Function capacity expansion of nano-optics via multiplexed metasurfaces

Throughout history, the study of light has captivated humans due to its important role in shaping our perception of the world, enabling efficient communication, manufacturing complex structures and devices, and influencing various aspects of science and technologies. Metasurfaces, which consist of two-dimensional subwavelength nanostructures that can locally manipulate the proprieties of light including amplitude, phase, polarization, and angular momentum, provide an unprecedented means to control optical waves in a prescribed manner. Different functionalities, such as structured light generation, holograms, and flat lenses, have been realized by metasurfaces. The working platform of metasurfaces can be broadly classified into two categories. The first represents the conventional scenario, where light impinges on and interact with metasurfaces in free space. The second, a relatively recent development, involves confining light within a planar waveguide in a two-dimensional fashion. In both cases, it is critical to advance the practical applications of metasurfaces by broadening their functionalities, allowing them to exhibit diverse responses under varying working conditions. Toward this end, in my thesis, I will explore the utilization of metasurfaces to enhance the degrees of freedom (DOFs) and expand functionality channels of both platforms through the multiplexing of different input/output combinations such as the polarization, angle of incidence, wavelength, etc.Conventional design approach for planar metasurfaces involves multiplexing segmented chips, each serving distinct purposes and encoded into a single device. Thus, new design methods that can integrate more channels into a single metasurface while also ensuring precise control over each channel is highly desirable. For example, with the complete engineering of multiple polarization channels, we will significantly push the boundaries of nano-printing multiplexing by mapping images into different locations in the Fourier space. Our proposed Polarization-Encoded Lenticular Nano-Printing (Pollen) method has the potential to significantly advance modern optical applications such as optical encryption, optical data storage, augmented reality (AR), and virtual reality (VR). Not only can we manage to control the grayscale images on the surface of the device, which are directly related to the amplitude profile, but we can also preciously control the wavefront of the light. An inverse design algorithm for generating complex structured light will also be discussed, so that the light scattered from metasurfaces could form complex spiral phase contours, including vortex beam arrays and perfect vector vortex beam arrays, corresponding to different incident angles. This approach can potentially advance various fields, including optical communication, optical manipulation, and so on. On the other hand, on-chip metasurfaces, which refer to metasurfaces integrated with photonic waveguides, are highly desirable due to their compact footprint and flexible functionality. To explore the benefit of both free space optics and integrated photonics, we can combine the on-chip metasurface with a planar waveguide to couple out the guided mode to the free space while controlling their wavefront and polarization. We aim to broaden the range of functions achievable by on-chip metasurfaces by leveraging the principles of multiplexed metasurfaces. Through precise control of geometric parameters, including the rotation angle and displacement of V-shaped nanoantennas, we have developed a novel on-chip metasurface design capable of multiplexing up to four functions, such as focusing and vortex beam generation. These multiplexed metasurfaces leverage polarization states and waveguide mode propagation directions as additional degrees of freedom. Furthermore, we have explored mode division multiplexing (MDM), a crucial technique for high-volume optical transmission, where multiple waveguide modes can transmit within the same waveguide without crosstalk. Our research also delves into on-chip metasurfaces for mode conversion, allowing on-demand conversion between waveguide modes with orthogonal polarizations. Specifically, by manipulating the orientation of individual C-shaped nanoantennas, we achieve mode conversion and focusing effects for cross-polarized electric fields within the waveguide. We have successfully demonstrated the design and simulation results for 16 scenarios of wideband transverse magnetic (TM) to transverse electric (TE) mode conversion, achieving mode purity levels of up to 98%, along with on-chip lenses operating at a wavelength of 1550 nm. The dissertation is organized as follows: In Chapter 1, we introduce the application and working principle of meta-devices and various multiplexing methods. Moving forward, Chapter 2 and Chapter 3 focus on free space metasurfaces and applying various multiplexing and optimization techniques to expand their functional capabilities. Chapter 4 and Chapter 5 focus on on-chip metasurfaces and their role in controlling scattered light within multiplexed channels and the MDM systems, respectively. Finally, Chapter 6 outlines prospects in this field and elaborates on our forthcoming research directions. --Author's abstract