Electrically switchable metasurface for beam steering using PEDOT

Switchable and active metasurfaces allow for the realization of beam steering, zoomable metalenses, or dynamic holography. To achieve this goal, one has to combine high-performance metasurfaces with switchable materials that exhibit high refractive index contrast and high switching speeds. In this work, we present an electrochemically switchable metasurface for beam steering where we use the conducting polymer poly(3,4-ethylene-dioxythiophene) (PEDOT) as an active material. We show beam diffraction with angles up to 10{\deg} and change of the intensities of the diffracted and primary beams employing an externally applied cyclic voltage between -1 V and +0.5 V. With this unique combination, we realize switching speeds in the range of 1 Hz while the extension to typical display frequencies in the tens of Hz region is possible. Our findings have immediate implications on the design and fabrication of future electronically switchable and display nanotechnologies, such as dynamic holograms.


Introduction
The manipulation of optical wavefronts via lenses, mirrors, or any other optical devices is present in all our everyday lives. This includes, e.g., bulky lenses made from glasses, Fresnel lenses in smartphone illumination optics, or spatial light modulators and liquid crystals in displays. However, emerging display technologies such as dynamic holography or augmented and virtual reality require ever-increasing pixel densities and thus new smart optical methods with the ability to manipulate optical wavefronts, beam paths, polarization, or similar actively on ultra-small length-scales [1][2][3]. For the realization, the research field of plasmonics has gained significant interest over the last years as plasmonic nanoparticles allow to focus, manipulate, or steer light on the nanometer scale at subwavelength dimensions [4][5][6]. The combination of optically active and externally switchable materials with plasmonics into hybrid nanosystems has increased their applicability even further and opened the door towards active plasmonically-driven light manipulation [7]. Especially, plasmonic metasurfaces, that are, artificial sheet materials with sub-wavelength thickness, allow the realization of flat optical components with unique optical properties [8][9][10][11][12][13]. Furthermore, the combination with phase change materials inaugurates active plasmonic optical applications such as active beam switching [14,15], zoom lensing [16], dynamic holography [17], dynamic plasmonic color displays [18,19] and many other. One possibility to realize these hybrid nanosystems is the fabrication of nanoparticles directly from phase change materials such as magnesium [20][21][22][23], palladium [24][25][26][27], and yttrium [28]. On the other hand, phase change materials such as polyaniline (PANI) [29,30], germanium-antimony-tellurium-based film films (GST) [31][32][33][34], liquid crystals [35,36], or VO2 [37,38] are widely used to combine with nanoparticles to allow active switching. However, widespread and commercial applications are, so far, hindered by several limiting factors such as material degradation, slow switching speeds, low optical contrast (refractive index shifts), and similar. Even more, most material phase transitions are only accessible via variations in, e.g., temperature or exposed gases and not electrically.
Here, we present an optically active system to perform switchable beam steering realized via a novel hybrid metasurface. It consists of a unique combination of gold plasmonic nanoantennas and the electrically switchable and conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) [39].
We show that tuning of geometric parameters allows switching angles and beam diffraction of up to 10°, while the efficiency of the hybrid metasurface and thus the intensity of diffracted light can be actively controlled via an externally applied cyclic voltage between only -1 V and +0.5 V. An increase of the scan-rate of the cyclic voltammetry reveals switching frequencies close to 1 Hz while an extension to typical display frequencies in the tens of Hz range is possible, only limited by the measurement components. In order to design the plasmonic nanostructures, we used the transient solver of CST Microwave Studio Suite in a wavelength range between 600 nm and 1200 nm. Figure 1a shows the material stack and geometries of the simulated system. We simulated a single unit-cell with open boundary conditions in the z-direction to emulate free space and periodic boundary conditions in xand y-direction to simulate an infinite array of antennas. To optimize the on/off behavior, we performed two simulations-one series with the refractive index of the reduced PEDOT and one sequence with the oxidized PEDOT. With a parameter scan, the geometry was optimized to find an antenna geometry, which leads to a significant shift of the plasmonic resonance when the refractive index of the PEDOT changes. Resulting amplitude and phase spectra of the transmission coefficient for cross-and co-polarization can be found in Figure S1 in the Supplementary Information. The resonance shift allows us to turn on and off the anomalous refraction of the plasmonic antenna array at a specific wavelength. The refractive index of PEDOT was taken from Stockhausen et. al. [39]. Further, the refractive index of gold was taken from Yakubovsky et. al. [40].
The final metasurface consists of gold nanoantenna arrays with progressively rotated elements along one axis [41], as shown in the SEM images in Figure 1b layer which is deposited by electropolymerization [42,43]. This PEDOT layer undergoes a significant refractive index change in the visible spectral range when the applied voltage is varied by cyclic voltammetry [44] and thus results, in combination with the metasurface, in electrochemically activated switchable beam steering. Please note that modifying the deflection angles or similar would require an active change of the metasurface parameters (antenna geometry, spacing, etc.) or the combination of multiple metasurfaces. So far, this is not possible with our hybrid metasurface design. See Figure S2 in the Supplementary Information for details on the electrodeposition and structural change of PEDOT. A schematic drawing of our main setup to perform angle-resolved imaging is shown in Figure   2a. It consists of a modified transmission microscope (Nikon Eclipse TE2000-U) in combination with a 6 home-built k-space imaging module [27]. We use a tunable laser (NKT Photonics SuperK Extreme) as an illumination source, set at a wavelength of λ = 750 nm. To obtain right-circularly polarized (RCP) light we use a polarizer (Thorlabs LPVIS100) and a broadband quarter-wave plate (QWP) (B. Halle RAC 5.4.20). As we image in k-space, the condenser is set highly defocused to allow for the best possible normal incidence on the metasurface. The metasurface is placed at the sample position in the front  Figure S2 in the Supplementary Information. The effect of beam steering is obtained via the refractive index shift of the PEDOT layer during cyclic voltammetry. This change in refractive index causes the plasmonic resonance of the metasurface to shift and thus the efficiency of the beam diffraction to vary. The spectral response of our hybrid metasurface is shown in Figure 3a. For spectral measurements, we replace the k-space imaging module in Figure 2a with a grating spectrometer (Princeton Instruments SP2500i) and a Peltier-cooled frontilluminated CCD camera (Acton PIXIS 256E). In the oxidized state (red curve), we find a broad plasmonic resonance with a centroid wavelength around λc,O = 760 nm. In contrast, the plasmonic resonance in the reduced state (blue curve) is red-shifted by approximately 85 nm to a centroid wavelength around λc,R = 845 nm. Overall, this matches the spectral shift expected from the refractive index shift of PEDOT [42,44]. Furthermore, a spectral transmission peak around λ = 720 nm is found in the reduced state.

Results and Discussion
This peak originates from the overlay of the plasmonic resonance dip at λc,R = 845 nm and the intrinsic PEDOT material transmission dip (absorption peak) around λ = 600 nm. This material resonance is also visible in the transmission spectra of only the electropolymerized PEDOT layer in the oxidized and reduced state, which can be found in Figure S3 Figure 3b depicts the voltage cycling between -1 V and +0.5 V with a scan rate of 8 20 mV/s. We find that the centroid wavelength cycles between the oxidized and reduced state and that the PEDOT and thus the hybrid metasurface responds immediately to changes in the voltage. The reduced and oxidized states are separated from each other which allows for stable cycling between two distinct optical states. Furthermore, the switching is reproducible over several cycles with no indication of any degradation. As our hybrid metasurface is designed to allow for switchable beam steering in the visible spectral range, we now turn our attention to angle-resolved k-space imaging, as it is illustrated in  Figure 4a and b for the oxidized and reduced state, respectively (illumination wavelength λ = 750 nm). The lower graphs depict the spatially resolved intensity in k-space, whereas the upper graphs show the corresponding integrated intensities. Overall, we find a gaussian beam profile of both beams. Please note that the intensities are obtained by converting the CCD camera images from sRGB to linear RGB. In the oxidized state in Figure 4a, the diffracted beam I1 has a higher intensity in comparison to the main beam I0. In contrast, this relation inverts for the reduced state in Figure 4b, as here the diffracted beam has a lower intensity than the main beam. We find that the total intensity of both beams increases, which results from the overall higher transmittance of the hybrid metasurface in the reduced state at λ = 750 nm (compare Figure 2a). The diffraction of this designed hybrid metasurface in k-space is kx = sin θx = 0.085, which corresponds to a diffraction angle of θx = 4.9°. By changing the superperiod of the metasurface we can increase the diffraction angle to θx ≈ 10°. The k-space and temporal response in dependence of the applied voltage of this modified metasurface are shown in Figure S4 in the Supplementary Information.
For potential applications as light modulators, it is important to determine the modulation efficiency of our metasurface. In detail, we determine two quantities necessary to quantify our metasurface relative to existing literature. The first parameter, the basic efficiency where 1 , 1 , and 0 , 0 are the intensities of the diffracted and main beam in the oxidized and reduced state, respectively. and denote the contrasts in the oxidized and reduced state, respectively. A complete transfer of the intensity from the main beam to the diffracted beam during switching would result in a =1. For our metasurface in Figure 4, we obtain a value of = 0.05, whereas for the modified metasurface in Figure S4 we obtain = 0.01. These values are small compared to other active metasurface designs in literature [46], which most likely originates from the comparably small refractive index shift of PEDOT in the visible spectral range.
The intensity ratio I1/I0 of our initial metasurface during cyclic voltammetry is plotted in Figure   4c. Please note that due to the reduced intensity of the main beam I0, the absolute value of the intensity ratio I1/I0 is in fact smaller as discussed above for the calculation of the efficiency of the metasurface. A further increase of the intensity ratio and thus diffraction efficiency might be possible by considering other metasurface designs with modified design parameters (metasurface material, antenna geometry, operating wavelength, variation of active material, and many more). Again, we find that the hybrid metasurface quickly reacts to voltage changes. In the oxidized state after t = 24 s, the ratio has a sharp maximum whereas in the reduced state after t = 104 s we find a flat plateau around the minimum. The observed temporal behavior of the intensity ratio clearly shows that we can actively control the intensity of both beams. The intensities of the individual beams during cyclic voltammetry are plotted in Figure S5 in the Supplementary Information. The variations in the beam intensities do not originate from a simple transmittance change of the PEDOT, which would result in a temporal constant intensity ratio I1/I0, but rather from a change in the diffracted intensity due to its refractive index change and subsequent tuning of the metasurface plasmonic resonance. We find from the individual beam intensities in Figure S5 that it is, in fact, possible to vary the diffracted beam intensity individually while the intensity of the main beam remains constant. Thus, the "gained" intensity in the diffracted beam needs to result in a reduced intensity in higher-order beams and background (mostly diffuse scattering). Consequently, we are able to vary the diffraction-efficiency of the hybrid metasurface via the applied voltage. Selected k-space images of this beam steering process are depicted in Figure 4d.  Furthermore, several other factors contribute to define the switching speed, which is discussed in the Supplementary Information.

Conclusion
In conclusion, we have demonstrated a novel approach for an optically active as well as externally and electrically switchable hybrid metasurface. We presented a unique combination of a metasurface comprising gold nanoantennas and electropolymerized PEDOT which allowed for an electrochemically activated switchable beam steering. The feasibility of this combination for potential nanophotonic applications was demonstrated via a detailed investigation of the optical and temporal properties of the hybrid metasurface. We used Fourier-space imaging to reveal switching and diffraction angles of up to 10° with excellent conservation of beam profiles in the first diffraction order.
A temporal investigation of the intensities of main and diffracted beams showed that we are able to actively control the efficiency of the metasurface and thus the intensity of the diffracted and primary light. We reach switching frequencies around 1 Hz while the extension to display frequencies is only limited by the measurement components and not intrinsically by the optical and electrical properties of our hybrid metasurface. Overall, our approach finds immediate implication in the design, fabrication, and realization of optically active nanophotonic systems that are electrically switchable.
Our results will help to develop future optical technologies such as virtual and augmented reality as well as dynamic holography.