In this study, we present an experimental demonstration of metasurfaces capable of achieving light beam deflection at angles exceeding 75 degrees [1]. These advanced metasurfaces are engineered using high-aspect-ratio gallium arsenide (GaAs) nano-resonators fabricated on double-sided polished GaAs substrates. Operating in the reflective mode at a visible wavelength of 650 nm, the metasurfaces exhibit capabilities in manipulating both the direction and spatial structure of incident light beams. A key innovation in our design lies in the ability of these metasurfaces not only to redirect incoming light to large angles but also to encode complex phase profiles into the reflected wavefront. By incorporating vortex beam (VB) structured light with topological charges of up to 8, we demonstrate the generation of doughnut-shaped emission patterns with large-angle deflection [2]. The structured emissions clearly exhibit the helical phase fronts and central phase singularity characteristic of optical vortices, confirming the metasurfaces' capability to handle complex beam profiles. High-angle light steering is a critical challenge in flat optics, particularly for applications requiring compact, on-chip solutions for beam routing, optical interconnects, and spatial light modulation [3]. Conventional optical elements often require bulky geometries to achieve wide-angle deflection, making integration into miniaturized systems difficult. In contrast, our GaAs-based metasurfaces achieve wide-angle reflection, offering a promising route toward the miniaturization of high-performance photonic systems. The use of GaAs as the material platform brings several advantages. First, GaAs possesses a high refractive index in the visible spectrum, which facilitates strong light-matter interaction and efficient phase modulation at subwavelength scales. Second, the fabrication of high-aspect-ratio GaAs nano-structures allows precise control over the optical response, enabling the design of metasurfaces with high efficiency and sharp angular selectivity. Third, GaAs offers excellent compatibility with mature semiconductor fabrication processes, which is advantageous for scalable and cost-effective device manufacturing. To further investigate the versatility of these metasurfaces, we explored their broadband performance across a range of visible wavelengths. Our measurements reveal that the metasurfaces maintain strong deflection and phase control across a bandwidth of over 50 nm, highlighting their potential for use in applications requiring spectral tunability, such as multi-wavelength imaging, broadband holography, and wavelength-multiplexed communications. The successful integration of vortex beam generation with high-angle steering extends the functionality of metasurfaces into the domain of structured light manipulation. Vortex beams, characterized by their orbital angular momentum (OAM), are of increasing interest in a variety of fields, including optical trapping, microscopy, quantum information processing, and high-capacity optical communication. By demonstrating the co-generation of OAM beams and their deflection at steep angles, our work opens new avenues for designing compact OAM-based devices and systems. Moreover, the experimental results in this study represent a significant step forward in the development of multifunctional metasurfaces capable of combining beam steering, wavelength control, and complex field shaping. This multifunctionality is critical for the next generation of flat optics, where integrating multiple optical functions into a single layer can dramatically reduce system complexity and size while enhancing performance. In summary, we have demonstrated that high-aspect-ratio GaAs metasurfaces can serve as powerful tools for achieving high-angle light deflection and structured light generation, with the added benefit of broadband operation. These findings contribute to the growing body of knowledge in the field of nanophotonics and pave the way toward compact, efficient, and multifunctional optical platforms. As metasurface technology continues to mature, we anticipate that such devices will play an increasingly important role in applications ranging from augmented reality displays and laser beam shaping to advanced sensors and quantum photonic circuits.

The third-generation semiconductors, gallium nitride (GaN) and silicon carbide (SiC), have attracted extensive attention due to their exceptional material properties and potential in high-performance electronic and optoelectronic devices. Among them, GaN is especially fascinating for high-frequency applications. This advantage arises from the formation of a two-dimensional electron gas (2DEG) at the AlGaN/GaN heterojunction interface, which enables high electron mobility and supports device architectures such as high electron mobility transistors (HEMTs). By contrast, SiC is renowned for its superior thermal conductivity, wide bandgap, and high critical electric field, which together inspire its capability for handling high power density and ensuring robust thermal management. When these two materials are combined—namely, GaN epitaxially grown on SiC substrates—the resulting heterostructure becomes a highly attractive platform for realizing high-power and high-frequency HEMTs. Such devices are promised to play a critical role in next-generation wireless communication systems, power electronics, and radar applications.

Although the lattice constant mismatch between GaN and SiC is relatively small, the dislocation density in conventional GaN-on-SiC epitaxial structures remains on the order of 10^9 cm^−2. This high density of threading dislocations and related crystalline defects significantly degrades device performance, limiting the reliability, efficiency, and lifetime of HEMTs. Traditional approaches to mitigate defect density mainly rely on optimizing epitaxial growth parameters through methods such as buffer layer engineering, substrate miscut angle control, and multi-step growth processes. However, these approaches are both time-consuming and highly costly, demanding extensive iterative experimentation that is impractical for large-scale production. Consequently, innovative substrate engineering concepts have been explored as an alternative pathway to address these challenges.

One promising strategy is the use of patterned SiC substrates to improve GaN epitaxial quality. By introducing micro-/nano-sized structures onto the SiC surface, epitaxial strain can be more effectively distributed, and dislocation propagation can be hindered, leading to a significant reduction in defect density within the active GaN layer. Building upon this concept, our team has recently developed a new class of engineered substrates, referred to as meta-substrates. These are fabricated by creating periodic meta-structures directly on 4H-SiC substrates, designed to enhance defect annihilation, suppress dislocation propagation, and tailor strain relaxation during GaN epitaxy. Unlike conventional patterned substrates, the meta-substrate approach offers new degrees of freedom for crystal quality optimization.

In this presentation, we report on the complete HEMT device fabrication processes carried out on these SiC meta-substrates, including ohmic and Schottky contact formation, passivation strategies, and gate metallization. Also, we evaluate the radio-frequency (RF) performance of the fabricated devices, highlighting their advantages in terms of cut-off frequency and device performance when compared with conventional GaN-on-SiC HEMTs.

The results confirm that meta-substrates provide a viable pathway for advancing GaN-on-SiC technologies, enabling scalable and cost-effective solutions for high-power and high-frequency electronics. This work demonstrates how advanced substrate design can unlock new performance benchmarks in GaN-based HEMTs. We believe that the meta-substrate concept represents not only a significant advancement in semiconductor device fabrication but also a strategic opportunity to accelerate the deployment of GaN/SiC HEMTs in emerging applications such as 5G/6G communications, electric vehicles, and next-generation radar systems.