**3. Metamaterials in Photonic Devices**

Over past few years, researchers have been searching for highly integrated light source. The photon transparency in metallic photonic crystals which were doped with nanoparticles have been studied by Mahi R Singh; his study used new optical devices like switches and optical transistors [149]. There is a high demand for the development of practical optical photonic devices and its application systems for controlling absorption/transmission/reflection of electromagnetic waves. It was observed that by adjusting

the graphene fermi level a tunable metamaterial attains electronically reconfigurable terahertz reflection, absorption, and transmission in an effective manner [150]. In this view, various designs for free electron radiation emitters have seen light, which includes light wells [151,152], graphene based plasmonic, and dielectric based undulators [153,154], metallic nano gratings [155], etc. Along with the advancement of narrow linewidth miniaturized lasers [156] and highly integrated electron accelerators [157,158], the progress in nanoscale undulators paves the way toward the realization of X-ray light sources and on-chip extreme ultraviolet (EUV) sources. These advanced sources have promising applications in various fields such as natural sciences, engineering, and medicine [159,160]. Owing to strong confinement, low losses and dynamic tunability, the graphene plasmons are much suitable for manipulation of light matter interaction [161–165]. In addition, highly confined plasmons interact effectively with the matter. Nevertheless, small transverse extent of polaritonic field of such highly confined plasmons limits the interaction. This type of limitation exists in the light matter interaction in micro/nanoscale photonic wavelengths. Such limitations can be conquered by introducing metamaterials. Amongst, various kinds of materials graphene exhibit outstanding optical properties are highly appealing for optoelectronics and energy conversion applications [166–173]. However, the low optical absorption (2.3%) and ultrathin nature (3.4 Å) of monolayer graphene over a broadband wavelength limits its ability to provide sufficient optical modulation that restricts the performance in optical applications [174,175]. However, the graphene based-metamaterials consisting alternating dielectric layers and graphene can enhance the optical modulation which can be useful in advanced photonic devices [176–178]. Recently, Pizzi et al. [179] reported the enhancement of electron–plasmon interaction area by graphene metamaterials. They found that the output intensity is scaleup by a factor of 580 with respect to single graphene layer. For example, for 5 MeV electrons, a single layer metamaterial having 50 µm length and 50 layers, and a beam of current 1.7 <sup>µ</sup>A can generate 1.5 <sup>×</sup> <sup>10</sup><sup>7</sup> photons. This is due to the ability of the graphene multilayer structures to support MRPs (multilayer resonant plasmons) [180–186]. Further, the layered conducting structures can also be generated the visible Cerenkov radiation [187]. In addition, they reported that through allowing large amount of electron beams, the graphene multilayers produces the significant improvement in output intensities. Based on the conductivity of the graphene, there will be optimum layers in the metamaterial that enhances the intensity of the output. Further, by varying the resonating modes at different laser frequencies, the proposed metamaterials can generate multiple X-ray harmonics, which are used in time-resolved X-ray spectroscopy for ultrafast imaging of chemical reactions and electronic state transitions [188,189]. In addition, Yang et al. [190] suggested a low-cost and transfer free, solution-phase technique for the fabrication of multilayer graphene-based metamaterials. This consists of alternating monolayer graphene oxide/graphene and dielectric layers. They reported that the optical properties of the prepared graphene-based metamaterials can be tuned dynamically by controllable laser mediated conversion. Graphene-enabled active metamaterials may provide new platform for dynamical manipulation of light matter interactions [191]. Figure 3 shows the schematic representation of dynamic process for in situ phototunable graphene based metamaterial (Figure 1 of Ref. [190]). In addition, the laser patterning leads to functional photonic devices like ultrathin flat lenses embedded in the lab-on-chip device. In general, these ultrathin flat lenses maintain consistency and shows subwavelength focusing resolution at ambient environment without any observable degradation compared with the original lens. Therefore, these graphene-based metamaterials provide a new insight for widespread applications in on-chip integrated photonic devices.

**Figure 3.** The representation of dynamic process for (**a**) initial, (**b**) schematic and (**c**) final state of in situ phototunable graphene based metamaterial (Figure 1 of Ref. [190]).
