Optical Brewster Metasurfaces Exhibiting Ultrabroadband Reflectionless Absorption and Extreme Angular Asymmetry

Impedance mismatch between free space and absorptive materials is a fundamental issue plaguing the pursuit of high-efficiency light absorption. In this work, we design and numerically demonstrate a type of nonresonant impedance-matched optical metasurface exhibiting ultrabroadband reflectionless absorption based on the anomalous Brewster effect, which is denoted as an optical Brewster metasurface here. Interestingly, the Brewster metasurface exhibits a type of extreme angular asymmetry: a transition between perfect transparency and perfect absorption appears when the sign of the incident angle is changed. Such a remarkable phenomenon originates from the coexistence of traditional and anomalous Brewster effects. Guidelines for material selection based on an effective-medium description and strategies such as the integration of a metal back-reflector or a folded metasurface are proposed to improve the absorption performance. Finally, a gradient optical Brewster metasurface exhibiting ultrabroadband and near-omnidirectional reflectionless absorption is demonstrated. Such high-efficiency asymmetric optical metasurfaces may find applications in optoelectrical and thermal devices like photodetectors, thermal emitters, and photovoltaics.

Figure 1

(a) Schematic layout of an ultrabroadband reflectionless optical Brewster metasurface exhibiting extreme angular asymmetry. Metasurface consists of a periodic array of metal films embedded in a dielectric host. (b) Upper panel, ultrabroadband perfect transparency due to the BE under an incident angle of ${\theta }_i={\theta }_B$. Lower panel, ultrabroadband perfect absorption due to the ABE under ${\theta }_i=-{\theta }_B$.

Figure 2

(a) Simulated normalized magnetic field, $H_x/H_0$, distributions under illumination of TM-polarized Gaussian beams with ${\theta }_i={\theta }_B={55.6}^{\circ }$ (upper) and ${\theta }_i=-{\theta }_B=-{55.6}^{\circ }$ (lower) at ${\lambda }_0=800\mathrm{nm}$. Brewster metasurface consists of tilted $\mathrm{Cr}$ films in a ${\mathrm{Si}\mathrm{O}}_2$ host. Relevant parameters are $a=100\mathrm{nm}$, $t=10\mathrm{nm}$, $d=400\mathrm{nm}$, and $\alpha =34.4^{\circ }$. (b) Reflectance, (c) transmittance, and (d) absorptance as functions of the incident angle and working wavelength.

Figure 3

Reflectance under ${\theta }_i=\pm {\theta }_B=\pm {55.6}^{\circ }$, transmittance under ${\theta }_i={\theta }_B={55.6}^{\circ }$, and absorptance under ${\theta }_i=-{\theta }_B=-{55.6}^{\circ }$ when (a) separation distance, a, is changed with fixed $t=10\mathrm{nm}$, and (b) thickness of $\mathrm{Cr}$ films, t, is changed with fixed $a=100\mathrm{nm}$. (c) Schematic graph of an optical Brewster metasurface with random a and t in different transversal positions. a varies randomly over the range from $0.9a_0$ to $1.1a_0$ with $a_0=100\mathrm{nm}$, and t varies randomly over the range from $0.9t_0$ to $1.1t_0$ with $t_0=10\mathrm{nm}$. (d) Simulated $H_x/H_0$ distributions when a TM-polarized Gaussian beam is incident from air onto the random metasurface under ${\theta }_i={\theta }_B={55.6}^{\circ }$ (upper) and ${\theta }_i=-{\theta }_B=-{55.6}^{\circ }$ (lower). Working wavelength in (a)–(d) is 800 nm.

Figure 4

(a) Absorption spectra under ${\theta }_i=-{\theta }_B=-{55.6}^{\circ }$. Metasurface is the same as that in Fig. 2 except for the metal-film materials. (b) Absorptance under ${\theta }_i=-{\theta }_B=-{55.6}^{\circ }$ at ${\lambda }_0=800\mathrm{nm}$ when the thickness, d, is increased. (c) Normalized decay rate of light in the metasurface as a function of ${\epsilon }_m^{\mathrm{\prime }\mathrm{\prime }}$ (black solid line). Vertical dashed lines denote values of ${\epsilon }_m^{\mathrm{\prime }\mathrm{\prime }}$ of $\mathrm{V}$, $\mathrm{W}$, $\mathrm{Ti}$, $\mathrm{Mo},$ and $\mathrm{Cr}$. (d) Absorption spectra under ${\theta }_i=-{\theta }_B$. Metasurface is the same as that in Fig. 2, except for the dielectric host materials. ${\theta }_B$ changes accordingly for different dielectric hosts. (e) Absorptance under ${\theta }_i=-{\theta }_B$ at ${\lambda }_0=800\mathrm{nm}$ with increasing thickness d. (f) Normalized decay rate as a function of ${\epsilon }_d$ (black solid line). Vertical dashed lines denote values of ${\epsilon }_d$ of ${\mathrm{Si}\mathrm{O}}_2$, ${\mathrm{Ta}}_2{\mathrm{O}}_5,$ and ${\mathrm{Ti}\mathrm{O}}_2$.

Figure 5

Illustration of an optical Brewster metasurface with (a) metal back-reflector and (d) folded metasurface. (b),(e) Simulated $H_x/H_0$ distributions when a TM-polarized Gaussian beam is incident under ${\theta }_i={\theta }_B={55.6}^{\circ }$ (upper) and ${\theta }_i=-{\theta }_B=-{55.6}^{\circ }$ (lower) at ${\lambda }_0=800\mathrm{nm}$. Relevant parameters are $a=100\mathrm{nm}$, $t=10\mathrm{nm}$, $d=800\mathrm{nm}$, and $\alpha =34.4^{\circ }$. In (b), the back-reflector is made of $\mathrm{Cr}$. In (e), the bottom half is a folded metasurface. (c),(f) Absorptance as functions of the incident angle and working wavelength with regard to the metasurfaces in (b),(e), respectively.

Figure 6

(a) Schematic graphs of gradient metasurfaces with varied tilt angles of metal films satisfying $\alpha ={\theta }_i$ (upper) or $\alpha =-{\theta }_i$ (lower) everywhere in a dielectric host. Simulated magnetic field distributions when an electric dipole source is placed between two metasurfaces in the ${\mathrm{Si}\mathrm{O}}_2$ host at (b) ${\lambda }_0=800\mathrm{nm}$ and (c) ${\lambda }_0=400\mathrm{nm}$. Upper (lower) metasurface satisfies the condition $\alpha ={\theta }_i(\alpha =-{\theta }_i)$ everywhere.