Efficient wide-angle broadband blazed gratings enabled by metasurfaces

Optical metasurfaces have recently attracted increasing interest, owing to their ability to manipulate the phase, amplitude, and polarization of optical fields on a subwavelength scale.^1–5 Compared with conventional optical components that are bulky and heavy, metasurface-based devices, with compact size and the ability to effectively manipulate optical fields, have been considered as potential planar alternatives to realize various functionalities, such as optical trapping,^6,7 meta-holography,^8–10 high-resolution focusing,^11–15 vortex beam generation,^16–18 dynamic beamforming,^19–21 multiplexing,^22,23 and sensing.²⁴ 

As important optical components, blazed gratings play a critical role in spectroscopy and displays, owing to their efficient diffraction of light in a desired direction. However, it is difficult to achieve a large deflection angle, because of the shadowing effect that originates from the sawtooth geometry. This hinders the efficient wide-angle operation of devices. In contrast to conventional blazed gratings, which produce a nearly continuous phase change of 2π, gradient metasurfaces impart a discrete phase response with a staircase approximation by arranging nanostructures to realize beam steering. Initially, V-shaped antennas¹¹ based on the multiresonance effect and consisting of two nanorods were proposed. However, the resulting designed metasurfaces are inefficient, because the antennas support not only the anomalous reflected light beams, but also normal ones. To overcome this problem, a gap-plasmon-based gradient metasurface with a metallic ground plane on the back has been proposed.²⁵ This can reflect the normal incident light to the desired diffraction order efficiently. Plasmonic metasurfaces suffer from undesirable losses and it is difficult to achieve efficient transmission with them, and therefore all-dielectric gradient metasurfaces with high index contrast and low intrinsic loss have been explored, using approaches based on the transmission phase^26–28 and the Pancharatnam–Berry (PB) phase,^29,30 respectively. The first approach has been realized by utilizing nanoposts with different sizes, with each post operating as a low-quality-factor Fabry–Pérot resonator. The second approach relies on spatially varying the orientations of the nanoposts, and every nanopost operates as a half-waveplate. In addition, a double-phase modulating dielectric metasurface that can separate x and y linearly polarized light has been reported.³¹ Meanwhile, inverse design by topology optimization has been applied to metagratings, with the aim of efficient deflection at a 75° angle with an efficiency of 75%³² It should be noted that these good performances have been achieved only under normal incidence at a specific wavelength, but little as been reported about gradient metasurfaces for wide-angle broadband operations, although many critical applications in practice require a large incident or refraction angle and a broad working bandwidth as well if possible.

In this paper, we demonstrate a wide-angle broadband all-dielectric gradient metasurface with an extreme incident angle of up to 80°, which is achieved by optimizing wide-angle phase shifts and transmissivities of the unit cells. An average efficiency of 64% and a maximum diffraction efficiency of 72% are measured when the incident angle is varied from −80° to 45° at 1.55 μm. The proposed gradient metasurface also has a broad bandwidth (1.45–1.65 μm). Average efficiencies of about 56% and 51% can be obtained experimentally over the same range of incident angles at 1.48 and 1.64 μm, respectively. The results show that the optimized nanostructures can be flexibly used to construct various metasurface devices such as metagratings and metalenses with superior wide-angle efficiency.

To manipulate light wavefronts, a set of nanoposts with the same lattice constant P to achieve 2π phase coverage is designed for the metasurface-based devices. In general, phase shifts and transmissivities of unit cells are analyzed at normal incidence. The nanoposts can achieve desired phase and amplitude responses within a small angular range near normal incidence, but these may quickly degrade as the incidence angle increases. This stems from the fact that phase shifts and transmissivities of nanoposts on the substrate undergo obvious changes. To obtain an efficient wide-angle metasurface, the responses of unit cells should be checked at a widely varying incident angle.

On the basis of the principle of the transmission phase, we design and optimize a set of unit cells. The period P and height H of the unit cell are 0.8 μm and 1.1 μm, respectively. The subwavelength period is chosen to suppress high-order diffraction. The unit cells are formed by two identical silicon nanoposts spaced by a gap g and placed on a quartz substrate, and their geometric parameters are defined in Fig. 1(a). The unit cells can behave as a slot waveguide. Owing to the refractive index discontinuity, this structure confines the energy of the light field within the gap region. Such a structure can mitigate material dispersion and avoid the resonance effect by confining the light in the air gap to allow the construction of efficient wide-angle broadband metasurface-based devices.³³ In the simulation, the source is placed in the substrate. We choose an x-polarized plane wave with wavelength 1.55 μm. The sizes of the nanoposts are optimized to cover a phase shift from 0 to 2π with an incremental phase of π/4 under different incident angles. The optimization procedure is as follows. First, a set of unit cells that can cover a phase shift of 2π and have high transmissivity is optimized at an incident angle of 40° by changing the parameters L, W, and g. The ranges of parameter variation should be in line with the requirements imposed by the processing conditions. Second, the phase shifts and transmissivities of the selected unit cells are simulated from 0° to 60° with an interval of 10°. Any unit cell with low transmissivity is discarded, and the unit cell corresponding to the phase value is reoptimized. The geometric sizes of the selected eight nanoposts are shown in Table I. In addition, another set of square posts, which only respond under normal incidence, is used to construct a gradient metasurface for comparison, as shown in Fig. 1(d). The lengths L are set as 210, 263, 283, 300, 311, 325, 340, and 395 nm, with the same period and height. It can be seen from Figs. 1(b) and 1(c) that that the optimized nanoposts exhibit high transmissivities and nearly parallel phase curves across incident angles ranging from 0° to 60°. Figures 1(e) and 1(f) show the phase shifts and transmissivities, respectively, of the eight square posts. It should noted that low transmissivities and abrupt phase shifts appear at large incident angles, resulting in poor wide-angle performances.

The proposed gradient metasurface consists of periodically arranged supercells along the x and y direction, and each supercell contains eight optimized unit cells to cover the phase change from 0 to 2π linearly shown in Fig. 2(a). Figures 2(b)–2(d) show the simulated E_x field distributions in the x–z plane under illumination by x-polarized light at incident angles of −85°, −45°, and 15°, respectively. Figure 2(e) shows the electric field component E_z of the transmitted light in the x–z plane at an incident angle of 45°, because in this case E_z becomes a major field component. It can be observed that the wavefront is uniform for different incident angles. This confirms that the gradient metasurface has a good wide-angle performance. The normalized far-field intensity profiles at an incident angle of 45° for both gradient metasurfaces are shown in Fig. 2(f). It can be seen that most incident light is refracted to the expected angle of 71° by the gradient metasurface formed by the optimized unit cells. By contrast, the metasurface consisting of square posts diffracts the incident light into undesirable diffraction orders and has an obvious wavefront distortion at an incident angle of 45°, as shown in Fig. 2(g).

Figure 3(a) shows the efficiencies of the gradient metasurfaces formed by the optimized and unoptimized nanoposts at 1.55 μm. The efficiency is defined as the power ratio of the desired diffraction order to the incident light. It can be seen that the efficiency remains above 70% as the incident angle is varied over the wide range from −80° to 45° for the optimized metasurface. According to the generalized Snell’s law, the corresponding refraction angle is from −48° to 71°. By contrast, the gradient metasurface consisting of the unoptimized square posts only works well in a very small angular range near the normal direction and degrades quickly otherwise.

It is also crucial to examine the performance of the metasurfaces when they are operated away from the target wavelength, because both the wavelength and the refractive index of the material change, affecting the phase shifts and transmissivities of the unit cells. Therefore, we explore the wideband characteristics of the designed gradient metasurface. Figure 3(b) shows the efficiencies of the gradient metasurface at various wavelengths. We find that the metasurface has a broad working bandwidth and achieves high efficiencies above 60% from 1.45 μm to 1.65 μm. The phase shifts and transmissivities at 1.48 μm are shown in Figs. 3(c) and 3(d), respectively. It can be seen that the optimized unit cells still retain high transmissivities and nearly constant phase gradients, which ensures good broadband properties of the gradient metasurface, although there are some fluctuations in transmissivity.

The fabrication process is illustrated in Fig. 4. The metasurface is fabricated by a standard electron beam lithography (EBL) technique and inductively coupled plasma reactive ion etching (ICP-RIE) using equipment from Oxford Instruments. First, a 1.1-μm-thick amorphous silicon (a-Si) layer is deposited on a quartz substrate using plasma-enhanced chemical vapor deposition [Fig. 4(a)]. The a-Si is coated with a layer of photoresist (ZEP520A) as a mask [Fig. 4(b)]. Then, the pattern is exposed using EBL [Fig. 4(c)]. After that, the pattern is transferred to the sample using ICP-RIE [Fig. 4(d)]. Finally, the sample is cleaned by dry etching to remove residual photoresist from the surface [Fig. 4(e)].

Scanning electron microscope (SEM) images of the fabricated sample with dimensions of 1 × 1 mm² are shown in Fig. 5(a). The experimental setup is shown in Fig. 5(b). Light from a tunable laser (1.48–1.64 μm) is collimated and polarized. It is then focused on the sample, which is placed on a rotation stage and rotated to change the incident angle. An optical power meter is used to measure the incident light power and the power of light diffracted to the desired order.

Figure 5(c) shows the experimental results for efficiency at a wavelength of 1.55 μm. The average efficiency is 64% at incident angles ranging from −80° to 45°, which indicates that the proposed gradient metasurface achieves high diffraction efficiencies. A maximum diffraction efficiency of 72% into the desired diffraction order is obtained. The measured efficiencies at other wavelengths are shown vs the incident angle in Fig. 5(d), from which it can be seen that the gradient metasurface enables nearly constant broadband efficiency over a wide range of incident angles. The average efficiencies are 54% and 51% at wavelengths of 1.48 and 1.64 μm, respectively. Compared with the simulation results, the measured efficiency is slightly lower, because, as can be seen from the far-field profiles of the transmitted intensity in Fig. 5(e), there are other diffraction orders around 15% of the incident energy in total, although most of the power is deflected to +1 order as expected. We believe that this arises from fabrication imperfections and thus deviations in the responses of the unit cells.

Table II gives a detailed comparison of our work with previously reported gradient metasurfaces in terms of incident angle, refraction angle, material, wavelength, and efficiency. We note that a wide bandwidth is associated with low-efficiency values at some wavelengths and needs to be re-evaluated. By contrast, the proposed gradient metasurface can achieve wide-angle broadband beam steering, enabling efficient manipulation of wavefronts.

We have demonstrated a wide-angle broadband gradient metasurface. It consists of all-dielectric double rectangular posts with high transmissivities and has an almost unchanged phase gradient for different incident angles. The average diffraction efficiency is above 64% for incident angles in the range from −80° to 45° at 1.55 μm. In addition, the proposed gradient metasurface has a broad working bandwidth (1.45–1.65 μm). Compared with previously studied gradient metasurfaces, the designed metasurface based on optimized unit cells operates at a greater bandwidth and larger incident angles. It allows for precise control over the phase of the light field across a wide field of view, enabling efficient manipulation of wavefronts even at large incident angles. This capability is crucial for applications such as beam shaping, steering, and focusing in systems where incident angles vary significantly. For example, in wireless communication systems, the proposed metasurface could be used to improve signal transmission and reception by shaping and directing beams over a wide range of angles. In radar systems, could facilitate beam scanning and agile beamforming, enhancing detection and tracking capabilities. In imaging applications such as medical imaging or surveillance, it could allow the creation of high-resolution images by controlling the propagation of electromagnetic waves.

From a broader point of view, the proposed set of unit cells exhibits a unique advantage in manipulating light over a wide range of incident angles with high efficiency, and it not only allows for flexible operations of gradient metasurfaces as demonstrated here, but also paves the way for the construction of other wide-angle or large-field-of-view components.

We acknowledge support by the Advanced Integrated Optoelectronics Facility at Tianjin University.

The authors have no conflicts to disclose.

The data that support the findings of this study are available from the corresponding author upon reasonable request.

Chunshu Li received the B.S. degree in electronics science and technology from Yanshan University, Qinhuangdao, China, in 2018. He is currently pursuing the Ph.D. degree in optical engineering with Tianjin University. His research interests include photonic integrated devices and metasurfaces.

Yongjun Guo received the B.S. degree in mechanical design, manufacturing, and automation from Chang’an University, Xian, China, in 2011. He received the M.S. degree in microelectromechanical systems and nanotechnology from Northwestern Polytechnical University, Xian, China, in 2014. He received the Ph.D. degree in optical engineering from Tianjin University, Tianjin, China, in 2023. His research interests include optical antennas and LiDAR.

Yang Wang received the B.S. degree from Changchun University of Science and Technology in 2019, during which she was engaged in research into the transmission and communication characteristics of Bessel beams in atmospheric turbulence and ocean turbulence. She is currently pursuing the Ph.D. degree in optical engineering with Tianjin University, conducting research on metasurface-based multiplane light converters.

Yuhao Guo received the B.S. degree in electronic science and engineering from Tianjin University, Tianjin, China, in 2015. He received the Ph.D. degree in optical engineering from Tianjin University in 2020. His research interests include dispersion engineering, frequency comb generation, supercontinuum generation, and metasurfaces.

Lin Zhang (Member, IEEE) received the B.S. and M.S. degrees (Hons.) from Tsinghua University, China, in 2001 and 2004, respectively, and the Ph.D. degree from the University of Southern California (USC), USA, in 2011. He worked as a Postdoctoral Researcher with the Massachusetts Institute of Technology (MIT), USA. Since 2015, he has been a Professor at Tianjin University, China. He has published over 310 peer-reviewed journal articles and conference papers, including 55 invited articles and two book chapters. He has 20 patents issued. His H-index is 39. His research interests include integrated nanophotonics, on-chip nonlinear ultrafast phenomena, microresonator devices and system applications, chip-scale optical interconnects, sensing, and photonic crystal fibers. Dr. Zhang is a Member of the IEEE Photonics Society and a Senior Member of the Chinese Society for Optical Engineering and Optica (formerly OSA).