Transmission Resonances of Metallic Compound Gratings with Subwavelength Slits

Transmission metallic gratings with subwavelength slits are known to produce enhanced transmitted intensity for certain resonant wavelengths. One of the mechanisms that produce these resonances is the excitation of waveguide modes inside the slits. We show that by adding slits to the period, the transmission maxima are widened and, simultaneously, this generates phase resonances that appear as sharp dips in the transmission response. These resonances are characterized by a significant enhancement of the interior field.

Figure 1

Scheme of the simple and compound gratings.

Figure 2

Zero-order transmittance as a function of the wavelength for a normally incident, $p$-polarized plane wave impinging on a metallic grating with $a/d=c/d=0.08$ and $h/d=1.14$. The refraction index of the metal is $\nu =0.15+i24.9$. The different curves correspond to different numbers of slits in the period: $J=1$ to 5.

Figure 3

Phase difference of the magnetic field at adjacent slits as a function of the wavelength for the same structure considered in Fig. 2, for $J=3$ and $J=5$.

Figure 4

Phase of the magnetic field for a normally incident $p$-polarized plane wave impinging on a metallic grating with $a/d=0.08$, $h/d=1.14$, and $J=3$. The refraction index of the metal is $\nu =0.15+i24.9$. (a) $\lambda /d=1.372$, (b) $\lambda /d=3.00496$. The inset in part (a) shows the region where the field is calculated.

Figure 5

Comparison between the $J=3$ curve in Fig. 2 (solid line) and the transmission response of a grating with a single slit of width $a/d=0.24$, which is 3 times that corresponding to the solid curve (dashed line). The dotted curve corresponds to $J=3$ and $a/d=0.15$.