# Dynamical Manipulation of Surface Plasmon Polaritons
## 6 Discussion and Perspectives  
Since the pioneering works of polarization-controlled SPP excitation by the [PERSON] group [48] and [PERSON] group [49], active SPP focusing by the [PERSON] group [57] and [PERSON] group [36], dynamic switching of SPP vortex by the [PERSON] group [21] and [PERSON] group [70], and dynamically controlled nondiffracting SPP beams by the X. Zhang Group [42], many researchers have devoted efforts to dynamic modulation of SPP and lots of fascinating works have been accomplished. We have briefly reviewed some of the achievements in the domain of dynamical SPP manipulation. According to the functionality, in this review, the dynamical SPP devices are classified in four categories: SPP excitation, SPP focusing, SPP vortex, and SPP nondiffracting beam. Other functionalities, such as dynamical SPP holography [77; 78] and controlled Cherenkov SPP wakes [47; 93], are demonstrated as well. Overall, modulating the wavelength, amplitude, phase, and polarization of the excitation light is the basic underlying principle of most dynamical SPP devices. The amplitude and phase can be controlled with SLMs and the wavelength-based modulation usually requires broadband light sources, which are complex and costly. On the contrary, the polarization states of light can be easily adjusted with half wave plates and quarter wave plates. With carefully designed subwavelength structures, almost all the functionalities of the SPP devices can be manipulated through polarization. Therefore, modulating the polarization of excitation light is a more convenient and effective way to actively control SPP field. Currently, most polarization-controlled SPP devices rely on changing the handedness of circularly polarized light. Further study may take advantage of linearly polarized light, elliptically polarized light, or light with space-variant state of polarization. Coherence is also one of the fundamental degrees of freedom of light [94; 95]. In the above discussion, we have mainly focused on the modulation of monochromatic SPP field. The spatial coherence of monochromatic SPP is conserved during propagation [96; 97]. Theories for partially coherent polychromatic SPP field have been developed recently [98; 99; 100; 101; 102; 103]. The SPP coherence characteristics can be actively controlled by tailoring the spectral and spatial coherence of incident light [98]. Particularly, [PERSON] et al. introduced and studied a class of structured polychromatic SPP field--partially coherent axionic SPP field [100; 101]. Theoretical analyses show that vectorial axionic SPP field possesses high structural stability and broad statistical versatility. Despite the lack of interference for the uncorrelated axionic SPP field, coherence lattices of varying forms can be obtained due to statistical similarity. These studies indicate that SPP coherence engineering is of fundamental importance in controlling the spatial, temporal, and polarization properties of the SPP field. Besides, 2D materials such as graphene and MoS\({}_{2}\) whose optical properties can be electrically tuned have been introduced as a new platform to dynamically modulate SPP [104; 105; 106; 107]. The strong coupling between plasmon and material excitons [108; 109; 110] has been applied to plasmon enhanced spectroscopy. Combining the 2D materials, the modulation of excitation light and the design of subwavelength structure should enable a more flexible control over SPP fields in the future. Compared with static SPP devices, dynamical SPP devices are multifunctional and more efficient, which makes them a powerful tool in applications like on-chip communications [10; 11; 83; 111], polarization analyses [70; 112; 113], particle manipulation [69; 114; 115], and super resolution imaging [38; 116]. This manuscript is written by [PERSON] and [PERSON]. The discussions and revisions of the manuscript are carried out by [PERSON], [PERSON], and [PERSON] All authors read and approved the final manuscript. This work is supported by the National Natural Science Foundation of China (grant nos.