Demonstration of 2D Optoelectronic THz-Wave Beam Steering

This paper outlines the design, performance and application of a highly-integrated stacked wavelength selective switching (WSS) technology platform based on 4k liquid crystal on silicon (LCOS) devices and two-dimensional (2D) beam steering.

Integrated optical phased arrays (OPAs) based on arrayed waveguide gratings (AWGs) enable two-dimensional (2D) beam steering through wavelength tuning. Achieving a high vertical resolution for practical applications remains challenging due to the waveguide phase errors associated with the large AWG required. Here, we present a high-resolution 65-channel AWG-assisted OPA with 5 inputs. Ultra-low loss silicon nitride waveguide technology and transverse magnetic mode operation are employed for waveguide-phase-error mitigation. A vertical resolution of 0.32° using a single input is attained with a beam width of 0.55° × 0.07° and an alias-free horizontal field of view (FOV) of 30°. By utilizing all 5 inputs, the vertical resolution can be enhanced to 0.06°. Real-time 2D beam steering over a 25° × 5° FOV is demonstrated. To our best knowledge, this work demonstrates the highest vertical resolution reported for an AWG-based OPA.

The alignment of a receiver with a pencil beam in a wireless optical power transfer (WOPT) system employing a resonance beam charging (RBC) technology limits the establishment of a resonance cavity. Accurate tracking necessitates precise and dependable monitoring, which requires the exact placement of transmitting and receiving devices. Herein, we present a concept of a two-dimensional (2D) beam steering mechanism for RBC-based WOPT systems utilizing dispersed laser beams. The proposed approach allows a significant improvement, including reduction of scanning times and minimization of errors, in relation to conventional pencil-beam-based systems. Experimental results reveal 14% faster acquisition time efficiency, an 18% improvement in pointing accuracy, and a 24% enhancement in tracking accuracy. These results establish the prerequisites for the implementation of dispersed beam steering in the RBC-based WOPT system. This capability empowers the system to charge movable devices and Internet of Things devices consistently in smart factories.

We propose a novel concept for the implementation of 2-dimensional (2D) optical phased arrays (OPAs) with end-fire waveguides as antenna elements (AEs), and we present its theoretical model and experimental proof. The concept is based on the use of 3-dimensional (3D) photonic integrated circuits (PICs) with multiple waveguiding layers on the PolyBoard platform. In their simplest form, the 3D PICs comprise AEs at different layers, vertical and lateral couplers for the distribution of light among the AEs, and phase shifters for the execution of the 2D beam scanning process. Using the field equivalence principle, we model the radiated field from the single-mode waveguide of the platform at 1550 nm, and we find that the expected beam width is 12.7°. We also investigate the perturbation that is induced into propagating fields inside parallel waveguides in proximity, and we conclude that waveguide spacings down to 6 µm can be safely used for development of uniform OPAs in the PolyBoard platform. For OPAs with 6 µm pitch and 4 AEs, we find that the maximum steering angle is 14.0° and the expected angular clearance, wherein the main radiation lobe is higher than any grating lobe by at least 3, 6 and 10 dB is 10.8°, 7.6° and 2.8°, respectively. Based on our simulations, we design and fabricate single- and 2-layer PICs with 1 × 4 and 2 × 4 OPAs. The lateral pitch of the OPAs ranges from 10 down to 6 µm, while the vertical pitch is 7.2 µm. We experimentally characterize these OPAs and validate the potential of the 2-layer PICs for 2D beam scanning on the azimuthal and elevation plane. The beam profiles and the main scanning parameters such as the maximum steering angle and the relative intensity between the main and the grating lobes are found in excellent agreement with our simulations.

We propose a promising method to develop flexible, compact, and tunable light-activated film diffractive optical elements (FDOEs) with exceptional diffraction efficiency, by integrating liquid crystal (LC) geometric phase-based diffractive optical elements (DOEs) with a specifically designed light-activated LC polymer (LCP) film.Arbitrary film bending induced by UV/Vis irradiation is realized through precise mesogens arrangement within the LCP film, enabling 1D and 2D beam steering, as well as dynamic and reversible switching between structured and Gaussian lights after cooperating with the DOE design.Furthermore, remarkable fatigue resistance, solvent resistance, and thermal stability are demonstrated, providing a solid material platform for advanced optical applications.

We present high-performance integrated optical phased arrays along with first-of-their-kind light detection and ranging (LiDAR) and free-space data communication demonstrators. First, record-performance optical phased array components are shown with low-power phase shifters and high-directionality waveguide grating antennas. Then, one-dimensional (1-D) 512-element optical phased arrays are demonstrated with record low-power operation ( $ 1 mW total), large steering ranges, and high-speed two-dimensional (2-D) beam steering ( $ 30 $\mu$ s phase shifter time constant). Next, by utilizing optical phased arrays, we show coherent 2-D solid-state LiDAR on diffusive targets with simultaneous velocity extraction at a range of nearly 200 m. In addition, the first demonstration of 3-D coherent LiDAR with optical phased arrays is presented with raster-scanning arrays. Finally, lens-free chip-to-chip free-space optical communication links up to 50 m are shown, including a demonstration of a steerable transmitter to multiple optical phased array receivers at a 1 Gb/s data rate. This paper shows the most advanced silicon photonics solid-state beam steering to date with relevant demonstrators in practical applications.

Optical wireless communication (OWC) is gaining momentum as a key enabler for next-generation indoor Internet of Things (IoT) systems, offering high-bandwidth, low-latency, and interference-free connectivity by leveraging its inherent line-of-sight nature. Its versatility makes it attractive for a wide range of dynamic applications, including flexible interconnects in data centers, high-speed data delivery to mobile devices, and communication with cooperative robots. However, conventional solutions still face critical challenges. Visible light communication and positioning (VLCP) is constrained by limited modulation bandwidth and a trade-off between localization precision and data rate, while laser-based OWC systems often suffer from alignment sensitivity, making them less effective in dynamic or mobile scenarios. To overcome these barriers, we present a dynamic localization and two-dimensional (2D) beam scanning communication system based on a single tunable MEMS vertical-cavity surface-emitting laser (MEMS-VCSEL) integrated with a corner cube reflector (CCR). This compact, coaxial optical architecture enables precise real-time tracking and continuous beam alignment without relying on external sensors. A Search-and-Communicate mechanism facilitates autonomous target acquisition and link maintenance. Experimental validation demonstrates sub-3 mm tracking error, stable communication over a 3-meter communication link achieving data rates up to 3.4 Gbps-while the target is in motion.

Optical phased arrays (OPAs) with the ability of dynamic beam-steering hold great promise for LiDAR applications. In contrast to mechanical scanning, OPAs have the advantage of being capable of agile beam pointing, allowing the multifunctional operation in LiDARs. In this paper, we propose a liquid crystal (LC) tunable OPA to achieve 2D beam steering. This approach enables a LC-tunable beam control in a second axis in addition to an OPA-tuning, thereby achieving 2D beam steering at a single wavelength. The LC-tunable OPA is fabricated using a standard Si photonics process and a LC process common for commercialized LC displays and LC on Si (LCOS) devices. We have fabricated samples of LC-tunable devices and LC-tunable OPA devices for 1D and 2D steering respectively and demonstrated a LiDAR using the 1D steering device. By taking advantage of non-mechanical movement of the device, a target tracking is realized by detecting a person from a camera image and irradiating the steering beam in that direction with more than 10 frames per second. With a faster 2D steering, a 3D target tracking even for multiple targets could be realized in the future.

Optical phased arrays (OPAs) have received considerable attention as solid-state beam scanners. However, conventional OPAs that actively control the phase difference between arrays are characterized by excessive power consumption for high-precision beam emission. In this study, we fabricated an OPA comprising Bragg grating and arrayed waveguide grating (AWG). Multi-mode waveguide is used in AWG to reduce the effect of manufacturing error. This device realizes wide and high-resolution two-dimensional beam steering only by sweeping wavelength. FWHM of the emitted beam is 0.534° × 2.27°, and the steering range is 43.9° × 13.5° with 1/64 of the power consumption of conventional OPA.

Controllable manipulation of self-organized dynamic superstructures of functional molecular materials by external stimuli is an enabling enterprise. Herein, we have developed a thermally driven, self-organized helical superstructure, i.e., thermoresponsive cholesteric liquid crystal (CLC), by integrating a judiciously chosen thermoresponsive chiral molecular switch into an achiral liquid crystalline medium. The CLC in lying state, in both planar and twisted nematic cells, exhibits reversible in-plane orthogonal switching of its helical axis in response to the combined effect of temperature and electric field. Consequently, the direction of the cholesteric grating has been observed to undergo 90° switching in a single cell, enabling non-mechanical beam steering along two orthogonal directions. The ability to reversibly switch the cholesteric gartings along perpendicular directions by appropriately adjusting temperature and electric field strength could facilitate their applications in 2D beam steering, spectrum scanning, optoelectronics and beyond.

In this work, we present two-dimensional beam steering in the near-infrared using a SiN integrated circuit, containing optical phased arrays. Beam steering was achieved over a range of 17.6° × 3°, at a fixed wavelength of 905 nm. The first dimension was steered via phase differences between the optical phased array channels. The second dimension was accessed by actively switching between various optical phased array sub-devices containing output diffraction gratings with different periods. The characterisation was performed on a wafer-level test station.

Free-space beam steering plays an important role in ultrafast switching and scanning applications. Optical phased array (OPA) is commonly used for this function. Here we propose an alternative method to realize two-dimensional large-angle beam steering for single-wavelength light on Si platform. The steerer consists of an electrically controlled 1×N cascaded add-drop ring resonator and N off-chip emitters. Each output of the resonator connects with one emitter to emit light to free space at a certain direction. Single-wavelength light coupled in the input bus waveguide of the cascaded ring resonator can be switched among these N outputs by means of tuning the refractive index of the cascaded rings electrically and then be emitted to different directions in far field, which behaves as beam steering apparently. The steering can be one- or two-dimension, depending on the arrangement of these N off-chip emitters on the chip. Moreover, the proposed steerer can be combined with the conventional OPA for more precisely beam steering. A 1×9 cascaded ring resonator was fabricated on hydrogen amorphous silicon platform for proof of concept demonstration.

On-chip optical phased arrays (OPAs) are the enabling technology for diverse applications, ranging from optical interconnects to metrology and light detection and ranging (LIDAR). To meet the required performance demands, OPAs need to achieve a narrow beam width and wide-angle steering, along with efficient sidelobe suppression. A typical OPA configuration consists of either one-dimensional (1D) linear or two-dimensional (2D) rectangular arrays. However, the presence of grating sidelobes from these array configurations in the far-field pattern limits the aliasing-free beam steering, when the antenna element spacing is larger than half of a wavelength. In this work, we provide numerical analysis for 2D circular OPAs with radially arranged nano-antennas. The circular array geometry is shown to effectively suppress the grating lobes, expand the range for beam steering and obtain narrower beamwidths, while increasing element spacing to about 10 μm. To allow for high coupling efficiency, we propose the use of a central circular grating coupler to feed the designed circular OPA. Leveraging radially positioned nano-antennas and an efficient central grating coupler, our design can yield an aliasing-free azimuthal field of view (FOV) of 360°, while the elevation angle FOV is limited by the far-field beamwidth of the nano-antenna element and its array arrangement. With a main-to-sidelobe contrast ratio of 10 dB, a 110-element OPA offers an elevation FOV of 5° and an angular beamwidth of 1.14°, while an 870-element array provides an elevation FOV up to 20° with an angular beamwidth of 0.35°. Our analysis suggests that the performance of the circular OPAs can be further improved by integrating more elements, achieving larger aliasing-free FOV and narrower beamwidths. Our proposed design paves a new way for the development of on-chip OPAs with large 2D beam steering and high resolutions in communications and LIDAR systems.

We present experimental studies of extraordinary terahertz (THz) transmission in subwavelength plasmonic hole arrays patterned on both metals and doped semiconductors. Transmission efficiency higher than unity was achieved when it was normalized to the area occupied by the holes. The effects of hole shape, dielectric function of metals and surrounding media, polarization dependence, and array thickness on the enhanced THz transmission in both metals and plasmonic semiconductors were demonstrated by use of THz time-domain spectroscopy. Extraordinary THz transmission was also realized in metallic arrays having thickness of only one-third of the skin depth. THz surface plasmon resonance has potential applications in THz imaging, biosensing, interconnects, and the development of next generation photoconductive antennas for THz generation and detection.

Future Unmanned Aerial Vehicle (UAV)-assisted wireless communication systems are expected to utilize wide bandwidths available at terahertz (THz) frequencies to enhance system throughput. To compensate for the severe path loss in the THz band, it is essential to have a multitude of antennas in the UAV to generate narrow beams for directional transmission. However, narrow beams severely limit its spatial coverage, which greatly affects the efficiency of large-scale access UAV-assisted THz systems. Moreover, the combination of massive antennas and large bandwidth at THz makes the misalignment of the beams caused by beam squint non-negligible and also high energy consumption. UAV-assisted communication technology can effectively increase spatial coverage and provide reliable LoS communication links. In addition, reducing the number of radio frequency (RF) chains while ensuring the number of transmitted data streams and space division multiplexing capability is also an effective way to reduce energy consumption in the UAV communication. In this paper, a single RF chain uniform planar array (UPA) with true-time-delays (TTDs) is equipped on the UAV to achieve two dimensional (2D) beams and split spatial beams to improve transmission efficiency. We analyze the 2D beam squint of the UPA and design a time-delay phased UPA for UAV-assisted THz communication systems. By introducing TTDs between the single RF chain and phase shifters, the beam squint can be controlled flexibly by introducing the delay between each antenna. When TTDs are arranged in both the horizontal and vertical dimensions, the coverage of the beams becomes more complicated compared to uniform linear arrays (ULA). Simulation results show that the proposed time-delay phased UPA can achieve better performance in both single-beam and multi-beam modes for single user and multi-user scenarios compared with conventional phased UPA, respectively. In addition, we propose frequency division beam multiple access (FDBMA) multi access technology, which achieves more efficient multi access by reusing resources from different frequency beam pairs. Finally, the results also show that the enlargement of the beamwidth through the proposed FDBMA strategy can also increase the performance in multi-user scenarios.