2.1 Design of the optical characterization of the liquid crystal metadevice

Figure 1(a) illustrates the schematic of bidirectional high-speed optical wireless communication system with tunable large FOV between the station and users assisted by LC metadevice. The proposed LC metadevice consists of an LCoS and a set of LC metasurfaces, which fully utilizes the tunability of LCoS and the controlled angular magnification of LC metasurface set simultaneously. The LC metasurface set includes an LC metalens L ₁ integrated with an LC quarter-wave plate (QWP) and another LC metalens L ₂, whose focal lengths are denoted as f ₁ and f ₂. As shown in Figure 1(b), the two metalenses work as a convex and a concave lens, spaced apart by the sum of focal lengths f ₁ + f ₂ to be confocal. In this way, the collimated incident beam will be outputted with a deflection angle magnified by M times, and M equals to −f ₁/f ₂. As a linear polarized signal beam is cast onto the metadevice, it is firstly modulated by LCoS, which is capable of 2D beam scanning within a small FOV. Later, the beam transmits through the LC meatasurface set, where the deflection beam is transferred into a left circular polarized (LCP) one through the LC QWP and is linearly and continuously magnified in the deflection angle and 2D scanning FOV through metalenses L ₁ and L ₂. Finally, the signal beam is sent to the dynamic moving users.

Figure 1:

Schematic of the LC metadevice–assisted OWC system with tunable large FOV. (a) High-speed OWC system using LC metadevice with tunable and magnified FOV. (b) Principal diagram of the LC metalenses to enlarge the FOV. (C) Unit cell of the sandwich LC metasurface structure.

For better application, conventional lens counterparts with equivalent NA around 0.2 are not taken into consideration for its significantly curved surfaces, increased thickness, greater weight, and complexity of aberration correlation, while metalenses boasts flat surface, thin and compact factor, precise pointwise phase modulation, and high degree of design freedom, as well as capability to mitigate the aberration. The proposed LC metasurfaces are composed of the sandwich structured unit cells, which generally contain two silicon substrates, an oriented film, and the key functional LC layer as shown in Figure 1(c). The three LC metasurfaces are designed by different principles with precise and continuous phase regulation without increasing the processing complexity. The LC QWP is designed by accumulating the thickness of birefringent LC layer to obtain a propagation phase difference of π/2 or its odd multiples between ordinary light and extraordinary light. And the LC metalenses are designed via Pancharatnam–Berry phase modulation by varying the orientation angle γ of pixeled LC directors under LCP illumination to produce 2γ phase shifts, realizing a full 2π multilevel phase shift for beam steering. For polarization transform and angle magnification, the orientation angle distribution of LC directors of LC QWP is uniform, while that of the two metalenses are both characterized with quadratic profiles, whose corresponding phase distribution is as follows:

where r, λ, and f are the distance to the metalens center, the design wavelength, and the focal length, respectively. It is worth mentioned that the LC metalenses function as expected only when incidence onto them is LCP light, so LC QWP is essential. Otherwise, if the incidence is right circular polarized (RCP), the phase shift will be inversed so that L ₁ and L ₂ will get opposite focal length, respectively, which breaks the confocal state of the two metalenses.

In physical realization, the LC QWP is integrated onto the front substrate of L ₁ to form a three-substrate structure, as shown in Figure 1(a). Given the limited deflection angle of LCoS, the effective apertures are set to be 6?mm for L ₁ and 4?mm for L ₂. The focal length f ₁ is designed as 21?mm and f ₂ is −7?mm at a working wavelength of 1,550?nm, so the magnification factor M is 3. To ensure at least 10 phase levels for the edge periods, the two lenses are designed with different pixel sizes, 0.91 × 0.91?μm² for the front lens L ₁ and 0.47 × 0.47?μm² for the back lens L ₂, so L ₁ has 6,593 pixels in diameter and L ₂ has 8,510 pixels in diameter.

Figure 2(a) shows the photograph of the packaged LC metasurface set. The fabricated three LC metasurfaces are separated into two parts: a three-substrate structure containing LC QWP, and L ₁ shown in the left inset, and a two-substrate structure containing L ₂ depicted in the right inset. Figure 2(b) shows the optical polarizing microscopic images of LC QWP, L ₁, and L ₂, respectively. At the bottom-right corner of the insets in each image, partial orientation distributions of LC directors are displayed. As can be seen, the LC QWP has a uniform orientation distribution, while L ₁ and L ₂ have different but similar orientation distributions. It indicates the different design principles of them.

Figure 2:

The fabricated LC metasurface set. (a) Physical diagram of the packaged LC metasurface set. The insets show the two fabricated bare samples. (b) Polarizing microscopic images of QWP, L ₁, and L ₂. The corresponding partial orientation distribution of LC directors is shown in the insets. Scale bar: 0.1?mm.

Next, we experimentally characterized the optical performance of the LC metadevice for tunable and large FOV. A laser beam at the operation wavelength of 1,550?nm through a collimator illuminated the reflective plane of LCoS with an incident angle of 45°, which enabled a configurable FOV of 7°, from −3.5° to 3.5°. The measured results for the linear angle magnification ability are shown in Figure 3(a), where the fitting line with a slope of 3 matches well with the experimental results. In other words, for the beam incident onto the LC metadevice rather than only incident onto the LCoS, the deflection angles at the output ports were magnified by three times, thus realizing a continuously adjustable FOV of 21°. The efficiencies were measured using an IR spatial optical detector as illustrated in Figure 3(b). For different incident angles, the LC metasurface set shows a stable and high transmission efficiency above 83?%. The overall efficiencies of LCoS are determined by the reflectivity and the modulation efficiency. At the incidence wavelength of 1,550?nm with LCoS-aligned polarization, the reflectivity of LCoS is 66.7?%, and the modulation efficiency decreases rapidly with increasing incident angles. It is because the initial reflection angle is larger than the working reflection angle of LCoS, which is a compromise of the excessive occlusion caused by the bulky package of the LC metasurface set. This can be eliminated by reducing the packaging footprint.

Figure 3:

Characterization of the LC metadevice. (a) The measured results for the linear angle magnification ability. The slope of the fitting line (dotted line) is 3. (b) The transmission efficiency of LC metasurface set (black line) and the modulation efficiency of LCoS (red line) for different incident angles. The dotted line marks the efficiency of LC metasurface set of 83?%. (c) Tracks of a moving light spot modulated by LCoS only, on IR sensitive cards 0.25?m away. (d) Tracks of a moving light spot modulated and enlarged by LCoS and LC metasurface set, on IR sensitive cards 0.25?m away. (e) Point-to-multipoint beam steering displayed on IR sensitive cards 0.25?m away without LC metasurface set (upper) and with LC metasurface set (lower).

To evaluate the continuous 2D beam-steering function of the LC metadevice, we recorded the tracks of a deflected light spot, which is dynamically modulated by LCoS only or by the whole LC metadevice on IR sensitive cards 0.25?m away from the LCoS, as illustrated in Figure 3(c) and (d). The gray arrows indicate the moving paths of the light spots. Notably, it clearly indicates the good continuity and flexibility of 2D beam steering of LC metadevice, in addition to the linear magnification of 2D FOV. The complete videos of 2D beam-steering processes are provided in the Supporting Information. Apart from point-to-point beam steering, the proposed LC metadevice can also be utilized for point-to-multipoint beam steering. Figure 3(e) shows the four-beam steering displayed on IR sensitive cards 0.25?m away from LCoS, where the upper inset is multibeam steering tuned by LCoS only, while the lower inset is multibeam steering modulated by the whole LC metadevice. As can be seen, the expanded tunable FOV by LC metadevice brings more feasibilities for optical broadcasting system with large FOV.

2.2 Experiments of bidirectional high-speed optical wireless communication assisted by liquid crystal metadevice

We applied the fabricated LC metadevice to a bidirectional OWC system for proof-of-concept. Figure 4(a) illustrates the configuration of the experimental system for LC metadevice–assisted bidirectional point-to-point indoor OWC system (more details about the experimental setup of the system are provided in Section S2, Supporting Information).

Figure 4:

Experimental system configuration and results for the LC metadevice–assisted bidirectional coherent OWC system at the data rate of 200?Gbps for both links. (a) Schematic of the OWC system. (b) Measured BERs and (c) received powers of downlink at different output angles and different wavelengths when the source power is fixed at 12?dBm. (d) Measured BERs and (e) received powers of the downlink and uplink at different output angles and wavelength of 1,545?nm when the source power is fixed at 10?dBm and 2?dB is compensated at the receiving end. The dotted line marks the FEC threshold of 2.4 × 10⁻².

For the downlink transmission, six wavelengths spanning across S, C, and L band were used as optical carriers to explore full band characteristics of the system. A LiNbO3 in-phase and quadrature-phase (IQ) modulator was driven by an arbitrary waveform generator (AWG) with sampling rate of 120 GSa/s to provide 200?Gbps 16 quadrature amplitude modulation (QAM) signals with a length of 50,000, and each of the six wavelengths carried 200?Gbps signals. After being amplified by an erbium-doped fiber amplifier (EDFA), the optical power outputted from collimator 1 with a beam waist diameter of 1.2?mm was ensured to be ≈12?dBm. To become linear polarization so as to maximize phase modulation efficiency of the polarization-dependent LCoS, the beam was also tuned by a polarization controller (PC) before collimator 1. Thereafter, the beam was illuminated onto the LC metadevice, where the LC metasurface set was positioned closely after the LCoS and aligned its untuned reflected beam axis. As the measured optical loss of LC metadevice was above 2.47?dB, the optical power after the LC metadevice was below 10?dBm, which is under eye-safe operation power. After a transmission distance of 1.5?m in free space, a collimator (collimator 2) with a beam waist diameter of 1.2?mm installed on the multiaxis stage was utilized to couple the optical signal into a single mode fiber. An additional PC was employed to tune the optical signal with single polarization, thus ensuring the single polarization channels of the following digital storage oscilloscope (DSO). The optical signal was converted into electrical signal by the coherent receiver, which was sent to DSO with 256?GSa/s. Finally, the offline digital signal processing (DSP) was executed for BER calculation.

Figure 4(b) shows the downlink BER curves of various 1D unilateral output deflection angles ranging within 10° at different wavelengths of S + C + L band. A wider FOV up to 20° × 20° can be naturally achieved due to the 2D tunability of LCoS and the symmetricity of opposite diffraction orders. For wavelengths of C band (1,530?nm, 1,545?nm, 1,555?nm) and shorter L band (1,570?nm), the LC metadevice presents similar performance for various output deflection angles, and the BER values are below the FEC threshold of 2.4 × 10⁻² marked with a dotted horizontal line. But for S band (1,520?nm) and longer L band (1,585?nm), the BER values go bad. Figure 4(c) manifests the received power curves of deflection angles and wavelengths corresponding to Figure 4(b), where received powers at the wavelength of 1,530?nm, 1,545?nm, 1,555?nm, and 1,570?nm have similar tendency, while at 1,520?nm and 1,585?nm, the received power go below −20?dBm quickly. This can be contributed to the limited working bandwidth of LCoS device.

For the uplink transmission, the optical wireless system approximates optical reciprocity but has an additional EDFA at the receiving end, as illustrated in Figure 4(a). To ensure the eye safety, the output power of collimator 2 was slightly below 10?dBm. Another EDFA was added at the uplink receiving end to compensate for the 2?dB difference between downlink and uplink transmitting optical powers. As PC can randomly modulate polarization of light in fiber, an LCP beam was transmitted backward from the collimator 2 by tuning the PC at the transmitting end. After passing the LC metasurface set, the beam was transformed into the linear polarized one just aligning with the most modulation efficient polarization of LCoS device. Therefore, the beam was reciprocally transmitted into collimator 1.

Due to the approximately reciprocal optical paths, the optical losses of the uplink transmissions are nearly equal to that of the downlink. Here, we take the user with wavelength of 1,545?nm, for example, using the same modulation format and rate as the downlink ones. Figure 4(e) shows the optical power losses of the uplink and downlink at different output angles at 1,545?nm. It is easy to see that the values of uplink and downlink power losses are close to each other. Take 2° output angle as an example, the total optical power losses are 26.6?dB for downlink and 26.4?dB for uplink. They both include five parts. The first part is caused by the 66.7?% reflection efficiency of LCoS device, which results in a 1.76?dB loss. The second part is the modulation efficiency of LCoS, about 64.3?%, introducing a 1.92?dB loss. The third part is transmission loss of metalens doublet, about a 0.71?dB loss. The fourth part is the coupling loss between the two collimators. For downlink, considering 3 times beam size shrinkage caused by the LC metasurface set, a 1.2 beam waist of collimator 2, a 1.5?m propagation distance, and the wavelength of 1,545?nm, the calculated theoretical downlink coupling loss is 5.26?dB. For uplink, considering 1.1 times beam size expansion caused by the free space propagation, 3 times beam size expansion caused by the LC metasurface set, a 1.2 beam waist of collimator 1, and a 100?mm propagation distance from the LC metasurface set to collimator 1, the theoretical uplink coupling loss is 5.11?dB. The last part is about 16.9?dB (26.4–1.76–1.92–0.71–5.11 = 16.9), which may be attributed to the optical misalignment and the fixed interfacial loss of devices in the optical path. The main loss difference of both links is due to the coupling loss, so the downlink and uplink share nearly the same receiving power. Figure 4(d) shows the BER curves of various deflection angles in the downlink and uplink at the wavelength of 1,545?nm. As can be seen, the uplink BER has similar tendency and values as those of the downlink. The uplink and downlink have approximately the same transmission performance. Therefore, it is feasible to realize a bidirectional high-speed optical wireless communication with tunable large FOV using LC metadevice–assisted system.

5.1 LC metasurface fabrication

The process for fabricating the three LC metasurfaces can be mainly divided into two parts: the preparation of the oriented film and the photopatterning of the designed orientation profiles. Initially, silicon substrates were cleaned with deionized water and isopropyl alcohol, later ultraviolet ozone bathed, and then spin-coated with azo-dye. After heat-cured for several minutes, the azo oriented films were attached firmly, which could be oriented under polarized blue light and guided the orientation of LC directors through intermolecular anchoring energy. Secondly, to realize the designed orientations, a maskless direct laser writing lithography system was introduced, mainly composed of a light source, a polarization controlling module, a reflector, and an optical focusing component. The designed orientation profiles were photopatterned on the oriented film, and later LCs were spin-coated on the oriented film and ultraviolet irradiated. Then the LCP-oriented films with designed orientations were formed. As the films were fragile, substrates with antireflection film were attached on the top to protect them from external impact. For ease of optical operation, the LC QWP and L ₁ share the middle substrate to integrate together.