Metasurfaces are artificially structured thin films with unusual properties on demand. Different from metamaterials, the metasurfaces change the electromagnetic waves mainly by exploiting the boundary conditions, rather than the constitutive parameters in three dimensional(3D) spaces. Despite the intrinsic similarities in the operational principles, there is not a universal theory available for the understanding and design of metasurface-based devices. In this article, we propose the concept of metasurface waves(M-waves) and provide a general theory to describe the principles of them. Most importantly, it is shown that the M-waves share some fundamental properties such as extremely short wavelength, abrupt phase change and strong chromatic dispersion, which make them different from traditional bulk waves. It is shown that these properties can enable many important applications such as subwavelength imaging and lithography, planar optical devices, broadband anti-reflection, absorption and polarization conversion. Our results demonstrated unambiguously that traditional laws of diffraction, refraction, reflection and absorption should be revised by using the novel properties of M-waves. The theory provided here may pave the way for the design of new electromagnetic devices and further improvement of metasurfaces. The exotic properties of metasurfaces may also form the foundations for two new sub-disciplines called "subwavelength surface electromagnetics" and "subwavelength electromagnetics".
In this paper,we demonstrate an auto accurate alignment method to align mask-substrate in the prototype of plasmonic lithography(PL),which is essential for multilayer nanostructure fabrication with high resolution,low cost,high efficiency,and high throughput,such as circuit manufacturing and other applications.We obtained an alignment signal with sensitivity better than 20 nm by using the Moiréfringe image.However,only using the Moiréfringes cannot guarantee the alignment of the mask and the substrate because the Moiréfringe repeats itself when the mask and substrate are offset by a fixed displacement.To eliminate the ambiguity,boxes and the crosses alignment marks are designed beside the grating marks on the substrate and the mask,respectively.A two-step alignment scheme including coarse alignment and fine alignment is explored in the auto alignment system.In the stage of coarse alignment,the edge detection algorithm based on Canny operator is adopted to detect the edges image effectively.In the process of fine alignment,Fourier transform based on Moiréfringe image is obtained to improve the alignment accuracy.In addition,experimental results of overlay indicate that PL can obtain sub-100 nm alignment accuracy over an area of 1 cm^2 using the proposed two-step alignment scheme.Via the substrate-mask mismatch compensation,better stages and precise environment control,it is expected that much higher overlay accuracy is feasible.
Minggang LiuChengwei ZhaoChangtao WangXiangang Luo
