Figure 1: Schematic diagram of meta-axicon fabrication using DUV photolithography and 313 samples fabricated with a diameter of 2 mm from an 8-inch wafer.
1. Pre-reading
Metasurfaces are optical components composed of nanostructure array with subwavelength spacing, providing multi-functionalities within thickness of several hundred nanometers. With the ability to manipulate the amplitude, phase, and polarization of light, metasurfaces are expected to be used as essential optical components in various fields such as cameras, drones, satellites, and sensors. However, conventional patterning methods like electron beam lithography are time-consuming and unsuitable for large-scale fabrication, and therefore researchers are introducing DUV (Deep-Ultraviolet) photolithography methods for efficient mass-production. With the introduction of these wafer-scale lithography used in CMOS platform, the commercialization of metasurfaces is accelerated recently.
Prof. Park’s group from Chungnam National University has fabricated large-area meta-axicons designed to modulate both phase and polarization simultaneously, collaborating with Office of Nano Convergence Technology in National Nano Fab Center (NNFC).
With DUV photolithography, 313 meta-axicons with a diameter of 2 mm and a numerical aperture of 0.4 that is operated in the near-infrared were mass-manufactured on an 8-inch quartz wafer as shown in Figure 1(a). The fabricated meta-axicon is designed to form a Bessel beam with circular polarization based on geometric phase principles, and was made of a hydrogenated amorphous silicon (a-Si:H). By measuring a beam shape, a depth of focus (DoF), and a polarization state of light transmitted through meta-axicons, we proved that fabricated samples sustained a Bessel beam shape with circular polarization well, and the DoF corresponded to around 2.3 mm.
The introduction of well-established semiconductor production technologies such as DUV lithography to fabricate metasurfaces with circular polarization is expected to open up new application of metasurfaces because it is CMOS-compatible, cost-effective, and mass-productive.
2. Background
To date, numerous metasurfaces with various functions have been proposed in the research phase. Metasurfaces can replace existing optical components into lighter and thinner ones and are expected to make complex optical systems compact in various photonic applications. Therefore, metasurfaces have been attracting much attention in various optical and photonic fields. However, for metasurfaces to become more practical and fully replace traditional optical systems, improvements are needed in integration with other components, fabrication of metasurfaces with large size, and cost-effectiveness etc.
Recent studies have demonstrated the mass-production of large-area metasurfaces, which can serve as a foundation for commercializing metasurfaces. A prominent method being applied is a CMOS platform technology. Thanks to the rapid growth of the semiconductor industry, nanostructure patterning techniques on silicon, glass, and other materials have advanced significantly. Consequently, CMOS platform technologies have reached a remarkable economical level, and the usage of these platforms allows the cost-effective mass-production of large-scale metasurfaces. Additionally, numerous studies on the mass-production of metasurfaces through nanoimprinting techniques have been reported and they are increasing the possibility of metasurface commercialization.
Researchers demonstrated a large-scale meta-axicon lens that generates circularly polarized Bessel beams by using CMOS platform technologies. Our achievement is part of ongoing research on mass fabrication of metasurfaces on CMOS platforms and is expected to accelerate the mass-production of metasurfaces with various type using CMOS technology in the future.
3. Innovative research
The design method of geometric phase-based metasurfaces that can modulate polarization and phase simultaneously has been proposed as one of the representative methodologies for metasurface design. By introducing an anisotropic structure as a meta-atom, the incident polarization can be converted by quarter or half of the wavelength, and the phase of the transmitted light can also be changed as much as desired by rotating this anisotropic meta-atom. This enables the design of a metasurface that simultaneously controls phase and polarization.
In this context, the researchers realized a metasurface-based axicon lens that can form circularly polarized Bessel beams in the near-infrared range based on this geometric phase. During this process, the team optimized Stokes parameters by finely adjusting the size according to the rotation angle and demonstrated that the polarization conversion efficiency was improved because this optimization minimized a decrease in conversion efficiency that occurs when anisotropic meta-atoms are rotated on a square lattice.
Figure 2: Optimization results obtained by using Stokes parameters.
Additionally, to achieve the size of the metasurface at the mm scale, the CMOS platform was applied, demonstrating that it could be realized with 313 metasurfaces on an 8-inch wafer. To validate the optical properties of the meta-axicons produced on this CMOS platform, beam imaging experiments were conducted to verify the Bessel beam formation capabilities of the axicon lens. The designed meta-axicon, fabricated with a 2mm diameter at a numerical aperture (NA) of 0.4, was confirmed to operate well within a 2.5% error margin at the theoretically calculated Depth of Focus (DoF) value of 2.29 mm.
Moreover, by adjusting the polarization of the input beam, the polarization dependence of the fabricated meta-axicons was investigated. To further demonstrate the uniformity of the mass-produced meta-axicons at the wafer scale, the Depth of Focus (DoF) values of the meta-axicons were measured from the center to the edge of the wafer, proving their homogeneity. This indicates that the meta-axicons can sufficiently perform their functions when produced in large quantities on the CMOS platform.
Figure 3: NIR experimental demonstration of forming a Bessel beam with circular polarization.
4. Applications and perspectives
The meta-axicons based on geometric phase obtained from the rotation of anisotropic structures can simultaneously implement circular polarization and Bessel beams. Since these meta-axicons can be designed to have various DoF according to the application, and can be mass-produced cost-effectively at the wafer scale, they can be applied such as high-precision laser drilling or optical trapping and tweezing. Additionally, this research shows an optimization method that can yield higher polarization conversion efficiency when designing meta-atoms. This design method can enrich the library of meta-atoms when designing geometric phase-based metasurfaces with various wavelength ranges.
These research results are published online with the title “Large-scale fabrication of meta-axicon with circular polarization on CMOS platform” in Nanophotonics.
The authors of this article are Gyu-Won Han, Jaewon Jang, Minsu Park, Hui Jae Cho, Jungchul Song, and Yeonsang Park, with the first two contributing equally to this work. Yeonsang Park is the corresponding author of this work. Prof. Yeonsang Park’s research group is affiliated with the Department of Physics and the Institute of Quantum Systems, Chungnam National University, Daejeon 34134, Korea.
