# Dynamical Manipulation of Surface Plasmon Polaritons
## 1 Introduction  
Arising from the collective oscillations between the incident phonons and electrons at the dielectric/metal interface, surface plasmon polaritons (SPP) exhibit intriguing properties of large in-plane momentum, subwavelength field confinement, and strong field enhancement [1; 2; 3], which makes it promising in applications ranging from sensors [4], solar cells [5], super-resolution imaging [6; 7; 8], optoelectronic circuits [9; 10; 11], and more. Modulating the propagation and distribution of SPP is indispensible in these applications. Most optical devices in the 3D free space can find their 2D counterparts in the SPP platform. SPP focusing [12; 13; 14; 15; 16; 17], SPP refection [18; 19; 20], SPP vortex [21; 22; 23; 24], SPP Airy beam [25; 26; 27], SPP Bessel beam [28; 29; 30], SPP logic gates [31; 32], and unidirectional SPP excitation [33; 34; 35] have been accomplished by elaborately designing the shape and position of dielectric or metallic structures. Nevertheless, the functionalities of SPP devices are generally single and static.  
Dynamic light modulation is always desired in different branches of optics. In the free space, the amplitude and phase of light can be readily modulated with spatial light modulator (SLM). For SPPs, however, no such modulator exists. Fortunately, the amplitude and phase of SPPs are closely related to the excitation light. The amplitude of SPPs is proportional to the intensity of the incident light and the phase is the same as that of the incident light. Based on this feature, the distribution of SPPs can be dynamically manipulated by modulating the excitation light [36; 37; 38; 39; 40; 41; 42; 43; 44; 45]. Another distinctive feature is that the amplitude and phase of the excited SPPs depend heavily on the polarization states of incident light. Particularly, for circularly polarized light, the SPPs generated by subwavelength slits are imprinted with a spin-dependent Pancharatnam-Berry (PB) phase which can be tuned by changing the orientation angle of the slits [46; 47]. Polarization-controlled dynamical SPP excitation [48; 49; 50; 51; 52; 53; 54; 55; 56], SPP focusing [46; 57; 58; 59; 60; 61; 62; 63; 64; 65; 66], SPP vortex [67; 68; 69; 70; 71; 72; 73], SPP nondiffracting beams [74; 75; 76], and SPP holography [77; 78] have beenrealized. Besides, the other degrees of freedom of light--including the wavelength, topological charge, and the spatial frequency--can be taken advantage of to actively control the SPP field [79; 80; 81; 82; 83; 84; 85; 86] as well.  
In this paper, we review the achievements in a branch of plasmonic-dynamical SPP modulation. In the main part, different approaches adopted to actively manipulate SPP excitation, SPP focusing, SPP vortex, and SPP nondiffracting beams are explained. The underlying physics of dynamical SPP devices are summarized and comparisons of the different approaches are presented in Section 6. In passing, the basic properties of SPPs and the detection of SPP fields which can be found in [1; 2; 3] are not involved in this review. The non-equilibrium properties of localized surface plasmon generated around single metal nanoparticle, such as ultra-fast nonlinear dynamics [87; 88] are not included either.