Metalens integrated photonics

This thesis investigates the optical metasurfaces utilized for lensing (metalens) and their potential to integrate with photonic circuits. In recent years, a plethora of studies have been done on metalenses, especially in the visible region. However, there are still rooms to discover the metalenses working in the mid-IR and ultraviolet (UV) spectra. Due to the applications in these regions, from biosensing in mid-IR to lithography in UV, the development of metalenses operating in these ranges is essential. One aspect of metalenses that requires further investigation is to analyze the deviation of the measured focal spot from the simulated value, which can affect the focusing quality. As this deviation becomes smaller, a more homogeneous airy-disk pattern can be achieved. Unlike the previous studies that have suggested the Fresnel number is the only relevant factor for the possible focal shift, we numerically show that for mid-IR metalenses, the numerical aperture (NA) of metalenses is an essential parameter to predict the focal spot deviation from the simulated value. We also introduce a critical NA (between 0.55-0.65) in which the deviation of the simulated and measured focal spot can reach zero value. The other aspect of metalenses addressed in this thesis is to find a fine-tuning method to multiply the number of focal sites, which is a practical concept for polarization filtering applications. The single-material-base metalenses are numerically investigated in the UV regime. Moreover, a design method to have full spatial control on phase, polarization, and focal spots is carefully implemented to attain multifocal metalenses. These single-material metalenses are composed of Al$_2$O$_3$ nanoblocks on the Al$_2$O$_3$ substrates, which can facilitate the fabrication process. Finally, by taking advantage of focal shift prediction and multifocal design principle, we propose a metalens-integrated photonic device. On one side of such device, a metalens structure is incorporated, and a plasmonic antenna array is patterned on its other side. We numerically study this device and show that this new compact design could offer an excellent lens-free illumination mechanism for plasmonic structures. Also, as it is possible to focus the incident light on the plasmonic structure, the near field intensity is significantly increased in this device. In addition, utilizing multifocal metalenses on the backside also allows us to illuminate multiple chips on the surface with different polarization modes. Considering the plasmonic structure applications for biosensing purposes, we can increase the sensitivity and selectivity of biodetection in these integrated devices while avoiding complicated measurement techniques.