Active Huygens’ metasurface based on in-situ grown conductive polymer

1 Introduction

Active metasurfaces, often alternatively termed as tunable or reconfigurable metasurfaces, are rapidly emerging as a major frontier in photonic research and have launched tremendous breakthroughs in modern optics [1], [2], [3]. Compared with their passive counterparts, active metasurfaces consist of ultrathin planar arrays of subwavelength active nanoantennas, whose optical responses can be dynamically modulated on-demand. Over the past decade, active metasurfaces have made a significant impact on the new development of beam deflectors [4], [5], [6], [7], [8], spatial light modulators [9], [10], [11], [12], varifocal metalenses [13], [14], [15], dynamic holograms [16], [17], [18], [19] and many others. The active tuning schemes mainly rely on varying optical properties of the nanoantennas or their surrounding materials through chemical reactions [19], [20], [21], mechanical displacements [22], [23], [24], electrical switching [16], [17], [25], [26], [27], thermal modulation [28], [29], [30], [31] and all-optical switching [32], [33], [34]. Among these modulation schemes, electrical switching is of particular interests because it promises compact integration of meta-optics with miniaturized on-chip electro-optic systems, which can be readily incorporated into electronic smart devices for practical applications [1], [2], such as displays, augmented/virtual reality (AR/VR) glasses, dynamic holograms, optical sensing, beam steering and light detection and ranging (LiDAR).

To this end, a number of active metasurfaces have been implemented by using different electro-active materials, such as chalcogenide phase-change materials (PCMs) (26, 27), III–V semiconducting materials [5], [35], [36], ionic conducting materials [37], [38], metallic polymers [6], [14]. Previous active metasurfaces exhibit only distinct subsets of key performance metrics [1], including switching speed, cycling duration, electro-optical controllability, modulation contrast, optical efficiency and operation voltages, as shown in Figure 1a. However, these performance attributes are hardly met simultaneously, making them almost impossible for any practical use. For instance, even though PCM metasurfaces can provide a switching speed of up to 2 MHz [27], they require high operation voltage and large cell manipulation [26]. III–V semiconductors offer excellent switching duration and controllability for intermediate states, but suffer from low modulation contrasts due to the small tuning range of the material intrinsic refractive index [5]. Recently, conductive polymers have shown many desired properties for electrically active metasurfaces, such as large variation of refractive index, fast switching speed, superior cycling stability and low operation voltages [39], [40], [41], [42]. However, previous active metasurfaces purely based on conductive polymers are disadvantaged by low diffractive efficiency, due to the geometric phase design and the weak resonant nature of the nanostructured polymer [6], [14]. Moreover, the fabrication of the conductive polymer relying on spin-coating prevents long-term switching durability since the contact issue between the polymer and the substrate. While active metasurfaces incorporating conductive polymers as active surrounding materials have shown superior switching performance [16], the implemented geometric phase-based plasmonic nanoantenna design yields suboptimal diffraction efficiency in optical frequencies.

Figure 1:

Comparison of active metasurface performance and the concept of electrically active Huygens’ metasurface composed of PANI-modulated dielectric nanoantennas. (a) Comparison of different electrically active metasurfaces in performances. The performance is evaluated relatively as defined in the reference [1]. The evaluation of each electro-active materials is based on representative references: chalcogenide PCMs [26], [27], III–V semiconductors [5], [36], ionic conducting materials [37], [38], metallic polymers [6], [7], [8], [9], [10], [11], [12], [13], [14]. (b) Schematics illustrating the electrical switching of an individual active Huygens’ nanoantenna based on conductive polymer. The polymer is switched to oxidized state at +0.6 V (left), and to reduced state at −0.2 V (right). The switching time between the states is 15 ms, corresponding to a refresh rate of over 60 fps. (c) Simulated and measured transmission spectra (left panel) of a metasurface made of homogeneous nanoantennas (height H = 140 nm, radius R = 110 nm, periodicity P _x = P _y = 450 nm) and the calculated phase retardation (right panel) of the nanoantennas with different radii in the oxidized state. (d) Simulated and measured transmission spectra (left panel) and the calculated phase retardation (right panel) in the reduced state. The inset chemical structures show the conversion of the polymer between the oxidized and reduced states. The inset images show the simulated field distributions of the electric dipole (ED, red) and the magnetic dipole (MD, blue) in the oxidized state, as well as the spectrally overlapping electric and magnetic dipoles (ED + MD, yellow) in the reduced state, at their corresponding resonant wavelengths of 694 nm, 765 nm and 785 nm, respectively.

In this work, we introduce an electrically active Huygens’ metasurface based on the in-situ grown conductive polymer, polyaniline (PANI), and experimentally demonstrated the overall active performance. We combine the superior electro-optical response of PANI with the dielectric Huygens’ nanoantennas by an electrochemically in-situ grown manufacture methods, allowing for the strengthened cycling stability and the optimization of electro-optical modulation. Our active Huygens’ metasurface exhibits a high modulation contrast of 1400 % and a diffraction efficiency up to 28 %, which is 25 times higher than the previous polymer-based metasurface [6]. The solid contact between the nanoantennas and the grown PANI facilitates mechanical and electrical durability, enabling a superior cycling duration of over 2000 switching cycles without noticeable degradations. The intrinsic dynamic properties of PANI endow the active metasurface with fast switching speed of 60 fps and hysteresis-free controllability within a low operation voltage range from −0.2 V to +0.6 V. Unlike previous active metasurfaces performing well only in subsets of performance metrics (Figure 1a), our active metasurface holds great promise towards practical applications that needs overall switching performance.

2 Concept of electrically active Huygens’ nanoantennas

Our active Huygens’ metasurface is composed of dielectric silicon nanodisks surrounded by a layer of PANI, resting on an indium-tin-oxide (ITO) substrate that acts as an electrical contact (Figure 1b). The cylindrical shape of the nanoantenna is chosen to provide polarization independence and the period of the nanoantennas is fixed to 450 nm in both x and y direction so that the unit cell is subdiffractive. The state of the polymer is collectively controlled by voltage tuning via electrochemistry, where an applied voltage range between +0.6 V and −0.2 V (vs. a reference electrode) can continuously tune the polymer into states in between the oxidized and reduced states. In detail, the redox reaction triggered by the applied voltage induces an alternating delocalization of the π-electrons in the polymer chain of PANI, resulting in a large variation of the refractive index with a maximum Δn = 0.6 at 780 nm (Figure S1, Supporting Information).

The core concept of our design is the spectral tuning of the electric dipole (ED) and magnetic dipole (MD) of the nanoantenna by electrical modulation of the surrounding polymer refractive index. Figure 1c and d depict the spectral response of nanoantennas at different applied voltages. In the PANI oxidized state, the simulated and measured metasurface transmission spectra show that the ED and the MD are spectrally separated, resonating at 694 nm and 765 nm, respectively. When the polymer is switched to the reduced state, the two dipole resonances redshift to 785 nm and overlap with each other. As suggested by the principle of dielectric Huygens’ metasurfaces [43], the spectrally overlapping resonances of ED and MD signifies the fulfillment of Huygens’ condition, which produces a full-range phase shift from 0 to 2π combined with high transmission. Importantly, the phase modulation functionality of the nanoantenna is switched on at an applied voltage of −0.2 V, exhibiting strong phase shift (Figure 1d), while at +0.6 V, the nanoantenna exhibits a phase retardation well below 2π-phase coverage and can be considered switched off (Figure 1d). The variation of the phase shift in the nanoantennas is determined by the interference of the ED and MD resonances, where the spectral responses of the two resonances are determined by the nanoantenna geometry and the refractive-index contrast between the nanoantenna and the surrounding environment [43]. Therefore, by harnessing the environmental tuning scheme and carefully choosing nanoantenna geometries for optimum phase engineering, switchable metasurfaces with high efficiency and greatly improved switching contrast can be realized.

3 Metasurface design for electrical beam steering

As a proof of concept, we implemented an active metasurface for optical beam steering, where the incident light can be deflected into a fixed angle on-demand by electrical control. The metasurface design is based on an eight-element gradient phase geometry using the calculated nanoantenna phase profiles at the reduced state for an incident wavelength of 785 nm (Figure S2). This design constructs a modulator between binary states. It allows for transmitted beam deflection at the reduced state, while maintains the undeflected beam at the oxidized state (Figure 2a and b). The chosen structures have an average transmittance of 0.6. The metasurfaces were fabricated via electron-beam lithography (EBL) patterning and reactive-ion etching (RIE), followed by electrochemical polymerization for the growth of the polymer. Figure 2c shows a scanning electron microscopy (SEM) image of the fabricated bare dielectric metasurface on a transparent conductive ITO/glass substrate. The metasurface is designed such that the steered beam couples to the +1 diffraction order at the polymer reduced state, and to the 0 order at the polymer oxidized state. The design principle is validated by simulated electric field distributions (Figure S3), where the transmitted spatial phase profile supports diffraction at +1 order at an operation voltage of −0.2 V and strongly suppresses the diffraction at +0.6 V. Detailed far-field analysis of the metasurfaces reveals the switchable beam steering performance with high switching contrast at a diffraction angle of 12.7°, meanwhile the intensity of the 0 order can be readily modulated (Figure 2d).

Figure 2:

Metasurface design for electrical beam steering. (a–b) Schematic illustrating the concept of electrical beam steering using a polymer-integrated Huygens’ metasurface at the applied voltage of +0.6 V and −0.2 V, respectively. (c) Tilted SEM image of the bare dielectric metasurface with eight-element gradient nanoantennas design (highlighted structures). Scale bar: 1 μm. (d) Simulated far-field transmitted intensities of +1, 0, −1 diffraction orders for the electrical beam steering metasurface.

4 In-situ polymer growth and electrical beam steering performance

To experimentally validate our electrical beam steering metasurfaces, electrochemical growth of PANI was implemented on the fabricated metasurface in-situ in the optical measurement setup, where a high-speed monochromatic camera was used to monitor the diffraction pattern during the polymer growth (Figure 3a). A three-electrode system integrated with the optical measurement setup was employed for PANI growth and electrical switching in an aqueous electrolyte using a custom-built electrochemical cell (Figure S4, see also Section 7 for further information). Since PANI is switched between −0.2 V and +0.6 V and can be electrochemically polymerized under an applied voltage of +0.8 V on an ITO electrode [44], [45], a cyclic voltammetry with a voltage range from −0.2 V to +0.8 V was chosen to carry out PANI growth and electrical switching simultaneously (Figure S5, see also Supplementary Note 1 in Supporting Information), thus allowing for in-situ optimization of the electrical beam steering performance.

Figure 3:

In-situ measurement and optimization of beam steering performance. (a) Schematic of the in-situ optical measurement setup for the active beam steering metasurface. A high-speed monochromatic camera was used to record the diffraction pattern during the polymer growth process. (b) Normalized intensities of +1, 0 and −1 diffraction orders during the polymer growth. (c) Figure of merit (FOM) of beam steering between 0 and +1 diffraction orders at different coating cycles obtained from the measured intensities in (b). (d) Measured camera image (top) and diffraction intensity profile (bottom) at an applied voltage of +0.6 V. (e) Measured camera image (top) and diffraction intensity profile (bottom) at an applied voltage of −0.2 V. (f) Camera images of the diffraction orders at different input voltages. (g) Diffraction intensity profiles at different input voltage extracted from (f). The data in (d)–(f) were measured on the beam steering metasurface at an optimized polymer coating cycle number of 56.

The diffraction images were monitored throughout the entire polymer growth process. Figure 3b shows the intensities of +1, 0 and −1 diffraction orders as function of the PANI coating cycle. In the first 25 coating cycles, the variation of the diffractive intensities is very small due to the insufficient amount of the grown PANI. By increasing the coating cycle number and thus the PANI thickness, the effect of the grown PANI layer becomes large enough to enable switching of the diffraction intensities. As predicted by the design, the diffraction intensity of the +1 order reaches its minima and maxima at the oxidized state and the reduced state, respectively, while the 0 order shows an opposite response. The intensity contrasts of the +1 and 0 diffraction orders at the two states are dictated by the optical phase variation and the intrinsic optical absorption of PANI. Further increasing the PANI thickness allows for the optical absorption of PANI to become more influential, as evidenced by the decreased diffractive intensity and switching contrast in the +1 order. The overcoated PANI also leads to a phase mismatch in the nanoantennas, where the switching of the +1 and 0 diffraction orders becomes synchronous and solely dependent on variation of optical absorption of PANI.

Quantitative evaluation of the electrical beam steering performance is accomplished by a figure-of-merit (FOM) generally used for beam deflectors, which is based on the switching contrasts between the +1 and 0 diffraction orders at two states [28]. The FOM can be represented as:

where C is the switching contrast between two diffraction orders, I ₀ and I ₁ represent the intensities of the 0 and +1 diffraction orders, respectively, and the superscripts of ox and red denote the oxidized and reduced states, respectively. The value of FOM, which falls between 0 and 1, describes the capability of a beam deflector to route energy into certain angles, where FOM = 1 represents the ideal case. The calculated FOM for different coating cycles indicates that the beam steering performance is optimized when the FOM reaches 0.6 with a polymer coating cycle number of 56 (Figure 3c). In comparison, the obtained FOM from a chalcogenide PCM-based beam steering metasurface [26] is 0.42, and 0.38 from a liquid crystal-based one [9]. Figure 3d and e present the diffraction images at the optimized thickness and the corresponding intensity profiles, at +0.6 V and −0.2 V, respectively, and the results are in excellent agreement with the simulation. The modulation contrast defined as the intensity ratio of the +1 order between the two states, is over 1400 % and can be further increased by increasing the PANI thickness. The thickness of the PANI layer at the optimized condition is measured to be 200 nm (Figure S6), which is only 1/7 as thin as the thickness of the embedding active layer in liquid crystal-based active metasurfaces [9], [17]. The PANI layer grown on the metasurfaces has completely covered the entire resonators with a fiber-like surface structure, as shown in the SEM image (Figure S7). So far, the diffraction efficiency of the +1 order at the reduced state is 28.8 %. In comparison with the beam steering metasurfaces purely based on conductive polymers [6], [14], our design has a 25 times higher diffraction efficiency. The optical loss increases when the PANI is switched to oxidized state, which accounts for up to 70 % of the transmitted energy of the reduced state (Figure S8). The diffraction efficiency can be further improved by decreasing the polymer thickness together with optimizations of the nanoantenna geometry and materials.

In addition, intermediate PANI states, which possess gradually changing refractive indices due to the partially delocalized π-electrons, can be accessed via voltage tuning. By operating the voltage between the oxidized and reduced states, we can continuously modify the diffraction intensity and the intensity ratio between the +1 and 0 diffraction orders, as demonstrated in the diffraction images (Figure 3f) and the intensity profiles (Figure 3g) at different applied voltages. Notably, the electrical switching demonstrates high-quality reversibility. With a cycling voltage between −0.2 V and +0.6 V, the electrical switching is fully reversible with a remarkable stability on our active metasurface, as shown in the Movie S1 which includes the diffraction images, diffraction intensities and the cyclic voltammograms for a total of 9 switching cycles in real-time. We further analyzed the hysteresis behavior of the electrical switching (Figure S9). Our electrically active metasurfaces can be operated in a nearly hysteresis-free manner. This allows switching to the intermediate states without a memory effect that could possibly deteriorate the accuracy of optical response by electrical modulation, which is crucial for realizing active metasurfaces for precise on-demand control of phase and amplitude.

5 Electrical switching speed and durability

To evaluate the switching speed of our active metasurfaces, the temporal optical response is measured under an abrupt alternation of the input voltages between the on (−0.2 V) and off (+0.6 V) states, as presented in Figure 4a and b. The rise time (τ _rise) of the switch-on process and the fall time (τ _fall) of the switch-off process, defined as the time required for the modulated intensity to rise or fall between the 10 % and 90 % switching window, are 14.1 ms and 11.7 ms, respectively. The limiting factor of the switching speed is the electrical characteristics of the cell and the electrochemical system, where the potential switching time for polyaniline can be within 200 μs [47]. We further analyzed the switching stability under the high-speed and low-speed operation configuration. Under the high-speed working condition, the metasurface is operated at alternating input voltages between the on and off states with a voltage duration time (Δt) varied from 50 ms to 5 ms (Figure 4c). Such intense switching conditions could possibly lead to mechanical or electrical failures due to the volume expansion, physical exfoliation or irreversible reactions of polymer films [46]. However, remarkably, when Δt approaches the switching time τ _rise and τ _fall, the accessible range of the optical response approaches the 10 %–90 % switching window at Δt = 15 ms, equivalent to 66 fps, as shown in Figure 4d. Lowering Δt below 15 ms decreases the accessible optical range, but does not significantly influence the reversibility. In the low-speed switching condition, the metasurface is operated at Δt = 3 s and shows no decrease in signals within the operating window. The electrical switching also exhibits no substantial degradations in the switching contrast over 2000 switching cycles (Figure 4e). The key characteristic supporting the superior cycling stability is that, when electron injection or extraction takes place by voltage application, the highly conductive PANI is able to redistribute delocalized π-electrons along the polymer chain and keep mobility without structural degradation [47], [48], even after 10⁷ switching cycles [49]. This ensures the stability when deploying intense high-speed electrical modulation, and the long-term switching contrast. Moreover, the solid contact between the nanoantennas and the PANI film, which is directly grown on the metasurface, provides additional mechanical and electrical stabilities.

Figure 4:

Electrical switching speed and durability. (a–b) Temporal responses for the switch-on and switch-off processes, respectively. The rise time (τ _rise) of the switch-on process is 14.1 ms and the fall time (τ _fall) of the switch-off process is 11.7 ms. The rise time and the fall time are defined as the time required to switch between 10 % and 90 % of the optical response, depicted as the gray area, under an abrupt alternation of the applied voltage (green and red areas). (c) Input voltage schemes with different voltage duration times (Δt) in a range from 50 ms to 5 ms. (d) Temporal optical response at different input voltage frequencies. Each scheme lasts 10 s. The percentage numbers indicate the maximum accessible range of the optical response. The recorded slightly inconsistent maxima and minima are due to frame drops of the camera. (e) Switching durability of the electrically active metasurface under Δt = 3 s. The electrical switching is highly reversible and consistent without noticeable degradation over 2000 switching cycles. The optical response in this figure corresponds to the +1 order at an optimized coating cycle of PANI.

6 Conclusions

In this study, we have introduced and experimentally realized an active Huygens’ metasurface based on the in-situ grown PANI with an overall switching performance. We have achieved electrical switching on the beam steering metasurface with large modulation contrast of over 1400 %, hysteresis-free electro-optical controllability, high diffraction efficiency of 28 %, fast switching speed of over 60 fps and superior switching duration over 2000 switching cycles, under an operation voltage range only from −0.2 V to +0.6 V. We have demonstrated that the active Huygens’ nanoantenna design boosts the optical efficiency compared to the nanoantenna purely based on polymers. In addition, we have shown that PANI is an excellent candidate of electro-active materials for active metasurfaces, owing to its large tuning range of refractive index, fast switching speed, electrical properties and superior physical and chemical stability. The manufacture of conductive polymer by in-situ growth on metasurface is straightforward and more reliable, which can be processed in a low-cost, large-area and scalable manner. The concept of our electrically active Huygens’ nanoantenna design can be broadly extended to a variety of active metasurfaces that are potentially capable of programmable and dynamic manipulation. From an industrial standpoint, the overall switching performance, together with the scalable, reliable, cost-effective polymer manufacturability, provide promising opportunities to achieve electro-optical metadevices for emerging applications, forging the path of active metasurfaces towards commercial success.

7 Materials and Methods