Polarization and Wide Incident Angle Insensitive Metal Free Superwideband Absorber For Terahertz and Infrared Spectrum

A circular slot loaded octagonal metal free graphite based absorber for superwideband applications at terahertz and infrared spectrum is proposed. The absorber comprises a circular slot loaded octagonal graphite slab at the top layer, silicon dioxide as substrate and square graphite slab as the bottom layer. This absorber is operating over a frequency spectrum of 6 THz to more than 80 THz (ratio bandwidth of > 12:1 and fractional bandwidth of > 172.093%) for absorption more than 90% with unity peak absorption. The absorber dimensions are 5 µm × 5 µm × 7.04 µm. The thickness of the substrate is only ∼ λo/26 and the periodicity of absorber is ∼ λo/10 at 6 THz. The performance of the absorber is found to be insensitive to changes in polarization angle (Φ). Furthermore, the performance of the absorber is observed to be unaffected by incident angle (θ) variations from 0 o to 60 o . The metal-free geometry along with insensitiveness to Φ and 0 o ≤ θ ≤ 60 o makes the proposed absorber suitable for compact terahertz/infrared micro/nanoscale systems.


I. Introduction
Terahertz (THz) spectrum has received much interest from academia and industry in the last two decades for its vast applications in communication, imaging, medical, radar, sensing, spectroscopy, and other elds. The eld of absorbers has been determined to be the most practical use of the THz spectrum. Researchers have recently reported several single and multilayered geometry-based terahertz absorbers having narrowband, multi-band, wideband, and frequencydependent recon gurable performance . Different techniques leading to the excitation of multiple resonances are used to design multi-band absorbers. The merging of these multiple resonances leads to wideband performance. Multilayered or multiple stacked systems provide wide bandwidth; however, they are challenging to implement. The easy to implement single layer structures provide limited operating frequency range. Therefore, a major challenge is to design a broadband absorber having single layer of substrate without any stacking of metallic/graphene layers.
Another challenge faced during absorber designing is the limited performance at higher frequencies due to the metals' limited electrical properties and high-temperature sensitivity, which is mainly responsible for incident power absorption in metal/dielectric multi-layered absorbers [27,28]. Few researchers have reported absorbers based on vanadium oxide, a temperature-sensitive & phase change material [21,29]. They provide desired wideband performance even at high temperatures, but these high temperatures affect the functioning of other nano-scale components. All the abovediscussed broadband absorbers have complex fabrication techniques, thick geometries, and temperature-dependent performance. Thus, there is a need to design broadband absorbers whose fabrication is easy, the substrate thickness is negligible, and the performance is temperature independent. According to the available literature, metal-free absorbers like graphite/dielectric-based absorbers and semiconductor grating-based absorbers have provided temperatureindependent performance [26, 30,31]. The major reason for the usage of graphite in exible electronics components like THz antennas and high-frequency absorbers is its better temperature stability [21]. These broadband metal-free absorbers can cover only a small portion of the terahertz spectrum. The future requirements require the coverage of both terahertz and infrared spectrum. This paper presents the design and analysis of a metal-free graphite/dielectric-based super wideband absorber. The combination of circular slot-loaded octagonal graphite slab, silicon dioxide dielectric material, and square graphite slab resulted in an operating frequency range from 6 THz to more than 80 THz with the absorption of more than 90% for the normal incidence of electromagnetic waves. In two frequency bands of 7.85-13.05 THz and 22.47 THz to more than 80 THz, the absorption is found to be more than 95%. In the frequency spectrum of 25.35-28.71 THz, 44.68-74.65 THz, and 79.52 THz to more than 80 THz, the absorption is more than 99%. The proposed absorber's overall volume is signi cantly lesser than several dielectric, metamaterial, and grating-based structures. The advantageous features of planar metal-free geometry with miniaturized dimensions, low volume, insensitivity to polarization angle, and wide incident angle range make this absorber compatible with the future nanoscale super wideband terahertz and infrared systems.

Ii. Absorber Design
The proposed antenna geometry, shown in Fig. 1, is a three layered structure. In the proposed absorber, a 1.54 µm thick silicon dioxide substrate is sandwiched between two graphite slabs. The top graphite slab is octagonal in shape. It is loaded with a central circular slot. During the simulation of the proposed absorber structure by using nite integration technique (FIT)-based CST Microwave Studio (CST MWS), periodic boundaries are xed along x-and y-axis. Along the zaxis i.e. transmission direction, open boundary conditions are xed. This absorber is designed in three stages i.e. initially only graphite ground is analyzed. In second stage, the combination of graphite ground and substrate is analyzed. In the last stage, the circular slot loaded octagonal graphite, substrate material and graphite ground plane are analyzed together.

Iii. Results And Discussion
The magnitudes of re ectance (S 11 ) and transmittance (S 21 ) for all absorber designing stages are shown in Fig. 2. It is observed that for each stage the magnitude of transmittance is almost zero i.e. S 21 ≅ 0. The re ectance for rst stage is decreasing linearly from 1 to 0.7 in the frequency range of 1 to 80 THz without any resonance. For second stage, two resonance at the frequencies of 22.79 and 69.85 THz with |S 11 | of 0.55 and 0.23 are observed. In the last stage, the re ectance is observed to be sharply decreasing from 1 to 0.1 with three resonances. For the frequency range of 6.33 THz to more than 80 THz, the re ectance magnitude is less than 0.  Fig. 2 (c). For rst stage, a maximum absorption of 50% is achieved. In second stage, two peaks having absorption of 70% and 95% are observed at resonance frequencies. In the last stage, absorption of more than 90% is achieved in the frequency spectrum of 6THz to more than 80 THz. Figure 3 illustrates a good agreement between the absorption in both modes i.e. transverse electric (TE) and magnetic (TM) for the proposed absorber.
To analyze the polarization sensitiveness of this absorber, the polarization angle (Φ) is varied from 0 o to 90 o . Due to the geometrical symmetry, this absorber is insensitive to the variations in polarization angle as shown in Fig. 4. Table 1 illustrates that the proposed absorber has widest bandwidth (6 THz to more than 80 THz) along with minimum volume of 176 µm 3 among the compared structures. The proposed absorber is covering terahertz frequency spectrum from 6-10 THz and infrared region from 10 THz to more than 80 THz whereas the operating band of other absorbers is covering only a narrow portion of the terahertz spectrum. In addition to this, this absorber is polarization angle and wide incident angle insensitive.   Absorption of proposed absorber with variations in incidence angle in TE mode Figure 6 Absorption of proposed absorber with variations in incidence angle in TM mode