Selective dual-band metamaterial perfect absorber for infrared stealth technology

We propose a dual-band metamaterial perfect absorber with a metal–insulator–metal structure (MIM) for use in infrared (IR) stealth technology. We designed the MIM structure to have surface plasmon polariton (SPP) and magnetic polariton (MP) resonance peaks at 1.54 μm and 6.2 μm, respectively. One peak suppresses the scattering signals used by laser-guided missiles, and the other matches the atmospheric absorption band, thereby enabling the suppression of long-wavelength IR (LWIR) and mid-wavelength IR (MWIR) signals from objects as they propagate through the air. We analysed the spectral properties of the resonance peaks by comparing the wavelength of the MP peak calculated using the finite-difference time-domain method with that obtained by utilizing an inductor–capacitor circuit model. We evaluated the dependence of the performance of the dual-band metamaterial perfect absorber on the incident angle of light at the surface. The proposed absorber was able to reduce the scattering of 1.54 μm IR laser light by more than 90% and suppress the MWIR and LWIR signatures by more than 92%, as well as maintain MWIR and LWIR signal reduction rates greater than 90% across a wide temperature range from room temperature to 500 °C.

where the parameters are defined in Tables S1 and S2. Figure S1. Equivalent ICC of a disk in an MIM structure 1 .

Geometrical parameter or material property
Description a0 Period of a unit cell in the circular disk-ring metamaterial Mutual capacitance, m1 Gap capacitance, Kinetic inductance, Similar to the interaction between the incident light and a circular disk, the interaction between the incident light and a circular ring in the MIM structure can be represented using the ICC illustrated in Fig. S2(a). The periodic unit cells are related to one another in parallel via gap capacitances, and within each unit cell, there is a series circuit, which includes mutual inductances, kinetic inductances, and mutual capacitances. to Lenz's law, a magnetic field is confined within the dielectric space between the current flows in two metal layers 2 . Therefore, mutual inductance is created between the lower and upper metal layers.
Due to the vibrations of the free electrons, caused by the currents on the metal surfaces, kinetic inductances are generated on the lower and upper metal plates. As they depend upon the shapes of the surfaces in which the currents flow, the kinetic and mutual inductances are related in series, as shown in Fig. S2(a). Unlike the mutual inductance in the lower plate, which depends only on whether the symmetrical currents flow in a direction opposite to the current in the circular ring, the kinetic inductance in the lower layer is induced throughout the area in which the current flows. Therefore, the characteristics of the kinetic inductance on the lower plate are identical to those in the circular disk array, while those of the kinetic inductance on the upper metal plate are different, due to the air space in the circular ring geometry that is related to the inner radius. To perform resonance wavelength predictions, we derived the total impedance of the ICC for the circular ring shown in Fig. S2(a), which can be expressed as 3 where the parameters are defined in Table S3. When the impedance is zero, like that of free space, the incident light is almost perfectly absorbed by the circular ring structure. We set the outer radius of the circular rings to 600 nm and the unit cell period to 1.34 μm. Finally, we determined that the inner radius is 500 nm when the magnetic polariton resonance wavelength is 6.2 μm.

ICC parameter
Description cring Non-uniform charge distribution coefficient, which was determined to be 0.08 in this study by performing an FDTD simulation of the circular ring structure lring Length of the surface current on a circular ring,  Table S3. Parameters in the ICC for the circular ring structure.

Usability of the perfect absorber to be fabricated using single lithography step
For the fabrication process using single lithography step, we additionally designed a dual-band metamaterial absorber with the same thicknesses for the metal disk and ring. Considering the independent resonance characteristics of the circular ring and disk, we fixed the design parameters of the ring for the MP2 peak and fixed the dimensions of the disk such that the thicknesses t1 and t2 were the same (100 nm). The spectral properties of the dual-band perfect absorber designed for the single lithography step (solid line) is displayed along with the atmospheric absorption spectrum (dotted line) in Fig. S3.
On comparing the wide spectral property in Fig. S3 with that in Fig. 4, we found no significant difference in the spectral properties of the two perfect absorbers designed for double lithography step and single lithography step. When the various spectral properties of the perfect absorber, designed for single lithography step, was investigated closely, it was found that the absorptivity at 1.54 μm is much sensitive to the variations in incident angle. The dependences of absorptivity and resonance wavelength on the incident angle are plotted in Figs. S4 (a) and (b) for the peaks of 1.54 μm and 6.2 μm, respectively. It is noticeable that the absorptivity of the 1.54 μm resonance peak is very sensitive and sharply decays as a function of the incident angle. Compared with that for the perfect absorber designed for double lithography step, the decay rate in Fig. S4 (a) is 1.8 times larger than that in Fig. 5 (a). For the laser guided missile, in which the receiver and detectors are combined, the performance of IR stealth technology depending on the sensitivity of incident angle is negligible 23 . However, if the detector is separated from the receiver, the absorptivity at a wide incident angle may be considered as an important performance 23 .
Fortunately, however, except for the incident angle dependence in the 1.54 μm band, both the designs exhibit almost the same performances.