Terahertz Broadband Polarization Conversion for Transmitted Waves Based on Graphene Plasmon Resonances

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.

Tunable Transmissive Terahertz Linear Polarizer for Arbitrary Linear Incidence Based on Low-Dimensional Metamaterials

“…The total transmission will be enhanced if constructive interaction takes place in the cavity formed between the 1st and the 2nd/3rd MML and the cavity formed between the 2nd/3rd MML and the metallic gratings. In this case, the proposed structure is similar to the one studied in our previous work [ 22 ], and polarization rotation around 90° is anticipated. However, there remains the problem of how well the 1st MML works as a linear polarizer, which is important for the efficiency of the cross-polarization conversion.…”

“…The refractive indices of the two dielectric layers and the substrate are all n 1 for simplicity. The boundary matrix T top at the surface of the proposed structure can be calculated as: where is the transfer matrix describing the 2nd and 3rd MMLs by considering them as a whole [ 22 ]. σ u (ω) and σ v (ω) are the effective conductivities in the x - and y -coordinates, respectively, ε 0 is the dielectric constant in vacuum, and c is the speed of light in vacuum.…”

“…M upper and M lower are the transfer matrices of the upper and lower dielectric separations, M uv → xy and M xy → uv are the transfer matrices describing the coordinate transformation, M top is the transfer matrix describing plasmon resonances of the 1st MML at the surface of the proposed structure, and T bottom describes the electric field at the upper surface of the metallic gratings. The details of these transfer matrices and boundary matrix have been given in our previous work [ 22 ]. Here, the spacing between the 2nd and 3rd MMLs (~100 nm) is ignored because it is far less than the relevant wavelength (~ 300 μm).…”

“…In our previous work [ 22 ], it was pointed out that plasmon resonances or Drude absorption should not be excited simultaneously with the same frequency and strength in the u - and v -coordinates. Therefore, the 2nd and 3rd MMLs cannot be in the “ON” state (the MML is electrically biased and has plenty of charge carriers) simultaneously.…”

“…However, since the phase difference and the amplitude of two orthogonal components change as the frequency of the incidence changes, the bandwidths of such transmissive polarization rotators are usually limited [ 7 , 8 , 9 , 10 , 11 , 12 , 13 , 14 ]. Metallic gratings can help to enhance the bandwidth and the transmissivity [ 16 , 17 , 18 , 19 , 20 , 21 , 22 ]. Electromagnetic waves transmitted through or reflected by the metamaterials with unwanted polarization are reflected by metallic gratings and interact with the metamaterials again, so the transmission window grows taller and wider.…”