Flat-optics enable the miniaturization of many traditional bulk photonic devices routinely used in optical modulation and detection in telecommunication systems, biosensing and microscopic imaging in biomedical research, and light detection and ranging (LiDAR) used in automobile, military and surveillance applications. The backbone of typical flat-optic devices are the metasurfaces comprising structured nanoparticle lattices embedded in flat layer of traditional dielectric or semiconductor optical materials. The metasurface lattices can create optical resonances by exploiting different aspects of the light-matter interaction, e.g., light absorption, radiation, scattering and diffraction by the nanoparticles array. Such resonances are essential for the efficient optical interactions performed by the flat-optic devices, for example, enhancing nonlinear second-harmonic generation for optical frequency modulators, or enhancing light absorption in photodetectors. This Ph.D. dissertation reports the mechanisms of exciting and tailoring the metasurface lattice-controlled resonances using metal nanoparticle arrays. Exhibiting localized surface plasmon effects, metal particles can dramatically enhance the light field intensity under resonance conditions. Nevertheless, by nature, metal particles concurrently exhibit high absorption, radiation, and scattering losses, which cannot be sufficiently suppressed by the localized surface plasmon resonances. Almost two decades ago, researchers theoretically estimated that the benefits of the plasmonic field enhancement could still be harnessed by suppressing the scattering loss by organizing such lossy metal particles in a periodic lattice formation. In contrast to the low-Q localized resonances, such an engineered lattice arrangement could excite high-Q nonlocalized resonances, which are now often called as lattice plasmon or surface lattice resonances. Notwithstanding, the efforts on the experimental validation of such a concept were not succeeding as per expectation in terms of the resonance Q-factors. Thus, prior to the work accomplished in this dissertation, it was largely believed by the photonics community that, it is the 'lossy' plasmonic metal particles that do not allow to excite the high-Q resonances as per the minimum requirements in the practical flat optic applications. As a primary contribution to my Ph.D. dissertation, we successfully debunk that myth. In our work, we systematically proved that the non-localized lattice resonances can still be excited in 'lossy' metal nanoparticle arrays. Precisely, we improved both the design of the metasurface lattices and their fabrication and characterization techniques to eventually observe the high-Q lattice resonances as per the theoretical prediction. Our primary success later inspired us to analyze the systems more profoundly to make them suitable for different types of practical applications, which ultimately resulted in additional secondary successful projects described in my Ph.D. dissertation. The success of these projects would allow us in the future to utilize the nonlocalized plasmonic metasurface lattice-controlled resonances in a diverse range of flat integrated photonics applications, such as free-space light modulation and detection, which may rely on the nonlinear or electro-optical light-matter interaction in the flat thin-film region. We believe that the outcome of this dissertation will pave the way to designing and manufacturing efficient flat meta-optic devices for real-life applications, particularly in the telecommunication and medical sectors for the utmost betterment of human civilization.

This thesis investigates the potential of flat optics as a solution to the problem of bulky and expensive optical components in producing lightweight and wearable optoelectronic devices. The research addresses scalability challenges in structure fabrication, design of broadband operating devices, and increasing operational and transmission efficiency in the visible range. It focuses on the experimental part of the challenge. The study evaluates various design approaches, including inverse designs using optimization techniques as well as the use of machine learning algorithms. The thesis aims to explore a path toward high efficiency, wide bandwidth, functional response, and scalable fabrication in flat optics using semiconductor nanostructures. The results demonstrate the potential of using semiconductor nanostructures to engineer efficient, scalable, and broadband optical components in obtain light processing through flat surfaces.

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