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Metasurfaces have had a profound influence on the global photonic design landscape over the last decade due to their ability to exercise abrupt phase and polarization changes for light manipulation. The ever-increasing complexity and need for miniaturization in modern photonic devices have called for the design of multifunctional meta-devices. However, the limitation of different functionality switching mechanisms in the visible range has hampered the translation of multifunctionality to the visible spectrum. This thesis focuses on the design of several multifunctional metasurfaces for the visible range utilizing phase change materials (PCM).
The first part of this thesis investigates a novel reconfigurable broadband metasurface that can be switched between an achromatic metalens and a broadband absorber in the visible range. This switchable functionality of the meta-device originates from the PCM (VO2)-based multi-stage unit cell. The unit cell was designed to produce switchable ab- sorption characteristics in the visible band with near-perfect broadband absorption for metallic VO2 (off state) and good transmission for its insulating phase (on state). This novel unit cell structure helped overcome the small variation of optical properties be- tween the two VO2 states in the visible regime, an inherent limitation of phase change materials (PCM) in general, which hindered the realization of visible frequency broad- band absorption switching in previous works. The proposed metasurface, constructed from the designed unit cells, shows excellent achromatic focusing in the on state with a focal shift of less than ±5% and a maximum intensity contrast of 21.1dB between the on and off states in the 678nm to 795nm band in the FDTD simulation environment. This is the highest switching ratio among previously reported broadband metalens-absorber platforms in any frequency band. These results suggest the suitability of our metasur- face platform as optical switches in many novel applications. Also, this is the first report of a reconfigurable broadband bi-functional metasurface in the visible range.
The second part of this work proposes a novel methodology for combining four dis- tinct broadband functionalities, namely achromatic beam deflection, wavelength beam splitting, achromatic focusing, and broadband absorption, in the same metasurface with properly aligned operating bands at the visible regime. This functionality switching of the meta-device originates from the Sb2S3 and VO2-based multi-stage unit cell. The unit cell was designed to produce both broadband amplitude and phase switching of re- flected light between the four states of the meta-unit, overcoming the inherent small op-
tical contrast between the phase change material (PCM) states by virtue of the cascaded Fabry-Perot cavities. This allowed overcoming the limitations of other functionality switching mechanisms for visible frequencies. A tandem neural network architecture has been used to inverse design the final metasurfaces, satisfying the wide range of re- quirements involving reflection amplitude and phase corresponding to the four functions while efficiently navigating the vast design space. The supervised training of both the reverse and forward networks was conducted using the labeled dataset formed through numerical simulation. The optical characteristics of two inverse-designed metasurfaces have been evaluated as test cases for two different sets of design parameters in the four states. Both structures demonstrate the four desired broadband functionalities while closely matching the design requirements in the simulation environment, suggesting their potential in visible-range portable medical imaging devices like the vis-OCT sys- tem. Such a meta-device with four or more different broadband functionalities has not been reported in previous literature for any design band.
The findings in this thesis work are expected to pave the way for novel multifunctional devices for miniaturized complex nano-photonic platforms for visible frequencies and in any spectral band in general. |
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