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Abstract
Recently, light absorbers have attracted great attentions due to their promising in applications in functional optoelectronic devices. Herein, we theoretically propose and numerically demonstrate a new absorber platform, which consists of a 280-nm-thick photonic nonlinear waveguide film covering on the metal grating structure. Strong reflection inhibition and absorption enhancement is achieved in both the forward and backward directions, which indicates potential novel performances since the previous reports only achieved absorption in one side due to the using of opaque metal film substrate or the reflective mirror. The anti-reflection bands or the absorption peaks at the shorter and longer wavelength ranges are related to the excitation of the propagating surface plasmon resonance by the slit-assisted grating and the cavity mode by the slit in the metal film. Strong differential manipulation is realized for the double-face absorbers via the all-optical operation. Moreover, the operation wavelengths for the double-face light absorber can be modified strongly via using an asymmetric dielectric medium for the coating films. These new findings pave approaches for subtractive lightwave modulation technology, selective filtering, multiplex sensing and detection, etc.
© 2022 Optica Publishing Group under the terms of the Optica Open Access Publishing Agreement
1. Introduction
Plasmonic resonant nano-structures have been demonstrated for a lot exciting optical phenomena, such as the enhanced solar absorption and photovoltaics [1–3], optical sensing and detection [4–6], spectral filters [7–9], and color generation and displaying [10,11]. Most of these impressive applications are with strong relevant to the unique properties of the electromagnetic field enhancement via the localized or propagating surface plasmon resonances, which can be feasibly tuned by the structural features of the metal resonators. Recently, light absorbers have been achieved via the metal-insulator-metal metamaterial, which was with the capability to support strong absorption by the electric and magnetic resonances [12]. Since then, extraordinary optical absorption or the so-called perfect absorption have been investigated in a wide range via different structural systems [13–17]. The typical way is to form the metallic resonators layer on the surface of dielectric buffer layer, which is then supported by an opaque metal mirror [12,18–20]. In addition, metallic film structures together with the phase-change material [21] and graphene [22] and even the varactor diodes [23] have used to pave typical ways for tunable absorbers. In these absorbers, the bottom thick metal film is a standard component with the purpose to wholly cancel the transmission.
Otherwise, all-metal light absorbers were proposed for strong resonant field distributions in the surface area, which is desirable for sensing and detection [24]. Grating or the resonators array were widely used in these systems to produce the lattice resonances and their hybridized coupling [18]. A thick metal substrate layer is also needed for these situations. Metallic gratings have also used for applications in biosensing [25], plasmonic Bragg reflection [26]. Meanwhile, silicon subwavelength gratings were widely used for metamaterials and couplers [27–29].
In addition, super absorption was achieved via the resonators based metasurface, which was coated by the distributed Bragg reflector [30–32] and coupled resonant metasurface [33,34]. The reflector can extremely cancel the transmission. These kinds of absorbers were based on the resonators and the mirror substrate, which therefore only produced the high light absorption in single directional channel, let alone the high reflection or the low absorption at the rear side direction. For instance, high absorption can be only achieved in the forward direction to the resonators metasurface for the incident light. In contrast to this, high reflection occurs when the incident light is along to the backward direction since there is a mirror formed by the metal substrate or the distributed Bragg reflector. As a result, only one-side high absorption can be achieved for the conventional absorbers.
In this work, we propose and demonstrate a platform for ultra-thin double-face light absorbers via etching the metal film with subtractive cavities in its two surfaces. Moreover, nonlinear medium is introduced as the coating layer component. Based on the laser pumping at different spatial positions, the absorber can produce distinct spectral responses: one side absorber can be almost maintained while the other one shows remarkable scaling in the wavelength range. The all-optical manipulation can therefore deepen the operation of the double-face absorption. Otherwise, the double-face absorption spectra can be remarkably shifted to achieve a large displacement distance at the wavelength ranges via choosing different dielectrics to act as the waveguide films. These impressive findings can pave applications in high-compact and sub-wavelength optoelectronic devices [35,36].
2. Result and discussion
Figure 1 presents the schematic of the proposed double-face efficient anti-reflection and absorption platform, which consists of a metal film with the slits on the surfaces and the coating dielectric layers on the both sides. The slits array on the top surface is only with one slit cavity in the unit cell for the periodic array. Otherwise, two slits are introduced in the bottom surface for the resonant unit. The differential surface geometry features then introduce different surface plasmon resonances. As the schematic shown in Fig. 1, under the forward (FR) and backward (BR) excitations, the metal-grating surfaces show the light reflection and absorption behaviors related to the structural characteristics. For instance, the FR excitation means the illumination is along the negative z-direction onto the surface with only one slit in the top surface of the metal film during the single period. As a result, the single metal-grating system can be used for the double-face light flow manipulation, which shows a novel response in contrast to the one-side light absorption by the conventional metallic absorbers [17,20,37]. In order to achieve a high light absorption, we set the metal film to be 100 nm thick. Gold is used as the metal due to its high stability. The material permittivities of gold are obtained from the experimental data [38]. The dispersion of the materials is also included during the simulation. The meshing size for the meta-surface range is set to be 3 nm along the y-direction and 5 nm along x-direction. Otherwise, the size of the simulation domain in the y-direction is about 8µm. In the other area, the system is set to be a high accuracy order and the minimum mesh step is down to 2.5e-5 µm. The slit is with the width of 20 nm and the height is set to be 50 nm. The period of the array is 700 nm. The displacement δ for the slit at the bottom away from the slit at the top layer is set to be 125 nm.
The material in the slit is the Kerr nonlinear medium (such as, LiNbO3 crystal) [39] with the refractive index n = n0 + n2, where the n0 is 2 and the n2 is 5×10−3 µm2/W (related to the nonlinear susceptibility χ(3)∼1×10−8 esu, 1 esu = 1 cm3/erg, 1 erg = 1×10−7 J) [40,41]. Based on the nonlinear coating layer, it is then feasible to tune the refractive index of the dielectric medium following the definition of n = n0 + In2, where the I is the intensity of the pumping laser. As a result, an all-optical manipulation can be realized for the artificially adjusting of the double-face absorption.
Meanwhile, in order to keep the absorber platform satisfactory for the high-compact components and devices, we just use a 140 nm nonlinear layer to coat on the metal-grating substrate, indicating a sub-wavelength multi-functional light flow manipulating model. Finite-difference time-domain method is employed for calculating reflection, transmission, absorption and the optical field distributions. Periodic boundary conditions are used along the x-direction to reproduce the array. Perfectly matched layers are used as the boundary conditions at the light input and output sides in the z-direction to cancel the additional scattering. For the study on the cases of the forward and backward manipulations, a plane wave source with linear polarization along x-direction is used as the incident illumination.
As for the model, the experimental fabrication can be realized via the well-developed atomic layer lithography [42] and the laser lithography methods [43]. These techniques have been widely used to form the nano-gap or nano-slit arrays. For instance, sub-5-nm gaps have been realized for optical field confinement [42]. Split-wedge antennas with sub-5-nm [44] and coaxial nano-apertures with 10 nm gap [45] were also demonstrated for optical trapping and nano-focusing. Otherwise, in order to keep the structure being simple enough, the relatively large gap size with the width above 20 nm is used in this model. For the coating film, the physical depositions such as the sputtering technique or electron beam evaporation can be used to realize the waveguide layer.
Figure 2(a) shows the spectral reflection, transmission and absorption for the system under the forward illumination. It is observed that two reflection dips occur in the near-infrared range. In the same wavelength range, slight transmission bands are observed, suggesting the relatively weak transmission for this grating based metal film structure. The 200-nm-thick metal film with just 50-nm-height slits in its both surfaces is the main reason for the obtained weak transmission. For instance, except the slits in the film's surface, a continuous metal layer with the normal thickness of 100 nm still exists in the middle area, which is much larger than the skin depth. These features eventually ensure the weak transmission. Spectral absorption A can be achieved with the definition of A = 1 - R - T, where the R and T represents the reflection and transmission. As a result, two absorption peaks are obtained. At λ = 956.5 nm, the A reaches 88.8%, suggesting a high absorption. At λ = 1423 nm, the A reaches 85.7%. It should be noted that the high absorption is realized by a relatively thin metal film.
Fig. 2. (a), (b) Spectral reflection, transmission and absorption for the forward and backward illuminations, respectively. (c) Reflection curves under the forward (FR) and backward (BR) illuminations.
As shown in Fig. 2(b), two absorption peaks are also achieved when the light is illuminated along the backward direction. In particular, the absorption reaches 97.4% for the peak at λ = 982.5 nm. It suggests a near-unity absorption. At the longer wavelength range, the peak is also with the A up to 65.5%. These features confirm the realization of the double-face anti-reflection and high absorption supported by the single metal film structure. Moreover, as the reflection curves shown in Fig. 2(c), the dual-band anti-reflection indicates noticeable displacement for each other in the wavelength range. For instance, the wavelength shift is up to 26 nm for the absorption band at the shorter wavelength range. These features confirm that the double-face spectral responses are with selective behaviors to the structural geometries and also hold the feasibility for differential operation regions.
Figure 3(a) shows the resonant field distributions for the absorption peaks when the light is excited along the forward direction. At λ = 956.5 nm, electric field is not only confined in the slit but also located in the dielectric film, which is also confirmed by the magnetic field distribution pattern. These features suggest the excitation of the propagating surface plasmon resonance by the array. At λ = 1423 nm, electric and magnetic fields are all strongly concentrated in the slit area, indicating the excitation of the gap cavity resonance [46–49].
Fig. 3. (a), (b) Normalized electric and magnetic field intensity distributions for the absorption peaks under the forward and backward illuminations, respectively.
The field distributions for the resonances occurred under the backward excitation are shown in Fig. 3(b). It presents the similar characteristics to that of the surface plasmon resonance in grating and the cavity mode in photonic structure for these two absorption bands. Therefore, the anti-reflection bands or the absorption peaks at the shorter and longer wavelength ranges are related to the excitation of the propagating surface plasmon resonance by the slit-assisted grating and the cavity mode by the slit in the metal film. In addition, it should be noted that the resonant field distributions are strongly related to the geometry features for the forward and backward surfaces in the metal film, which directly leads to the different optical responses under the FR and BR illuminations. The different geometrical features in the rear sides eventually lead to the differential resonant behaviors.
Via tuning the polarization and incident angles under the forward excitation, the reflection evolutions are shown in Fig. 4. It is observed that the dual-band anti-reflection becomes weaker when the polarization angle is increased from 0° (TM) to 90° (TE). It suggests the resonant modes are polarization-dependent. The results in Fig. 4(a) also indicate the way to tune the spectral intensity via the polarization state. Figure 4(b) shows the reflection mapping when the incident angle is changed from 0° to 40° under the TM polarization. For the reflection dip at the shorter wavelength range, two bands are observed under the oblique excitation. For the resonant mode at the longer wavelength range, it is nearly stable under the tuning of the incident angle, suggesting the angle-insensitive absorption. These properties also confirm that the propagating surface plasmon resonances and the cavity mode are the main contributions for the obtained anti-reflection bands.
Fig. 4. (a) Reflection mapping picture for the system under a tuning of polarization angle. (b) Spectral reflection evolution for the TM polarization (electric field along x-axis) under a tuning of incident angle.
Following, we do further study on the manipulation for this double-face reflection inhibition system via changing the material properties and structural characteristics. Figures 5(a) and 5(b) show the reflection curves when the illumination is along the forward and backward direction as a function of the refractive index of the up medium layer. The operation is realized via using a laser pumping onto the top nonlinear dielectric. For instance, the index can be increased to 2.005 when a laser intensity is increased from 0 to 1.0 W/µm2. It is clearly observed that the large spectral shift for the dual-band anti-reflection under the forward case. Nevertheless, the reflection bands for the backward situation are almost stable during the tuning process. In order to clearly show the spectral properties, Figs. 5(c) and 5(d) present the plotted wavelength positions for the anti-reflection bands (λ1, λ2) under the forward and backward illuminations as a function of the index of the top dielectric layer. In contrast to the flat curves of the situation under backward illumination, remarkable spectral shifts are obtained for the forward situation. For instance, at λ1, a relatively large shift of 7 nm is achieved for the forward case while the position for the backward situation is stable. At λ2, the wavelength shift (Δλ) amplitude for the forward situation (Δλ=10.5 nm) is ∼420% to that of the backward situation (Δλ=2.5 nm). These features confirm the strong differential manipulation (different responses of the bidirectional absorption) for the double-face absorbers via the all-optical operation. It should be noted that such optical operation is carried out based on the artificially modified change to the surrounding medium for the plasmonic structure and eventually leads to the manipulation on the resonant response. Thereby, other potential applications such as the electro-optical manipulation and multidimensional manipulation of wave fields [50–54] can also been realized in the similar operation process via using the nonlinear optical or phase-change materials.
Fig. 5. (a), (b) Forward and backward reflection curves for the system under a slight change of the up nonlinear dielectric via the laser irradiation, respectively. (c), (d) Resonant anti-reflection positions for the modes (λ1, λ2) as a function of the refractive index of the dielectric, respectively.
The related spectral intensity changes for the reflection after the all-optical operation is shown in Fig. 6. In comparison with the curves without the laser pumping, the reflection curve of the forward illumination shows a large amplitude close to 0.5 when a 0.02 change of the index is introduced by the pumping intensity of 4.0 W/µm2. The reflection intensity change is only ∼ 0.1 in the same process for the spectrum under the backward illumination. These properties also show the differential manipulation on the spectral intensity for the double-face resonant structure via the laser pumping operation.
Fig. 6. (a), (b) Reflection intensity changes after the all-optical manipulation for the forward and backward situations.
Figure 7(a) shows the reflection evolution under the forward illumination via tuning the film thickness of the dielectric medium. With increasing the film thickness, the spectral dips show red-shifts. Moreover, the spectral intensity for the dips also show noticeable changes. Figure 7(b) presents the plotted reflection intensity. For the mode at λ1, the reflection decreases quickly when the dielectric medium is increased from 10 nm to 90 nm. Then, the reflection is maintained in a low intensity. As for the mode at λ2, the reflection retains at a weak intensity range for the film thickness less than 90 nm and then increases noticeably. It mainly results from the enhanced optical field coupling and confinement for the propagating surface plasmons by the dielectric waveguide with a relatively thick film. Nevertheless, for the cavity mode (λ2), the thick coating dielectric film with a relatively large refractive index leads to the additional reflection.
Fig. 7. (a) Reflection curves for the system under different thicknesses of the dielectric film. (b) Plotted reflection intensity for the dips (λ1, λ2) as a function of the film thickness.
Reflection responses for the system under different values of spatial displacement (δ) for the slit away from the center are shown in Fig. 8(a). The anti-reflection can be maintained well if the displacement is less than 25 nm. Increasing the δ from 50 nm to 75 nm, two reflection dips are observed at the longer wavelength range, suggesting a spectral splitting occurred via the strong near-field coupling by the top and bottom slits. When the δ is close to 125 nm, that is, the top slit is close to the position of the one slit at the bottom surface, the coupling effect disappears and the grating resonance is also strongly rebuilt. Keep increasing the δ to a large value, the slits in the top and bottom surfaces will be again changed from a strong coupling effect to a weak coupling evolution. Figure 8(b) shows the wavelength positions for the dual-band anti-reflection via tuning the slit width. It is observed that the positions for the two bands both show a periodic fluctuation when the slit width is increased at the step of 10 nm. Moreover, the anti-reflection is observed to be occurred at the shorter wavelength ranges under the larger slit width. These features indicate the spectral manipulation in the frequency region by the slit cavity width. The strong cavity resonances and the their coupling to the surface plasmons are the main contributions for this photonic adjusting phenomenon.
Fig. 8. (a) Reflection curves for the slit with different spatial displacement (δ) values. (b) The plotted positions of the reflection dips as a function of the slit width.
In order to clearly show the feasible way for achieving large differential frequency spectrum for the double-face absorber, a further study on the asymmetric dielectric materials for the front and back coating films is carried out. As shown in Fig. 9(a), the reflection curves for the FR and BR situations are plotted by changing the bottom dielectric index from 1.45 to 2.45 while keeping the top layer with a stable index of 2.0. It is observed that the reflection curves for the FR are nearly stable when we change the index of the bottom medium. However, the reflection curves for the BR show a remarkable shift in the spectrum. Large spectral displacement of the position is therefore achieved. Figure 9(b) shows the plotted positions for the anti-reflection modes (λ1, λ2) of the FR and BR as a function of the index for the bottom medium. The wavelength positions for the modes under the BR situation are strongly shifted. For instance, the position is increased from 822 nm to 1129.5 nm for the mode λ1 of the BR case. Nevertheless, there is only 2 nm red-shift for the reflection mode λ1 under the FR situation. For the reflection mode λ2, the spectral red-shift for the BR situation reaches 674.5 nm while the red-shift is only 1 nm for the FR situation. Thereby, the operation wavelengths for the double-face light absorber can be modified strongly via using an asymmetric dielectric medium for the coating films.
Fig. 9. (a) Reflection curves for the system via tuning the refractive index of the bottom film while keeping the up film with the index of 2.0. (b) Plotted positions of the resonant reflection dips at different index values.
Finally, comparison on the structural features and the absorption characteristics with other reports in the bidirectional perfect absorption is summarized in Table 1. It is observed that the typical bidirectional perfect absorbers were formed by the multi-layered metallic nano-structure films together with the dielectric layers [55,58,60,61]. The metal films are with the capability to support resonant absorption under the FR or BR situations. Usually, these absorbers presented the similar absorption behaviors under the FR and BR excitations. In this work, we used a single metal film intercalated with slits in the surfaces to realize a bidirectional perfect absorber, which can even introduce different absorption features for the FR and BR illumination. Moreover, the dynamic all-optical tunable absorption is also realized.
Table 1. Comparison with the typical bidirectional perfect absorbers
3. Conclusion
In conclusion, we have proposed and demonstrated a novel double-face anti-reflection and high absorption platform, which consists of a dual-grating meta-surface structure coated with ultra-thin dielectric medium layers. The metal film etched with slit cavities in both surfaces was utilized for the excitation of surface plasmon resonances. Additionally, Kerr nonlinear dielectric medium is used to coat the metallic meta-surface and simultaneously forms the optical field confinement component. The absorber can support the differential spectral responses under the forward and backward illuminations, suggesting a simultaneous spectral manipulation for the both directions. Moreover, the double-face absorption can be artificially tuned by the all-optical operation and shows the differential spectral responses for the forward and backward situations. Moreover, the double-face anti-reflection spectra can be largely scaled in the wavelength range via using the different dielectrics as the waveguide layers for the metal grating, which finally produces the far separated resonant modes for the forward and backward illuminations. These new findings can pave approaches for subtractive lightwave technology, selective double-face filter, all-optical modulators, etc.
Funding
National Natural Science Foundation of China (62065007, 11804134, 11664015, 51761015); Natural Science Foundation of Jiangxi Province (20202BBEL53036, 20202BAB201009).
Disclosures
The authors declare that they have no competing interests.
Data availability
Data underlying the results presented in this paper are not publicly available at this time but may be obtained from the authors upon reasonable request
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