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Abstract
Metalenses with dynamically controllable smart materials have attracted growing attention for next-generation integrated intelligent optical and photonics systems. Here, we incorporate an electrochromic material into the Au nanorod-based metalens, and demonstrate that the intensity of the focused visible light can be controlled by an external voltage without using any mechanical moving parts, while the focal length and the focal spot size remain stable. Furthermore, a large modulation depth (>90%) is obtained with relatively low power consumption ($\thicksim$ 3 mW/cm2). Such ultracompact electrical-controlled metalenses may enable miniaturization and integration of various optical components and systems such as optical imaging, optical sensing and optical communications.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Ultracompact optical elements promise substantially significant prospects in modern optics and photonics, such as imaging, sensing, quantum communication and optical manipulation. Metalens, one of important device application of metasurfaces, uses a two-dimensional planar micro/nano structure array to focus light of various wavelengths in a certain arrangement [1–7]. These ultrathin lenses are lightweight and very easy to integrate, which means it can greatly reduce the size and complexity of the optical system [8–10]. However, once these respective nanostructures are fabricated, their optical features cannot be reversibly modified. For practical applications, it is highly desirable that the metalenses can be dynamically controlled at a high speed to realize sufficient handling and adaptability, particularly in the field of micro-optics (e.g., micro-imaging, beam-shaping and integrated optics on chip) where versatility is required [11–16].
Currently, there are two main approaches to achieve dynamic controllable metalenses. One is based on reconfigurable metasurfaces whose geometries or properties can be actively varied via various mechanical mechanisms, such as microelectromechanical systems, stretchable substrates and dielectric elastomer actuators [15–20]. However, there are still some demerits such as large size, low response speed and excessive power consumption in the practical applications. The other approach is to integrate the metalenses with smart materials. The optical properties of certain smart materials, such as varactors, phase change materials, graphene, liquid crystals and transparent conducting oxide materials, can be actively tuned by applying external excitation [21–28]. By combining the nanoblocks of metalens with smart materials, the optical response of the metalens would be sensitive to its dielectric surround and can hence be actively controlled as a result. Obviously, this approach can make devices and systems more compact, efficient and convenient. To date, dynamic controllable metalenses for intensity modulation at the terahertz band or near-infrared band have been reported with these smart materials [24–28]. Although these commonly used materials are well adapted for the microwave and near-infrared bands, they suffered from limited tuning of global optical absorption, reflection, or transmission, especially in the visible wavelength range. Therefore, the search for alternative smart material, which could be combined with nanostructures and operate at visible frequency range, becomes more important. Since electrochromic materials (ECMs) have been well exploited for displays in the visible wavelength range [29–34], the optical properties of electrochemical polymers can be regulated by injecting or extracting electric charges (ions or electrons) under an external electric field. They have also been shown to exhibit high tuning ability, excellent reconfigurability and great bistable performance [35,36]. These features allow ECMs to be one of the potential materials for achieving a controlled electro-optical response with appropriate nanostructuress [37–43], and thus provide the possibility to realize the tunability of the metalens.
In this work, we demonstrate an electrical-controlled metalens with tunable focusing intensity by integrating ECMs and metasurface. The ECMs is formed by combining the poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS), ion-gel electrolyte, transparent electrode, and the metasurface is composed of Au nanorod unit cells. Firstly, we employ the finite-difference-time-domain (FDTD) method to simulate the transmission responses of the multilayer structure with different reaction states of PEDOT:PSS film. It shows that the reaction state and the optical parameters of the attached PEDOT:PSS films can be controlled with different bias voltages. Secondly, the effects of the reaction states of PEDOT:PSS on the power of focused light, focal length, and focal spot size of the electrical-controlled metalens are experimentally studied. Finally, the modulation depth, switching speed, and power consumption of the device are also investigated by the change of PEDOT:PSS reaction states. Our results show that the intensity of focused visible light can be well controlled by the designed electrical-controlled metalens.
2. Results and discussion
2.1 Design of electrochromic Au nanorod-based metalens
As shown in Fig. 1(a), the electrical-controlled metalens consists of six parts: (1) SiO2 substrate, (2) Indium-Tin-Oxide (ITO) transparent top/bottom electrode layer, (3) ultra-thin Au nanorod layer, (4) ECM covering and filling layer, (its chemical structure is shown in Fig. 1(b)), (5) ion–gel electrolyte transport layer, and (6) polyethylene terephthalate (PET) transparent top layer. The typical structural units are shown in Fig. 1(c). Here, the thickness of the electrode layer is H1 = 225 nm which provided both excellent electrical conductivity (square resistance ≤ 8 Ω) and high transmission (≥ 90%). The metalens consisted of Au nanorods, and the thickness H2 of the Au nanorod was 80 nm. The period P of each unit cell of the Au nanorod array was 320 nm. The length L and width W of all the Au nanorod were 260 nm and 100 nm, respectively. The rotation angle (RA) of each block was denoted by φ, as shown in the inset of Fig. 1(d). A right-handed circularly polarized (RCP) incident plane wave with 623.8 nm wavelength propagated along the z-axis. According to the Pancharatnam-Berry phase method, an abrupt phase change of −2φwill occur with a rotation angle, φ, for the left-handed circularly polarized (LCP) wave transmitted through a rectangular block [44,45], while the Au nanorod is rotated 180°, the 2π phase distribution modulation can be achieved for the transmitted LCP wave. The PEDOT:PSS film was chosen as the ECM layer for its optimal properties such as its high contrast, fast response speed, low power consumption and great bistable properties [37,46]. Especially in the visible region, strong modulation of optical properties has been achieved by electrically tuning the reaction level [37,47–49]. Through printing technology, a continuous film can be formed by ECM. After high-temperature baking, the ECM layer forms into a solid-state, and the metalens formed by the nanoscale blocks can be perfectly wrapped into it. Then, an ion-gel electrolyte overlaid the ECM layer and fit snugly to the top electrode.
Fig. 1. (a) Configuration of the proposed intensity tunable metalense. (b) Chemical structures of PEDOT:PSS in the reduced and oxidized form. (c) Side view of a unit-cell structure. (d) and (e) Numerical simulation results of transmission phase and magnitude of RCP scattering light wave at 632.8 nm under normal incidence as a function of angle φ. Inset of (d) shows top view of a unit-cell structure, inset of (e) shows the numerical simulation results of z-component of electric field distributions with two different reaction state of PEDOT:PSS film.
The FDTD method and Matlab were used for the electrical-controlled metalens simulations. For the unit cell simulations in FDTD, periodic boundary condition and plane wave excitation were applied to simulate the transmission phase and magnitude responses in the infinite array environment. The material property of the Au in the visible region was acquired from the Material Library of FDTD, which is called Gold (Johnson)(Optical). The materials of PET, ITO, and ion-gel electrolyte were self-defined in FDTD according to the optical property in the light spectrum of red; the corresponding relative refractive indices were 1.65 + i0, 1.86 + i0.009 and 1.695 + i0.01 (determined by spectroscopic ellipsometer), respectively [47]. The refractive index (n) and extinction coefficient (k) values for PEDOT:PSS film can be derived by spectroscopic ellipsometer. The complex refractive indices of PEDOT:PSS film at 632.8 nm in the oxidized and reduced states were 1.436 + i0.02033 and 1.485 + i0.12466, respectively [49]. A significant difference in the extinction coefficient k between the two states was observed. First, the electric field of the unit cell with different reaction states was numerically calculated as shown in the inset of Fig. 1(e). When the PEDOT:PSS film is in an oxidized state, the loss of incident light is relatively small, and the transmission magnitude of the designed unit cell is high. It is evident that the transmission magnitude of the unit cell was significantly reduced by changing PEDOT:PSS film into a reduced state, which could be attributed to energy dissipation during the transmission process. Next, we analyzed the transmission characteristics of the unit cell. As sketched in Fig. 1(d) and Fig. 1(e), the transmission phase and magnitude response of the unit cell were investigated at oxidized and reduced states of the PEDOT:PSS, respectively. Upon biasing, the PEDOT:PSS changed from an oxidized state to a reduced state, a parallel-shift of phase curves were detected and the transmission magnitude decreased significantly.
For the array simulations in Matlab, array theory was used to calculate the scattering field. A metalens array can be realized using the designed unit cells, which are defined as different rotation angles to construct a parabolic phase profile as shown in Eq. (1) [44]:
2.2 Fabrication and characterization
As shown in Fig. 2(a), using electron beam lithography (EBL) and lift-off techniques, the Au nanorod-based metalens were fabricated on a SiO2-ITO substrate, and the PEDOT:PSS was uniformly coated by printing techniques. The detailed steps are as follows: First, a 225-nm-thick ITO layer was deposited on a silicon substrate in sequence using magnetron sputtering technology. Second, the PMMA-A4 photoresist was deposited via spin coating and baked at 180 °C for 90 seconds. After electron beam exposure, the sample was then developed in a solution of isopropyl alcohol (IPA) and 4-Methyl-2-pentanone (MP) of IPA: MP = 3:1 for 100 seconds, subsequently rinsed with IPA for 30 seconds and blow-dried with a nitrogen gun. Third, a 5-nm-thick titanium layer and an 80-nm-thick Au layer were deposited via an E-beam evaporator. A lift-off process was implemented in the acetone solution to form the required metalens pattern. Fourth, the PEDOT:PSS aqueous dispersion from Clevios Company was used. The concentration of PEDOT:PSS in the dispersion solution was 1.3wt.% and the molar ratio of PEDOT to PSS was 1:6. The anionic surfactant, with a ratio of 0.3wt.%, was used to reduce the surface tension of dispersion and to improve the quality of spin-coated thin films. The solution was dispersed by ultrasonication and filtered by a 0.45 μm filter before use. Fifth, the PEDOT:PSS was evenly coated using printing techniques and thermal annealing at high temperature (120 ℃) for 15 mins. Sixth, Symmetric solid state devices were fabricated by sandwiching together PEDOT:PSS film and ITO electrode deposited on PET using a transparent conducting ion gel electrolyte (poly2-acylamido 2-methyl propane sulfonic acid). SEM images of the PEDOT:PSS coated metalens with a designed focal length (320 μm) are shown in Fig. 2(b)-(e), the effective area of the sample had a diameter of 300 μm, and the fabricated geometrical size of the unit cell was 260 nm × 100 nm × 80 nm, which meets the predesigned requirements. The numerical simulation results indicate that the tolerance to fabrication error of the nanostructure is about 10 nm (see detail in Supplement 1). The Au nanorod were perfectly matched with the PEDOT:PSS, and the mixed structure was completely uniform. By using different printing process parameters, the thickness of the PEDOT:PSS film can be easily altered. Here, a proper thickness of about 800 nm was chosen for the PEDOT:PSS film.
Fig. 2. (a) Fabrication steps of the tunable metalense. (b) and (c) Top view, (d) and (e) side view of SEM images of the metalens coated with PEDOT:PSS film.
An optical measurement device was built to experimentally verify the tunability of the electrical-controlled metalens as shown in Fig. 3(a). The light source was a He-Ne laser (Thorlabs, HNL020L) with a wavelength of 632.8 nm. After passing through the linear polarizer (LP) and the quarter-wave plate (QWP), an RCP light can be obtained. The RCP light was then irradiated onto the electrical-controlled metalens and transformed into LCP light. The focal spot was magnified by a microscope objective (Olympus, MPLFLN, ×100, NA = 0.9). Through another combination of a QWP and an LP, the quality of the generated LCP light was checked and confirmed. Finally, the focal spot of pure LCP light was captured by a CCD camera (QHY 183M, pixel size is 2.4 μm × 2.4 μm). As shown in the inset of Fig. 3(a), during the experiment, the sample was stably fixed on the high-precision mechanical displacement platform, and the upper and lower electrodes of the sample were connected to the positive and negative electrodes of the electrochemical workstation (EW, CHI600E, test accuracy: 1 mA, scanning accuracy: 0.002 s) through wires, respectively. Different operating voltages can then be applied to control the redox reactions states of PEDOT:PSS by an EW. To investigate the focal properties of the electrical-controlled metalens, a negative voltage (-1.5 V) was continuously applied to the sample for 10 s to completely oxidize the PEDOT:PSS film. After the state is stable, the operating voltage was removed. At the same time, a z-scan experiment was performed by moving the electrical controlled metalens along the z-axis from 0 to 360 μm in intervals of 0.29 μm. Then the different forward voltages (0.5 V, 1 V, 1.5 V, 2 V, 2.5 V) were applied to the sample for 10 s to make the PEDOT:PSS film react in different reduced states, respectively. For each forward voltage, we repeated the scanning process. To avoid the spontaneous reduced reaction of the samples caused by contact with the gel electrolyte, a negative voltage (-1.5 V) was continuously applied to the sample for 5 s to make the PEDOT:PSS films completely oxidized before each reduction.
Fig. 3. (a) Diagram of measurement setup of the fabricated tunable metalense. Inset shows the physical picture of sample fixed on a precision displacement stage. (b) Measured longitudinal intensity distributions of the cross-polarized light on the xz plane, (c) Measured intensity distributions on the focal plane, (d) Transverse intensity distributions on the focal plane for the tunable metalense, (e) Measurement transmission efficiency (red) and relative modulation depth (blue) of tunable metalense under different operating voltages.
During the experiment, photos of the transmission light of the electrical-controlled metalens were taken at different planes in stable, completely oxidized states and stable reduced states, and the entire experimental process was under the same illumination condition. We reconstructed the longitudinal profiles of the focal light from the captured CCD images. The longitudinal intensity distributions of the cross-polarized field on the Y-Z plane with different operating voltages of -1.5 V, 0.5 V, 1 V, 1.5 V, 2 V, and 2.5 V at 632.8 nm are displayed in Fig. 3(b). To show the intensity variations, all the intensity values were normalized by the maximum value of the intensity distribution at -1.5 V. It can be found that the focal length for 632.8 nm is about 315 μm. The slight differences between the experimental and the theoretical results may arise from the nano-fabrication process or measurement deviation. As the operating voltage increased, the focal intensity decreased gradually while the focal length was almost unchanged, which agrees with the theoretical results predicted by Eq. (1).
As shown in Fig. 3(c), the intensity distributions of the crosspolarized scattered light field on the focal plane were also measured at different operating voltages. It can be found that the cross-polarized scattered wave was focused on the focal plane with a focal spot size of 735 nm under an operating voltage of -1.5 V. Here, the focal spot size was defined as the full-width half maximum (FWHM) value. Obviously, the focal spot with the strongest focal intensity can be obtained when the operating voltage was -1.5 V and as the operating voltage increased from -1.5 V to 2.5 V, the focal intensity weakened gradually while the position of the focal spot was almost unchanged. The measured transverse intensity distributions of the electrical-controlled metalens with the selected operating voltages are shown in Fig. 3(d). All the curves were normalized with the highest value. It can be found that the focal intensity weakened and the FWHM maintained at about 735 nm while the operating voltage increased from -1.5 V to 2.5 V. Here, the numerical simulation results indicate that the focusing performance of the tunable metalens is sensitive to the incident angle of the input light (see detail in Supplement 1). In our future work, we will further explore new methods to design and prepare angle-independent tunable metalenses, because improving the angle independence of the tunable metalens is of great significance. The measured transmission efficiency of the electrical-controlled metalens with the selected operating voltages is shown in Fig. 3(e) (red line). Here, the efficiency was defined as the ratio of the optical power focused on the electrical-controlled metalens to the incident optical power irradiated onto the electrical-controlled metalens [2,4,18]. It is clear that the measured transmission efficiency of the electrical controlled metalens decreased gradually while the operating voltage increased, which indicates that the power of focused light can be electrodynamically controlled. Due to the high loss caused by the interaction between electromagnetic waves and Au nanorods, the ON-state (oxidized state) transmission efficiency of the TML demonstrated here is relatively low. To quantify the active tunability, the measured relative modulation depth for focusing light is defined by Eq. (2):
Furthermore, the electrical characteristics of the electricalcontrolled metalens were also investigated. Figure 4(a) shows the reaction current curve as a function of reaction time under different operating voltages. Here, a series of positive voltages (from 0.5 to 2.5 V, with an increment of 0.5 V) and a fixed negative (-1.5 V) voltage were applied to the sample alternately for a well-defined period (10 s). The change in the current with the reaction time was scanned by the EW. As the operating voltage increases from 0.5 V to 2.5 V (black dotted line), the peak value of reaction current increases (red line), and such change in electrochemical behavior is completely reversible. It should be noted that a symmetric behavior can be also observed between oxidized and reduced reactions. Figure 4(b) shows the zoom-in view of the dotted frame in Fig. 4(a). It is clear that when the operating voltage is applied to the PEDOT:PSS thin film, the reaction current rapidly decreases with the reaction time and then stabilizes after reaching the threshold. Here, to obtain the average power consumption (APC) of electrical-controlled metalens under different operating voltage, we integrated the current over time to determine the amount of reaction charge and then calculated the APC according to Eq. (3):
Fig. 4. (a) Reaction current curve as a function of reaction time. (b) Zoom-in view of the dotted frame in Fig. 4(a) for clarity. (c) Reaction charge and average power consumption. (d) Switching time of the device under different operating voltages.
3. Conclusion
In conclusion, we have developed an electrical-controlled metalens based on the electrochromic Au nanorod operating at visible frequency. The experimental results show that the electrical-controlled metalens can effectively focus circularly polarized light and the power of focused light can be dynamically controlled by applying different operating voltages while the focal length and the focal spot size are constant. Large power modulation depth (>90%) of the electrical-controlled metalens has been realized with relatively low power consumption (∼ 3 mW/cm2). This highly integrated dynamic controllable metalens may enable new applications in optical sensing, optical displays, optical imaging and optical communications.
Funding
National Key Research and Development Program of China (Grant Nos.2016YFA0301200); Natural Science Foundation of Shandong Province (ZR2018MF029); Science and Technology Planning Project of Shenzhen Municipality (JCYJ 20170818101939090); Jiangsu Provincial Key Research and Development Program (BE2018006-3); Key Technology Research and Development Program of Shandong (2017GGX10111); Fundamental Research Fund of Shandong University (2016WLJH44).
Acknowledgments
The authors thank the CAS Interdisciplinary Innovation Team, NANO-X Workstation and Nano Fabrication Facility of Chinese Academy of Sciences, Jiangsu Province, Suzhou City, Suzhou Industrial Park.
Disclosures
The authors declare no conflicts of interest.
Data Availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request.
Supplemental document
See Supplement 1 for supporting content.
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