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Abstract
This work demonstrates a one-dimensional unidirectional terahertz (THz) absorber with thermal switching from broadband to narrowband and tunable multiple narrowband absorption with vanadium dioxide (VO2)-graphene-based defective photonic crystal. The thermal tuning of defect layer switches the phase of VO2 and obtain multiple narrowband optical absorptance with 70-90% peak at 4.12, 4.86 and 5.23THz respectively with a Q factor around 291 for 4.86 THz peak. The thermal dependent Q factor of the stack varies from 19 to 291 with phase transition from metal to the insulating state of the defect. The optical non-magnetic THz unidirectional absorber has switchable propagation functions within the metallic phase from non-absorption to higher peak absorption with forward and backward propagated wave. The asymmetrical dual defect layer with dual absorption peaks can be switched and the wavelength can be changed by changing the distance between the two peaks. Multiple graphene-based VO2 defects have increased peaks of narrowband absorption. This novel phase changing material (PCM) based asymmetric defective photonic layer can tune the defect layer for optimum and adjustable absorption at THz range and non-magnetic reciprocal and unidirectional structure with temperature dependent dual band switchable, which leads to this structure for terahertz wireless communication systems as well as other THz sensing devices.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Terahertz metamaterial absorbers have attracted the attentions of a diverse group of researchers due to their high switchable bifunctional properties and tunable bandwidth [1–3]. Several types of metamaterial absorbers with varying frequency bands, absorption widths, and absorption peak counts have been proposed one after the other [4,5]. The performance of most presently available passive metamaterial-based structures for electromotive force absorbers is assessed after they have been constructed. In this way they suffer from one clear drawback: inflexible tuning of their resonance frequency and absorption strength. Active, adjustable electromagnetic metamaterial absorbers are needed to handle the ever-increasing complexity of present electromagnetic applications. Metamaterial absorbers that can be adjustable has less practical usage [4]. The majority of research in the terahertz band range has been on absorbers combined with metamaterial for analysis of absorption strengths. There is a dearth of research on absorbers that function well in terms of absorption type conversion (wide to narrowband) [5–7]. As a result, metamaterial absorbers with customizable absorption shapes and a wide adjustment range are required for experimental process. Various methods are tried to achieve tunable and switchable terahertz metamaterial absorptive devices. Easy fabrication, low cost, and high compatibility became the main concern for designing such devices. One dimensional (1D) Photonic crystals (PCs) are artificially nonuniform structure [8,9]. Dielectric constant modulation in various directions is the basis for PC design and fabrication. PCs have grown in significance as a result of their role in limiting and regulating the spread of electromagnetic radiation. The existence of certain spectrum frequencies known as photonic band gaps (PBGs) may cause this phenomenon to occur. The gap formation is dependent on geometrical structure and optical properties of the material used in it [10]. As a consequence, customizing PCs offers new opportunities for scientific study and technical application development. Biosensors, wave guide sensors, optical transistors directional reflectors are made with the principle of band gap formations. These photonic gaps can be modified with the variation of optical properties including refractive index change of different materials. Therefore, properties of various materials differ due to the shift of permittivity. In case of semiconductors, concentration of carriers also changes as it is dependent on the optical properties. However, the usage of dispersive materials in certain optical applications may be restricted as they can spread and render pulses. A defect layer in the 1D PC provides excellent photonic correlation, making it an ideal pass band filter for possible applications in mode interaction and selectivity [11]. Wide band gap, incident photon sensitivity, and polarization insensitivity are design goals for absorbers and filters. The borders of the Bragg band gap are often blue shifted as the inclining angle rises in dielectric 1D PCs. For traditional 1D PCs, researchers have suggested numerous successful ways for obtaining broad-band and wide-angle responses, such as retaining frequency or incident-angle domain 1D PC heterostructures [12,13] or quasi-period 1D PCs [14,15]. In addition to the above, researchers have advanced 1D metamaterial-based PCs with dispersion less and band gaps that does not depends on any polarization, such as a 1D PC with alternating layers of positive and negative-index media [16,17], shifting layers of material and real part of permittivity and permeability of structure works as metal atrial [18–20] and a phase-variation-compensated 1D PC [18–21]. Aside from the foregoing attempts, customizable 1D PCs have gotten a lot of interest recently. Graphene and phase-change materials (VO2 and Ge2Sb2Te5) are often employed in tunable 1D PCs [22–25].Due to the fast and changeable modification of their operational states by peripheral inflammations such as thermal and optical methods, phase change materials (PCMs) play an important role in this contrast [23,25]. As a consequence of the phase shift, the electric, thermal, and optical characteristics of the material alter dramatically. Different PCM such as liquid crystals, chalcogenide glasses like Ge-Sb-Te, associated oxides like vanadium dioxide (VO2), and topological insulators like bismuth selenide (Bi2Se3) have been utilized for optical switching purpose. When heated to higher temperatures, VO2 transitions from its insulator phase to its metallic phase, which is why it is the most well-known PCM [26]. Monoclinic VO2 shows metal to insulator transition (MIT) by changing into rutile phase at a specific temperature known as critical temperature (Tc) around 341K. At rutile phase, VO2 behaves like metallic and all other optical, electrical and thermal properties varied from the monoclinic state [27]. At an ambient temperature around 341K, VO2(R) works in metallic and these transition shifting are modelled numerically by filling fraction which is temperature-dependent (i.e., the metallic volume fraction of VO2). Researchers have previously demonstrated band-pass filters or absorbers when VO2 is placed as a fault layer underneath the insulating phase or as a surface coating beneath the metal phase in traditional 1D dielectric PCs [23]. Defect mode tuning is sometimes utilized in 1D PCs using graphene sheets [28,29]. The lateral side of the faulty 1D PC is covered with a graphene layer in Ref. [28], and the recurrence of defect modes is regulated by altering the chemical potential of graphene through voltage level. The transmission characteristics of a faulty one-dimensional PC with graphene nanolayers between high and low dielectrics were examined in Ref. [30].It is shown that by varying the chemical potential inside the THz area, the defect mode frequency may be adjusted. In this paper, a defective photonic THz absorber is proposed with phase change material VO2 along with dual graphene monolayer working as nanocavity for the structure. The thermal biostability switches the structure from narrowband to broad band absorber with increased Q factor. The resonant frequency also varies and shifts towards higher frequency. The increase in defect layer thickness is inversely proportional to the narrowband Q factor. The absorption peaks blue moved as the incidence angle rose in Transverse electric (TE) and Transverse magnetic (TM) modes. The optical non-magnetic THz unidirectional and non-reciprocal absorber may transition from non-absorption to greater peak absorption with forward and backward propagating waves. The metallic phase of VO2 shows near-perfect absorption peaks at oblique incidence angles in TE polarization modes. Many graphene-based VO2 defects have increased narrowband absorption peaks. This innovative PCM-based faulty photonic layer has tunable THz absorption and a non-magnetic reciprocal and unidirectional structure with temperature-dependent dual band watchability.
2. Mathematical analysis
Figure 1 shows one-dimensional(1D) defective photonic crystal (DPC) model investigated in this paper. It is consisting of alternating layers of SiO2 and Si with different number of periods in x- direction The refractive index and thicknesses of SiO2 and Si are represented by (${n_a},{d_a}$) and (${n_b},{d_b}$) respectively. The single layer defect is embedded with graphene and VO2 with permittivity and thicknesses of graphene and VO2 are represented by (${\varepsilon _g},{d_g}$) and (${\varepsilon _v},{d_v}$) respectively. The values of the silicon and SiO2 layer has a fixed permittivity of 3.9 and 11.7 according to the literature [30]. The thickness of mono layer graphene is considered as 0.34nm. A resonant frequency is considered as 5 THz and the thickness of SiO2 and Si has taken quarter wave length conditions with 288nm and 185 nm respectively. The permittivity of the graphene is dependent on conductivity which is described by kubo formulization. The numerical equation designates the surface conductivity of graphene (${\sigma _g} = {\sigma _{intra}} + {\sigma _{inter}}$) in two different factors [31],
Fig. 1. A schematic diagram of asymmetric defective photonic crystal with graphene embedded VO2 layer working as defect.
Here $\omega $ is the alternation frequency, ${{\mu} _c}$ is the chemical potential, Г is dispersion or scattering rate, $T$ is the absolute temperature, $e$ is the electron charge, ${k_B}$ is the boltzmann ’s constant. In higher frequencies like terahertz region, 2D materials such as graphene acts as Drude materials and the intra band mode of conductivity is important in such frequency. For experimental setup, large dimensions of monolayer use chemical vapor deposition (CVD) method. For the proposed structure, a characteristic value of $\Gamma $= 1THz and room temperature (T = 300K) are considered. Graphene working as anisotropic material has two different permittivity and within plane, permittivity can be expressed as [32],
Here ${\textrm{d}_\textrm{g}}$ is the thickness of the graphene. The final resultant refractive index for the graphene becomes ${n_g} = \sqrt {{\varepsilon _g}} $.
Figure 2(a) depicts dependency of the refractive index of graphene at THz frequency with two different chemical potential (${\mu _c}$) of 0.4eV and 0.8eV respectively. Higher frequencies around 2 THz, both real and lossy parts of indies are dependent to the chemical potential. This principle can be used to analysis terahertz sensor design as well as to the tunable photonic band gaps, peaks in absorptions and transmission spectrum.
Fig. 2. (a) Refractive index of graphene monolayer. Blue line indicates real part and green line indicates complex part (b) Real permittivity of VO2. Blue line indicates insulator phase and green line indicates metal part.
For PCM, VO2 changes its insulator to metallic phase at around 68°C which makes it a good candidate as a defect material. The VO2 optical spectrum can be observed by drude-lorentz dispersion model [33,34]
Figure 2(b) indicates the real permittivity of VO2 film in the terahertz region under 30°C (insulator phase) and 90° C (metallic phase). The temperature plays an important role in the transition of VO2 from an insulator to a metal phase, which alters the optical response of the structure through a dependent dielectric function, known as the thermo-optical effect, when the structure is heated. The temperature of VO2 changes by 4 K during its transition from the entirely dielectric state to the fully metallic state [37]. An insulating VO2 coating layer remains embedded with metallic VO2 particles during the phase transfer state which is observed at ${T_c}$ and ${T_c} + \Delta ({{T_c} = 341K,T + \Delta = 345K} )$ for increasing ${T_{V{O_2}}}$ and ${T_c} = 333K$, ${T_c} + \Delta = 337K$ for decreasing ${T_{V{O_2}}}$. The condition of the composition in the transition period can be simulated with a mixing fraction, $f(T )$ such that $f({{T_c}} )= 0$ and $f({{T_c} + \Delta } )= 1$.
The absorption of light through the proposed structure can be measured by Transfer matrix method(TMM) for multilayer structure as [26]
Here ${\textrm{Q}_{11}}$ and ${Q_{21}}$ present the components of the overall transfer matrix ${Q_{TOTAL}}\; $ of the considered structure. ${Q_{TOTAL}}$ relates the individual transfer matrices of constituent layers are multiplied to observed the full system with incident and reflected photons in electric and magnetic fields for TE or TM mode. ${Q_{TOTAL}} = {Q_1}{({Q_a}{Q_b})^{{N_1}}}({Q_g}{Q_v}{Q_g}){({Q_a}{Q_b})^{{N_2}}}{Q_0}$. Here ${N_1} = {N_2}$ is the periodicity of the PC, ${Q_1}$ and ${Q_0}$ are defined as ${Q_1} = \frac{1}{2}\left( {\begin{array}{cc} 1&{ - \frac{1}{{{q_0}}}}\\ 1&{\frac{1}{{{q_0}}}} \end{array}} \right)$ and ${Q_0} = \frac{1}{2}\left( {\begin{array}{c} 1\\ { - {q_s}} \end{array}} \right)$ where ${q_0} = \frac{{\cos (\theta )}}{{c{\mathrm{\mu} _0}}}$ and ${q_s} = \frac{{{k_z}}}{{\omega {\mathrm{\mu} _0}}}$ for TE mode. For TM mode the variables become ${q_0} ={-} \frac{{\cos (\theta )}}{{c{\mathrm{\mu} _0}}}$, ${q_s} = \frac{{{k_z}}}{{ - \omega {\alpha _{0{\alpha _s}}}}}$ respectively. Here ${\mathrm{\mu} _0},{\alpha _0}$ are permeability, permittivity of vacuum, ${k_z}$ is z component of wave vector and ${\alpha _s}$ is permittivity of final layer.
3. Results and discussion
3.1 Impact of graphene and VO2 as the defect layer
The inclusion of a defect layer in the 1DPC changes the overall refractive index of the structure which consequently leads to shifting spectrum with a narrow band of light known as defect mode. This defect mode has a strongly localized field around the defect region in the photonic band gap (PBG). Figures 3(a), (b) and (c) show the absorptance(A), reflectance(R) and transmittance(T) spectrum of the proposed structure with insulator defect mode(f = 0) of VO2. Higher chemical potential value of graphene shifts the defect state of a 1DPC towards higher frequency in insulator mode of VO2-based structure [35]. For this reason, the chemical potential (${\mathrm{\mu} _c})$ of graphene monolayer considered as 0.9 eV for Figs. 3(a), (b) and (c). The periodicity is taken as ${N_1} = {N_2} = 5$ for defect mode-based structure and in case of without defect structure, periodicity N1 = 10. The effects of the defect modes are observed with no defect in the structure (blue line) compared with only VO2 defect (green line) and graphene-VO2 defect (red line). Figures display the distinct features of the quasi defect over the structure. There is a Bragg band gap with only photonic structure (SiO2/Si)N1 is present. Figure 3(a) shows the absorptance spectrum has no effects with only photonic and VO2 defect present as there are no localization of photon. In the presence of dual defect graphene layered embedded with VO2 at thickness of 28$\mathrm{\mu} m$, three peaks of optical absorptance with 50-90% peak at 4.12, 4.86 and 5.23THz respectively are observed. This is due to surface defect in between graphene and PC creating photon localization which leads to enhance overall absorption of the structure [38]. Fabry-Perot (FP) resonance has made the electric field intensity distributed in different concentration for different resonant frequencies. For this reason various absorption peaks are obtained within the defect layer of the structure [39].In case of reflectance and transmittance spectrum, there are defect modes in presence of only VO2 defect within Bragg band gap in the structure. Now, graphene-VO2 based defect layer is used for tuning the resonance frequency of the defect mode. The presence of graphene defect layer with VO2 has changed permittivity of the defect layer. Figure 3(b) shows that the resonant frequency of the defect mode shifts to a higher frequency.
Fig. 3. THz absorber with insulating phase of VO2 comparison with no defect (blue line), only VO2 defect (green line) and graphene-VO2 defect (red line) for (a) absorptance (b) reflectance (c) transmittance (d) Switching effect from insulator (red line) to metal phase (green line) (e) impact of chemical potential of defect in insulator phase (f) variation of peak of absorption with chemical potential.
Figure 3(d) shows the absorption of the structure can achieve broadband absorption when VO2 is in the metallic state. Absorptance exceeds ∼80% in IP with the frequency range of 4.82-4.84 THz. When the phase shifts from IP to MP, the designed absorber transits form narrows to broad absorption at the change of bandwidth of about 19% shift. Thus, the structure is switched from a broadband absorber to a narrowband absorber. The mixing ratio (f) of the VO2 changes effective refractive index of the VO2 layer which had an impact on the switchable of this dual band. When mixing ratio shifts from IP to MP, the absorption spectrum with Q factor of the over structure shifts from 19 (metal phase) to 292 (insulator phase). Figure 3(e) shows the impact of chemical potential on the structure in IP. The increment of chemical potential of mono layer graphene increases the real part of refractive index of graphene. As a result, broader band gap in the interface of graphene and PC is formed due to the enlarged diversion of refractive indices in the interface. Figure 3(f) shows impact of chemical potential on the narrow band absorption. When the fermi level of graphene increases from 0.7 eV to 1.0 eV, the defect state progressively transfers from 4.78 THz to 4.82 THz with a step of 0.133 THz/eV and 94% of optical absorption has increased with increment of the graphene fermi level compare to graphene monolayer absorption.
Figure 4 (a) shows the heating mode operation of the structure and Fig. 4(b) depicts the cooling mode function with normal incident of light. The structure exhibits a moderate absorption at resonance peaks, which increases dramatically as the temperature rises over 340 K, when the VO2 shift from insulator to metal. The impact of temperature on the absorption spectrum on both metallic and insulating mode of defective layer as a function of wavelength is observed in Fig. 4(a) and (b). The structure has a strong absorption at resonant wavelengths which is maintained if the VO2 are heated below the critical temperature of 335 K. (left panel of Fig. 4). At 335 K, when the VO2 starts to shift phases, there is near to zero absorption in the transient period with a modest absorption of approximately 0.45 emerges after the critical condition. This weak absorption is exacerbated by raising the temperature. In addition, by cooling down the structure to 330 K it reversed the transition to the insulating state. The overall absorption spectrum is dramatically decreased lower value. For heat mode the range for narrowband is within 320-335K and cool mode it drops down to 328K. In case of broadband absorption, the metallic mode starts from 330K (cool mode) and 340K (heat mode) has a wide range of optical absorption in the THz range.
Fig. 4. Temperature dependent absorption variation with normal incident angle under (a) heating mode of VO2 (b) cooling mode of VO2.
3.2 Variations in the thickness of the defect layer
Defect layer thickness has an important impact in a defective 1DPC structure. Figure 5 shows performance of THz absorber with the variation of layer thickness. For the case of VO2 in the insulator phase, Fig. 5(a) shows number of peaks of absorption (with more than 50 percentile) increases with increment of defect layer thickness. Within 100µm range maximum of 5 peaks are observed. Although peaks of the absorption increase with increase of defect thickness but it decreases the Q factor. Figure 5(b) shows the increased thickness of the defect layer has decreased the overall absorption Q factor of the narrowband. An optimum thickness value of defect state is required for high peaks of absorption, enhanced light absorption and high Q factor. The optimum thickness for the proposed structure has peaked as 28µm. This thickness is taken throughout all the simulation and calculation of the article. Figure 5(c) shows the impact of Q factor in the phase changing factor or mixing ratio(f). If it is in IP (f = 0), it has a high factor with narrowband absorption. The increase f with phase shifting towards metallic drops down the Q factor as low as 18.5 within the range of 4-5 THz. Figure 5(d) shows impact on the absorption spectrum over the change of periodic number of the photonic crystal. It shows the increased number of photonic layers has reduced the absorption spectrum of the structure. With the increase of layers, overall transmittance of the structure will increase [40] and as a result the absorption will decrease with layer numbers.
Fig. 5. (a) Defect layer variations with number of peaks of absorption at frequency range within 4-5 THz and absorption percentile is above 50% (b) defect layer thickness variation with peak of absorption and Q factor of the device at frequency range within 4-5 THz and absorption percentile is above 50% (insulator phase) (c) Q factor dependency on mixing ratio of VO2 (d) periodic number variation of N1 = N2 = 5 (red line) and N1 = N2 = 15 (blue square line) over absorption of the structure.
3.3 Incident angle effect on different polarizations
The absorptance spectrum has impact on incoming incident angle along with different polarization of light. Figure 6 shows all the variations of incident angle under TE and TM mode in the proposed THz PC absorber.
Fig. 6. Incident angle dependency under insulating phase of (a) TE and TM mode (b) variations of peaks of absorption (red lines) in TE mode under insulation phase. (c) Absorption dependency on metallic phase of TE and TM mode (d) variations of peaks of absorption (red lines) and peak absorption frequency (blue lines inlet) in TE mode under metallic phase. Polarization dependency of the structure observed with TE/TM mode within two different angle (40° and 80°) within (d) insulating phase (f = 0) and (e) metallic phase (f = 0.95).
Figure 6(a) shows variation of angle incident within insulating phase of VO2 for both polarization mode. Multiple narrowband absorption spectrum is observed with polarization insensitive performance of the structure. Polarization independent absorbers are useful for electromagnetic screening compared to conventional polarization sensitive ones. Figure 6(b) shows the peak values of the absorption range within wide range of incident angle. The peak value stays around 0.9-0.8 within incident angle of 0°-50° in TE polarization.
Figure 6(c) depicts absorption variations at metal state of VO2 with both polarization mode. Figure 6(d) shows the peak of absorption variations with incident angle and increases with increment of incident angle. The inlet of Fig. 6(f) shows the shift of peak frequency with higher angle. Figure 6(e) and (f) shows variations of absorption with different incident angle with both polarization mode.
3.4 Analysis of the structure as unidirectional and non-reciprocal THz perfect absorber
Terahertz unidirectional devices are the extended counterpart of classical optical devices. To design the proposed structure work as non-magnetic THz nonreciprocal unidirectional devices and THz functionality-switchable devices two structurers are observed. The peak and the wavelength of the PBG within defect mode are adjusted by tuning different parameters.
Parameters such as polarizations, thickness of the defect layers and incoming angles has a big impact on defect tuning of the structure.
Symmetricity of the structure also has a deep impact on unidirectionality of device. Asymmetrical structure with (Si/SiO2)N1/(G/VO2/G)/(Si/SiO2)N2 and (Si/SiO2)N1/(G/VO2/G)/ (SiO2/Si)N2 as symmetrical structure is observed with the forward or backward propagation along the +/− x direction. Both structures show shift of absorptions with increase of chemical potential. Figure 7(a) shows when VO2 is in insulating stage ($T = {30^0}C)$, the forward propagation condition with TE polarization of symmetric structure shows high absorption peak at 4.72 THz. The peak is higher compared with other structures. In case of metal phase of VO2 ($T = {90^0}C)$, under normal angle of incoming photons at TE polarization, both symmetric and asymmetric structure in forward propagation has high absorption at around 4.2 THz in defect state bandgap. In both cases, for Fig. 7(a) and (b) the chemical potential is taken as 0.9 eV. Backward propagation shows no absorption within defect band gap. For this reason, forward propagated either symmetric or asymmetric structure of PC in metallic phase has unidirectional THz absorber. Alternatively in insulating phase the absorption from a low level to nearly 90% can be modulated by changing forward and backward propagation in a symmetric PC based defective structure at around 4.2 THz. Number of peaks of absorption varies with vary of forward and backward propagations. The TE polarization and angle dependent recital of the unidirectional THz absorber is observed within asymmetric structure. The absorption varies from zero to high value with backward propagation within the band gap. Figures 7 (c) and (d) show the effect of increment of chemical potential on the structure. The blue shift only occurs at the edges of insulation phase of the VO2 (Fig. 7(c)). However, no such shift is observed for absorption with chemical potential in metallic phase (Fig. 7(d)). These switching and unidirectional features of absorber has impacts on THz optical switching and filters.
Fig. 7. Absorption spectrum with symmetric and non-symmetric stacks under forward and backward incoming direction at (a) insulting (b) metallic phase in normal incoming angle. (c) Absorption spectrum with symmetric backward insulating phase for different values of chemical potential of graphene (d) Absorption spectrum with symmetric forward metallic phase for different values of graphene’s chemical potential.
3.5 Defect structure modification
A wavelength variance narrowband absorber with enhanced absorption can be achieved by defect mode adjustability. To achieve such phenomenon, a defect mode PC(DPC) design can be incorporated. In this paper, we introduced two patterns of defective PC to gain high Q factor along with multiple peak absorption. Figure 8 shows two different combinations of graphene-VO2 based defect structure. Figure 8(a) shows dual defect mode with Q-period distance between them. P-period and R-period are photonic dielectric periodic sequence. Figure 8(b) depicts multiple graphene-based defect mode 1DPC with N1 period, N2 period are periodic sequence number for dielectric and multiple graphene layer-based defect mode.
Fig. 8. The schematic diagram of (a) dual defect with different periodic number (b) multiple graphene-based defect mode.
The proposed two different patterns of structure are shown in Figs. 9(a) and (b). Dual defect layer with structure as (Si/SiO2)P/(G/VO2/G)/(SiO2/Si)Q/(G/VO2/G)/(Si/SiO2)R and multiple graphene layer addition with stack as (Si/SiO2)N1/(G/VO2/G)N2/(Si/SiO2)N1 . Figure 9(a) shows the dual defect mode of the proposed structure. Dual defect absorption spectrum appears in the PBG of the structure. Peak-to-peak distance of two defect mode shifts with the increased number of layers between two defects and enhanced the optical absorption to near perfect. Figure 9(b) shows the impact on metal phase and shows increment of layers in between the defect has no impact on the absorption but dual defect has introduced dual peak in the PBG of the structure.
Fig. 9. Absorption spectrum of dual defective structure under different periodic numbers (P =5, Q = 4, R = 5 red line and P = 5, Q = 20, R = 5 dotted green line) with (a) insulating phase, TE mode and (b) metallic phase, TE mode. (c) . Absorption spectrum of multiple graphene based defective mode with (a) insulating phase, TE (b) metallic phase, TE mode.
Figure 9(c) shows the impact of absorption spectrum with multiple numbers of graphene defect layers under insulating phase of VO2 and TE mode of the structure. Taking N1 = N2 = 5 and changing the layer number, increased optical absorptance in insulating phase are observed. Figure 9(d) shows that metal phase defect layer has no impact on the absorption of the structure with the increased layers of graphene-based defect. Table 1 categorize the possible defect combinations with graphene based PCM defect under different conditions. A comparison of the proposed work with reported works are summarized in Table 2.
Table 1. Summary of different defect states and modification in proposed structure:a
Table 2. Comparison of 1D PCs at THz range
4. Conclusion
In this work, a graphene-VO2 based defective photonic crystal shows unidirectional THz absorber with thermal switching from broadband to narrow band. Under the insulating phase of VO2 acting as a defect, the proposed structure features multiple narrowband optical absorption peak at 4.12, 4.86 and 5.23THz respectively with a Q factor around 291 for 4.86 THz peak. With dual band switching, the absorptance may be modified in both heat and cool mode functions of the PCM. With the phase change from metal to insulating state of the defect, the thermally dependent Q factor of the stack varies from 19 to 291. Temperature, chemical potential, incidence angle with dual polarization, and the periodic number of PC in both defective phases are used to observe the optical features of the defect layer. The rise in defect layer thickness is inversely related to Q factor and proportional to many absorption peaks. In both TM and TE modes, the absorption peaks have blue shifted as the incidence angle has increased. With forward and backward propagated waves, the optical non-magnetic THz unidirectional absorber has switchable propagation functions within the metallic phase from non-absorption to higher peak absorption. The findings also reveal that in TE polarization modes, the metallic phase of VO2 exhibits near-perfect absorption peaks at oblique incidence angles. The wavelength of the symmetrical dual defect layer with dual absorption peaks can be varied by adjusting the distance between the two peaks. The peaks of narrowband absorption have been boosted by many graphene-based VO2 flaws. This novel PCM-based defective photonic layer can be tuned for optimum and adjustable absorption in the THz range, as well as a non-magnetic reciprocal and unidirectional structure with temperature-dependent dual band watchability, making it ideal for terahertz wireless communication systems and other THz sensing devices.
Acknowledgments
This work has been done in the department of Electrical and Electronic Engineering, Bangladesh University of Engineering Technology (BUET). The authors thankfully acknowledge the support and facilities provided by BUET.
Disclosures
The authors declare no conflict of interest.
Data availability
Data underlying the results presented in this paper are available upon request.
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