Dielectric metasurfaces designed based on bound states in the continuum (BICs) exhibit extremely high quality factors, high sensitivity, and low loss, making them suitable for surface-enhanced infrared absorption and refractive index sensing. It has been demonstrated in some literatures that the symmetric property of the elliptical unit cell of the BICs mode can be broken by increasing the rotation angle, thereby achieving high-quality factor resonant structures. By varying the rotation angle, major and minor axis lengths of the ellipse, and the period of the bilayer elliptical unit, unique resonant properties can be achieved even without considering the existence of BIC. In this work, we have designed a deep learning-based transmission spectrum prediction network by combining the parameters of the elliptical unit. This network can replace traditional electromagnetic simulation calculations to quickly obtain the transmission spectrum of the target structure. While simulating 3000 sets of elliptical unit using the finite-difference time-domain method on an i7- 12700H processor requires 14 hours, using our neural network yields a transmission spectrum with prediction accuracy better than 10-3 in less than 6.9 seconds, significantly improving the design efficiency. The network takes the image of metasurface unit as input data and couples the scaling factor that affects the period of the metasurface into the image data, making it possible to train and predict the spectra with different structures and ratios, effectively improving the generalization ability of the network.
In traditional metasurface structure design, it heavily relies on electromagnetic simulations to obtain transmission and phase spectral, followed by empirical adjustments. This iterative trial-and-error process, especially when dealing with multi-objective optimization tasks, demands intensive and time-consuming computations, which to a certain extent restricts the development of the metasurface research field. In this paper, a proposed method achieves rapid prediction of spectral responses corresponding to structural units by seeking analytical solutions within the constructed neural network model. The proposed deep learning-based method for predicting transmission and phase spectral of metasurface units consists of metasurface unit dataset construction and a ResNet-based network framework. In the dataset construction approach, an overhead view of the unit structure is extracted and transformed into a binary image, where scaling factors are coupled into the two-dimensional image to increase dimensionality. This enables the representation of different structures such as square pillars, elliptical cylinders, and varying sizes of metasurface units using the same data format, significantly enhancing network generalization. Within the network framework, ResNet is employed to predict the real and imaginary parts of the S21 parameter, which are then inverted to obtain transmission and phase information. The progressive training method employed in combination with this framework yields high prediction accuracy. The deep learning-based method for predicting transmission and phase spectral of dielectric metasurface units, as revealed in this paper, achieves a 7200-fold increase in prediction speed compared to traditional electromagnetic
In this research, a key optical component for multi-degree-of-freedom (MDOF) surface encoder was designed, fabricated and evaluated. In a MDOF grating interferometry system, there are four diffraction beams from the scale grating and reference grating. For further modulation, these beams will propagate more than 100 mm, which makes paralleling these beams necessary. In previous researches, separate prisms and a home fabricated diffraction device by combining four separate one-axis line gratings in a glass substrate have been demonstrated. However, large power loss and assembly complicity makes this technique less competitive. For solving this problem, this research proposed a new lens module, which is an improved type prism, quadrangular frustum pyramid. The prism is designed in such a way that these four reflected beams from the grating are symmetrically incident into the prism through the upper surface, total reflected on the inner sides of the prism, and then paralleling propagate through the bottom surface. A prism that allows an incident beam diameter of 1 mm and four paralleling beams with a 10 mm distance between the two diffraction beams along one direction was designed, fabricated and tested. Testing results based on an entire grating interferometry system verified that the proposal in this research is greatly effective in beam paralleling in terms of less power loss and high paralleling and greatly reduce the assembly complicity, which will eventually be beneficial for grating interferometry application.
In this paper, an orthogonal type two-axis Lloyd’s mirror interference lithography technique was employed to fabricate two-dimensional planar scale gratings for surface encoder application. The two-axis Lloyd’s mirror interferometer is composed of a substrate and two reflective mirrors, which are placed edge by edge perpendicularly. An expanded and collimated beam was divided into three beams by this interferometer, a direct beam and two reflected beam. The unnecessary beam section was blocked by a home designed filter. These two reflected beams interfere with the direct beam respectively, generates perpendicularly cross patterns for forming two-dimensional scale gratings. The two reflected beams also interfere with each other and generate an undesired grating pattern along a 45 degree direction, which influence the pattern uniformity in a certain degree. The undesired grating pattern can be eliminated by polarization modulation, yet which will influence the grating area. Theoretical and experimental study was carefully carried out to evaluate the fabrication quality with and without polarization modulation. Two-dimensional scale gratings with a 1 μm period in X- and Y-directions were achieved by using the constructed experiment system with a 442 nm HeCd laser source. Atomic force microscope (AFM) images and diffraction performances verified that the two-axis Lloyd’s mirror interferometer with a small undesired interference between two reflected beams under an order of nominal value of 0.1 can provide a better fabrication result for scale gratings application.
A new type of plane optical lens, the Fresnel-grating lens with variable-line-space grating surface based on dual pointsource holographic technique, is demonstrated, optimized and fabricated in this research. The Fresnel-grating lens integrates the function of a diffraction grating and a Fresnel lens, and provides collimation, focusing as well as dispersion effects at the same time, which simplifies the optical structure and fits perfectly for a miniature spectrometer. The variable-line-space grating surface can enhance the modulation ability of the spectrometer system A mathematical model is established based on the least wave-change principle and the system parameters are optimized using optics simulation software ZEMAX. The fabrication procedures for the Fresnel-grating lens with soft lithography are illustrated in detail, and a dual-point holographic exposure optical path to form the grating surface as well as a mold to cast the Fresnel-grating lens are constructed. Samples of Fresnel-grating lens with the size of 16mm×16mm×5mm are fabricated and preliminarily tested. The period on the central part of fabricated grating surface is 926±2nm and the results show great consistency with the simulation. With further testing in the spectrometer system, the Fresnel-grating lens with a variable-line-space grating surface holds considerable potential in the miniature spectrometer area.
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