The emerging meta-holograms rely on arrays of intractable meta-atoms with various geometries and sizes for customized phase profiles that can precisely modulate the phase of a wavefront at an optimal incident angle for given wavelengths. The stringent and band-limited angle tolerance remains a fundamental obstacle for their practical application, in addition to high fabrication precision demands. Utilizing a different design principle, we determined that facile metagrating holograms based on extraordinary optical diffraction can allow the molding of arbitrary wavefronts with extreme angle tolerances (near-grazing incidence) in the visible–near-infrared regime. By modulating the displacements between uniformly sized meta-atoms rather than the geometrical parameters, the metagratings produce a robust detour phase profile that is irrespective of the wavelength or incident angle. The demonstration of high-fidelity meta-holograms and in-site polarization multiplexing significantly simplifies the metasurface design and lowers the fabrication demand, thereby opening new routes for flat optics with high performances and improved practicality.

We have developed a simple imprinting technique that allows patterning over a nonflat substrate without the need for planarization. In this process, a polymer film is spin coated onto the mold and then transferred to a patterned substrate by imprinting. By selecting polymers with different mechanical properties, either suspended structures over wide gaps or supported patterns on raised features of the substrate can be obtained with high uniformity. It is found that imprinting at a temperature well above the glass transition temperature (Tg) of the polymer causes the thin residue film between features to dewet from the mold, which can greatly simplify the subsequent pattern transfer process. Multilayer three-dimensional polymer structures have also been successfully fabricated using this new imprinting method. The yield and dimensional stability in the multilayer structure can both be improved when polymers with progressively lower Tg are used for different layers. Compared to existing techniques for patterning on nonflat substrates, the current method has a number of advantages, including simplicity, versatility, high resolution, and low pattern distortion.

A reversal imprinting technique was developed in this study. A polymer layer was first spin coated on a patterned hard mold, and then transferred to a substrate under an elevated temperature and pressure. The reversal imprinting method offers an advantage over conventional nanoimprinting by allowing imprinting onto substrates that cannot be easily spin coated, such as flexible polymer substrates. Another unique feature of reversal imprinting is that three different pattern-transfer modes can be achieved by controlling the degree of surface planarization of the mold after spin coating the polymer resist as well as the imprinting temperature. “Embossing” occurs at temperatures well above the glass transition temperature (Tg) of a polymer; “inking” occurs at temperatures around Tg with nonplanarized polymer coating surface on the mold; and “whole-layer transfer” occurs at temperatures around Tg but with a somewhat planarized surface. These three imprinting modes have been quantitatively correlated with the surface planarization of the mold after polymer coating and the imprinting temperature.