Ultraviolet surface plasmon polaritons (UV SPPs) have important and extensive potential applications in SPPs nanolithography, SERS, LEDs, solar cells, photocatalysis, and so on. Recently, more and more attention has been paid to extend plasmonic properties from the visible and near-IR band into the ultraviolet range. However, the low efficiency to generate UV plasma is an enormous challenge and unidirectional excitation of SPPs is considered as a reliable way to solve the problem. Here we theoretically propose a structure with an asymmetric nano-slit cavity that serves as an efficient unidirectional coupler for deep UV plasmons, especially for the wavelength of 248nm, which is a typical light source used in the SPPs nanolithography. The theoretical results based on FDTD algorithm show that by optimally designing the asymmetric nano-slit cavity the extinction ratio can reach 16.8 dB and can be further increased to 28.0 dB when assisted by a nano grating. Additionally the direction of the proposed unidirectional excitation is controllable by adjusting the length of nano-slit cavity.
Surface plasmon (SP) properties of more than dozen of metal materials have been comparative analyzed in ultraviolet (UV) band (436nm, 365nm) which are the typical light source used in SP lithography. We excited the SP wave by Otto configuration. The electromagnetic field in the structure were calculated by the full-vector FDTD algorithm. The SP resonance angle and the quality factor of the resonance peak were extracted. The results showed that the calculated SP resonance angle is approximately the same as the angle expected by the SP theory , and the error is no more than 5%. This gave an evidence that the SP theory is still applicable in the UV band although the theoretical model of permittivity, such as Drude, Lorentz and Debye, is ineffective at this time. We choose some metals with smaller errors to research the quality factor of SP resonance peaks. The results showed that even if the excitation angle was similar, the Q-factor was different which mean that the propagation length of the same SP wave was different. It can provide references for material selection when designing SP devices in UV band.
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