Broadband light absorption using a multilayered gap surface plasmon resonator

Abstract

A broadband visible light absorber composed of multiple metal-dielectric-metal (MMDM) layers is proposed and numerically investigated. Dielectric layers of different thicknesses in the MMDM structure lead to multiple plasmon resonances at different wavelengths; as a result, efficient broadband absorption can be achieved under optimized conditions. We found that an average simulated absorption of 93 % was obtained over the entire visible spectrum of 400–700 nm by controlling the geometric parameters. Furthermore, the origin of the broadband absorption was studied, and the effects of the diameter and pitch of the pattern on the absorption were investigated. Our proposed structure with a periodic array of circular patterns represents a novel candidate for future applications in photovoltaic cells and thermal emitters.

1 Introduction

Metal nanostructures for exciting surface plasmons (SPs) have received much attention because of their unique properties for controlling the electromagnetic field at subwavelength scales [1–3]. In particular, these plasmonic nanostructures used to manipulate the electromagnetic field in the subwavelength regime have great potential for achieving the enhanced light absorption; thus, they are promising candidates for applications in photovoltaic devices [4, 5] and thermal emitters [6]. Many theoretical and experimental studies have been performed to design and realize near-perfect absorption using plasmonic nanostructures such as metal nanoparticles, periodic gratings and metal-dielectric-metal (MDM) thin layers [7–10]. However, most studies have focused on the absorption of the electromagnetic field within a narrow range of wavelengths, while only a few reports have studied broadband light absorption in the visible range. Broad absorption throughout the entire visible wavelength is important for photovoltaic and thermophotovoltaic cells. Recently, Aydin et al. [11] reported that their proposed plasmonic resonator, with a nanostructured top layer composed of a silver film of trapezoidal arrays, exhibited broadband polarization-independent light absorption throughout the entire visible range of 400–700 nm. In addition, Nielsen et al. [12] demonstrated that efficient absorption of visible radiation can be realized using a periodic array of differently sized patterns. Although these few previous reports proposed an efficient broadband absorber in the visible range, the anisotropic shaped or differently sized patterns involved in these absorbers require extremely precise control in device fabrication. In addition, Wang et al. [13] numerically proposed a tunable broadband perfect absorber with a planar multilayer structure for the isotropic pattern, which may simplify the fabrication process; however, the absorption band of the tunable absorber does not cover the entire visible range. Therefore, in this study, we propose a periodic array of a multilayer gap surface plasmon (GSP) absorber with a circularly shaped pattern for efficient broadband light absorption. This absorber consists of multiple metal-dielectric-metal (MMDM) layers, in which the dielectric layers have different thicknesses. The different thicknesses of the dielectric layers in the MMDM layers lead to hybridized magnetic resonance modes among the metal-dielectric-metal layers, resulting in multiple plasmon resonances. As a result, the controlled multiple plasmon resonances induce efficient broadband absorption over the entire visible range. Simulation results of the absorber conducted using a finite difference time domain (FDTD) method show that the average absorption over the visible regime of 400–700 nm is more than 93 %. To the best of our knowledge, this is the first report of a periodic array of an MMDM structure with dielectric layers of different thicknesses exhibiting enhanced broadband absorption over the entire visible range.

2 Design and simulation

The unit cell of the proposed MMDM plasmonic absorber is shown in Fig. 1. The cell is composed of multiple Au–SiO₂–Au layers with different SiO₂ thicknesses. In our study, the thicknesses of the SiO₂ layers in the MMDM structure are t _d1 = 20, t _d2 = 40 and t _d3 = 60 nm, and the sandwiched and top Au layers have a fixed thickness of 20 nm (t _Au2). In addition, the absorber has a diameter D of 290 nm, a pitch P of 420 nm and a thickness t _Au1 of 200 nm. Numerical simulations of the proposed absorber were performed using Lumerical, a commercial FDTD Maxwell equation solver. Simulations were performed for the unit cell of the periodic structure, where the x and y planes have periodic boundary conditions and the z plane has a perfectly matched layer (PML). The plane waves are normally incident to the structure along the (−) z direction, and the reflection (R) was detected with a power monitor located behind the radiation source of the plane waves. The absorption was calculated as 1−R because there is no transmission in our design. In all of the simulations, a cubic mesh with a size of 2.5 nm was employed, which is sufficiently fine to provide accurate calculations. Material properties with respect to the wavelength were obtained from [14].

Fig. 1

Unit cell structure of the proposed MMDM absorber composed of multiple Au–SiO₂–Au layers with different dielectric thicknesses. The overall structure is a periodic array of the unit cell. The inset represents the simulation domain in the x–z plane

3 Results and discussion

The simulation results are shown in Fig. 2, where the absorption spectra of the MMDM absorber (Fig. 2a) are compared with that of a conventional MDM absorber (Fig. 2b). It is clearly shown that the MMDM absorber exhibits highly efficient broadband light absorption, whereas the conventional MDM absorber displays only narrowband light absorption. For the conventional MDM absorber, the resonance wavelength increased as the SiO₂ thickness decreased; as a result, the conventional MDM structure has resonance wavelengths of 654, 597 and 580 nm for SiO₂ thicknesses of 20, 40 and 60 nm, respectively. Generally, in a GSP resonator, the electromagnetic field can be effectively controlled by modifying the dimensions of the dielectric gap and the diameter of the structure as follows [12, 15].

Fig. 2

Absorption spectra of a the proposed MMDM absorber and b a conventional MDM absorber

$$ D\frac{2\pi }{\lambda }n_{\text{eff}} = m\pi - \phi $$

(1)

where D is the diameter of the structure, λ is the resonance wavelength, n _eff is the effective index for the GSP, m is the order of the GSP and ϕ is the phase obtained upon reflection at the bottom Au layer [12, 15]. Thus, the resonance wavelength is proportional to the effective index at the fixed diameter. Furthermore, the effective index becomes larger as the thickness of the dielectric layer (t _d) is reduced, as described below [16, 17].

$$ \tanh \left( {\frac{{\sqrt {\beta^{2} - k_{ 0}^{2} \varepsilon_{\text{d}} } t_{\text{d}} }}{2}} \right) = \frac{{ - \varepsilon_{\text{d}} \sqrt {\beta^{2} - k_{0}^{2} \varepsilon_{\text{m}} } }}{{\varepsilon_{\text{m}} \sqrt {\beta^{2} - k_{0}^{2} \varepsilon_{\text{d}} } }} $$

(2)

where k ₀ is the wave vector of light in free space and ε _m and ε _d are the relative dielectric constants for the metal and dielectric materials. Re(β/k ₀), representing the effective refractive index (n _eff) in the dielectric layer, increases significantly as the gap thickness decreases. Accordingly, the resonance wavelength is inversely proportional to the thickness of the dielectric layer in the MDM, based on Eqs. (1) and (2).

In this work, we propose the induction of broadband absorption by combining MDM layers with different dielectric thicknesses, as shown in Fig. 2a, because the resonant absorption wavelength is a function of the dielectric thickness in the MDM structure. As expected, the MMDM absorber exhibits three GSP resonances at 680, 648 and 601 nm for thicknesses of 20, 40 and 60 nm, respectively, as shown in Fig. 2a. Compared to the conventional MDM structure, the resonance peaks corresponding to the respective SiO₂ thickness were red shifted in the MMDM absorber. Importantly, it should be noted that the average light absorption for wavelengths of 400–700 nm was greater than 93 %, and the local absorption at a wavelength of 541 nm had a near-perfect absorption of 99.8 % at the MMDM absorber.

To investigate how light is absorbed in the MMDM structure, the field distributions for the different resonant wavelengths are displayed in Fig. 3. As shown in Fig. 3, the first-order resonance at 680 nm strongly occurs in the first SiO₂ layer. It is clear that the confined field is very strong in the first SiO₂ layer, while the confinement of the field is very weak in the second and third SiO₂ layers. Similarly, the second-order resonance at a wavelength of 648 nm primarily occurs in the second SiO₂ layer, while the field confinement in the third SiO₂ layer begins to occur at 601 nm. This result is due to the hybridized mode of the magnetic resonances occurring in the respective SiO₂ layers [18, 19]. It is well known that anti-parallel currents are induced in the top metal pattern and the bottom metal layer for a MDM structure [19]. The anti-parallel current creates a magnetic moment, strongly interacting with the incident light, and eventually, magnetic resonance can be generated in the dielectric layer [18, 19]. In our structure, because the sandwiched and top Au films are thin enough to allow light to be transmitted through the Au films, the magnetic polaritons in the three dielectric layers interact strongly with each other. Therefore, the multiple plasmon resonances occurring in the MMDM structure are associated with the hybridized magnetic resonance coupled in the three dielectric layers. Furthermore, we suggest that the hybridized mode leads to a red shift of the resonance peaks corresponding to the SiO₂ thickness in the MMDM structure, as compared to that in the conventional MDM structure. Interestingly, the absorption at 541 nm is greater than 99 %; however, this absorption is primarily due to the enhanced intrinsic losses in the Au of the MMDM absorber rather than the GSP resonance [12].

Fig. 3

y-component magnitude of the magnetic field $\left| {H_{y} } \right|$ and the magnitude of the electric field $\left| E \right|$ in the xz plane for resonant wavelengths of (a1, a2) 680 nm, (b1, b2) 648 nm, and (c1, c2) 601 nm

As described in Eq. (1), the diameter of the absorber is one of the critical parameters controlling light absorption. The electric and magnetic fields in the unit cells are strongly influenced by the dimensions of the absorber [13]. Figure 4 shows the absorption spectra with respect to the diameter of the MMDM absorber for P = 420 nm. It is clear that the resonance wavelengths decrease rapidly as the diameter decreases, as a result of Eq. (1). Notably, the resonance peaks exhibit a different degree of shifting as a function of the diameter. That is, the blue shift of the first resonance peak due to the decreased diameter occurs relatively quickly, while that of the second and third resonance peaks occurs relatively slowly. For this reason, the respective resonance wavelengths become closer together as the diameter is reduced and can eventually overlap. Our calculation shows that the average absorption for wavelengths of 400–700 nm reaches a maximum value of 93 % at a diameter of 290 nm. It is noted that for diameters less than 290 nm, the oscillations of the resonance peaks almost disappear due to the overlapped resonance wavelength; however, the average absorption for the visible wavelength region of 400–700 nm decreases due to the reduced bandwidth.

Fig. 4

Absorption spectra as a function of the diameter of the MMDM absorber. The inset shows the average light absorption for wavelengths of 400–700 nm as a function of the diameter. The pitch (P) of the MMDM structure was fixed at 420 nm

Figure 5 shows the absorption spectra with respect to the pitch of the MMDM absorber for D = 290 nm. In contrast to the effect of the diameter, the wavelengths of the resonance peaks do not show a critical dependence on the pitch, even though the first resonance peaks show a slight blue shift and the second peaks become relatively red shifted with increasing pitch. Instead, it is observed that the intensities between the first and second peaks follow a trade-off relation with respect to the pitch. For example, for P = 380 nm, the intensity of the first peak is relatively weaker than that of the second peak. However, as the pitch increases, the intensity of the first peak becomes strong, while that of the second peak becomes weaker. In the case of P = 460 nm, the second peak completely disappears. Moreover, the absorption drop occurs within a small wavelength range, and the drop wavelengths become longer as the pitch increases. We presume that this result occurs because the filling factor (D/P) of the fixed pattern decreases as the pitch increases, resulting in increased reflectance from the thick bottom Au film.

Fig. 5

Absorption spectra as a function of the pitch of the MMDM absorber. The inset shows the average light absorption for wavelengths of 400–700 nm as a function of the pitch. The diameter (D) of the MMDM structure was fixed at 290 nm

In addition, we found that the difference in the average absorption between the polarization angles of 0° and 45° is less than 0.1 % (data not shown) due to the symmetric arrangement of the isotropic pattern, indicating that our MMDM absorber exhibits polarization-independent absorption spectra.

Finally, we discuss solar cell applications. If the dielectric layers were replaced by a semiconductor with a high absorption coefficient, the efficiently trapped photons between the metal layers would be absorbed by the semiconductor, allowing photocarrier generation. Although the light absorbed by the metal layer would be lost as heat, the light trapped by the plasmonic nanostructures would lead to a considerable reduction in the photoactive layer thickness, without a loss of efficiency [11].

4 Conclusion

In conclusion, we proposed an efficient broadband visible light absorber composed of multiple metal-dielectric-metal structures. We demonstrated that dielectric layers with different thicknesses in the MMDM structure play an important role in inducing broadband absorption with multiple plasmon resonances. The wavelength and intensity of the respective resonance peaks showed a strong dependence on the diameter and pitch of the pattern, respectively, and as a result, the MMDM absorber had an average absorption of 93 % under appropriate geometrical conditions. The periodic, circularly shaped pattern of the MMDM absorber allows an easy fabrication process; thus, our proposed absorber shows potential for achieving blackbody radiation and for enhancing light absorption in thermal emitters and photovoltaic devices.