One-way absorption of terahertz waves in rod-type and multilayer structures containing polar dielectrics

One-way absorption can be obtained at terahertz frequencies in low-profile rod-type and multilayer dielectric structures with broken spatial inversion symmetry, which contain either a rod layer or an ultrathin homogeneous layer made of a polar dielectric. Perfect absorption for one of the two opposite incidence directions and perfect reflection for the other one are observed at the edge of the polaritonic gap in a wide range of the incident angle variation, when the thickness of the entire structure is of the order of the incident wavelength. Moreover, this regime appears in a wide frequency range, in which the forward-to-backward absorption contrast is strong. The exploited mechanism is connected with the parameter adjustment that enables the location of the polaritonic gap of the polar dielectric, of which the lossy part of the structure is made, inside the stop band arising due to the periodicity of the lossless part of the structure that is made of a nondispersive dielectric. It also exploits absorption enhancement in the lossy part by backing it with the highly reflecting lossless part, which has alternating stop and pass bands.

Figure 1

(a) Schematic illustrating perfect one-way absorption in structures with broken structural symmetry; examples of the structures composed of both polar (lossy) dielectrics and nondispersive (lossless) dielectrics in one configuration with (b) the same and (c) different shape of the components that are made of dielectrics of two types; P and ND denote the parts that are fully made of a polar dielectric and a nondispersive lossless dielectric, respectively; (d) permittivity of GaAs: $\text{Re}{\epsilon }_P$, solid line, and $\text{Im}{\epsilon }_P$, dashed line.

Figure 2

(a) $T$ and $R$ for rod arrays made of nondispersive dielectric at $M=2$ (solid line) and $M=3$ (dashed line), with ${\epsilon }_{\mathrm{ND}}={\epsilon }_{\infty }=10.9$; (b) $T$, $R$, and $A$ for rod arrays made of GaAs at $N=1$ (solid line) and $N=2$ (dashed line); $d/a=0.4$, $\theta =0$; two insets (in each plot) schematically show geometry of a single period in the $x$ direction, for both structures.

Figure 3

$T^{\to }$, $R^{\to }$, and $A^{\to }$ (a), (c) and $T^{\leftarrow }$, $R^{\leftarrow }$, and $A^{\leftarrow }$ (b), (d), for two composite structures with (a), (b) $M=1$ and $N=1$, and (c), (d) $M=3$ and $N=1$; $d/a=0.4$, $\theta =0$; solid line: $T^{\to }$ and $T^{\leftarrow }$; dashed line: $R^{\to }$ and $R^{\leftarrow }$; dash-dotted line: $A^{\to }$ and $A^{\leftarrow }$.

Figure 4

(a) $A^{\to }$ and (b) $A^{\leftarrow }$ vs frequency for four composite structures with $N=1$: solid line - $M=1$, dashed line - $M=2$, dotted line - $M=3$, dash-dotted line - $M=4$; $d/a=0.4$, $\theta =0$; the lines for $M=2$, $M=3$, and $M=4$ coincide at the maximum in plot (a); four insets in plot (a) show geometry within a single period in the $x$ direction, for the structures with $M=1$, $M=2$, $M=3$, and $M=4$, from the left to the right.

Figure 5

(a) $A^{\to }$ and (b) $C_A$ vs frequency for the composite structure with $M=3$, $N=1$, $d/a=0.4$. Left (a) and lower (b) solid line: $\theta =0$; dashed line: $\theta ={10}^{\circ }$; dotted line: $\theta ={20}^{\circ }$; dash-dotted line: $\theta ={30}^{\circ }$; right (a) and upper (b) solid line: $\theta ={45}^{\circ }$.

Figure 6

$T^{\to }$, $R^{\to }$, and $A^{\to }$ (a), (c) and $T^{\leftarrow }$, $R^{\leftarrow }$, and $A^{\leftarrow }$ (b), (d), for two composite structures with (a), (b) $M=1$ and homogeneous GaAs layer, and (c), (d) $M=3$ and homogeneous GaAs layer; $d/a=0.4$, $\theta =0$; solid line: $T^{\to }$ and $T^{\leftarrow }$; dashed line: $R^{\to }$ and $R^{\leftarrow }$; dash-dotted line: $A^{\to }$ and $A^{\leftarrow }$.

Figure 7

Same as Fig. 4 but for the P part that represents a homogeneous GaAs layer; four insets in plot (a) schematically show geometry of a single period for the structures with $M=1$, $M=2$, $M=3$, and $M=4$, from the left to the right.

Figure 8

$T^{\to }$, $R^{\to }$, and $A^{\to }$ (a) and $T^{\leftarrow }$, $R^{\leftarrow }$, and $A^{\leftarrow }$ (b) for the multilayer composite structure with a homogeneous GaAs layer placed at the upper interface, $\theta =0$; solid line: $T^{\to }$ and $T^{\leftarrow }$; dashed line: $R^{\to }$ and $R^{\leftarrow }$; dash-dotted line: $A^{\to }$ and $A^{\leftarrow }$.

Figure 9

(a) $A^{\to }$ and (b) $C_A$ vs frequency for the composite multilayer structure in Fig.  8. Left (a) and lower (b) solid line: $\theta =0$; dashed line: $\theta ={10}^{\circ }$; dotted line: $\theta ={20}^{\circ }$; dash-dotted line: $\theta ={30}^{\circ }$; right (a) and second upper (b) solid line: $\theta ={40}^{\circ }$; uppermost solid line (b): $\theta ={50}^{\circ }$.