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Abstract
This paper proposes a polarization-sensitive, metal-dielectric-metal (MDM) subwavelength grating structure based on surface plasmon resonance that achieves wide-angle, narrow-band, and high absorption in the long-infrared region. The resonance characteristics of the MDM structure, excited by magnetic resonance (MR), cause the transverse magnetic (TM) and transverse electric (TE) modes to polarize. A model of the inductor capacitor (LC) circuit is also presented. Structural simulations demonstrate a near-perfect absorption characteristic (99.99%) at 9 µm center wavelength. For TM polarization with incident angles ranging from 0° to 89°, the MDM grating structure produced absorption rates over 90%, 81%, and 71% for incident angles of 66°, 73°, and 77°, respectively. The absorption peaks in the long-wave infrared band can be adjusted by varying the duty cycle or period, without adjusting structural parameters. The spectral absorption curve shows a red shift and maintains high absorption, with wide-angle and narrow-band, across various azimuth angles (0–90°), during an increase in duty cycle or period. This method reduces the difficulty and complexity of micro-nano processing, and enables multiple absorbers in the long-infrared band (7.5–13 µm) to be processed and prepared on the same substrate surface.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Perfect optical absorption is an effective method to convert light energy into other forms of energy. Absorbers are commonly applied in several fields and help efficiently improve optical properties. In particular, the application of perfect absorption characteristics to detectors has attracted significant attention. In 1976, Maystre et al. first reported the phenomenon of total absorption in diffraction gratings [1,2]. In 1998, Ebbesen et al. studied the extraordinary transmission of periodic keyhole metal films at a specific wavelength [3] and progressive understanding of a metal grating structure at subwavelength dimensions. In 2008, Landy et al. conducted research on a perfect absorber based on artificial electromagnetic materials [4]. Additionally, in 2010, Liu et al. designed a metamaterial absorber using an infrared band for operational frequencies [5]. Absorbers in the 8–13 µm infrared atmospheric window band play an important role in the fields of infrared detection [6–8], photon detection [9,10], infrared imaging [11–13], infrared stealth [14–16], thermal emission [17], solar cells [18–20], biological sensors [21], astronomy [22], and spectroscopy [23]. Detector performance is enhanced by improving the absorption efficiency of the absorption layer of the infrared detector. Further, when combined with surface plasmon resonance, the absorbers can be applied to other fields, including sensing [24–27], filtering modulation [28–31], surface-enhanced Raman scattering [32,33], optical resonator [34], and Si-based detector [35]. Extending the working spectrum to the absorber can be applied to fields [36–40], including far-infrared terahertz spectrum that can strengthen the application of the phenomenon to bio-molecular, gas detection, medical diagnostics, spatial biosensing, far-infrared spectrometers, and detectors.
The performance of a perfect absorber is determined by the absorption rate. In addition, angular independence, broad-band or narrow-band spectra, and high absorption are important characteristics for determining appropriate applications. To improve the absorption efficiency of infrared spectrum detection systems, which rely on polarization with extended azimuth, a fast lens system with a small F number is generally required. However, this can limit the applications of slit-based spectrometers owing to their low efficiency and narrow-band angles. Most previous studies focused on broad-spectrum absorption in the long-wave infrared band [41–44], whereas only a few studies focused on examining narrow-band high absorption [45–47]. In Ref. [45], four types of metamaterial absorber (MA) structures have been proposed to achieve narrow band, single and double absorption in long-wave infrared bands. However, a center wavelength jump phenomenon occurs that makes it difficult to distinguish the working center wavelength. Under the incident angle of 0–90°, only the cross symmetric structure can achieve an ultra-high absorption rate, while the absorption efficiency of other structures remains poor in the wide incident angle range. In Ref. [14], two types of metal-insulator-metal (MIM) structures are proposed to realize narrow band single absorption and broadband absorption. The emissivity peak red-shifted from 7 µm to 14.8 µm, while the emissivity intensity increased from 0.78 to 0.98. A MIM structure for narrow band absorption of long wavelength infrared band has been proposed in Ref. [46]. In a wavelength range of 7 µm to 9 µm, the full width at half maximum (FWHM) of spectral curve broadens with the blue shift in wavelength and rapidly attenuates to both sides for peak absorptivity, when the structural absorption rate is modulated. Reference [14,46] discusses the absorption efficiency only under the condition of vertical incidence and does not consider different incident angles. Reference [47] employed a band-selective near-unity absorber based on the strong interference of light in an ultra-thin bi-layered system, comprising a metal-dielectric composite material known as cermet, and a plasmonic reflecting substrate. This structure produces the red shift phenomenon in a 5 µm to 9 µm band, and a FWHM value of the spectral absorption curve of above 2 µm, which is not conducive to the polarization spectrum and does not include polarization selectivity.
The enhanced physical absorption mechanism of the metal-dielectric-metal (MDM) structure has been explained in detail. The grating compensation method was applied to enhance the surface plasmon resonance. Additionally, the enhanced absorption of a long-wave infrared band was theoretically modeled by numerical simulations and finite element analysis. This method eliminates the problems previously associated with wide-angle, narrow-band high absorption in the long-infrared band. According to the resonance characteristics of the structure, the transverse magnetic (TM) and transverse electric (TE) modes of the incident light can be separated to realize polarization selection [48–53]. The MDM grating structure has near-perfect absorption efficiency for TM polarization, which was attributed to the magnetic resonance (MR) excitation and was further enhanced by the LC circuit model. The central wavelength of the absorption peak in the long-wave infrared band was adjusted by changing the duty cycle or period of the structure while maintaining its film parameters in the simulation and optimization stage. The spectral absorption curve shows a red shift with the increase in the duty cycle or period. Simultaneously, wide-angle, narrow-band high absorption was achieved under multiple azimuth angle (0–90°) conditions. The optical properties of this MDM structure significantly reduce the complexities of micro-nano processing, and allows efficient preparation and processing of multiple groups of different long-wave infrared narrow-band absorbers. Therefore, the absorber designed in this study has been equipped with the advantages of polarization sensitivity, wide-angle, ultra-wide azimuthal angle, and narrow-band high absorption, as well as facilitating an easily adjustable working central wavelength. Hence, the MDM structure is suitable for integrated applications in many fields, such as infrared polarization imaging and detection, thermal emission, biological sensors, and infrared polarization spectroscopy.
2. Structure design and simulation
Figure 1 shows the one-dimensional, subwavelength MDM grating structure based on the surface plasmon polariton principle and the propagation mode of the incident electromagnetic wave. The multilayer grating structure composed of seven layers: in a bottom to top sequence, starting with a metal aluminum (Al) film, and in between the zinc sulfide (ZnS) layer, zinc selenide (ZnSe) layer, and germanium (Ge) layer, was simulated by finite element analysis using the COMSOL Multiphysics software. Hence, the first three sandwiches are made of ZnS, ZnSe, and Ge. Al was selected for the metal film owing to its cost effectiveness and ease of processing. The permittivity of Al in the infrared region can be described by the Drude model:
where $\omega$ is the angular frequency of the incident plane wave, ${\omega _P}$ and ${\omega _c}$ are taken from [54]. The permittivity of Ge in the infrared region can be taken from [55]. The permittivity of ZnS and ZnSe in the infrared region can be taken from [56].
Fig. 1. Schematic of subwavelength MDM grating structure, for one-dimensional enhanced absorption. ($d = 2\mu m$, $f = 0.5$, $w = fd$, ${H_1} = 200nm$, ${H_\textrm{2}} = \textrm{900}nm$, ${H_3} = 70nm$, ${H_\textrm{4}} = \textrm{322}nm$, ${H_5} = 248nm$, ${H_\textrm{6}} = \textrm{182}nm$, ${H_7} = 76nm$.)
The geometric parameters of each layer are adjusted to obtain the optimal performance, and thus the width of the vertical cascade reactor is $w = fd$, the period of the structure is $d = 2\mu m$, the duty cycle is $f = 0.5$. The thicknesses of Al film in the metal layers are ${H_1} = 200nm$, ${H_3} = 70nm$, ${H_5} = 248nm$, and ${H_7} = 76nm$, respectively. The dielectric layers include ZnS, ZnSe, and Ge, with thicknesses of ${H_\textrm{2}} = \textrm{900}nm$, ${H_\textrm{4}} = \textrm{322}nm$, and ${H_\textrm{6}} = \textrm{182}nm$, respectively.
Figure 2 shows the spectrum curves for the absorption of TM and TE polarizations at normal incidence. The polarization direction of the TM wave is perpendicular to the grating lines, and that of the TE wave is parallel. When the TM and TE waves strike the surface at an angle (α), the electrical field distribution ensures that the TM waves are transmitted and the TE waves are blocked, because they use different coupling mechanisms to interact with the metal gratings. This contrast in the inherent polarization of TM and TE waves was shown in Fig. 2(a). Assuming that the transmission of the MDM structure was T (T=0), the absorption (A) was obtained via the reflectivity (R) using the relation A=1-R. The absorption of TM polarized waves at 9 µm was 99.99% and the MDM was thus a near-perfect absorber. In contrast, for TE, absorption was near 0%, regardless of wavelength. This structure can achieve a high absorption over a wide range of incidence angles, as shown in Fig. 2(b). The TM polarization spectrum curves range from 0–89°. At 9 µm the absorption was above 99% for 0–45° (the four lines for α = 0°, 20°, 40°, and 45° superimpose), and was over 90%, 81%, and 71% for angles of 66°, 73°, and 77°, respectively.
Fig. 2. (a) The absorption spectrum curves of TM and TE polarizations at normal incidence. (b) Spectrum curves for TM polarization, with incident angles 0–89°.
3. Results and discussion
The electromagnetic field distribution, at a resonant wavelength of 9 µm, was first simulated to determine the physical mechanism of polarization selection in the absorption-enhanced MDM structure. As explored in Section 2, the electromagnetic field with TM polarization displays localized resonance and absorption in a one-dimensional structure, while the electromagnetic field with TE polarization does not exhibit absorption. The electric field and magnetic field distributions of the TM and TE polarization resonances are shown in Fig. 3.
Fig. 3. At vertical incidence, (a) electric field and (b) magnetic field of TM polarization, and (c) electric field and (d) magnetic field of TE polarization.
As shown in Fig. 3(a), in the TM mode, a strong electric field exists on both sides of the spacer, particularly near the edge and corners of the topmost metal. The electric field vector circulates around the spacer. The magnetic field that forms the induced eddy current [57,58] was an order of magnitude larger than the incident magnetic field, which is limited to the first layer of the spacer. The distribution of the electromagnetic field exhibits a diamagnetic response, that is, a strong interaction between the magnetic field of the incident light and the magnetic moment generated by the circulating current. Strong resonance absorption occurs under the MR excitation. To explain the physical mechanism of the absorption effect, a model of the equivalent LC circuit predicted the MR state based on the distribution of electrical charge in the magnetic field [59–62]. The total impedance of the LC circuit can be expressed as follows:
By setting the total impedance to zero, a predicted resonant wavelength of 9 µm was obtained, which was in approximate agreement with the COMSOL Multiphysics simulated results. The MDM structure has the azimuthal wide-angle characteristic of TM polarization, because the magnetic field of the incident light was always perpendicular to the incident plane, irrespective of incident angle, and thus effectively generates the anti-parallel current. Notably, both sides of the LC circuit are periodic and infinite, and that the MR condition of the LC model was independent of the structural length along the magnetic field direction.
The proposed subwavelength MDM grating structure has the advantage of angle independence due to the long-infrared wavelength, wide-angle, narrow-band, enhanced absorption, and polarization selectivity. As the characteristics of the azimuth ultra-wide field angle are vital for practical application, the relationship between the spectral absorption function, azimuth angle β, and the incident angle α, with multi-angle incidence under TM polarization, was studied. Figure 5(a) shows that in TM polarization, with a wavelength of 9 µm, the absorption peak remains close to a 100% as the azimuth angle β changes from 0°, where the incident surface is perpendicular to the grating, to 90°, where the incident surface is parallel to the grating. When the incident angle α moves from 0° to 90°, for three fixed azimuth angles (β = 0°, 45°, 90°), the TM polarization peak absorption is consistent with the relationship between the wavelength and angle, as shown in Fig. 5(b). Therefore, the structure can maintain high absorption under arbitrary azimuth angles. This azimuth angle independence eliminates the limitations inherent in existing slit spectrometers and significantly improves the energy utilization rate of the spectrum detection system.
Fig. 5. (a) At vertical incidence, relationship between TM polarization absorption peak and azimuth angle. (b) For varying incident angle α, relation between TM polarization absorption peak, wavelength and incident angle for three fixed azimuth angles β (β=0°, 45°, 90°).
To efficiently process and prepare multiple groups of different long-wave infrared narrow-band absorbers, it is necessary to achieve wide-angle, narrow-band, and high absorption for multiple working wavelengths. In this study, the surface plasmon resonance was excited using the grating compensation method, and the enhanced absorption effects of the MDM grating structure were analyzed. The finite element analysis method was used for numerical simulation of the structure. The absorption efficiency and other optical properties of the structure were analyzed at different duty cycles or periods, while the values of the other membrane layer parameters remained constant. In Section 3.1, the analysis explains a method for changing the duty cycle without changing the structure period, and Section 3.2 details a method for changing the period without changing the duty cycle of the structure. In both cases, the thickness of each grating cascade layer was constant. Additionally, the MDM structure adjusted the central wavelength of the absorption peak in the long-wave infrared band, when only the duty cycle or period was changed, while other structural film parameters remained constant. The spectral absorption curve showed a red shift with an increase in duty cycle or period. Simultaneously, wide-angle and narrow-band high absorption can be achieved under the condition of multiple azimuth angles (0–90°).
To obtain the polarization spectra of different regions and wavelengths under the full array, the MDM structure was processed by the combined technologies of photolithography, etching, and coating. While the film thickness and materials of each structure remained constant, the center wavelength of the absorption peak was translationally adjusted, by changing only the duty cycle or period in the simulation optimization stage. In the full array area, the multiple strip micro-nano structures were coated with the same film layers and processed simultaneously. This method overcame the order of magnitude increased in the time taken for traditional coating as multiple bands were coated at different times, the challenge of relative parallelism between multiple bands was also overcome, which simultaneously improved processing efficiency while reducing the processing difficulty and complexity of spectroscopic polarization devices in the full array area.
3.1 Influence of duty cycle on absorption with fixed period and fixed thickness of each grating cascade layer
The enhanced absorption of the MDM grating structure was further simulated using the finite element analysis method. The red shift of the working center wavelength in the long-infrared band was realized by varying the duty cycle, when the period and thickness of each grating cascade layer were fixed. Under vertical incidence, the central wavelength from 7 µm to 11.5 µm, exhibited an approximately linear red shift with an increasing duty cycle (0.39–0.58), and overall absorptivity of over 90%, as shown in Fig. 6(a). When the incident angle reached 47°, the absorptivity exceeded 80%. Through analysis of the simulation data, the overall absorption of the working center wavelength from 7.5 µm to 10 µm was determined to be over 94%, as shown in Fig. 6(b). When the incident angle increased further, the absorption efficiency and resonant wavelength decreased. However, under the condition of a fixed working center wavelength, when the incident angles were 44° and 60°, the absorptivity remained over 90% and 80%, respectively.
Fig. 6. Under vertical incidence, (a) The working center wavelength and absorption with varying duty cycle values. (b) The overall absorption of working center wavelength for varying duty cycle values. (c) The relationship between TM polarization absorptivity and duty cycle with different wavelengths.
For TM polarization with incident angles 0–89°, the absorption rates of working center wavelengths as a function of wavelength and incident angle were shown in Fig. 7, with further details in Table 1.
Fig. 7. The relationship between TM polarization absorptivity and incident angle with different wavelengths. The central wavelengths are (a) 7.5 µm, (b) 8 µm, (c) 8.5 µm, (d) 9 µm, (e) 9.5 µm, (f) 10 µm, and (g) 10.5 µm.
Table 1. The relationship between TM polarization absorptivity and incident angle with different wavelengths
Under vertical incident conditions and when the azimuth varies from 0° to 90°, the absorptivity as a function of wavelength and azimuth were shown in Fig. 8. The MDM structure facilitates wide-angle, narrow-band, and high absorption with an ultra-wide field angle, thereby enhancing the spectral absorption rate in the long-infrared band, with different working center wavelengths. This provides a useful theoretical model for practical engineering applications.
Fig. 8. For TM polarization, with azimuth angle 0–90°, the absorptivity of the working center wavelength as a function of wavelength and azimuth angle. (a) 7.5 µm, (b) 8 µm, (c) 8.5 µm, (d) 9 µm, (e) 9.5 µm, (f) 10 µm, and (g) 10.5 µm.
The absorption efficiency was very high when the duty cycle increased from 0.3 to 0.8. Additionally, a secondary peak appeared in the duty cycle range 0.4–0.9, resulting in double absorption peaks in this interval, or the phenomenon of three absorption peaks occurred. Under the condition of vertical incidence, Fig. 9(a) determined the dual-wave absorption peaks generated at near- and mid-infrared bands, when the duty cycle was 0.29. The absorption peaks of the working center wavelengths were at 2 µm and 5 µm, with absorptivity of 81.54% and 85.98% respectively. Figure 9(b) determined the dual-wave absorption peaks generated in near- and long-infrared bands at a duty cycle of 0.42, under the condition of vertical incidence. The corresponding values for the absorption peaks were 2.5 µm and 7.5 µm, with absorptivity of 88.08% and 95.88%, respectively. Figure 9(c) determined the three wave absorption peaks in near-, mid-, and long-infrared bands at the duty ratio of 0.88, with associated absorption peaks at 2.5 µm, 4.5 µm, and 10.5 µm, and absorptivity of 86.78%, 83.84%, and 91.45%, respectively. If applied to a detector, the dual and the triple absorption peaks was determined by the area array detector partition, and a spectral response to short-, medium-, and long-wave targets could be achieved simultaneously, and thus enhance target detection.
Fig. 9. The IR wave absorption peaks under TM polarization and vertical incidence, for duty cycle values of (a) 0.29, produces near- and mid-infrared wave absorption peaks, (b) 0.42, produces near- and long-infrared wave absorption peaks, and (c) 0.88, produces three wave absorption peaks. In each case the corresponding electric field modulus is shown as an inset.
3.2 Influence of period on absorption with fixed duty cycle and fixed thickness of each grating cascade layer
With increasing period, the MDM structure also exhibited a red shift in the long-infrared band for different working center wavelengths. Further, the structure demonstrates improved stability, wide-angle, narrow-band, and high absorption with ultra-wide field angles, thereby producing enhanced spectral absorptivity, and hence potentially wider application. The absorptivity as a function of wavelength and period of TM polarization are shown in Fig. 10(a) and (b). Under vertical incidence, the central wavelength exhibited a red shift with increasing period, and the red shift is approximately linear in the wavelength range 7–14µm. From 1.8 µm to 2.1 µm, the absorptivity of over 92%, and from 3.5 µm to 4.9 µm the absorptivity of over 90%. The absorption rate of over 90% in the long-infrared range. The spectral absorptivity of the working center wavelengths in the long-infrared band of over 90% under the condition of vertical incidence, were shown in Fig. 10(c). Figure 11(a)–(k) determined the absorptivity as a function of wavelength and incident angle of TM polarization, where the working center wavelength was 8–13 µm and the incident angle was 0–89°. The findings were explored further in Table 2.
Fig. 10. Relationship between wavelength and period when the period is (a) 1.8–2.1 µm, and (b) 3.5–4.9 µm, (c) Relationship between wavelength and absorption for varying periods.
Fig. 11. The relationship between TM polarization absorptivity and incident angle with different periods. The periods are (a) 1.8 µm, (b) 1.9 µm, (c) 2.0 µm, (d) 2.1 µm, (e) 3.7 µm, (f) 3.9 µm, (g) 4.1 µm, (h) 4.3 µm, (i) 4.5 µm, (j) 4.7 µm, and (k) 4.9 µm.
Table 2. The relationship between TM polarization absorptivity and incident angle with different periods
Under vertical incident conditions and when the azimuth varies from 0° to 90°, the absorptivity as a function of period and azimuth are shown in Fig. 12.
Fig. 12. For TM polarization, when the azimuth angle changes from 0° to 90°, the absorptivity of the working center wavelength as a function of period and azimuth angle (a) 8 µm, (b) 8.5 µm, (c) 9 µm, (d) 9.5 µm, (e) 10 µm, (f) 10.5 µm, (g) 11 µm, (h) 11.5 µm, (i) 12 µm, (j) 12.5 µm, (k) 13 µm.
The absorption efficiency was very high when the approximately linearly increased in the period of 1.8–2.1 µm and 3.2–4.9 µm. Additionally, under the condition of normal incidence and constant duty cycle of the structure, the center wavelength of a specific period will produce double absorption peaks or three absorption peaks with high absorptivity. Figure 13(a) shows the dual-wave absorption peaks generated in mid- and long-infrared bands at a period of 4.5 µm. The absorption peaks and absorptivity of the two working center wavelengths were 4 µm and 12 µm, and 97.44% and 98.40%, respectively. Figure 13(b) shows the dual-wave absorption peaks generated in near- and long-infrared bands at a period of 5.9 µm. The absorption peaks and absorptivity of the two working center wavelengths were 2 µm and 6 µm, and 99.87% and 77.53%, respectively. Figure 13(c) shows the three wave absorption peaks in near-, mid-, and long-infrared bands at a period of 3.1 µm. In this case, the absorption peaks and absorptivity of the three working center wavelengths were 2.5 µm, 4.5 µm, and 8.5 µm, and 69.34%, 94.61%, and 80.02%, respectively.
Fig. 13. The IR wave absorption peaks under TM polarization and vertical incidence, for periods of (a) 4.5 µm, mid- and long-infrared double wave absorption peaks are produced, (b) 5.7 µm, near- and mid-infrared double wave absorption peaks are produced, and (c) 3.2 µm, three wave absorption peaks are produced. In each case the corresponding electric field modulus is shown as an inset.
4. Conclusion
This paper presents a polarization-sensitive MDM grating structure with subwavelength dimensions based on the surface plasmon resonance principle. The structure possesses the advantages of wide-angle, narrow-band, and high absorption in the long-infrared band. Using MR resonance, the structure obtains near-perfect (99.99%) absorption characteristics at 9 µm center wavelength. Under the TM polarization condition, the absorption spectrum curve of the incident angle ranges from 0–89°, and the absorption rate is above 90%, 81%, and 71% when the incident angles are 66°, 73°, and 77°, respectively. Alongside this, the absorber works well, irrespective of the azimuth angle range. Using the finite element method and changing only the duty cycle, while keeping the central wavelength in the range of 7.5–10 µm, TM polarization incidence angles of 44° and 60° produced absorption rates above 90% and 80%, respectively. When only the period is changed, and the central wavelength is in the range of 8–12.5 µm, TM polarization incidence angles of 37° and 49° produced absorption rates above 90% and 80%, respectively. Similarly, under the condition of vertical incidence, the double and triple absorption peaks generated narrow-band, high absorption peaks in the near-, mid-, and long-infrared regions.
The study demonstrates that the MDM structure can adjust the central wavelength of the absorption peak in the long-wave infrared band, when only the duty cycle or period is varied and other parameters of the structural films remain unchanged. With an increase in the duty cycle or period, the spectral absorption curve shows a red shift. Simultaneously, wide-angle and narrow-band high absorption can be achieved with varied azimuth angles (0–90°). This method reduces the difficulty and complexity of micro-nano processing and can simultaneously process and prepare multiple groups of different long-wave infrared narrow-band absorbers, efficiently. Therefore, by utilizing the characteristics of this structure, multiple narrow-band absorbers in the long-infrared band (7.5–13 µm) can be processed and prepared on the same substrate surface. Thus, optical absorbers could be fabricated with wide-angle and narrow-band high absorption peaks in the long-infrared band.
Funding
Foundation of Equipment Pre-research Area (JZX7Y20190254047001); National Defense Basic Scientific Research Program of China (JCKY2018203B036, JCKY2019110); National Key Research and Development Program of China (2016YFB0501000, 2016YFB0501002).
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that supports the findings of this study are available within the article.
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