An electron bunch passing through a periodic metal grating can emit Smith-Purcell radiation (SPR). Recently, it has been found that SPR can be locked and enhanced at some emission wavelength and angle by excitation of surface plasmon (SP) on the metal substrate. In this work, the generation of a convergent light beam via using the SP-locked SPR is proposed and investigated by computer simulations. The proposed structure is composed of an insulator-metal-insulator (IMI) substrate with chirped gratings on the substrate. The chirped gratings are designed such that a convergent beam containing a single wavelength is formed directly above the gratings when an electron bunch passes beneath the substrate. The wavelength of the convergent beam changes with the refractive index of dielectric layer of the IMI structure, which is determined by the frequency of SP on the IMI substrate excited by the electron bunch. Moreover, reversing the direction of electron bunch will make the emitted light from the proposed structure to switch from a convergent beam to a divergent beam. Finally, the formation of a convergent beam containing red, green and blue lights just above the chirped gratings is also demonstrated. This work offers potential applications in the fields of optical imaging, optical beam steering, holography, microdisplay, cryptography and light source.
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