Wide band ultrathin polarization insensitive electric field driven metamaterial absorber

Optics Communications 443 (2019) 186–196

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Wide band ultrathin polarization insensitive electric field driven metamaterial absorber Amarveer Singh Dhillon a ,∗, Divesh Mittal b a b

Faculty of Graduate Studies and Research, Electronics System Engineering, University of Regina, Regina, SK, Canada Department of Civil Engineering, Punjabi University, Patiala, Punjab, India

ARTICLE

INFO

Keywords: Electric dipole resonance Fabry–Perot Magnetic dipole resonance Polarization insensitive Ultra-thin Wideband

ABSTRACT We demonstrate a novel wideband ultrathin polarization insensitive electric field driven metamaterial based dually stacked petal resonator (DSPR) perfect absorber for cloaking, THz imaging and sensing, solar cell, stealth technology, and remote sensing applications. The designed DSPR absorber is composed of a polyimide substrate and gold. The DSPR perfect absorber exhibits 98% absorption fractional bandwidth of 12.8% over a frequency range of 0.591 THz to 0.672 THz. The compact unit cell has 0.29𝜆𝑜 ∗ 0.29𝜆𝑜 ∗ 0.06𝜆𝑜 size, where 𝜆𝑜 is the lower absorption peak wavelength. Owing to the symmetric geometry, the DSPR absorption response is polarization insensitive over an extensive range of 90◦ for both Transverse Electric (TE) and Transverse Magnetic (TM) polarizations. An excellent absorption behavior is perceived over a wide oblique angle of incidence of 75◦ for both TE and TM polarizations. For an oblique angle of incidence, the proposed structure has interesting features to TE and TM polarizations. A thorough parametric optimization is investigated which elucidates the DSPR response as a function of structural parameters. The physics behind the absorption mechanism is explored by examining the surface current distribution, magnetic field distribution, electric field distribution, power loss density and normalized impedance. Field distributions reveal the existence of Fabry–Perot reflections and electric dipole resonances.

1. Introduction Metamaterials [1] are artificial subwavelength [2] composite electromagnetic materials. Metamaterials possess exotic and abnormal properties that are not conceived naturally, for example, negative refractive index [3], negative permittivity [4], negative permeability [4], antiparallel group and phase velocity [4], etc. Owing to the metamaterial characteristics, immense applications and phenomenons have been discovered such as cloaking [4], super lens [5], stealth technology [6], Electromagnetic Induced Transparency (EIT) [7] etc. Metamaterial based devices leverage prodigious capabilities to engineer the performance by precisely tailoring the geometrical parameters and material properties [8]. An ordinary metamaterial construction is a dielectric spacer sandwiched between periodically patterned metallic structures and a metallic sheet [9]. Metamaterials contributed substantially in development and miniaturization [10] of diverse devices such as absorbers [11–13], filters [14], solar cells [15], chiral materials [15] etc. Metamaterials and micro–nano fabrication technologies have enabled the transition of absorbers from microwave [16,17] domain to THz [18–21] and further to high frequency regimes [22]. Over past few years, variety of single band [23–25], multi-band and wide band absorbers [26–32] have been proposed. Stacking multiple layers [33,

34] and enclosing or cascading scaled multiple resonators [35,36] in a unit cell are 2 potential ordinary techniques to achieve multi-band and wideband responses, absorption bandwidth enhancement [36], unity absorption response [37,38] etc. Recently, researchers have introduced the tunability feature by employing materials like STO [39], Graphene [40], etc. The challenge associated with these techniques is the unit cell size, which yields complications in the fabrication process and results in discrepancies between the simulation and experimental responses. Also, the fundamental challenge is to achieve wideband characteristic sustaining the ultrathin configuration, polarization independence [40,41] and wide incident angle features [42]. The development of wearable [42] absorbers for conformal applications is also an emerging research regime. The following sections of the paper are organized as Section 2 illustrates the DSPR unit cell construction and simulation environment. Section 3 comprehensively discusses the performance analysis of the proposed ultrathin DSPR metamaterial absorber. The DSPR response is investigated in terms of electric field distributions, magnetic field distributions, current distributions, power loss density and impedance matching which rigorously justify the physics of the absorption mechanism. The parametric dependency of DSPR absorption spectra is also

∗ Corresponding author. E-mail addresses: [email protected], [email protected] (A.S. Dhillon).

https://doi.org/10.1016/j.optcom.2019.03.032 Received 30 December 2018; Received in revised form 9 March 2019; Accepted 11 March 2019 Available online 18 March 2019 0030-4018/© 2019 Elsevier B.V. All rights reserved.

A.S. Dhillon and D. Mittal

Optics Communications 443 (2019) 186–196

Fig. 1. Prospective view of the (A) proposed DSPR metamaterial absorber, (B) Absorber I, and (C) Absorber II and (D) Petal orientation on 2D plane. Table 1 DSPR unit cell dimensions.

validated. Section 4 explains the comparison between the literature and proposed work in order to assess the DSPR absorber potential. Finally, the conclusion is elucidated in Section 5. 2. DSPR metamaterial absorber design In this paper, we illustrate an ultrathin polarization insensitive Dually Stacked Petal Resonator (DSPR) metamaterial absorber. DSPR is constructed of a polyimide substrate and gold conductive material. The petal-shaped resonators are patterned on the top surfaces of the vertically cascaded resonators. The stacking yields Metal–Dielectric– Metal–Dielectric–Metal (MDMDM) configuration. The employed polyimide substrate has a relative permittivity (𝜖𝑟 ) and loss tangent (tan(𝛿)) of 3.4 and 0.09 [41], respectively. The gold used for the petal shaped resonators has a conductivity of 4.561*107 S/m [33]. As per the aforementioned configuration of the proposed DSPR, the first and third layers are petal resonators and the second and fourth layers are polyimide spacers. Figs. 1A, B, C and D depict the design of the proposed wide-angle ultrathin polarization insensitive DSPR absorber, absorber I (bottom), absorber II (upper) and petal orientation on a 2D plane, correspondingly. The bottom reflecting layer and both petal resonators are consisting of 500 nm thick gold. The square shaped, stacked 19 μm thick upper and 10 μm thick bottom polyimide dielectric spacers have 𝐿s = 𝑊 s = 75 μm. The proposed structure is 0.06𝜆𝑜 thick, where 𝜆𝑜 is the wavelength corresponding to the lower absorption peak (0.6034 THz) which manifests the ultrathin thickness characteristic of the proposed DSPR absorber. The dimensions of the petals are listed in Table 1. The diagonally oriented petals are designed along the vertical (y-axis) and an offset of 45◦ is applied. Mathematically, the absorption intensity can be computed as following:

Parameter

Value (μm)

Coordinates

Value

a b f g h i

30 5 10.5 30.7 10 30.5

c d e j

0 −30 32 30

impedance plot is presented in the following section to justify the impedance matching over the wide band operation. The impedance matching is accomplished via appropriate absorber design. The bottom surface of the ultrathin structure is gold laminated which functions as a perfect reflecting plane and blocks the radiation leakage across the absorber. Therefore, transmission, i.e. (|𝑆21 (𝜔)|2 ) component in the above equation reduces to zero and the equation becomes: 2 𝐴 (𝜔) = 1 − ||𝑆11 (𝜔)||

The designing, simulation and performance analysis of the proposed DSPR metamaterial absorber is carried out in the CST Microwave Studio 2014. Floquet and open boundary conditions are applied on the boundary planes orthogonal to the x, y and z axes, respectively. 3. Simulation results The DSPR metamaterial absorber is structured by vertically cascading two petal shaped resonators based metamaterial absorbers. The stacked absorbers are distinctly parametrically optimized to attain the unity absorption response for both TE and TM polarizations. Figs. 2A and B explain the absorption response of the absorber I and II for TE and TM polarizations and DSPR reflectance and absorption spectra for TE polarization, respectively. It can be observed from Fig. 2A that the absorber I has an absorption peak at 0.664 THz having absorption

2 2 𝐴 (𝜔) = 1 − ||𝑆11 (𝜔)|| − ||𝑆21 (𝜔)||

The reflected power intensity (|𝑆11 (𝜔)|2 ) from the DSPR surface exposed to EM radiations is mitigated by matching the absorber impedance with free space impedance. The free space normalized 187

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Optics Communications 443 (2019) 186–196

Fig. 2. (A) Absorption spectrum of Individual absorbers (absorber I and II) for TE and TM polarizations (B) Absorption and reflectance spectrum of the DSPR absorber for TE polarization.

Fig. 3. Absorptivity spectra at oblique angle of polarization (𝜙) for (A) TE polarization and (B) for TM polarization.

intensity of 99.93%. It has 90% absorption fractional bandwidth of 5.66% over a frequency range of 0.65 THz to 0.68 THz. Also, the TE and TM absorption responses of the absorber I are exactly same. From Fig. 2A, we can say that the absorber II has an absorption peak at 0.736 THz with an absorption level of 94.19%. It has 90% absorption fractional bandwidth of 3.18% over the frequency range of 0.72 THz to 0.74 THz. Fig. 2A elucidates that the TE and TM response of absorber II has reflected slightly different response in terms of absorption peak and absorption intensity. The TM polarization response conveys that the absorption peak of the absorber II has undergone blue shift i.e. shifted towards higher frequencies which is at 0.744 THz with 94.62% absorption intensity and 90% absorption fractional bandwidth of 3.36% over a frequency range of 0.73 THz to 0.76 THz. Fig. 2A explains that the 90% absorption fractional bandwidth of the TM response is 5.35% higher than the TE absorption response. This interesting characteristic broadens the application domain of the DSPR metamaterial absorber. The investigation incurred that absorber II has different responses for TE and TM polarizations because of the geometry of the petals. The dimensions of the unit cell listed in Table 1 reflect that petals of absorber II are not same in size whereas petals of absorber I are same in size. This concept also defines that the absorption characteristics are a function of petal size.

As the stacking yields a new structural configuration, the proposed layout is optimized such that the DSPR exhibits wide band ultrathin polarization insensitivity and wide incident angle characteristics. From Fig. 2B, the proposed DSPR metamaterial has perfect near unity absorption response over a wide frequency range between 0.591 THz to 0.672 THz having 98% fractional bandwidth of 12.83% for TE polarization. The wide band response has absorption peaks at 0.66 THz and 0.60 THz with absorption intensity of 99.52% and 99.53%, respectively. Considering 90% absorption level as a reference, DSPR absorber has a fractional bandwidth of 18.50%. The TM polarization response is not presented because it is identical to the TE polarization response except that the absorption spectra has trivial blue shift with a slight increase in 98% and 90% absorption fractional bandwidth which are 12.98% and 18.8%, respectively for TE response. The influence of the alteration in the DSPR structure parameters will be demonstrated comprehensively in the parametric analysis section. The polarization insensitivity is one of the crucial traits of the DSPR metamaterial absorber. Figs. 3A–B illustrate the absorption response at oblique polarization angles for TE and TM polarizations, correspondingly. The DSPR maintains excellent response with negligible variation in the absorption spectrum and intensity over a wide polarization angle (𝜙) of 90◦ . We concluded that the polarization insensitivity is resulted 188

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Fig. 4. Absorption vs frequency response at oblique angle of incidence (𝜃) for (A) TE polarization and (B) TM polarization.

Fig. 5. Surface current distribution at 0.6034 THz on (A) Absorber I surface (B) Absorber II surface and (C) ground plane.

from the symmetric geometry of the DSPR metamaterial absorber. For TE polarization, both absorption peaks have the absorption intensity of above 99% at 𝜙 = 0◦ , 15◦ , 30◦ , 45◦ and 90◦ . At 𝜙 = 60◦ , the lower peak absorption intensity reaches approximately 99.8% and the highest peak sustains absorptivity of above 98.85%. For TM polarization, both peaks have absorptivity of above 99% at 𝜙 = 0◦ , 15◦ , 45◦ and 90◦ . At 30◦ , the higher peak intensity exceeds 98.99% and the lower peak maintains 99.75% absorbance. At 60◦ , the higher and lower peak absorption exceeds 98.8% and 99.82%, respectively. Regardless of negligible variations in response spectrum, the DSPR has excellent polarization insensitivity performance. In practical applications, the absorber might be exposed to EM waves at oblique angles of incidence (𝜃). Therefore, the performance of the proposed DSPR metamaterial absorber is examined at oblique 𝜃 for both TE and TM polarizations. Fig. 4A reflects that for TE polarization, DSPR has an absorption intensity of above 80% over a wide angle of incidence of 60◦ . At 𝜃 = 60◦ , the absorption response has 2 absorption peaks at 0.674 THz and 0.585 THz with absorptivity of 93.5% and 94.8%, respectively. At 𝜃 = 75◦ , both absorption peaks have intensities above 75%. Comprehensively analyzing the absorption response variation reveals an interesting feature that as 𝜃 increases the wide band response converges to a dual band response. For TM polarization, represented in Fig. 4B, also the DSPR has a very interesting response. Until 𝜃 = 60◦ , the absorption spectrum sustains an absorption level of above 90%. At 𝜃 = 75◦ , the overall wide band response has above 70% intensity, the higher peak intensity almost touches the 90% absorption level and the lower peak absorption level is above 80%. The thorough examination shows that for TM polarization,

DSPR has a wide band response with significant absorption intensity over a wide angle of incidence of 75◦ . As presented in Fig. 4B, we also determine that as 𝜃 increases, the absorption spectrum undergoes a blue shift i.e. towards higher frequencies. Thus, we can say that DSPR can be exploited for dual band applications as well as for wide band applications. The electric field distribution, magnetic field distribution, surface current distribution, power loss density and normalized impedance characteristics are studied to explore the insight of the absorption mechanism. Figs. 5A, B and C demonstrate the surface current distribution on the absorber I, absorber II and bottom gold laminated (ground) surfaces at 0.6034 THz for TM polarization. Fig. 5A shows that petal II has surface currents on its upper and bottom surface which are found antiparallel at the center region and parallel on the petal surfaces. Also, the current intensity at the center of the bottom surface of petal II is high. From Figs. 5A and B it can be observed that the absorber I and II surface current loops are directed in the same directions. Figs. 5B and C reflect that the surface currents on the petal II and bottom metallic film are antiparallel which forms the magnetic dipole resonance. It signifies that at lower absorption peak, petal I and ground plane are magnetically excited. Another important observation is that both petals have high surface current intensity at the petal edges. Using petal I surface current distribution, we can examine the electric field driven behavior which signifies that the surface currents on the edges of the horizontal and diagonal petals are guided in the same direction with maximum concentration whereas the surface currents on the vertical petal edges are antiparallel. The surface current intensity on absorber II is substantially less in comparison to absorber I. Figs. 6A–H show 189

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Fig. 6. Absolute H-field distribution at 0.6034 THz (A) on petal I surface (xy plane), (B) on petal II surface (xy plane), (C) Bottom polyimide substrate (xy plane), (D) Stacked polyimide substrate (xy plane), (E–H) inside the DSPR absorber (z plane) at different instances and (I) Plane to visualize distribution inside DSPR.

Fig. 7. Surface current distribution at 0.656 THz on (A) Absorber II surface (B) Absorber I surface and (C) ground plane.

Fig. 8. H-field (absolute) distribution at 0.656 THz (A) on petal I surface (xy plane) (B) on petal II surface (xy plane) (C) Bottom polyimide substrate (xy plane) D) Stacked polyimide substrate (xy plane), (E–H) inside the DSPR absorber (z plane) at different instances.

the magnetic field distribution on the petal I surface, petal II surface, substrate I, substrate II and inside the DSPR absorber along the plane shown in Fig. 6I. The region enclosed in a dotted circle in Fig. 6D shows the existence of magnetic dipole at the intersection of petals. These tiny magnetic dipoles are observed at all intersections of both petals (refer to Fig. 6C and D). Figs. 6E–H explicitly manifest the magnetic excitation between petal I and the ground plane. Fig. 6I shows the plane employed to determine the field distributions inside the DSPR. Figs. 7A, B and C present surface current distribution on the petal surfaces and the ground layer at 0.656 THz resonant frequency for TM polarization. It can be seen evidently from Figs. 7A and B that the petal I and II surface currents are antiparallel which results in magnetic dipole resonance. The petal II and ground surface currents are directed laterally. The petal I and II surface currents have high

intensity in comparison to ground plane surface currents. Figs. 8E–J elaborate the magnetic excitation between petal I and II. Similar to Fig. 6D, Fig. 8B also shows the magnetic dipole on the petal II due to the orientation of surface currents. It can be observed that the surface currents are circulating on the ground plane, which also leads to magnetic dipole resonance. Thus, the antiparallel and circulating currents are potentially responsible for magnetic resonance at 0.656 THz. Figs. 9A and B illustrate the electric field distribution on the surface of absorber I and II of the DSPR metamaterial absorber upon its exposure to TM polarized wave at 0.6034 THz. Figs. 9A and B rigorously justify the above discussion on surface current distribution which is at 0.6034 THz. Petal I has highly concentrated surface currents and the charges are accommodated at the petal tips (refer Fig. 5). Thus, petal I has 6 electric dipoles. From Figs. 9A and B, we deduced 190

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Optics Communications 443 (2019) 186–196

Fig. 9. Electric (absolute) distribution at 0.6034 THz (A) on petal I surface (B) on petal II surface (C) Bottom polyimide substrate (D) Stacked polyimide substrate, (E–J) inside the DSPR absorber (z plane) at different instances.

Fig. 10. E-field (absolute) distribution at 0.656 THz (A) on petal I surface (xy plane) (B) on petal II surface (xy plane) (C) Bottom polyimide substrate (xy plane) (D) Stacked polyimide substrate (xy plane), (E–L) inside the DSPR absorber (z plane) at different instances and (M) Plane to visualize distribution inside DSPR.

Fig. 11. Power loss density at 0.6034 THz (A) on the surface of absorber I (xy plane), (B) on the surface of absorber II (xy plane), (C–F) inside the DSPR structure at different instances.

that the concentration of the electric field is maximum on the petal surfaces which are oriented along the direction of the electric field and reduces on the surface of the petals which are directed at an oblique angle with respect to the orientation of the incident electric field. Figs. 9C and D demonstrate the electric field distribution on the bottom and stacked polyimide substrate. Figs. 9E–J depict the electric field distribution inside the DSPR absorber at different instances and can be visualized that fields attenuates on propagating to a certain distance inside the polyimide. The strong electric fields are associated

with petal II because it has high charge concentration. From the electric field intensity distribution inside the upper polyimide, we can say that both absorbers are electrically coupled which might be responsible for the shift in the absorption peaks of the DSPR absorber in comparison to the absorption response of the individual bottom absorber I. Similarly, Figs. 10A and B depict the electric field distribution on the top surface (x-y plane) of the absorber I and absorber II, respectively, for TM polarized waves at 0.656 THz. The electric field is distributed on both petal I and II surfaces and follows an identical fashion to that of 191

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Optics Communications 443 (2019) 186–196

Fig. 12. Power loss density at 0.656 THz (A) on the surface of absorber I (xy plane), (B) on the surface of absorber II (xy plane), (C–F) inside the DSPR structure at different instances.

0.6034 THz which is e-field strength reduces on moving towards the center. Figs. 10C and D present e-field distribution on the substrate I and II, correspondingly. Figs. 10E–L show cross-sectional views of the electric field distribution inside the DSPR metamaterial absorber at 0.656 THz and ease the interpretation of field distribution inside the substrates I and II. We concluded that the higher absorption peak is originated from Fabry–Perot reflections because it seems like fields are trapped in the stacked polyimide and bounces back and forth between stacked and bottom petals. It is also evident that petal I and II are strongly electrically coupled. Also, moderate intensity fields attenuate on propagating to a certain distance inside the bottom substrate. Power loss density is also a vital parameter to explore the interpretation of absorption phenomenon. Figs. 11 and 12 present the power loss distribution on the surfaces of petal I and II and inside the stacked configuration at 0.6034 THz and 0.656 THz, respectively. Power loss distribution parsing elucidated that the electric dipoles on the petal I and II surfaces are responsible for the absorption phenomenon at 0.6034 THz and 0.656 THz. As elaborated in the electric field section, these electric dipoles originate from the surface currents and charge accommodation. The power loss distribution for both frequencies exactly follows the electric dipole profile. At 0.6034 THz, the petal I exhibits more losses and at 0.656 THz the petal II has more losses. We also explored the losses inside the DSPR construction and again the power loss distribution aligns with the electric field distribution. The normalized impedance plot shown below in Fig. 13 justifies that over the wide band absorption response, the impedance of the ultrathin DSPR metamaterial absorber is perfectly matched to the free space impedance. We can see in Fig. 13 that at higher and lower absorption peaks the real and imaginary part of the impedance reduces approximately to 1 and 0, respectively. Table 2 lists the precise normalized absorber impedance values at the resonant peaks and over the wide band response. An extensive parametric analysis has been done to explicit the influence of the structural parameters on the performance of the DSPR metamaterial absorber. The parametric examination incorporates alteration of petal dimensions, petal flaring angles, substrate thickness etc. The parametric optimization has played a crucial role in obtaining the optimum characteristics. The thickness of bottom substrate 𝑡1 has been varied, keeping all other parameters same as that of the proposed optimized DSPR metamaterial absorber. Thickness 𝑡1 is tailored from 𝑡1 = 5 μm to 19 μm. Fig. 14A represents that with 𝑡1 = 5 μm the DSPR absorption response becomes a single band with a resonant frequency at

Fig. 13. Normalized impedance of the proposed DSPR absorber.

0.72 THz. We can observe in Fig. 14A that as the thickness is increased by resolution of 1 μm the single band response becomes a wide band response. A prudent response evaluation yields that as the thickness 𝑡1 increases, the absorption peak experiences a red shift i.e. towards lower frequencies. As 𝑡1 = 9 μm, the second resonant frequency starts appearing. We found that as 𝑡1 increases from 5 μm to 10 μm, the existing resonant frequency’s absorption intensity hampers. However, as 𝑡1 increases to 11 μm, the absorptivity at both existing peak and emerging peak enhances significantly. From these observations, we concluded that as 𝑡1 is reduced the corresponding absorption peak (lower absorption peak) undergoes a blue shift and merges with the higher absorption peak and DSPR can be employed as the monopole absorber. The effect of the upper substrates thickness 𝑡2 is observed while all other parameters are kept as that of the optimized DSPR metamaterial absorber. As shown in Fig. 14B with 𝑡2 = 2 μm, the absorption response reduces from wide band response to a single band response and in terms of the response trajectory, it is identical to the response when 192

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Optics Communications 443 (2019) 186–196 Table 2 Normalized impedance. Absorption peaks/band (THz)

0.6034 (peak) 0.656 (peak) 0.591–0.672 (wide band)

Absorption intensity (%)

99.52 99.51 >98

Normalized impedance (Z) Real Z

Imaginary Z

0.921 0.976 0.78 to 0.76

0.105 −0.14 −0.11 to 0.07

Fig. 14. Absorption intensity analysis for different (A) absorber I thickness (B) absorber II thickness.

Fig. 15. Absorption vs frequency response of (A) HH and VV configurations (B) HV and VH configurations and (C) 4 petal configuration for TE and TM polarizations.

𝑡1 is tailored to 𝑡1 = 5 μm. The 90% absorption bandwidth is comparatively high but the absorption intensity at the resonant frequency is marginally less. Again, as 𝑡2 is increased from 2 μm to 10 μm with step size of 2 μm the absorption response dramatically improved exhibiting near unity absorption level over a wide frequency spectrum. On the basis of petal orientation, we have investigated 4 different possible DSPR configurations. Horizontal petals on both substrates (termed as HH), vertical petals on both substrates (called as VV), horizontal on the bottom and vertical on the upper substrates (labeled as HV) and vertical on the bottom and horizontal on the upper substrate (abbreviated as VH) are examined. Fig. 15A represents the TE and TM absorption response for HH and VV configuration. The HH configuration only responds to the TM polarization because as explained in the optimized DSPR response (Fig. 2) and electric field intensity distribution section, petals directed along the impinged E-field direction mainly contribute to the absorption mechanism. Further, depending upon the HH aligned petal dimensions, they have absorption peaks which end up with the 90% absorption fractional bandwidth of 13.77% having a frequency range of 0.61 THz to 0.71 THz. Over this frequency range, the peak absorption of 99.994% has been attained at 0.673 THz. Similarly, the VV configuration responds appropriately only to TE polarization. It has 90% absorption fractional bandwidth of 13.77%

with frequency range of 0.61 THz and 0.70 THz, which is exactly same as that of the HH configurations response. The absorption peak having above 90% absorption occurs at 0.668 THz with intensity of 99.996%. Therefore, the above explanation strongly justified that absorption response is a function of the orientation of the electric field and petals. From Fig. 15B, owing to the petal orientation in both horizontal and vertical directions, the HV configuration has a single band response for both TE and TM polarizations. The stacked vertical petals and bottom horizontal petals are responsible for responding to TE and TM polarizations, respectively. For TE polarization, the absorption peak occurs at 0.669 THz with an absorption level of 89.83%. With respect to TE polarization peak, the TM polarization response undergoes a red shift and occurs at 0.627 THz with intensity of 94.57%. The TM polarization response spectrum has 90% absorption fractional bandwidth of 3.17%. Similarly, TE and TM polarization absorption spectrums for VH configuration are depicted in Fig. 15B. The TM polarization has a single band response with absorption peak at 0.67 THz, having absorptivity of 90.23%. The TE polarization has 90% absorption fractional bandwidth of 2.97%. Thus, the E-field driven DSPR characteristic is rigorously verified. Eventually, as shown in Fig. 15C, DSPR metamaterial absorber with 4 petals on each substrate is designed. This configuration again follows 193

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Fig. 16. DSPR absorption spectra as a function of flaring angle (𝛹 ).

Table 3 DSPR response with different flaring angles. Flaring angle, 𝛹 (degree)

90% Absorption frequency 90% Fractional range (THz) bandwidth (%)

Response type

Response variance with respect to optimized DSPR response

0

0.5794–0.6844

16.67

Wideband

Red shift

5

0.5662–0.5976 0.60541–0.6636

5.398 9.181

Dual band

Red shift

15

0.5586–0.5872 0.6040–0.6541

4.99 7.97

Dual band

Red shift

25

0.5625–0.6628

16.43

Wide band

Red shift

35

0.5728–0.6868

18.17

Wide band

Red shift

the above discussion regarding the coplanar orientation of the electric field and DSPR petals. Comparing Figs. 15C and 2B, it is obvious that the appropriate orientation of diagonal petals is a crucial design attribute because it essentially enhances the absorption intensity as well as the absorption bandwidth of the DSPR absorber over a wide band of 0.591 to 0.672 THz. The DSPR response for different flaring angles (𝛹 ) is discussed below. The influence of the existence of the diagonal petals and the position of diagonal petals is demonstrated in Fig. 16. The response shown in Fig. 16 is for TE polarization. The TM polarization response is not presented because it is identical to the TE response. The position of diagonally aligned petals is illustrated in terms of flaring angle (𝛹 ). In the proposed optimized DSPR metamaterial absorber, the diagonal petals are oriented at 45◦ with respect to their adjacent vertical petals (refer to Fig. 1D). The flaring angle performance evaluation is observed at 𝛹 = 0◦ , 5◦ , 15◦ , 25◦ and 35◦ . Variation in 𝛹 from 0◦ to 15◦ yielded red shift with slight reduction in the absorptivity. Though the absorption level reduces, it is still above 90%. At 𝛹 = 25◦ also, the absorption spectrum undergoes a red shift with enhancement in the absorption level. At 𝛹 = 35◦ , the DSPR absorber behaves as a mono band absorber having an absorption peak at 0.573 THz. At 𝛹 = 45◦ , the near unity response has been determined over a wide frequency range. The Table 3 lists and compares the performance behavior of the ultrathin DSPR metamaterial for different flaring angles. Thus, from here we can say that by tailoring the petal orientation, it can be possible to customize the DSPR performance and required characteristics can be attained. The DSPR dependency on the length and width of the petals has also been explored. The lengths of the absorber I and II in the optimized DSPR, refer to Table 1, are varied simultaneously by resolution (h) of 2 μm. Figs. 17A and B illustrate that as the length of the petals increase, the absorption spectrum experiences red shift and the peak absorption intensity over the desired frequency range decreases gradually. However, even with increasing the length by 10 μm, the peak intensity is still above 95%. It has also been observed that the 90%

absorption fractional bandwidth increases significantly. Another important observation from Figs. 17A and B is that as the length increases, the wide band response (with respect to 90% absorption bandwidth) is transferred to dual band response. Thus, it can be possible to alter the DSPR geometry appropriately, adjust the simulation frequency range and design a multi band absorber for particular applications. Though the DSPR response type and trajectory for TE and TM polarizations in Figs. 17A and B are identical, but the numerical computation yields that the TM polarization response has higher 90% fractional bandwidth and also the peak absorption achieved by the TM response on varying the length is higher than the TE response. Similarly, the width of the petals has also been swept. The widths of the bottom and upper petals are altered from 1 μm to 6 μm and 5 μm to 10 μm, respectively with step size (t) of 1 μm. Figs. 18A and B elucidate that as the width increases, both, TE and TM polarizations, absorption spectrums undergo a blue shift and the absorption intensity also increases. We found that the combination of petal widths of 5 μm and 10 μm for the bottom and upper petals, respectively results in optimum response. Thus, these structural parameters are employed in the proposed DSPR metamaterial absorber design.

4. Comparison The proposed work is compared with the literature to evaluate the potential and capability of the DSPR metamaterial absorber. Table 4 reflects that stacking is the primary adoptable approach to enhance the absorption bandwidth. The monolayer absorbers exhibit very narrow band response with multiple absorption peaks. We concluded that DSPR has outperformed the literature in terms of peak absorption level, maximum polarization insensitivity and incidence angles. Thus, DSPR metamaterial absorber overcame the challenge of possessing all above traits in one novel structure. 194

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Fig. 17. Absorption spectra dependency on the petal length for (A) TE and (B) TM polarizations.

Fig. 18. Absorption behavior of the DSPR absorber as a function of petal width for (A) TE and (B) TM polarizations.

Table 4 Comparison between proposed work and literature. Reference

Structure configuration

Response type

Max. absorption intensity

Max. polarization insensitivity angle

Max. angle of incidence

[9] [10] [21] [29] [31] [33] [36] [40] [41] Our work

4 1 1 1 1 4 1 1 1 2

Broadband 3 distinct peaks 3 distinct peaks 2 distinct peaks 4 distinct peaks Broadband 2 distinct peaks Single peak 2 distinct peaks Wide band

>90% 99.1% >88% 95.5% >90% 98.3% 77.5% 99.51% 97.4% 99.53%

Insensitive (𝜙 = 70◦ ) Sensitive Insensitive (𝜙 = 90◦ ) 𝜙 = 30◦ Not discussed Insensitive (𝜙 = 90◦ ) 𝜙 = 45◦ Insensitive 𝜙 = 45◦ Insensitive (𝜙 = 90◦ )

75◦ 60◦ 50◦ 30◦ Not discussed 50◦ 45◦ 75◦ 45◦ 75◦

layers layer layer layer layer layers layer layer layer layers

(Narrow (Narrow (Narrow (Narrow

band) band) band) band)

(Narrow band) (Narrow band)

5. Conclusion

polarizations. It maintains excellent absorption response over a wide angle of incidence of 75◦ for both TE and TM polarizations. DSPR has reflected exotic characteristics in terms of transition from wide band to multi band response types. Interestingly, DSPR structure has the capability to mold the response by employing different configurations (HH, VV VH and HV), varying the structural dimensions, exposing DSPR to EM radiations at an oblique angle, etc. The insight study yielded the contribution of electric dipole resonances, Fabry–Perot reflections and magnetic dipole resonances. Comparison among the literature articles justified that DSPR owns features which are extremely difficult to attain simultaneously, which includes near unity absorption,

We emphasized on a novel E-field driven ultrathin polarization insensitive, wide angle DSPR metamaterial absorber for cloaking, THz imaging, THz sensing, solar cell, detection, stealth technology and remote sensing applications. The proposed DSPR metamaterial absorber is a composite of polyimide substrate and gold. The DSPR absorber exhibits a wide band near unity characteristics over the frequency range of 0.591 to 0.672 THz. It has 90% fractional bandwidth of 18.5%. Owing to the design symmetry, the DSPR has a polarization insensitive feature over a wide angle of 90◦ for both TE and TM 195

A.S. Dhillon and D. Mittal

Optics Communications 443 (2019) 186–196

ultrathin thickness, polarization insensitivity, wide angle of incidence, etc. The ultrathin thickness and response behavior concede that DSPR is a potential candidate for conformal and wearable applications.

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