Perfect interferenceless absorption at infrared frequencies by a van der Waals crystal

Traditionally, efforts to achieve perfect absorption have required the use of complicated metamaterial-based structures as well as relying on destructive interference to eliminate back reflections. Here, we have demonstrated both theoretically and experimentally that such perfect absorption can be achieved using a naturally occurring material, hexagonal boron nitride (hBN) due to its high optical anisotropy without the requirement of interference effects to absorb the incident field. This effect was observed for $p$-polarized light within the mid-infrared spectral range, and we provide the full theory describing the origin of the perfect absorption as well as the methodology for achieving this effect with other materials. Furthermore, while this is reported for the uniaxial crystal hBN, this is equally applicable to biaxial crystals and more complicated crystal structures. Interferenceless absorption is of fundamental interest to the field of optics; moreover, such materials may provide additional layers of flexibility in the design of frequency selective surfaces, absorbing coatings, and sensing devices operating in the infrared.

Figure 1

Concept of an (a) interference-based and (b) interferenceless perfect electromagnetic absorber.

Figure 2

Frequency dispersion of the real parts of the in-plane (${\varepsilon }_t$) and out-of-plane (${\varepsilon }_z$) hBN permittivity tensor components. Inset shows the schematic structure of the crystal.

Figure 3

Reflection by a semi-infinite hBN crystal. (a) and (b) Calculated reflectance spectra for $p$ and $s$ polarizations, respectively, for a series of different incidence angles. (c) The real and imaginary parts of squared cosine of the prefect absorption angle ${\theta }_0$. At two points denoted by A and B on the graph, ${cos}^2{\theta }_0$ becomes real and positive, corresponding to a real-valued incidence angle.

Figure 4

(a) and (b) The calculated reflectivity spectra of an hBN crystal for $p$ and $s$ polarization of incident light, respectively. (c) and (d) The corresponding measured spectra for 200-$\mu \mathrm{m}$-thick hBN slab on a metal substrate. The points A and B mark the position of perfect absorption points corresponding to those in Fig. 3.

Figure 5

Experimental reflectivity spectra for a series of incidence angles for (a) $p$ and (b) $s$ polarizations. The points A and B mark the position of perfect absorption points.