Metalens is one of the most promising applications for the development of metasurfaces. A wide variety of materials have been applied to metalenses working at certain spectral bands in order to meet ...the requirements of high efficiency and low-cost fabrication. Among these materials, wide-bandgap gallium nitride (GaN) is one of the most promising materials considering its advantages especially in semiconductor manufacturing. In this work, GaN has been utilized to fabricate the high-performance metalenses operating at visible wavelengths of 405, 532, and 633 nm with efficiencies up to 79%, 84%, and 89%, respectively. The homemade 1951 United State Air Force (UASF) resolution test chart has also been fabricated in order to provide resolvable lines with widths as small as 870 nm. As shown in the experimental results for imaging, the metalens designed at 405 nm can provide extremely high resolution to clearly resolve the smallest lines with the nano-sized widths in the homemade resolution test chart. These extraordinary experimental results come from our successful development in design and fabrication for the metalenses composed of high-aspect-ratio GaN nanoposts with nearly vertical sidewalls.
Abstract
The growth of wide-bandgap materials on patterned substrates has revolutionized the means with which we can improve the light output power of gallium nitride (GaN) light-emitting diodes ...(LEDs). Conventional patterned structure inspection usually relies on an expensive vacuum-system-required scanning electron microscope (SEM) or optical microscope (OM) with bulky objectives. On the other hand, ultra-thin metasurfaces have been widely used in widespread applications, especially for converging lenses. In this study, we propose newly developed, highly efficient hexagon-resonated elements (HREs) combined with gingerly selected subwavelength periods of the elements for the construction of polarization-insensitive metalenses of high performance. Also, the well-developed fabrication techniques have been employed to realize the high-aspect-ratio metalenses working at three distinct wavelengths of 405, 532, and 633 nm with respective diffraction-limited focusing efficiencies of 93%, 86%, and 92%. The 1951 United States Air Force (USAF) test chart has been chosen to characterize the imaging capability. All of the images formed by the 405-nm-designed metalens show exceptional clear line features, and the smallest resolvable features are lines with widths of 870 nm. To perform the inspection capacity for patterned substrates, for the proof of concept, a commercially available patterned sapphire substrate (PSS) for the growth of the GaN LEDs has been opted and carefully examined by the high-resolution SEM system. With the appropriately chosen metalenses at the desired wavelength, the summits of structures in the PSS can be clearly observed in the images. The PSS imaging qualities taken by the ultra-thin and light-weight metalenses with a numerical aperture (NA) of 0.3 are comparable to those seen by an objective with the NA of 0.4. This work can pioneer semiconductor manufacturing to choose the polarization-insensitive GaN metalenses to inspect the patterned structures instead of using the SEM or the bulky and heavy conventional objectives.
Metasurface-based components are known to be one of the promising candidates for developing flat optical systems. However, their low working efficiency highly limits the use of such flat components ...for feasible applications. Although the introduction of the metallic mirror has been demonstrated to successfully enhance the efficiency, it is still somehow limited for imaging and sensing applications because they are only available for devices operating in a reflection fashion. Here, we demonstrate three individual GaN-based metalenses working in a transmission window with extremely high operation efficiency at visible light (87%, 91.6%, and 50.6% for blue, green, and red light, respectively). For the proof of concept, a multiplex color router with dielectric metalens, which is capable of guiding individual primary colors into different spatial positions, is experimentally verified based on the design of out-of-plane focusing metalens. Our approach with low-cost, semiconductor fabrication compatibility and high working efficiency characteristics offers a way for establishing a complete set of flat optical components for a wide range of applications such as compact imaging sensors, optical spectroscopy, and high-resolution lithography, just named a few.
We experimentally demonstrate a highly efficient metasurface-based optical vortex beam (OVB) composed of high-aspect-ratio gallium nitride (GaN) meta-structures with an exceptional simulated absolute ...polarization conversion efficiency (APCE) of up to 98%. A flower-like interference pattern emerges at the converging distance of the device with the helicity switching in spiral and dislocation interference patterns beyond this point, as confirmed through meticulous Mach-Zehnder interferometer analysis. The device also performs broadband capabilities across visible wavelengths. Experimentally demonstrated, the annular shape adeptly expands its diameter with increasing incident wavelengths. This phenomenon is rooted in the fascinating anomalous refractive and reflective characteristics inherent to subwavelength-period metasurfaces.
In this work, a gallium nitride (GaN) metalens as a remote device has been applied to a commercially available white light-emitting diode (LED). We show the successful demonstration in fabricating ...the high-aspect-ratio GaN metalens capable of diffraction-limited focusing with an experimentally focusing efficiency up to 89% at the wavelength of 450 nm. The metalens can also resolve the subwavelength features as imaging. For the proof of concept, the rainbow-like phenomenon can be observed by using the remote GaN metalens to disperse the white light radiated by the white LED. The diode lasers working at various wavelengths have been employed to carefully verify the positions of colors in the rainbow-like profile. The results in this study can inspire the semiconductor manufacturing industry at integrating metalenses of various kinds and functionalities into the package of LED modules in the near future and prospect widespread applications in advanced solid-state lighting.
Optical bound states in the continuum (BICs), realizing substantial suppression of out‐of‐plane radiative losses, have been utilized to realize strong light confinement and optical modes with high ...quality‐factor (Q). Lasing actions with narrow linewidths based on optical BIC modes have been demonstrated in the near‐infrared and the visible ranges, but BIC‐based lasers in the ultraviolet (UV) region have not been reported. As light sources possessing wavelengths at the UV scale are essential in various fields, the strategy to design compact UV lasers based on high‐Q modes and directional emissions is highly desirable. Here, the first BIC‐based laser in the UV region is demonstrated by designing a 1D periodic resist structure on top of a GaN film. Using the symmetric‐protected BIC mode, the fabricated laser is having a directional single‐mode lasing emission with a small full‐width at half‐maximum of 0.10 nm and beam divergence of 1.5°. The lasing action is observed with a periodic structure area corresponding to a structure side length as small as 8 µm. Moreover, the wavelength control of the UV lasing is achieved by varying the period and temperature. This work provides strategies to design UV lasers having a small footprint together with narrow‐linewidth and out‐of‐plane emissions.
A bound state in the continuum (BIC) laser in the UV region by designing a 1D periodic resist structure on top of a GaN film is presented. The UV BIC laser shows a directional single‐mode lasing with a small linewidth of 0.10 nm and accurate emission wavelength tunability in sub‐nm steps.
The pursuit of device miniaturization and the generation of complex output responses has driven the development of optically tunable metasurfaces. Among these advancements, metasurface‐based vortex ...beams (VBs) have emerged as promising optically tunable knob technology due to their infinite output states through various orbital angular momentum (OAM) modes. In this study, the metasurface‐based VBs are successfully fabricated with exceptionally high OAM modes of 32 and 16, carefully examined using the Mach‐Zehnder interferometer. Throughout this study, these devices are referred to as infinite‐OAM meta‐knobs (IOMKs). The optical tunability of IOMKs is demonstrated by illuminating them with incident light possessing diverse degrees of freedom, including different polarizations and wavelengths. Furthermore, the interference nature of the IOMKs is experimentally explored by incorporating them into interference eraser measurements, providing an additional degree of freedom to the optical knob. In the interference eraser experiment, the broadband capability and the high reproducibility of the IOMKs with the OAM modes of 32 and 16 at three distinct wavelengths of 450, 530, and 610 nm is demonstrated. These findings represent a significant step toward understanding the potential applications of IOMKs in quantum optics and their promising role in the generation of complex output states.
The infinite‐OAM meta‐knobs (IOMKs) with exceptionally high OAM modes of 32 and 16 have been successfully fabricated. The optical tunability of the IOMKs has been demonstrated by illuminating them with incident light possessing diverse degrees of freedom, including different polarizations and wavelengths. These findings represent a significant step toward the important role of IOMKs in generating complex output states.
Actively tunable petal‐shaped pattern emissions have been successfully demonstrated by the cascade of two innovative rotational vortex metasurface arrays (RVMAs). To ensure accurate cascade, each ...RVMA comprises a central unit with the capability to converge light, surrounded by six units designed for emitting converging vortex beams (VBs) and converging perfect vortex beams (PVBs) with topological numbers (TCs) of 3, 6, and 9, respectively. Notably, although the number of spiral branches and the diameters of the VB and PVB are determined in accordance with specific design TCs, the diameters and the number of petals for the petal‐shaped beam emissions are actively adjusted through the rotation of the RVMAs. Interestingly, the superimposition of these VBs, PVBs, or both, consistently generates petal‐shaped beams with various petal numbers and diameters, highlighting the remarkable versatility of the approach. To demonstrate the proposed concept across wavelengths, indicating its broadband operation capability, RVMA performance in the visible spectrum is thoroughly investigated, ranging from 450 to 600 nm. The tunability of the cascade of RVMAs for petal‐shaped emissions featuring diverse diameters and petal numbers has been validated at visible wavelengths, affirming the broad capabilities of the concept presented in this study.
Actively tunable petal‐shaped pattern emissions have been successfully demonstrated by the cascade of two innovative rotational vortex metasurface arrays (RVMAs). The tunability of the cascade of RVMAs for petal‐shaped emissions featuring diverse diameters and petal numbers has been validated at visible wavelengths, affirming the broad capabilities of the concept presented in this study.
This work experimentally demonstrates the highly-efficient geometric and propagation metasurfaces for vortex beam emissions. These metasurfaces are respectively composed of high-aspect-ratio fin-like ...and cylindrical gallium nitride (GaN) meta-atoms. Remarkably, the optimized configuration of the fin-like GaN meta-atoms achieves a cross-polarization transmission efficiency of up to 99 %. Similarly, the cylindrical GaN meta-atoms exhibit an average co-polarization transmission efficiency of 97 %. Both metasurfaces, designed for vortex beam emission, exhibit annular intensity converging capabilities at distinct wavelengths in the visible. Notably, the geometric metasurface shows achromatic annular intensity distributions over a continuous wavelength range up to 100 nm, in sharp contrast to the propagation metasurface, which is subject to inherent wavelength dispersion limitations.
Vacuum ultraviolet (VUV) light plays an essential role across science and technology, from molecular spectroscopy to nanolithography and biomedical procedures. Realizing nanoscale devices for VUV ...light generation and control is critical for next-generation VUV sources and systems, but the scarcity of low-loss VUV materials creates a substantial challenge. We demonstrate a metalens that both generates-by second-harmonic generation-and simultaneously focuses the generated VUV light. The metalens consists of 150-nm-thick zinc oxide (ZnO) nanoresonators that convert 394 nm (~3.15 eV) light into focused 197-nm (~6.29 eV) radiation, producing a spot 1.7 μm in diameter with a 21-fold power density enhancement as compared to the wavefront at the metalens surface. The reported metalens is ultracompact and phase-matching free, allowing substantial streamlining of VUV system design and facilitating more advanced applications. This work provides a useful platform for developing low-loss VUV components and increasing the accessibility of the VUV regime.
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