Comsol Near Infrared Spectroscopy

Highly Lossy Metals

Fei Ding1, Jin Dai2, Yiting Chen1, Jianfei Zhu3, Yi Jin3 & Sergey I. Bozhevolnyi1 Radiation absorbers have increasingly been attracting attention as crucial components for controllable thermal emission, energy harvesting, modulators, etc. However, it is still challenging to realize thin absorbers which can operate over a wide spectrum range. Here, we propose and experimentally demonstrate thin, broadband, polarization-insensitive and omnidirectional absorbers working in the near-infrared range. We choose titanium (Ti) instead of the commonly used gold (Au) to construct nano-disk arrays on the top of a silicon dioxide (SiO2) coated Au substrate, with the quality (Q) factor of the localized surface plasmon (LSP) resonance being decreased due to the intrinsic high loss of Ti.

Figure: System Model & Simulation Flow for Comsol Near Infrared Spectroscopy

The combination of this low-Q LSP resonance and the propagating surface plasmon (PSP) excitation resonance, which occur at different wavelengths, is the fundamental origin of the broadband absorption. The measured (at normal light incidence) absorption is over 90% in the wavelength range from 900 nm to 1825 nm, with high absorption persisting up to the incident angle of ~40°. The demonstrated thin-film absorber configuration is relatively easy to fabricate and can be realized with other properly selected materials.

Figure: System Model & Simulation Flow for Comsol Near Infrared Spectroscopy

Thin-film radiation absorbers, which can completely absorb the incident light at specific frequencies within a few hundred nanometers, have drawn increasing attention due to their prospects in many fields of science and tech- nologies, e.g., sensing, thermal emission tailoring and solar energy harvesting1–5. However, thin-film broadband radiation absorbers are extremely difficult to realize due to natural constraints: very high absorption occurs only near resonances whose bandwidth is necessarily limited, while strong mismatch in dielectric properties with environment (e.g., air) causes high reflection at an interface. Recently, metamaterials have shown their unique capacity in controlling the propagation of electromagnetic waves6–10. Many thin-film metamaterial absorbers have been demonstrated to function as stable perfect absorbers from the microwave to visible, exhibiting wavelength scalability and insensitivity to the angle of incidence and/or polarization11–16. Nevertheless, the aforementioned metamaterial absorbers often tend to be efficient only within a narrow band owing to their resonance property, which might be not sufficient for many practical applications.

Figure: System Model & Simulation Flow for Comsol Near Infrared Spectroscopy

To achieve a large absorption bandwidth, a common and direct strategy is to merge several spectrally adjacent resonances by exciting various multi-sized resonators together17–22. But the number of the blended resonators is restricted due to the competition of neighboring resonances, and the absorption performance is highly sensitive to the structure size, which is difficult to control in fabrication. Thus the bandwidth broadening is still limited and the experimentally measured absorption efficiency is not very high. In addition to this, there have been other methods addressing this issue, such as adiabatic nanofocusing of gap surface plasmon modes excited by the scattering off subwavelength-sized wedges23,24 and the excitation of slow-wave modes in tapered alternating metal-dielectric multiple thin films25–28.

Figure: System Model & Simulation Flow for Comsol Near Infrared Spectroscopy

Despite these significant achievements, it remains challenging to realize efficient light absorbers with a large bandwidth, high efficiency and fabrication simplicity, since the fabrication of the aforementioned broadband absorbers involved focused ion beam milling (FIB) or complicated multiple steps of electron beam lithography (EBL), which are time-consuming and costly. In this paper, we propose and experimentally demonstrate thin, broadband, polarization-insensitive, and omnidirectional absorbers working in the near-infrared range, which of Materials and Nano Physics, School of Information and Communication Technology, KTH-Royal Institute of Technology, Electrum 229, 16440 Kista, Sweden. 3Centre for Optical and Electromagnetic Research, State Key

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are based on the simple and traditional metal-insulator-metal (MIM) configuration. Highly-efficient broadband absorption is ascribed to the excitation of low-Q localized surface plasmon (LSP) resonance supported by tita- nium (Ti) nano-disks, and the generation of propagating surface plasmon (PSP) resonance. Specifically, under normal incidence, the measured absorption of the fabricated sample realized by one step of EBL is over 90% in the spectrum ranging from 900 nm to 1825 nm. Moreover, numerical simulations show that the absorption per- formance is insensitive to the polarization of incident light, and high absorption persists when the incident angle is less than 40°. Such a planar device is much easier to fabricate compared with other broadband absorbers with complicated structures. Additionally, this design concept is general and can be applied to other frequencies with properly selected materials, such as highly-doped semiconductors in THz frequency.

Figure: System Model & Simulation Flow for Comsol Near Infrared Spectroscopy

Results And Discussion

Structure and design of the broadband absorber.

The Schematic Of The Broadband Near-Infrared Met-

amaterial absorber is depicted in Fig. 1. The absorber is composed of periodic Ti disk-shape nanoantenna array and a continuous gold (Au) film, separated by a silicon dioxide (SiO2) dielectric layer. The topmost disk nanoan- tennas are periodically distributed in both the x- and y-directions with a pitch of 600 nm. The other parameters of the disk nanoantenna are tm =​ 30 nm and d =​ 400 nm. The thickness of the middle SiO2 spacer layer is optimized to be 160 nm, and the bottom Au layer has a thickness of 100 nm, which is thick enough to block all the transmis- sion in the investigated wavelength range.

Simulation and experimental results of the broadband Ti-disk absorber.

We First Implement Three

dimensional (3D) full-wave simulations to verify the performance of the broadband absorber. The full-wave simulations are performed with the commercial finite element software Comsol Multiphysics (see the Methods section for details). Figure 1 shows the simulated structure under TM polarization (electric field is along the x-direction). The absorption is calculated from the scattering parameters as A(λ) =​ 1 −​ R(λ) −​ T(λ), where R(λ) represents the reflection, and T(λ) represents the transmission which is zero here. The obtained absorption spec- trum at normal incidence is shown in Fig. 2 (dashed red line), which clearly indicates that above 90% of the incident near-infrared light is efficiently absorbed in a rather wide wavelength range from 875 nm to 1840 nm.

The relative bandwidth of the absorber, defined as RWB =​ 2 ×​ (λl −​ λs)/(λl +​ λs), where λl and λs are the long and short limits of a wavelength range with absorption above 90% respectively, reaches about 71%. Figure 1.  Schematic of the perfect infrared broadband absorber and the incident light polarization configuration. The dimensions are p =​ 600 nm, d =​ 400 nm, tm =​ 30 nm, ts =​ 160 nm and dm =​ 100 nm, respectively.

Figure 2.  The simulated (red dashed line) and experimental (black solid line) absorption spectra for the fabricated sample. The inset shows the SEM image of a section of the sample.

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To experimentally validate the broadband absorption performance, we fabricated a set of broadband absorb- ers using the standard thin-film deposition techniques and electron-beam lithography (see the Methods section for details). The inset of Fig. 2 displays a scanning electron microscope (SEM) image of a part of the fabricated sample. All the absorbers cover an area of 75 μ​m ×​ 75 μ​m, which consist of 125 ×​ 125 unit cells. Reflection spectra of the fabricated samples are measured using a homemade linear reflection spectroscopy (see the Methods section for details). From Fig. 2, it is impressive that the measured absorption is above 90% in the wavelength range from 900 nm to 1825 nm (black solid curve). The experimentally achieved RWB is around 67.9%, which is superior to other reported absorbers based on multi-layered17,24 or multi-sized18–20 structures. Taking into account the imper- fect fabrication as well as the measurement error, there is remarkably good agreement between the simulated and experimental results.

To reveal the physical mechanism in the proposed perfect absorber, we calculate the electric and magnetic field (|Ex| and |Hy|) at the two absorption peaks, namely 914 nm and 1468 nm in Fig. 3. Overall, these two res- onances are both the electric dipole resonances excited on the nano-disks along the x-axis for TM polarization [Fig. 3(b,c,e,f)]. However, the distributions of the magnetic field in the cross-section [x-z plane in Fig. 3(g,h)] are significantly different. The short wavelength resonance at λ1 =​ 914 nm is considered to be the PSP resonance between continuous Au film and SiO2 spacer, where the magnetic field is not only strongly confined in the gap region underneath the nano-disks, but also strongly enhanced between the nano-disks. Otherwise, the long wave- length mode at λ2 =​ 1468 nm is a LSP resonance where the magnetic field is mainly concentrated within the gap between the topmost nano-antennas and the gold underlay29. Since Ti is very dispersive and has a relatively large imaginary part (see Supplementary Fig. S1 for details), the intrinsic absorption coefficient of Ti is very big. Thus the Q-factor of this LSP resonance is rather low, broadening the absorption bandwidth, which is different from the narrow band absorber made up of low-damping Au nano-disk array13,14 (see the following text for details).

Therefore, we can attribute the broadband perfect absorption to the combination effect of PSP and low-Q LSP resonances. Since optical impedance matching plays an important role in areas of metamaterial absorbers30, we retrieve the corresponding effective optical impedance to get further insight into the physics of our structure. The impedance of our structure can be calculated easily from the scattering parameter results of our numerical simulations, using

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where S11 and S21 denote the scattering matrix coefficients of normal incidence reflection and transmission in TM-polarization, respectively. Due to the presence of the thick metallic substrate, S21 can safely be set to zero. Figure 4 shows the simulation results for the normalized real and imaginary parts of the optical impedance for the Ti-disk broadband absorber. We can clearly observe that both the real and imaginary parts are matched to the Figure 3.  The normalized electric field Ex and magnetic field Hy distributions at two absorption peaks in both the x-y and x-z planes.

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respective vacuum values of one and zero in the wide absorption wavelength range, resulting in highly-efficient broadband absorption30. After the mechanism of this broadband absorber has been explained, the influence of some structure param- eters on the absorption can be easily understood. As discussed above, the absorption peak at λ2 =​ 1468 nm is related to the LSP resonance. Thus the diameter d of nano-disks may nearly determine the long wavelength limit of RWB with the other parameters fixed. Figure 5(a,b) show the corresponding simulated and experimental results with different d. As d increases from 380 nm to 420 nm, the LSP resonant wavelength is slightly red-shifted, resulting in an increase of RWB, which is consistent with previous study. Compared Fig. 5(b) with 5(a), there is Figure 4.  The normalized optical impedance of the Ti-disk broadband absorber at normal incidence.

Figure 5.  Influence of some structure parameters on the absorption performance and the corresponding absorption spectra of different materials within the absorber. Simulated (a) and measured (b) absorption spectra when the diameter d changes from 380 nm to 420 nm with the other parameter fixed. (c) Simulated absorption spectra with different period p while the other geometric parameters are fixed. (d) Absorption of different materials within this absorber.

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