On-chip 2D beam-steering with a liquid-crystal-tunable optical phased array

Optical phased arrays (OPAs) with the ability of dynamic beam-steering hold great promise for industrial applications. Although they potentially offer high-resolution, high-speed and wide-angle beam-steering, 2D beam-steering has only been achieved by 1D OPAs with wavelength-tuning or by 2D OPAs containing enormous numbers of elements, thereby signicantly increasing the system complexity and power consumption. Here we demonstrate a liquid-crystal-tunable Bragg-reector waveguide outcoupler integrated with a 1D OPA. The output beam angle is controlled by an electric-eld applied to the liquid crystal between the Bragg-mirrors, independently from steering along a second axis with the OPA. Our proof-of-concept demonstration with an 8-channel OPA shows 2D beam-steering with a 16° × 15° eld-of-view and 12 ms × 14 μs time-response. Our approach opens a practical way for scalable OPAs capable of single wavelength 2D beam-steering, as well as demonstrating a strategy for the integration of liquid crystal materials with silicon photonics.

Here we propose a LC-tunable Bragg-re ector waveguide outcoupler integrated with a 1D OPA for 2D beam-steering. The principles of the beam-steering mechanism are illustrated in Fig. 1. The structure of the outcoupler is similar to a VCSEL [25,26], but comprises LC as a tunable core sandwiched between two distributed Bragg re ectors (DBR) to support the guided modes (Fig. 1b). This mode is coupled out through the top DBR with a lower re ectivity compared to the bottom one. By applying an external electric eld to the LC core, the effective index of the guided mode is modulated, thereby dynamically changing the angle of the output beam from the outcoupler (Fig. 1c,d). The steering in the second dimension is obtained by building an array of outcouplers with controllable phases (Fig. 1b). This can be realized by connecting waveguide arrays with phase shifters (1D OPA) to the LC-tunable outcouplers (Fig. 1a).
Different from conventional techniques, our approach enables independent control of the LC and OPA for beam-steering in two directions. The OPA steering is generally faster (<100 ms using thermal phase shifters) than the LC-tuning (~10 ms), resulting in beam-steering over a 2D eld-of-view (FOV) within tens of milliseconds. Such a time response is su cient for various applications, including imaging-based fusion systems and image projections where the frame rate is typically ~30 frames per second.
More importantly, the LC-tunable 1D OPA consists of a small number of elements compared to a conventional 2D OPA, thus greatly simplifying the driving system and reducing power consumption. The 1D array generates only a few OPA grating-lobes, which could even be completely suppressed by reducing the antenna pitch [4,27], leading to a large power fraction in the main beam. The large antenna pitch used in 2D OPAs means that they suffer severely from a reduction in the main beam power caused by the presence of multiple grating-lobes [2,3,12], which poses challenges for long-range applications. Figure 2 shows the on-chip integration design of the LC-tunable 1D OPA based on silicon nitride waveguides operating at l = 940 nm (see Methods). The photonic circuit fabricated on the bottom DBR is coupled to the LC-tunable outcouplers, which are formed by a LC-lled core between the top and bottom DBRs. The outcoupler supports a high order DBR-guided mode in the vertical direction, while the lateral con nement is obtained by SiO 2 blocks acting as low-index claddings (Fig. 2c). This mode has a different propagation constant and eld pro le with respect to the input waveguide mode (Fig. 2b). To manage the optical transfer between these waveguides, a grating-based mode converter (MC) is introduced at the end of the input waveguide (Fig. 2a). Resonant mode conversion can be obtained via the phase matching condition p = l/(n w -n g ) [28], where p denotes the grating period, l the wavelength, n w and n g the effective index of the input and guided mode within the MC region. The exact resonance can be found by electrically tuning the LC index to ~1.6 in the MC (Fig. 2d). Further optimization of the mode conversion e ciency is achieved by adjusting the grating diffraction strength and the eld overlap between these modes [28].
To demonstrate LC-tunable beam-steering, we rstly fabricated and characterized a single Bragg-re ector waveguide outcoupler (Fig. 3). Fig. 3a shows the sample, which was fabricated using a modi ed CMOScompatible silicon photonics process, followed by integration of the LC ( Supplementary Fig. S1). The LCtunable outcoupler was formed by introducing the LC into the gap between the top and bottom DBRs (Supplementary Table S1 for the DBR design and Fig. S4 for the calculated re ectivity). The LC orientation was then altered by applying an external eld (Methods) as shown in polarized microscope images (Fig. 3b,c). The near-eld (far-eld) emission images are shown in Fig. 3d,e ( Fig. 3f,g), demonstrating that the measured FWHM beam divergences in the far-eld are close to the diffraction limited values. Fig. 3h shows the beam-steering performance as a function of the applied voltage. The measured steering range was ~30˚ with a beam divergence of ~0.25˚. The small variation in beam divergence was attributed to the scattering loss depending on the propagation constant of the guided mode in the LC core. From the measured steering range, our simulations indicate that the index-tuning range was 1.65-1.54, although the ideal tuning range provided by the birefringence is 1.68-1.52. This small discrepancy is because the movement of the LC inside the small core is limited ( Supplementary Fig.   S8).
We then demonstrated a LC-tunable OPA for 2D beam-steering (Fig. 4). The sample was prepared by adding LC material onto a Si-chip containing 1D OPA, consisting of a waveguide array and thermal phase shifters. Figure 4a shows a microscope image of the sample with an 8-channel array of LC-tunable outcouplers, and Fig. 4b shows an image of the near-eld emission. As shown in Fig. 4c, a beam spot in the far-eld was clearly visible after calibrating the initial phases ( Supplementary Fig. S6). Figure 4c demonstrates 2D beam-steering with a FOV of 16° × 15° achieved by tuning the phase shifters in the ydirection and the LC in the x-direction. The grating-lobes at ±15° due to the relatively large outcoupler pitch (3.6 mm) were generated only in the y-direction. The weak scattered light seen in the x-direction is likely due to the defects on the bottom of trench for the LC core ( Supplementary Fig. S5e). The FOV of 16o btained by the LC-tuning was smaller than that of the single outcoupler (30˚). This was caused by the limited movement of the LC inside the small core width (Supplementary Fig. S8). Solutions to these problems include the use of a LC slab-waveguide design ( Supplementary Fig. S9), a smaller OPA pitch to suppress the grating-lobes, and a larger LC area enabling improved LC control. The response time of the LC tuning was 12 ms, as shown in Supplementary Fig. S7b. The LC-tuning required extremely low power (<100 mW) since the liquid-crystal itself is essentially a capacitive load. The thermal phase shifters showed a response time of 14 ms and required 80 mW each for p phase shift ( Supplementary Fig. S7a).
In the future, more advanced phase shifters [13,18] may be used to reduce the power consumption, realizing low-power operation even for a larger scale LC-tunable OPA.
In conclusion, we have proposed a LC-tunable 1D OPA on a Si-chip for continuous 2D beam-steering at a xed wavelength. We have demonstrated LC-tunable beam steering using an integrated single LC-tunable Bragg-re ector waveguide outcoupler. By connecting the array of LC-tunable outcouplers to an 1D OPA, beam-steering in two-dimensions was additionally obtained. Our device with an 8-channel OPA demonstrates 2D beam-steering with 16° × 15° FOV and 12 ms × 14 μs response time at l = 940 nm. We believe the proposed LC-tunable OPAs hold great promise for industrial applications. Our LC integration method is applicable to conventional photonic integrated circuits and provides a new platform allowing the combination of the advantages of silicon photonics and the large refractive index modulation and low power operation of LCs.

Methods
Simulations. The nite-element method (Synopsys, FemSIM) was used to simulate the effective refractive indices and the mode pro les of the waveguides and the LC/Bragg-re ector waveguides, while the eigenmode expansion propagation method (Synopsys, ModePROP) was used to simulate the mode conversion.
Device design. The mode converter is designed for a wavelength of 940 nm with a refractive index of 2 for SiN waveguide core and 1.46 for the SiO 2 cladding. To reduce propagation loss, the input waveguides need to be separated from both DBRs, which contain high index layers. Therefore, we adopted an outcoupler design that has a large separation between the top and bottom DBRs. The width of the waveguide is 600 nm but is gradually expanded near the mode conversion to match the width of the LCcore. The core width is 10 mm for the single Bragg-re ector waveguide outcoupler and 1.6 mm for the LC-   On-chip integration of LC-tunable Bragg re ector waveguide outcouplers with an OPA. a, Cross-sectional schematic of a connection between the waveguide and the LC-tunable outcoupler in the xz-plane. A grating-based mode converter is placed at the end of the waveguide. Top and bottom electrodes are introduced to apply an external electric eld. The bottom electrode is patterned to drive the LC for mode conversion and beam-steering separately. b, The structure of the waveguide and c, the Bragg re ector waveguide in the yz-plane, and the corresponding simulated mode pro les. d, The calcu-lated effective index of the waveguide (nw) and the DBR-guided (ng) mode at the mode transfer region. The calculated mode conversion e ciency of the grating is also shown. Resonant mode conversion is achieved by tuning the LC index to ~1.6 where the phase matching condition is satis ed. e, The intensity pro le at the mode conversion area. The waveguide mode is e ciently transferred to the DBR-guided mode on the resonance.