancement. [4,8,[28][29][30][31] However, charge accumulation or depletion occurring at extremely small region has limited the efficiencies of such modulators. To overcome this inherent restraint, metal-insulator-metal (MIM) cavities including the nanoscale metal-oxidesemiconductor (MOS) configuration have been utilized in forming gap plasmon resonance, which couples to the ENZ region in TCO, leading to extremely high modal confinement that enhances light-matter interaction. [4][5][6]8,32] Based on such features, a number of reports have demonstrated dynamic control of phase, intensity, polarization, or spectrum of reflected light in MIM geometries by means of gate bias with high modulation speed. [4][5][6]8,9,28,32] In contrast to reflection-type gap Dynamic control of light based on gate-tunable metasurfaces has revolutionized traditional optoelectronic devices due to its unprecedented compactness and versatile functionalities. However, these devices are typically based on metal-insulator-metal geometries that enable field-effect modulation of only reflected light. Transmittance modulation techniques based on dielectric metasurfaces, despite their large modulation depth, have a disadvantage of low modulation speed due to high resistance of dielectric materials. Here, a high-efficiency transmittance modulator that enables high switching speed, as well as large modulation depth, is demonstrated using indium-tin-oxidebased metasurfaces. To realize these devices, the hybrid plasmonic waveguide mode is used which allows electromagnetic energy storage within the nanoscale permittivity-tunable region between metal and high-refractive dielectric layers. Experimental measurements reveal a change in the transmittance (≈33%) by applying 6 V gate bias, and a fast modulation speed (≈826 kHz of 3 dB cut-off frequency). This work provides a promising avenue for developing ultracompact optical components such as dynamic holograms, lenses with active focal lengths, or spatial light modulators.
plasmon resonators, electrically tunable transmittance modulators with high switching speed at near-infrared wavelength region have not yet been reported, to the best of our knowledge. Transmission modulation is essential for the implementation of ultracompact optoelectronic systems because additional optical elements such as beam splitters or polarizers to separate incident and reflected light from each other are required in reflectance modulators. Previously reported transmittance modulators based on dielectric metasurfaces exhibited strong resonances without absorption loss by metal, but the reduction of switching speed is inevitable due to high resistance of dielectric materials that lead to large RC (circuit resistance × circuit capacitance) time constant. [7,33] The development of a transmittance modulator with high switching speed would be beneficial in designing ultracompact optical components such as highresolution transmission-type spatial light modulators for realtime hologram display, lenses with reconfigurable focal lengths, and pixels for transparent display.
In this work, we propose electrically-tunable transmittance modulators that are compatible with fast AC gate bias at nearinfrared wavelengths. The devices are constructed from plasmonic metasurface which consists of indium tin oxide (ITO) layer as an active material on a high-index silicon waveguide. We utilize the coupled hybrid mode between the plasmonic and dielectric waveguide modes that enables energy storage across the ITO layer between the waveguide and metal. This strongly confined mode offers enhanced light-active material interaction, resulting in ≈33% change in the transmission. In respect to switching speed, our transmission modulator shows ≈826 kHz of the 3 dB cut-off frequency.

Results and Discussion
A schematic illustration of the proposed transmission modulator is depicted in Figure 1a. Au nanoslit array with 900 nm periodicity and 120 nm width is patterned on an Al 2 O 3 /ITO/ amorphous silicon (a-Si) stack grown on a quartz substrate. The thicknesses of each Au pattern and underlying Al 2 O 3 , ITO, and a-Si layers are 40 nm, 10 nm, 20 nm, and 140 nm, respectively. The metasurface based on a MOS structure features a charge accumulation layer within the dielectric spacer of each nano-resonator. In this configuration, the ITO layer serves both as an active material and electrode for applying voltage. Spatial distribution of accumulated carrier concentration in ITO varies depending on the applied bias, changing the complex refractive index of the region. [34] Under normal illumination with transverse magnetic (TM) polarization state, transmission can be electrically controlled via strong field confinement within the active material, enhancing light-matter interaction (Figure 1a). A scanning electron microscope (SEM) image of the fabricated device is shown in Figure 1b. Fabrication details can be found in Experimental Section. Au nano-antennas are electrically connected to the external Au electrode for voltage application.
In order to comprehend the change in refractive index and accumulated charges according to the applied bias, the carrier distribution in ITO layer was calculated with the Poisson equation using a one-dimensional MOS capacitor model consisting of Au, Al 2 O 3 , and ITO (The inset of Figure 1c). The carrier distribution is a function of the distance from the Al 2 O 3 / ITO interface for different applied voltages. In our calculations, it is assumed that the DC dielectric constant of gate insulator  Figure 1c (top), charge accumulation region at near oxide/semiconductor interface is generated by external electric field within about 2 nm from the interface. In our work, the range of voltage application is extended from 0 V to 8 V compared to Atwater group's [4] due to the two-fold increase in the insulator thickness, meaning that the device can withstand higher applied bias without dielectric breakdown. However, since a larger applied voltage is required to accumulate the same amount of charge, the maximum amount of charge accumulation would be similar. Figure 1c (bottom) shows the real part of the dielectric permittivity calculated for different applied biases as a function of the distance from the Al 2 O 3 / ITO interface at a target wavelength of 1500 nm. It is difficult to measure directly the permittivity of charge-accumulated region since its thickness is very thin. To solve this problem, above all, we obtained optical parameters such as refractive index n and absorption coefficient κ of ITO by ellipsometery measurement with no applied voltage. Due to the capacity of ITO to support high carrier density, the dielectric permittivity of ITO can be well fitted by the Drude model, which is given where the plasma frequency is ω p = (Ne/m*ε 0 ) 1/2 . Here, ε ∞ is dielectric permittivity at infinite frequency, Γ is damping constant, N is carrier density, and m* is electron effective mass. In this equation, all parameters can be determined by fitting the curve to obtained n and κ. Thus, the permittivity of ITO can be controlled by adjusting N, which is crucial mechanism of field-effect modulation based on ITO layer. N of ITO is adjustable by changing the ratio of Ar-O 2 during sputtering. This is because In 2 O 3 has point defects that consist of oxygen vacancies or the interstitial indium atoms, giving rise to free electrons in reduction environment. The related chemical equations of In 2 O 3 during the annealing process are indicated by Equations (1) and (2).
In Equations (1) and (2), V 0 + and In i + are oxygen vacancies and interstitial indium atoms, and e − is the carrier electron. If the rate of O 2 is higher during annealing process, the reverse reactions of Equations (1) and (2)  Note that real part of the dielectric permittivity of ITO layer is positive due to N = 3 × 10 20 cm −3 at target wavelength of 1500 nm with no applied bias according to the Drude equation. [5] When the applied bias increases from 0 V, the dielectric permittivity value of the ITO layer near the Al 2 O 3 /ITO interface decreases with the accumulation of charge. At bias above 2 V, the dielectric permittivity enters the ENZ region, and changes its sign upon further bias.
The electrically-tunable refractive index layer due to charge accumulation, however, is confined to the ultrathin region (within ≈3 nm, according to Figure 1c) at the Al 2 O 3 /ITO interface. To achieve a transmission modulator with large modulation depth, the following points should be fulfilled: i) strong field confinement within the active material for enhancing light-matter interaction and ii) reduction of unwanted absorption loss, or reflection by metallic structures to obtain sufficient transmittance. On this basis, we utilize a hybrid plasmonic waveguide configuration that consists of a a-Si layer separated  the metal layer by a nanoscale dielectric gap. [35] Si is not only transparent at the operating frequency, but also has a high refractive index, providing a larger effective mode index (see Supporting Information 1). The coupling between the plasmonic mode by surface plasmon polaritons (SPPs) excited at the metal/dielectric interface and a-Si waveguide mode enables electromagnetic energy storage, as in an optical capacitor. In both SPP and waveguide geometries, the electric-field components normal to the material interfaces are dominant, resulting in strong energy confinement in the gaps due to the continuity of the displacement field. The geometry-dependent behavior of the hybrid mode is demonstrated in Figure 3a, where the dependence of the effective indices of hybrid mode n hyb (t si , t o ), pure waveguide (without metal region) and SPP modes (with no waveguide), on the thickness of a-Si (t Si ) and Al 2 O 3 (t o ) layers in the proposed configuration (see Figure 1a) is shown (t s is fixed as 20 nm.). Evidently, pure SPP mode is invariable with t si or t o while pure waveguide mode depends solely on t si . When the t si is either very large or small, n hyb approaches that of pure waveguide or SPP mode, implying that the hybrid mode can be described as a superposition of the waveguide and the SPP modes. [35] The results show that the effective index of hybrid mode is always larger than those of the pure waveguide and SPP modes, which leads to large modulation depth (see Supporting Information 1).
The thickness of a-Si determines the operation wavelength in which the coupled hybrid mode is attained. Figure 3b demonstrates this in a map of transmission as a function of incident wavelength and the thickness of a-Si. The resonant wavelength of maximum transmittance red-shifts as the thickness of a-Si increases, resulting in the rise in effective mode index of hybrid mode as shown in Figure 3a. To obtain the resonance wavelength of 1500 nm, the thickness of a-Si was optimized to be 140 nm. As expect, full-field simulations show that highly confined hybrid modes can be excited between dielectric gap (Al 2 O 3 and ITO) by TM-polarized top illumination, enhancing light-active material interaction (Figures 3c,d). Figures 3e-f give an intuitive understanding of field-effect modulation, where the distribution of the z-component of the electric field (E z ) at different applied bias at the wavelength of 1500 nm is shown. As one can see from the field distribution of the magnified region in Figure 3d with no voltage bias (Figure 3e), ITO can be optically considered as a dielectric material like Al 2 O 3 , since its real part of the dielectric permittivity is positive. On the other hand, when the applied voltage is 8 V (Figure 3f), enhanced E z at the accumulation layer is observed due to the continuity of the normal component of electric displacement (ε ⊥ E z ) at the Al 2 O 3 /ITO interface. For this case, the charge accumulation layer in ITO becomes optically metallic, as the real part of the dielectric permittivity is negative.
Based on the complex dielectric permittivity of ITO, which is a function of position and applied voltage, the transmission spectra are numerically calculated for different applied biases as shown in Figure 4a (Supporting Information 2). The maximum transmittance at the resonance wavelength decreases with increasing applied bias. This is due to rise in reflectance by optically metallic property (the real part of dielectric permittivity is negative) of the charge accumulation layer, and the energy loss from the charge accumulation layer, of which the imaginary part of dielectric permittivity increases. Note that drastic transmission modulation is observed within voltage regions where the real part of dielectric permittivity is in the ENZ region. Namely, transmittance modulation between 4 and 6 V of applied bias is larger than those between 0 to 2 V, or 6 to 8 V. We also observe a blue shift of the resonance when the applied bias increases from 0 to 5 V, and a red shift for applied voltages larger than 5 V. This is because charge accumulation leads to the optically metallic state, thus, effective thickness of dielectric spacer (Al 2 O 3 and ITO) decreases and effective index of hybrid mode (Figure 3a) increases, resulting in the shift of resonance to longer wavelengths. Figure 4b shows measured transmission spectra for different applied voltages (see Supporting Information 3 for measurement setup). The voltage was only applied up to 6 V since the 10 nm Al 2 O 3 layer we fabricated undergoes electrical breakdown at around 6 V. The resonance shift, the maximum transmission, and the wavelength at the resonance show good agreement with simulation results. It is clear that the transmittance modulation is achieved with applied bias. The transmittance change at the resonant wavelength (defined as the difference between transmittance normalized to the transmittance without applied voltage, ΔT/T = [T(V)−T(0)]/T(0)) is ≈33% from 0 to 6 V of applied. The transmission change demonstrated in our device is comparable to previously reported results in reflectance or transmittance modulations. [4,5,8,19] To characterize the electrical properties of the fabricated sample, electrical impedance spectroscopy was measured by applying 10 mV AC input with frequencies ranging from 1 Hz to 1 MHz. The absolute value and phase angle of the complex impedance are shown in Figure 4c. Measured resistance is ≈210 MΩ at the DC environment, implying that the leakage current from the device is smaller than 1 nA. Over 100 Hz, capacitive behavior begins to occur, and the phase angle of the impedance becomes nearly −90 degrees. To demonstrate the dynamic control of transmission with fast AC input signals, an InGaAs amplified photodetector is used to detect the temporal changes of the transmittance upon application of 2 V bias (max: 2 V, min: 0 V) to the device with frequencies ranging from 100 Hz to 10 MHz. The inset in Figure 4d shows the applied voltage trigger (blue) for a square pulse with frequency of 500 kHz and the resultant transmittance (red). The modulation depth values (defined as the transmittance difference between the maximum and minimum transmittance) normalized to the value of the transmittance change obtained at 100 Hz frequency are shown in Figure 4d (Supporting Information 4). The frequency response shows a 3 dB cut-off frequency around 826 kHz. This result is in reasonable agreement with theoretical value of ≈865 kHz, estimated from the capacitance of the device measured as ≈115 pF and the resistance of ITO ≈1.6 kΩ. Our device controls transmission with large modulation depth and high switching speed, compared with other previously reported devices (Supporting Information 5).  Figure 3d shows the temporal transmission changes (red) upon voltage trigger (blue) from 0 to 2 V with frequency of 500 kHz.
Moreover, our device is expected to have a larger modulation depth, if the Al 2 O 3 is used as an insulator replaced with a lamination of Al 2 O 3 and HfO 2 , leading to a higher DC constant and larger breakdown voltage.

Conclusion
In summary, we have demonstrated an electrically tunable metasurface in the near-infrared wavelength region with high switching speed. The transmittance is controlled by gatetunable optical permittivity modulation in ENZ region. The hybridization of the fundamental mode of a Si waveguide with the SPP of a dielectric-metal interface enables strong field confinement within an active material. A transmittance change of ≈33% was measured by applying 6 V gate bias. A fast modulation speed of a 3 dB cut-off frequency around 826 kHz was also achieved. Considering that the current and previous research on intensity modulation with high switching speed is mostly focused on reflection-type devices, we emphasize that our approach towards improved transmittance modulators may contribute to the implementation of ultracompact optoelectronic and Si-based photonic systems. [36][37][38]

Experimental Section
Sample Fabrication: The proposed device was fabricated via standard thin film deposition processes and e-beam lithography techniques. Each layer was stacked in order from bottom to top. First, a 140 nm-thick amorphous Si layer was deposited by plasma-enhanced chemical vapor deposition (PECVD) on a quartz glass substrate (P5000, AMAT). Then, a 20 nm-thick ITO layer was sputtered on the silicon by DC magnetron sputtering in Ar-O 2 plasma. The target source was indium oxide (In 2 O 3 ) and tin dioxide (SnO 2 ) of a 90 to 10 weight percent ratio. Sputtering was carried out at a chamber pressure of 6.5 mTorr, with a sputtering power of 2 kW, while maintaining the ratio of the partial pressure of Ar-O 2 at 20:0.2. Next, a 10 nm of Al 2 O 3 was deposited by atomic layer deposition (ALD) method. Subsequently, Au nanoslit array was patterned by lift-off process on the stack. Lift-off was carried out by using bilayer e-beam resist to achieve undercut profile that was required for reliable nanopatterning. Positive resist PMMA was used for both layers in the bilayer process. Bottom PMMA (MicroChem Corp., PMMA 495K A2) and top PMMA (MicroChem Corp., PMMA 950K A2) have different molecular weight of 495 and 950 K, respectively, and all were 2% wt solution in anisole. The bottom PMMA was spin-coated at a speed of 2000 rpm for 45 s and baked at a temperature of 180 °C for 5 min. Successively, PMMA was once more spin-coated at a speed of 3000 rpm for 45 s and baked under the same conditions abovementioned. With this e-beam resist, the patterning was executed by e-beam lithography equipment (JEOL, JBX-6300FS) at an acceleration voltage of 100 keV with a dosage of 760 µC cm 2 . After the exposure process, the development of the resists was carried out by manually agitating in MIBK/IPA (1:3) for about 60 s. Then, Au layer of 50 nm was deposited by thermal evaporator (the rate was 0.1 nm s −1 ). Lastly, the sample was soaked in acetone for 24 h followed by sonication for 10 s to get rid of residuals.

Supporting Information
Supporting Information is available from the Wiley Online Library or from the author.