Large angle stable metamaterial for visible and infrared band absorption and thermal emitter inspired by fractal

Inspired by fractal, two units for infrared-band absorbing metamaterial are proposed in this paper. Unit I uses an additive method which is adding branches based on the cross structure, while Unit II adopts a subtractive method which is etching gaps in the material block. By comparing each construction step of the fractal, it can be confirmed that the fractal can improve the absorption effect without changing the period and thickness of the metamaterial. Then, the two units are put together, and the plane on which Unit 2 is placed is tilted to form an array. This operation is to explore better absorption effects and angle stability. Ultimately, the array achieves good absorption: under normal incident and the polarization angle equal to 45°, the array has an average absorptivity of 94.6 % in the range of 100–2183.3 nm. Under a large incident angle (45°), the TM and TE modes behave differently, but both maintain a fairly high absorptivity. The average absorptivity of TM waves is 96 %, and the average absorptivity of TE waves is 90.9 %, with the lowest point being 84 %. The high absorptivity in the infrared band and the angular stability allow the design to be used as a heat emitter or solar absorber. At 2000 K, this metamaterial can achieve 86.66 % thermal emitting efficiency. For sunlight, the spectral absorption efficiency reaches 94.67 %. Afterward, this work also uses the equivalent circuit to explain the mechanism of absorption, and uses parameter retrieval to obtain the effective impedance and permittivity of the metamaterial. The innovations of this work are listed. First, using the fractal concept to improve the absorption effect without changing the thickness so that the device has an advantage in thickness. Second, by placing some units on the slope, the absorption effect is improved and angular stability is obtained. The advantage of this method of combining two units is that the inclined placement makes the advantages of different units complementary and reduces the requirements for the design of a single unit. This work has potential applications in the fields of thermal radiation and solar energy collection.