We report on the experimental observation of chiral resonant modes via a free-space spin-preserving Fabry-Pérot cavity using Pancharatnam-Berry phase reflecting dielectric metasurface mirrors. Such meta-mirrors focus one spin state while diverging the other and preserve the helicity upon reflection.
Metasurface presents itself as a method to create flat optical devices that generate customizable wavefronts at the nanoscale. The traditional metasurface design process involves solving Maxwell’s equations through forward simulations and implementing trial-and-error to achieve the desired spectral response. This approach is computationally expensive and typically requires multiple iterations. In this study, we propose a reverse engineering solution that utilizes a deep learning artificial neural network (DNN). The ideal phase and transmission spectrums are inputted into the neural network, and the predicted dimensions which correspond to these spectrums are outputted by the network. The prediction process is less computationally expensive than forward simulations and is orders of magnitude faster to execute. Our neural network aims to identify the dimensions of elliptical nanopillars that will create the ideal phase response with a near unity transmission in a 20 nm wavelength interval surrounding the center wavelength of the spectral response. We have trained such a reverse DNN to predict the optimal dimensions for a birefringent metasurface composed of elliptical nanopillars.
Here we report on our recent experimental efforts towards the design, fabrication and characterization of various metasurface structures that would allow spatial and temporal control of photon emission from atomic ensembles, as well as state preparation of solid state and atomic quantum emitters. The emphasis is placed on the development of two distinct categories of structures: (i) Micro- and meso-scale free-space self-polarizing confocal cavities formed by dielectric metasurfaces. (ii) flat hyper-gratings fabricated on the surface of a diamond, which would make the radiation pattern from NV centers in the diamond to be highly directional so that the emitted photons can be collected with high efficiency.
Among different techniques developed for x-ray phase-contrast imaging (XPCi) coded-aperture and Talbot-Lau can potentially provide higher contrast and higher sensitivity in imaging soft and hard tissues. Absorption grating is the central component that makes these techniques different from other XPCi techniques. For hard xray imaging, compatible x-ray absorption gratings are a must so x-ray can be stopped at these absorption gratings to create the desired wave-fronts. For hard x-ray imaging or soft x-ray imaging with high-resolution detectors, the aspect-ratio of gratings feature size goes beyond 10:1 (height:width) – in other words thickness of absorbing part increases – which is technologically challenging through conventional UV lithography techniques. Although SU-8 photoresist along with x-ray lithography is an alternative to fabricate such a high-aspect ratio grating, a compatible UV lithograph-based fabrication process is more desirable as it reduces the cost and increases the throughput. In this work, we have broken down a high-aspect ratio x-ray grating design into multiple layers of lower-aspect ratio structures and employed only the conventional UV lithography. SU-8 photoresists are known for their multi-layer coating specification, which is used in this study. The new fabrication process proposed in this work results in a final high-aspect ratio x-ray absorption grating through accessible UV lithography with lower cost and scaling-up compatibility, thus every research group and industry can benefit from it. To the best of our knowledge, this is the first time a multi-layer x-ray grating design is proposed and reported. The output of this work can be used to perform large field-of-view high-energy coded-aperture x-ray phase-contrast imaging.
X-ray absorption gratings are the heart of coded-aperture and Talbot-Lau x-ray phase-contrast imaging (XPCi) techniques. The quality of imaging – visibility – in the aforementioned techniques is highly dependent on the quality of gratings. SU-8, an epoxy-based photoresist, is valued in Micro-Electro-Mechanical-Systems (MEMS) fabrication due to its excellent mechanical and optical properties. For fabricating x-ray absorption grating following the MEMS process, gold is the material most widely used to stop incident x-rays. One of the main challenges in fabricating a quality x-ray absorption grating is the adhesion between SU-8 molds and gold thin film, where due to poor adhesion of SU-8 to gold thin films and SU-8’s internal high level of stress, SU-8 molds delaminate, crack, or peel off during developing or electroplating steps. This problem becomes more challenging when a greater field of view gratings are required. Employing SU-8 on a gold substrate requires an adhesion layer to assure SU-8’s mechanical stability during fabrication steps. Without this layer, realizing a high-quality grating is challenging. In this work, we have investigated the adhesion quality of SU-8 to gold thin films for different adhesion promoter layers. We have employed a combination of a SU-8 thin film and a metallic-silane-based nanometer-thin film to improve the adhesion quality between SU-8 photoresist and a gold thin film substrate. We have tested the quality of adhesion using tape test and through a harsh (highly acidic) electroplating solution.
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