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Abstract
We propose and demonstrate a new kind of resonant absorber via introducing the nano-slit into a photonic film. The combination of the nano-slit cavity and the photonic waveguide provides a powerful way to manipulate the light behaviors including the spectral Q factors and the absorption efficiency. Ultra-sharp resonant absorption with the Q factors up to 579.5 is achieved, suggesting an enhancement of ∼6100% in contrast to that of the metal-dielectric flat film structure. Moreover, in comparison with the low absorption of 5.4% for the system without nano-slit, the spectral absorption is up to ∼96.6% for the nano-slit assisted photonic absorber. The high Q resonant absorption can be further manipulated via the structural parameters and the polarization state. The operation wavelengths can be tuned by the lattice constant. As the nano-slit introduced into the dielectric film, strong optical field confinement effects can be achieved by the cavity resonance via the nano-slit itself, and the guided resonant effect in the photonic waveguide cavity formed by the adjacent nano-slits. Otherwise, the photonic-plasmonic hybridization effect is simultaneously excited between the dielectric guided cavity layer and the metal substrate. These findings can be extended to other photonic nano-cavity systems and pave new insights into the high Q nano-optics devices.
© 2021 Optical Society of America under the terms of the OSA Open Access Publishing Agreement
1. Introduction
Metallic nano-gap cavities have been widely used in the design of the high-performance optoelectronic devices, especially for the enhanced plasmonic resonances and their optical fields manipulations. For instance, the gap plasmons concentrated by the two opposing metal surfaces have been used to introduce the deep sub-wavelength resonances [1–3]. Plasmonic nano-gap and nano-slit structures have been widely employed for enhanced Raman scattering technique [4–6] and optical rectification [7], and biosensors [8]. Open nano-slit and even the tilted nano-slit have also been used in the metal film structures for polarization-multiplexed plasmonic phase generation [9] and spatial field distribution [10], and plasmonic directional beam switching [11,12].
In 2008, a theoretical study indicated that the nanometric silver lamellar gratings could present strong light absorption in the visible range due to the quasistatic surface plasmon polaritons in the metal grooves [13]. Then, deeply sub-wavelength slit arrays [14] and gratings [15] have been used to adjust the light extinction and scattering. Owing to the efficient scattering of the free waves by the gap-surface plasmons in these very narrow and shallow metallic slits, strong reflection inhibition was achieved in a large spectral range. Recently, the strongly excited surface plasmons have further developed to study the coupling between the gap plasmons and the polariton surface plasmon resonances in the metal-dielectric-metal arrangements [16,17], which presented the multiple broadband absorption spectra. Based on the combination of the metal nano-slit array and the dielectric Fabry–Pérot resonator, broadband plasmonic absorption was achieved. The absorption peaks were with relatively low Q factors down to the single level due to the high loss by the metallic plasmons. For instance, omnidirectional nearly perfect absorber with the Q factor ∼10 was obtained with ultra-thin 2D subwavelength metal grating in the visible region [18]. Via using the periodic metallic nano-ribbon arrays in the metal-metal-dielectric structure, narrowband absorption with the bandwidth of 1.11 nm was achieved [19]. Broadband absorbers were also realized via the stacked metal-dielectric grating [20]. Resonant perfect optical absorption was observed in dielectric film supporting metallic grating structures [21]. Multi-layer dielectric grating and metal compound substrate was used for ultra-narrowband absorption [22]. As for these metallic nano-structures based absorbers, the intrinsic high loss due to the strong oscillations by the electrons inevitably leads to the low Q resonances. Recently, dual-band absorber formed by the asymmetric dielectric grating on the metal film was achieved [23]. Nevertheless, the resonant modes are also with a low Q factor level. Moreover, due to the large open cavity between the adjacent dielectric patches, the resonant fields are leaky into the space, suggesting the weak confinement of the optical flow.
In this work, we propose and numerically demonstrate a novel nano-slit enabled high Q resonant absorber based on a dielectric film supported by an opaque metal substrate. In contrast to the widely investigated models in the plasmonic metal films hollowed with nano-slits or nano-gaps, herein, a dielectric waveguide layer hollowed by an air slit cavity array is used as a multi-functional metasurface to introduce efficient optical field scattering and confinement. Based on the strong resonant coupling effect in the nano-slit and the propagated photonic cavity modes, and their interaction with the surface plasmons of the metal substrate, multiple sharp absorption bands are obtained. The spectral Q factors are with the level of tens of times to that of the system without the nano-slit. Moreover, a jump change from the weak absorption (∼0.05) to the near-unity light trapping (∼1) is achieved when the air nano-slit is introduced into the photonic film. Moreover, the absorption can be quantitatively tuned via the polarization states. The absorption properties including the Q factors and the efficiency can be manipulated via the structural parameters. The findings introduce new insights into the high Q resonant absorption via the nano-slit assisted photonic film and pave feasible ways to realize functional components [24,25].
2. Design and characterization
Figure 1 presents the schematic of the proposed high Q resonant absorber. It consists of a dielectric film with the nano-slit array in the top area and the thick metal substrate. The unique structural characteristic is the introduced nano-slit in the film. In the opinion of the real fabrication, it is relatively easy to realize this design via the combination of the film deposition and the well-developed atomic layer lithography [26–29]. The latter technique has been widely used to form the nanogap or nano-slit arrays. For instance, the proposed method of atomic layer lithography has been used for ultra-small nano-slits and nano-gaps [30]. The technique is with the capability for 5 nm scale fabrication. Via a new patterning technology based on atomic layer deposition and simple adhesive-tape-based planarization [31], vertically oriented gaps in opaque metal films with gap widths as narrow as 9.9 Å were realized on a 4-inch wafer. Nevertheless, the atomic layer deposition technique is needed during the fabrication. The planar nanofabrication technique is also another essential process for the physical way of sub-10 or sub-5 nm nano-structures [32]. Moreover, in order to keep the structure simple enough, the relatively large gap size with the width above 10 nm is used. The lattice period is set to be 500 nm. The height of the nano-slit is 100 nm. During the numerical study, finite-difference time-domain method is employed [33]. The dielectric permittivities of the silver are obtained from the experimental data [34]. The dielectric material is with the refractive index of 2.45. The dielectric medium is set to be the titanium oxide. The thickness of the dielectric layer is 300 nm. Periodic boundary conditions are used along the x-direction to reproduce the array. Perfectly matched layers are used along the z-direction. The silver reflector is used as the metal substrate. The silver substrate is 300 nm, which is thick enough to cancel the transmission. The reason for using the silver as the metal material is based on its relatively low absorption loss, which can introduce the relatively narrow resonances during the oscillations for the free electrons. A linear polarization light with the electric field along the x-axis is used as the light source.
Fig. 1. Schematic of the nano-slit assisted high-Q photonic-plasmonic absorber. The period P is 500 nm. The height h and the width w of the nano-slit are 100 nm and 10 nm, respectively. The silver substrate is 300 nm.
3. Result and discussion
As shown in Fig. 2(a), a series of ultra-narrowband reflection dips and absorption peaks are obtained for the proposed nano-slit assisted photonic-plasmonic absorber. The transmission (T) is wholly cancelled due to the opaque metal reflector. Therefore, the spectral absorption (A) can be obtained directly via the simplified definition with A = 1 - R (T = 0). Four absorption bands are observed. The spectral bandwidth is down to single digital level of the nanometer. In particular, the former three peaks show ultra-high Q factors with the values up to 549 (λ1), 579.5 (λ2), and 553 (λ3). Moreover, the spectral absorption reaches 91.5% (λ1 = 549 nm), 78.7% (λ2 = 591.5 nm), 96.6% (λ3 = 829.5 nm).
Fig. 2. (a) Spectral intensity of the proposed absorber and the related Q factors for the main resonant peaks. (b) Spectral response for the similar system without the nano-slit. The inset pictures are the electric field intensity distributions for the absorption bands.
As a comparison, the spectral responses of the system without the nano-slit, the flat dielectric-metal dual-layer film structure, are also investigated as shown in Fig. 2(b). Only slight reflection dips are observed. The spectral absorption peaks are with broad bandwidths, leading to the very low Q factors. The spectral absorption peaks at 623.5 nm (Q = 9.5, A = 5.4%) and 1023.2 nm (Q = 8.1, A = 2.6%) both show much lower performance for the Q and A than that of the proposed absorber. For instance, the spectral Q factor of the proposed nano-slit assisted absorber shows an enhancement level of 66 times to that of the system without nano-slit. The inset pictures show the normalized electric field intensity distributions for the two bands. It is clearly observed that the resonant guided modes with the 3th and 2th orders are excited for these absorption peaks [35,36].
In order to clearly know the intrinsic mechanism of the spectral responses for the nano-slit assisted absorber, the normalized electric and magnetic field intensity distributions of the peaks (λ1-λ4) are shown in Fig. 3. It is observed that strong electric and magnetic resonant behaviors are existed in the nano-slit and the other areas, which is very different to that of the field patterns of the guided modes by the film structure without the nano-slit (Fig. 2(b)). As for the proposed absorber, the resonant characteristics are different for the peaks at λ1-λ4. For instance, at λ1, the electric field is mainly confined in the nano-slit. A partial field is also observed in the adjacent area between the top and bottom dielectric layers. The magnetic field is observed to be strongly concentrated in the top layer cavity formed by the adjacent nano-slits. These features confirm that the lattice photonic resonance by the nano-slit array [13–16] and the cavity resonance in the top dielectric layer along the polarized direction are the main contributions for the absorption. At λ2, the electric field in the nano-slit is changed to be two parts at the top and bottom sides of the nano-slit. Moreover, the electric field is also observed at the top surface area of the dielectric film. The magnetic field distribution is with the similar pattern, indicating the 2th photonic cavity resonance occurred at the vertical direction of the nano-slit. Moreover, the high-order cavity mode by the top dielectric patch layer and the bottom metal film. At λ3, the electric and magnetic fields are both mainly distributed in the nano-slit area, which confirms the strong cavity resonance along the x-direction by the paired adjacent dielectric resonators. Moreover, the hybridized coupling between the photonic resonance in the middle dielectric layer and the surface plasmon resonance by the metal film substrate. At λ4, the electric field can be observed to be distributed at the top surface area of the silver substrate besides the ones distributed in the nano-slit. The magnetic field is mainly located in the top area of the silver substrate. These features suggest the propagating surface plasmon resonance by the metal film with the main contributions for the absorption band. In addition, for the other peaks, there is also with the positive relationship to the surface plasmon resonances by the metal film as the field located on the surface close to the metal shown in the patterns. Thereby, the introduced nano-slit can strongly rebuild the structural features and then produce plasmon-like photonic resonances, which eventually leads to the efficient absorption peaks with sharp lineshape.
Fig. 3. Calculated electric and magnetic field intensity distributions for the four absorption peaks when the absorber is illuminated under the linear polarization light with the electric field along the x-direction.
Figure 4(a) shows the absorption evolution of the absorber under a tuning of the incident angle. It is observed that the resonant absorption peaks are remarkably tuned during the oblique excitation. For instance, the absorption peaks are doubled due to the excitation of the diffraction modes (+1, −1) [37]. Otherwise, the absorption becomes weak when the incident angle is above 20°, particularly for the peaks at the shorter wavelength range. This could be the results of the reduced resonant field coupling effect by the nano-slit under the oblique excitation. However, the spectral ultra-narrow bandwidths for these peaks still remain, indicating the high Q absorption under large incident angle. The absorption responses under different polarization angles are shown in Fig. 4(b). It is observed that the original absorption peaks are continuously reduced while some of new peaks occur and become stronger and stronger. In order to clearly view the changes, Fig. 4(c) shows the curves under the polarization angle of 0° and 90°. With increasing the angle from 0° to 90°, the former peaks disappear and other new peaks emerge, indicating the different behaviors for the resonant modes under different polarization states. The emerging absorption peaks result from the excitation of the photonic cavity resonances by the metal-dielectric structure. Figure 4(d) shows the plotted absorption for the two peaks as a function of the polarization angle. It is observed that the absorption intensity is changed continuously. Moreover, it is found that the evolution process is similar to the classical Malus law. The curves are perfectly matched to the results followed by the Malus law R = R0 × cos2θ [38]. The R and R0 are the reflection at the angle of θ and 0°, respectively. That is, the high Q resonant absorption can be quantitatively tuned via the polarization state.
Fig. 4. (a) Absorption evolution of the absorber via tuning the incident angle under the TM polarization (electric field along x-axis). (b) Absorption evolution of the absorber via tuning the polarization angle under normal illumination. (c) Spectral absorption comparison between the states with the polarization angle of 0° and 90°. (d) Absorption intensity of the peaks marked in (c) as a function of the polarization angle. The curves of the ideal Malus law are also plotted.
The absorption responses for the system under a tuning of slit width are shown in Fig. 5(a). With increasing the width from 10 nm to 60 nm, the spectra show two main kinds of changes. One is the continuous reduction of the absorption intensity for the peaks (λ1-λ3) and the inverse process for the peak at λ4. That is, under a relatively large slit width, the resonant absorption becomes to be weak due to the reduced optical field confinement by the slit cavity. Nevertheless, as for the surface plasmon resonance of the metal film, the wider slit can introduce much more incident energy for electronic oscillations. The other one is the slightly increased bandwidths for these peaks. Figure 5(b) shows the plotted spectral Q factors for the peaks (λ1-λ3) under the different widths. It is observed that the Q factors show a fast decrease and then a slow reduction during the increase of the slit width.
Fig. 5. (a) Absorption curves under different values of the nano-slit width. (b) Plotted spectral nQ factors for the three main peaks (λ1-λ3) under different widths.
Figure 6(a) presents the absorption evolution via increasing the slit height from 20 nm to 200 nm. The spectral absorption efficiencies for these peaks are observed to be highly tuned. In particular, the absorption efficiency increases quickly when the height is tuned from 20 nm to 60 nm. That is, under a relatively large slit height, the resonant absorption becomes strong due to the enhanced optical coupling and confinement by the nano-slit. The absorption intensity for the main peaks (λ1-λ3) under different slit heights is plotted in Fig. 6(b). In the middle value range of the height, the absorption intensity is all with a high level for these peaks. For instance, the absorption at λ1 and λ3 are above 90% when the height is larger than 80 nm. Nevertheless, for the peak at λ2, a noticeable fluctuation is observed due to the cavity resonance by the adjacent photonic resonators, which is strongly related to the slit height.
Fig. 6. (a) Absorption evolution during the tuning of slit height. (b) Plotted intensity of the absorption for the peaks (λ1-λ3) as a function of the slit height.
The absorption scalable response under the tuning of the lattice period is studied as shown in Fig. 7. With tuning the period from 450 nm to 600 nm, the spectral absorption curves are observed to be shifted in the wavelength range while the absorption efficiency and the high Q factor remained well for these peaks. The noticeable red-shift of the resonant absorption peaks also paves a feasible way to artificially manipulate the operation wavelengths via the structural size. The reason for this tunable feature is the relationship between the resonant mode and the size of the photonic resonators formed by the adjacent nano-slits.
Finally, we do further study on the optical properties via changing the structural features of the nano-slit. As shown in Fig. 8, the spectral lineshapes remain well for the structures formed by different trapezium-like nano-slits. The trapezium-like nano-slits are set to be with the top and bottom widths of (10 nm, 5 nm), (15 nm, 5 nm), (20 nm, 5 nm), (25 nm, 5 nm). In comparison with the sharp absorption peaks in the proposed nano-slit (top width of 10 nm, bottom width of 10 nm) based absorber, the curves from the trapezium-like nano-slits also show the sharp and near-unity absorption peaks. For instance, the absorption peak λ2 at 591 nm is with the spectral A up to 99.95%. The spectral Q factor is also up to 402. The absorption peak λ1 at 547 nm is with the spectral A up to 95.98%. The spectral Q factor is also up to 437. The absorption peak λ3 at 824 nm is with the spectral A up to 95.20%. The spectral Q factor is also up to 471. The spectral absorption is even larger than that of the system formed by the uniform nano-slit. The spectral Q factors are slightly reduced. Nevertheless, the Q factors are still above 400, indicating the maintained sharp and narrowband absorption. The slightly reduced Q factors for the absorption peaks mainly results from the un-uniform cavity structure, which can reduce the optical field confinement effects. The findings ensure the high tolerance for the fabrication process, which can hold the proposed absorber with applications in the ultra-narrowband spectral techniques and devices such as the notch filters, modulators, and subtractive spectral sensing and detection.
4. Conclusion
In conclusion, we have proposed and numerically demonstrated a new high Q absorber platform via introducing nano-slit into the photonic dielectric film coupled by the metal substrate. The spectral Q factor is observed to be amplified by 66 times in comparison with that of the system without the nano-slit. Moreover, the optical field coupling and confinement have also been extremely improved, which directly lead to the near-perfect absorption in the resonant wavelengths. The nano-slit is found with the capability to rebuild the resonant field for the photonic-plasmonic hybridized system, which produces the combination of localized photonic resonance by the slit and the coupling photonic cavity modes together with the surface plasmon resonances by the metal film substrate. Furthermore, the resonant absorption can be manipulated quantitatively via the polarization state. The operation wavelengths can also been adjusted via the structural parameters. These findings pave new insights into the high Q resonant absorbers and hold potential applications in ultra-narrowband spectral techniques and devices.
Funding
National Natural Science Foundation of China (62065007, 11804134, 11664015, 51761015); Natural Science Foundation of Jiangxi Province (20182BCB22002, 20181BAB201015, 20202BBEL53036, 20202BAB201009, 2018ACB21005).
Disclosures
The authors declare that they have no competing interests.
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