Broadband light-matter interaction using subwavelength metasurface designs for multi-spectral camouflage and polarization conversion application

Abstract

Photonic metamaterials are one of the most promising solutions to achieve broadband optical response. Broadband response is an essential part of many technological advancements, from microscopy, lasers, communication, to energy harvesting, thermal management, camouflage, and many other applications. In this thesis, I have presented a noble metamaterial design for the multispectral camouflage application, which is fabricated, and the performance of the device is checked for the desired application. In addition, I have showcased the potential of the multistaged particle swarm optimization (MPSO) algorithm by applying it to design an ultra-broadband polarization converter. Due to the diverse detection mechanisms, it is important to make the optical response of a camouflage surface multifunctional, e.g., different reflection and emission properties in different frequency bands, to suppress its detectable signatures. A multispectral camouflage device requires low visible light absorption (to avoid solar induced heating), low reflection in short-wave infrared (SWIR) (0.9-1.7µm), low reflection at 1.06 µm and 1.55 µm against laser guiding, low emission in the mid infrared (MIR) (3.5-5, 8-12 µm), and high emission in the nontransmissive atmospheric window for radiative cooling. Various attempts with different material combinations have been made to achieve multispectral camouflage functionality, all of which could partially address a few spectral bands among all the bands described above. In this dissertation, I designed a noble metasurface comprising a layer of thick bottom Al followed by an ITO layer and a Si-Al grating on top that achieves the desired optical response in most of the multispectral camouflage bands. A highly stable device is fabricated using the nanofabrication techniques, such as electron beam lithography and physical vapor deposition, according to the design materials and geometric parameters. The spectral measurement of the fabricated device with optimum geometry shows a very low SWIR reflection (0.18 a.u.), low laser reflection (0.02 a.u. for 1.06 µm and 0.12 a.u. for 1.55 µm), low MIR average emission in trnasmissive window (0.24 a.u. average absorption between 3.5-5 µm, 0.05 a.u. average absorption between 8-12 µm), and high MIR average emission (0.7 a.u. average absorption) in the non-transparent atmospheric window (2.4-3.5 µm). Furthermore, exceptionally high SWIR absorption has been demonstrated to result from so-called Fano resonance. In this range, the structure yields double Fano resonance in absorption mode coupled to a Lorentzian that enables broadband absorption. On the other hand, polarization converters play a significant role by manipulating one of the fundamental characteristics of light and can enhance imaging contrast, especially for deep structures in highly scattering materials. Efforts have been made to achieve ultra-broadband polarization converters based on metasurfaces. However, the Conventional simulation-driven forward design approach demands a time-consuming optimization pathway, and the process can lead to suboptimal device performance. We implemented multi-stage particle swarm optimization to achieve optimum metasurface for ultra-broadband THz polarization conversion in transmission mode, as many real-world applications, i.e., THz communication systems, require polarization control in transmission mode. We f irst proposed a design that comprises a tri-layer metasurface structure, where a split-bridged disk-like resonator is sandwiched between two perpendicularly oriented grating layers separated by dielectric spacers. This configuration enables an efficient conversion of linearly polarized waves into their cross-polarized counterparts under normal incidence, achieving a high polarization conversion ratio exceeding 90% across the frequency range of 1.19–9.0 THz, corresponding to a relative bandwidth of 153%. Through the surface current distribution analysis at different operating frequencies, I found that the induced magnetic and electric dipole moments contribute to ultra-broadband polarization conversion.