Electromagnetic metasurfaces, assemblies of nanoantennas with subwavelength lateral size and ultrathin thickness, have emerged as a new frontier in the physics and engineering communities to provide extraordinary light-matter interactions. Metasurfaces transform the electromagnetic waves through local and space-variant phase responses within thin sheets, which open a new route for flat and compact wavefront engineering. Despite the success of bring novel physics, metasurfaces still face some issues which hinder their practical applications and commercialization. In this work, various types of metasurfaces are investigated for robust beam engineering with superior characteristics in terms of high efficiency, broad bandwidth and active tunability, while favorable for implementation.
Nanoantenna resonators usually gain limited phase response with poor efficiency due to the large impedance mismatching, leading them non-ideal metasurface components. We propose a plasmonic metasurface with the elements being three-layer cascaded plasmonic loops based on the transmission line theory. Arbitrary inhomogeneous phase control is achievable with efficiency of more than 80%. To bring metasurfaces into visible spectrum without suffering from Ohmic loss, the all-dielectric metasurface is investigated as an alternative with the highest efficiency to date. Complete 360 degree phase response is captured from the high-index disk resonators by tuning the magnetic and electric resonances through tailoring the disk geometry. The concept of all-dielectric metasurfaces shows great potential to enable practical device functionalities at high frequencies, which motivates us to study it one step further on conformal optical platforms. Ultrathin conformal metasurfaces are demonstrated for invisibility cloaking at 532 nm visible wavelength with superior performances.
To break the inherent narrow-bandwidth limitation accompanied with the rich phase control, truly achromatic metasurfaces are proposed to work in a broad bandwidth without performance degradation. The constitutive layered elements, either plasmonic or dielectric, is to mimic the low-pass filters with linear phase responses but to work at infrared and visible ranges with each layer behaving as a shunt capacitor or a series inductor. A plasmonic collimating lens and a dielectric focusing lens are demonstrated from 5.5μm to 7.5μm and in the whole visible band, respectively, without chromatic aberration.
Moreover, taking advantage of the chemical tunability, graphene metasurfaces with gradient index profile are realized by patterning two types of molecules on graphene to manipulate surface waves as desired. In addition, a reconfigurable leaky wave antenna with smart and simply one-dimensional biasing configuration over a graphene sheet is successfully evaluated for pencil beam radiation with dynamic control of beam direction and beamwidth.
Advisor: Professor Hossein Mosallaei
Professor Hossein Mosallaei
Professor Philip Serafim
Professor Ahmed Busnaina