Abstract
We applied the harmonic oscillator model combined with the transfer matrix method to study the polarization conversion for transmitted waves in metallic grating/plasmon-excitation layer/metallic grating structure in the terahertz (THz) region. By comparing the calculated spectra and the simulated (by the finite-difference-time-domain method) ones, we found that they correspond well with each other. Both methods show that the Drude background absorption and the excited plasmon resonances are responsible for polarization conversion. The transmission is close to 0 when the distance between the top/bottom metallic gratings and gated graphene is an integer multiple of half the wavelength of the incident wave (in the dielectrics), at which points the plasmon resonances are greatly suppressed by the destructive interference between the backward/forward electromagnetic waves and that reflected by the top/bottom metallic gratings. Away from these points, the transmission can be higher than 80%. The electron density and the excitation efficiency of the plasmon-excitation layer were found to be important for the bandwidth of the polarization conversion window, while the scattering rate was found to influence mainly the polarization conversion rate. Multi-broadband polarization conversion is realized by exciting plasmon modes between the 0 transmission points in the THz region.
Highlights
Polarization rotators are basic elements for THz applications because polarization is one of the fundamental properties that conveys valuable information of electromagnetic waves
The model shows that the polarization conversion is influenced by the modulation in amplitude and phase caused by both the plasmon resonances and Drude background absorption
We find that both the duty cycle of the plasmon-excitation region and the Fermi level of graphene are crucial for the relative bandwidth, while the carrier scattering rate will influence the polarization conversion rate (PCR)
Summary
Polarization rotators are basic elements for THz applications because polarization is one of the fundamental properties that conveys valuable information of electromagnetic waves. The model shows that the polarization conversion is influenced by the modulation in amplitude and phase caused by both the plasmon resonances and Drude background absorption. We find that both the duty cycle of the plasmon-excitation region and the Fermi level of graphene are crucial for the relative bandwidth, while the carrier scattering rate (or the relaxation time) will influence the PCR. The distance between the plasmon-excitation layer and the top/bottom metallic gratings will influence the PCR Based on these analyses, multi-broadband polarization conversion is realized by taking gated graphene as the plasmon-excitation layer. This study is helpful for the design of polarization rotators with similar sandwich structures, and it is beneficial for THz manipulation applications
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