Development of metasurfaces in imaging and light-matter interactions

Abstract

Metasurfaces, which are artificially created meta-atoms arranged in a single layer on a subwavelength scale, possess the ability to manipulate the light field in both amplitude and phase and have sparked considerable interest within the photonics community. By utilizing advanced experimental methods, such as nanofabrication and characterization, meta-devices have been developed and evaluated for numerous applications, including nonlinear signal modulation, photoelectric detection, optical imaging, and sensing. In the beginning section of the thesis, an overview of the primary research topics will be provided. This will include a background on second-order nonlinear effects, such as second-order generation and photocurrent generation using metasurfaces. Additionally, the concept of plasmon-induced transparency will be introduced. The fabrication techniques will be thoroughly discussed, with a focus on the sequence of thin-film depositions or growth, lithography, and etching. The wet etching method will be emphasized as it is utilized for creating free-standing gratings with high aspect ratios that can be adjusted. These gratings enable the development of functional devices, such as broadband focusing and imaging. Lastly, there will be a brief overview of some experimental techniques and simulation verification methods. In the subsequent chapters of the thesis, I will present research conducted in the aforementioned areas. Nonlinear metasurfaces are designed and optimized to exhibit resonances that match the fundamental wavelength. The nonlinear scattering theory is utilized to evaluate the modulation of second harmonic generation when varying twist angles exist between the artificial meta-atoms and the single crystalline molybdenum disulfide, resulting in a sine-like pattern. This is later verified through the use of a nonlinear characterization setup equipped with a femtosecond laser. Additionally, the study investigates photocurrent generation on gold nanoholes, which is another type of second-order nonlinear effect. The experimentally measured photocurrents align with the theoretical predictions and result from the excitation of surface plasmon polaritons, displaying diverse profiles for varying incident polarizations. Finally, metasurfaces with plasmon-induced transparency effects are designed, and their potential is demonstrated in surface-enhanced infrared absorption spectroscopy for molecular detection and sensing, assisted by complex-wave synthesis.