Resonant Polarization Conversion and Extraordinary Transmission in Photonic Crystal Slabs Covered with Metal

Full text: In this work we show that optical transmission through sub wavelength holes in a thin opaque metal layer can be substantially increased due to coupling of light to the eigenmodes of a 2D photonic crystal (PhC) located directly under the metal layer. The model system consists of a top gold layer and an underlying high-index dielectric substrate. A hexagonal lattice of cylindrical air holes is then 'etched' through the gold layer and to some depth through the substrate. The parameters of the PhC are a=3 μm, r=0.3a, h=4 μm, where a is the lattice constant, r is the radius of the pores, and h is the depth of the pores. The system is excited by a plane-wave incident normal to the surface. We define y to be the propagation direction; the incident wave is polarized in z-direction (Ez ≠0, Hx ≠0). The results of our 3D finite-difference time-domain (FDTD) calculations for the near field transmittance and reflectance are shown: We observe at least three pronounced dips in reflectance (and corresponding peaks in transmittance) which will be referred to as reflection dips 1, 2 and 3 corresponding to the notations. It is important to note, that for the given parameters 67% of the surface is covered by the reflecting gold layer. However, the reflectance at λ=6.84 μm (dip 3) is only 1%. The transmittance is 60% which means that the transmission efficiency, defined as transmitted power per unit area of the free surface (not covered by gold) is 180%. One can expect that each dip in reflection should correspond to an enhanced field below the gold layer. If we examine the distribution of the electric field within the PhC region we find that dips 1 and 3 correspond to a high amplitude of the Ey component (which is absent in the incident wave) and the minimum of the dip 2 corresponds to a directly transmitted wave (high amplitude of the original Ez-component). For a quantitative characterization of the polarization conversion the intensities of the field components integrated over the volume of the PhC were calculated. We have obtained ∫|Ey|2/∫|Ez|2=13.4 for the dip 3 and ∫|Ey|2/∫|Ez|2=13.8 for the dip 1. Another peculiarity of dips 1 and 3 is that the field has its maximal density just below the gold layer while for dip 2 the Ez-field is maximal in the middle of the pores. The yz cross sections of the absolute values of the corresponding electric field components are shown. By comparing reflectance spectra and field distributions obtained by the FDTD method with results of plane-wave method calculations we show that dips 1 and 3 appear due to the coupling of incident light to dipole eigenmodes of the PhC at the Γ-point of the TM (E-field is parallel to the pores) band structure. This is confirmed by the dependence of the positions of the dips on the radii of the pores and on the refractive index of the PhC. A physical model explaining the coupling to TM modes will be presented. Transmittance experiments are performed in GHz region by using a 6.6 mm thick ceramic plate with holes which is covered by a thin metal foil. The experiments confirm the existence of the transmission peak 3 showing, however, a lower value of the peak maximum. (author)