# Plasmonic and Dielectric Metasurfaces: Design, Fabrication and Applications
## 4 Discussion  
Although metasurfaces composed of plasmonic and dielectric units have only recently emerged as an active area of research, their potential as an approach to realize wavefront shaping devices has already been successfully implemented in flat optics, such as OAM beam generator, lens, wave plates, holograms, etc., for its superiorities of compact and flexible design. For instance, convenient optical elements are based on the refraction, reflection or diffraction of light. Their ability of wavefront shaping relies on propagation through media whose refractive indices can be designed to control the light path. As a consequence, those conventional optical elements suffer from thick materials, large volume, long diffraction distance and even low optical manipulation accuracy, such that it is difficult for them to be compatible with compact systems. Metasurface-based flat optical elements can potentially replace traditional ones. They are able to modulate the wavefront of the light beam by introducing a variety of nano-scaled electric dipole resonators replacing conventional propagation effect and therefore considerably reducing the whole volume of systems. Besides, the responses of metasurfaces rely on the characteristics of unit structures, such as structure dimensions and material types. It is relatively convenient to flexibly control the amplitude, phase and polarization of scattering light. However, there are still some challenges of metasurfaces. For plasmonic metasurfaces, they suffer from ohmic loss at optical wavelengths. One common solution to loss compensation  
Figure 27: **(a)** Ideal phase pattern to generate six-fold OAM multicasting (\(l=1\), \(4\), \(7\), \(10\), \(13\), \(16\)); **(b)** top view of the designed dielectric ellipse array to generate six-fold OAM multicasting by replacing the ideal phase pattern with corresponding resonators; **(c)** power distributions of the OAM channels generated by the ideal phase pattern and the dielectric ellipse array; **(d)** intensity distribution of six collinearly-superimposed OAM beams; **(e)** intensity distribution of undesired OAM channels after demultiplexing; **(f)** intensity distribution of multicasting OAM channels after demultiplexing. Reproduced with permission from [124], Copyright The Optical Society, 2015.  
is to introduce a gain medium in or around the plasmonic material. This solution can partially or fully compensate the loss in metals, but the incorporation of such active materials is very challenging [129]. Another challenge for plasmonic metasurfaces is that noble metals have a large plasma frequency and cannot be adjusted, making tunable or switchable metasurface devices hard to realize [130]. What is more, noble metals are not compatible with the standard semiconductor fabrication technologies, restricting metasurface devices to the proof-of-concept stage only. To deal with those issues, some recent research focuses on new materials platforms defining new, intermediate carrier density materials as the best candidates that have low loss, excellent tuning and modulation capabilities and that are compatible with established semiconductor processing technologies and integration procedures [131; 132; 133; 134]. In addition, metasurfaces are also facing the challenge of being reconfigurable. For metasurfaces, the unit structures must be subwavelength-scaled optical switches with a fraction of a square micrometer footprint and a volume of only approximate \(10^{-19}\) m\({}^{3}\). It is desired for the optical switches to be fast and energy saving. It is challenging for unit structures to satisfy the former two conditions. To solve the problem, one can alter the dimensions of the individual unit structures, or manipulate the near-field interactions between them. The nanomechanical device platform can provide one possible ideal solution [135; 136].