A simple absorber design which enables near-perfect absorption in the visible and near-infrared regions is presented. The absorber is an unpatterned metal/dielectric/metal triple-layer, e.g., a 20 nm-thick metal film as the top layer, a 250 nm-thick dielectric film as the middle layer, and a 200 nm-thick metal film as the bottom layer. It was found that the high-efficiency absorption at specific wavelengths is mainly due to the Fabry-Perot (FP) resonances in the dielectric middle layer which result in trapping of the resonant light in the middle layer and thus enhanced absorption efficiency.
© 2013 Optical Society of America
Light absorbers have attracted much attention for their great promise in a wide range of applications, such as solar cells, photodetectors, sensors, bolometers, nanoimaging devices and thermal emitters [1–5]. Since Landy et al. proposed in 2008 to achieve perfect absorption by using meta-materials with effect impedance equaling to free space, various kinds of super absorber have been proposed and studied [1–7]. However, the majority of current designs focus on building an elaborate metallic pattern on a thin (e.g., 20 nm) dielectric spacer film which is attached to a thick metal layer. It is worth noting that most reported absorber designs contain miniature and elaborate structural features (especially for the top metallic layer), setting manufacturing obstacles for the practical applications of super absorbers. In fact, currently most super absorbers working in the visible and near-IR regime are fabricated on a small area using expensive nanofabrication techniques, e.g., the e-beam lithography. Lately, simpler designs of perfect absorber based on layered structures (e.g., a lossy dielectric film on a metal film) have been proposed [8,9]. On the other hand, it should be pointed out that a simple triple-layered structure invented in 1952, the Salisbury screen (SS) which typically consists of a metallic ground plane, a lossless dielectric middle layer, and a thin glossy top layer [10,11], was one of the first near-perfect absorbers, originally designed for the military radar waves. However, the SS was soon surpassed by other designs for the disadvantage of bulky thickness. Here we propose a thin-film type of SS absorber working in visible and near-infrared regime, possessing great optical flexibility and fabrication convenience.
2. Structure model
The absorber design is schematically shown in Fig. 1, which consists of a metallic top layer, a dielectric middle layer, and a metallic bottom layer, with the respective layer thickness of d1, d2 and d3, and the respective relative permittivity of εr1, εr2 and εr3. The relative permittivity of metals is described using the Lortenz-Drude model [12,13]. Unless otherwise stated, in this report, d1, d2 and d3, are set to be 20, 250, and 200 nm, respectively, εr2 is set to be 3.42, and the top and bottom layers are made of Ag.
3. Theory study
As shown in Fig. 1, the proposed absorber structure can be divided into 5 zones. For Zones 0 to 4, for TE polarization, the electric field component and magnetic field component can be written as:14,15] can be adopted for the interface between Zones 2 and 3:Eq. (1) on the interface (z = -d1-d2) and the SIBC condition, we obtain:Eq. (4) becomes:Eq. (5) [15,16]:Eq. (6) and the modulus of Ffp, |Ffp|, are shown in Fig. 2(a) and Fig. 2(b).It can be seen from that the reflection dips and the |Ffp| peaks correlate well with each other, in spite of the small mismatch (~40 nm) between them. The observed small mismatch is possibly due to the slight phase shift of light in the top metal layer, which will be discussed later in this paper. Therefore, the reflection dips can be mainly attributed to the FP resonance in the middle dielectric layer. Here, the reflection coefficient R12 can be expressed as . |R12| and φ12 are the amplitude and phase shift of light reflected from the interface between Zone 1 and Zone 2. Calculation shows that |R12| is close to unity when the wavelength is bigger than 400 nm in Fig. 2(c). It can be seen that the reflection r is closely related to Ffp, suggesting that the middle dielectric layer acts as a FP resonance cavity when Eq. (9) is satisfied:Eq. (9) is satisfied in Fig. 2(d), the FP resonance enhances the electromagnetic field inside the middle dielectric layer, which results in enhanced light absorption in the interfacial region in the top and bottom layers that is adjacent to the middle dielectric layer, leading to high absorption and minimal light reflection from the device at the specific wavelengths. The mechanism unveiled here also shed light on the absorption mechanism of some previous studies that are based on metal/dielectric/metal structures [8,17].
Besides the mathematical method (MM) presented above, the Transfer Matrix Method (TMM) and the Finite-Difference Time-Domain (FDTD) method (using the commercial software EastFDTD) were also used to check the accuracy of the simulation results. It was found that these three methods are in good agreement (see Fig. 3). It should be pointed out that, for calculating structures with thick bottom layer thicknesses, the MM requires less computation time comparing to the numerical methods.
4. Results and discussion
The absorber structures with both the top and bottom metal layers made of the same type of metal, including Au, Ag, Cu, Al, Ni, are simulated (see Fig. 4).Among the different metals tested, all show multiple absorption peaks with the 1st, 2nd and 3rd order peaks located around 1200, 600 and 400 nm, respectively. Notably, Au, Ag and Cu show sharp and near-perfect absorption peaks in the visible and NIR region, demonstrating their potential application as perfect absorbers.
It should be pointed out that the working mechanism of the perfect absorber reported here is significantly different from another type that is being intensively investigated which is also based on a metal/dielectric/metal 3-layer structure. Comparing to the one reported here, the other type of perfect absorber usually features a micro-patterned top metal and much thinner (e.g., 20 nm) middle dielectric layer. More importantly, the absorption mechanism of the other type of triple-layer absorber is mainly attributed to the electric resonance of the top metal layer and magnetic resonance created by the top and bottom metal layers, whereas the middle dielectric layer mainly serves as a spacer layer. On the contrary, the high efficiency of the absorbers reported here is mainly attributed to the FP resonance of the middle dielectric layer. Simulation of the electric field distribution over the cross-section of the absorber (see Fig. 5) reveal that (1) most light is trapped in the middle dielectric layer, and (2) the bottom metal layer not only absorbs the transmitted light, but provide a strong reflection and form a Fabry-Perot cavity for the dielectric layer.
To reveal the function of each layer, the optical responses of the absorber with either the top metal layer or the bottom metal layer removed were studied (see Fig. 6). When either the top or bottom layer is missing, only low and broadened absorption peaks are present over the entire spectrum studied. This indicates the necessities of each layer and their synergetic effects for achieving high absorption.
Furthermore, it is found that the absorption response is highly dependent on the structural parameters of the absorber. The effects of d1 and d2 were studied. With d1 changes from 10 to 30 nm, interestingly, the absorption peaks blue shifted (see Fig. 7).This seemingly abnormal shifting behavior is related to the phase factor exp(2ik1zd1) in Eq. (4). It means the phase shift of light in the top metal layer affect the position of the absorption peaks. Regarding the peak height, it reaches the maximum d1 = 25 nm for the 1st order absorption peak. On the other hand, with d2 increases from 150 nm to 350 nm, the absorption peaks red shifted (see Fig. 8) due to the increased optical thickness of the middle dielectric layer. This is clear evidence that the absorption mechanism of the absorber studied here is different from the other type of triple-layer absorbers, for which the thickness increase of the middle dielectric layer leads to a blue shift of the absorption peaks [6,7].
For practical purposes, it will be useful to consider that the absorber middle layer possesses a complex refractive index (with absorption coefficient) that is wavelength-dispersive. Take silicon for example, two absorption spectra of an Ag/Si/Ag absorber were simulated (see Fig. 9): one plotted using the wavelength-dispersive complex refractive index of Si ; the other calculated with εr2 set to 11.7 for the entire wavelength range. It can be seen that with the absorption coefficient considered, lower absorption peaks with wider full width at half maximum (FWHM) are observed, possibly because the FP resonance is weakened by the lossy middle layer. The shifts of the absorption peaks are caused by the wavelength-dispersion characteristics of Si.
Study on the absorption behaviors with different incident angles show that the peak blue-shifted with increasing incident angle (see Fig. 10). Notice that the absorption intensity significantly reduces as the incident angle increases, e.g., the intensity of the 1st order peak at 1142 nm reduces to half at 20° for TE polarization and 30° for TM polarization, suggesting the angle sensitivity of the absorbers and its potential application for directional thermal emitters.
A simple type of thin-film absorber for visible and near-IR light is theoretically studied. These absorbers feature a simple metal/dielectric/metal 3-layer structure. Different types of metals, including Ag, Au and Cu, are well-suited for constructing this type of absorber with near-perfect absorption. Mechanism study reveals that light absorption is mainly resulted from the FP resonance in the middle dielectric layer. The absorption response is found sensitive to the thicknesses of the top and middle layers and the incident angle. The simple design of the absorbers reported here indicates particularly attractive manufacturing convenience for low-cost mass production, holding great promise for various practical applications.
This work was supported by the National Natural Science Foundation of China (Project 51202206), and the City University of Hong Kong (Projects 9667070 and 7003039).
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