Tunable metasurfaces for active nanophotonics

• Main Author: Omar Abdelrahman Mohamed Abdelraouf
• Other Authors: Wang Qijie
• Format: Thesis-Doctor of Philosophy
• Language: English
• Published: Nanyang Technological University 2023
• Subjects: Physics
• Online Access: https://hdl.handle.net/10356/169814
• Institution: Nanyang Technological University

The ability to control light propagation is a central key in optics. Conventional optical components rely on the concept of light diffraction and the gradual accumulation of the light phase as it propagates through the optical medium. However, the accumulated light phase is limited by the material thickness and its permittivity. Such a mechanism often results in very bulky optical components thicker than the operating light wavelength. Recently, metasurfaces have attracted significant attention owing to their possibility to manipulate the incident light properties such as amplitude, polarization, phase, and wavefront. Metasurfaces consist of small unit cells called meta-atoms, where each meta-atom can impose different light scattering properties based on its dimensions or orientations. As a result, metasurfaces have generalized the light diffraction law by precisely designing the optical response of each meta-atom inside metasurfaces, an abrupt light phase change is achieved and changes properties of transmitted, reflected, and absorbed light. This advantage of metasurfaces enables the shrinking of many bulky optical components into a thin layer of artificially engineered nanostructures. Since the optical response of metasurfaces depends on the dimensions, optical properties, and orientations of meta-atoms, achieving reconfigurability in metasurface performance will require tuning the optical properties or dimensions of these meta-atoms. Dynamic control of metasurface behavior is favorable to tune the novel optical performance of the passive metasurface or to achieve multi-functional optical devices as an ultimate goal. Tunable materials enable dynamic control of their optical properties by using external stimuli such as electrical, optical, thermal, chemical, magnetic, and elastic strain, to name a few. Integration of tunable materials inside passive metasurfaces results in the emergent field of tunable metasurfaces. In this thesis, we present our efforts toward achieving tunable metasurfaces to empower nanophotonics devices. We introduce a paradigm of tunable metasurfaces for dynamic performance of the electromagnetic waves for applications in nonlinear optics and light emission devices. By investigating the optical and thermal modulation mechanism to tune optical properties of phase change materials (PCMs) and designing tunable metasurface incorporating PCMs to tune the meta-atom response. We were able to design and experimentally demonstrate tunable third-harmonic generation (THG) in the visible spectrum based on amorphous silicon (a-Si) metasurface supporting Fano resonance. In addition, we experimentally showed tunable light sources based on the amplified spontaneous emission concept in visible and near-infrared regimes by designing two different tunable metasurfaces that support the bound states in the continuum resonance. Moreover, many thin-film structural colors have been demonstrated using the concept of intermediate states in PCMs. Lastly, we showed an all-optical fast reconfigurable phase tuning mechanism for a fast dynamic response. The ability to individually control the optical response of each meta-atoms has made tunable metasurface to be progressively ubiquitous by enabling a wide range of novel optical functionalities. The future prospect of the conducted research work can be done by incorporating atomically thin materials (2D materials) that support ultrafast carrier dynamics and large optical properties contrast for dynamic quantum optics applications such as tunable single-photon emitters and on-chip photonic systems like a neuromorphic chip.