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Abstract
Absorptive metamaterials composed of antimony-doped-germanium of different epsilon-near-zero wavelengths and germanium films are designed and fabricated for broadband perfect light absorption around the epsilon-near-zero wavelengths. By integrating two bilayers separated by a semiconductor spacer into a metamaterial structure backed by substrate and gold thick ground plane, we realize more than 93% P-polarized light absorption over a wavelength range of 1.8 microns for an angle of incidence of 40° and a flat-top 75% absorption over a wideband of 1.74 microns for an angle of incidence of 50° in the middle wave infrared region.
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1. Introduction
The ability of metamaterials to absorb light is a fundamental property that plays a critical role in a wide range of applications in optics and photonics. Subwavelength patterned metal-dielectric-metal structures have been utilized as a perfect absorber metamaterial. Such designs have been widely studied and verified in various spectrum regimes ranging from optical [1–7] to infrared [8–16] to terahertz [17,18] to microwave [19–23] frequencies. However, this type of perfect absorbers is characterized by its narrow bandwidth owing to the electromagnetic resonance nature. Perfect broadband light absorbers have been developed by employing complex structures of multiple resonance modes [3–6,9–14]. Nevertheless, they need highly complicated fabrication techniques. Inasmuch as light absorbers made of metal-dielectric-metal multilayer have benefits of easy fabrication unaccompanied with lithography complexities and the capability of integration with the majority of optical devices, this design has received a lot of researchers’ attention to investigate and realize broadband absorption exploiting the multiple resonances formed by metal-dielectric layer cavities. [24–29] 5-layers of SiO2/Ti backed by a Cu reflector resulted in average absorption of 98% over a wavelength range of 0.95 $\mu m$ centered at 0.725 $\mu m$, [27] two cascade optical nanocavities formed by 6-layers of SiO2/Ti achieved 97.97% absorption over a wavelength range of 1.3 $\mu m$ centered at 1.05 $\mu m,$ [24] and SiO2/Cr/SiO2 3-layers backed by an Al reflector realized 95% absorption over a wavelength range of 0.8 $\mu m$ centered at 0.8 $\mu m$ [28]. Moreover, the optical subwavelength nanoresonator has been used and enhanced to achieve an extreme light concentration with an optical cross-section comparable to the macroscopic one [30]. Zero-refractive-index media could also exhibit coherent perfect absorption [31–35] by utilizing photonic doping of absorptive defects [36] or embedding ultrathin conductive films [37], for instance. In this work, we modify the metal-dielectric-metal cavities by substituting the metal layer by hyperdoped semiconductor layer as an epsilon-near-zero (ENZ) material (Sb-doped Ge) and replacing the dielectric with semiconductor (Ge). That is to get the structure of hyperdoped Semiconductor-Semiconductor-hyperdoped semiconductor cavities. Moreover, we use two different ENZ layers with different ENZ wavelengths to achieve wideband absorption. Taking into account that the ENZ wavelength is the wavelength at which the real part of electric permittivity approaches zero [38–41]. Previously, semiconductor and ENZ material bilayer showed a strong absorption around the ENZ wavelength [42]; on the other hand, metamaterial of multilayers of dielectric and ENZ material gave the same behavior [43,44]. Therefore, in this paper, we purpose, optimize, and manufacture the semiconductor and ENZs thin film metamaterial for broadband near-perfect absorption. Two bilayers of different ENZ materials separated by a semiconductor spacer structure device is experimentally proven to realize more than 93% P-polarized light absorption over a wavelength range of $1.8\; \mu m$ centered at $5.3\; \mu m$ for an angle of incidence of 40° and a flat-top wideband ($1.74\; \mu m$) P-polarized light absorption ($> 75\%$) centered at $5.13\; \mu m$ for an angle of incidence of $50^\circ $. Moreover, our proposed sample fabrication is straightforward compared to the ordinary metal-dielectric-metal multilayer structure. Since our sample multilayers can be grown only using one technique (Molecular beam epitaxy) without breaking the vacuum and loading the sample in and out of the deposition chamber several times, contrary to the rest of metal-dielectric-metal metamaterials. Furthermore, its operation band in the middle wave infrared (MWIR) transparency widow with near-unity absorption is essential for an extensive range of applications such as night vision, fire alarm, military reconnaissance, environmental monitoring, passive cooling radiation into the outer space cold sink [45–48], imaging sensors, modulators, and photodetectors.
2. Experimental section
2.1. Simulation part
Finite-difference time-domain (FDTD) simulations were performed to calculate the reflectance and transmittance of the device and then optimize its structure (FDTD Solutions, Lumerical Solutions, Inc., Canada). A Perfect match layer (PML) was chosen as a boundary condition for the simulation region, which was overridden by Broadband Fixed Angle Source Technique (BFAST) for the boundaries perpendicular to the film's plane. A frequency-domain field and power monitor was placed posterior to the light source to measure the reflectance. In contrast, another monitor was located outside the substrate (or gold film if existing) to measure the transmittance. The substrate thickness was 50 µm, which is large enough in comparison to the wavelength (3-8 µm). A frequency-domain field profile monitor was placed normal to the film plane to get the electric near-field distribution. The dielectric constants provided by the software were used for Au, Ge, and Si, whereas newly imported constants were used for Sb-doped Ge films pursuant to the fitting results of the FTIR reflectance data. The magnitude of the light source electric field was set to be 1 V/m.
2.2 Experimental part
At first, two samples were carried out utilizing the molecular beam epitaxy (MBE) technique to extract the complex permittivity of Sb-doped Ge films. Therefore, two 2-inch double-side polished Si wafers were RCA cleaned and then loaded into the MBE system. After the degassing process, a silicon layer of 50 nm thickness was homo-epitaxial grown on each substrate individually to smooth the terraces after residual oxides removal. Then, two Sb-doped Ge films of thickness 1 $\mu m$ were grown on each substrate respectively under the same conditions except for the substrate temperature; it was 150 $^{\circ}{C}$ for the first and 250 $^{\circ}{C}$ for the second. They were grown under a constant rate of Ge deposition (0.511 ${Å} \left/ {s}\right.$) and Sb flux ($1.4 \times {10^{12}}\,c{m^{ - 2}}{S^{ - 1}}$). Our MBE system was equipped with (EVBB-63-5, MBE-Komponenten, Germany) electron beam evaporator for Si source and two knudsen cell sources for Ge and Sb (WEZ-63-35, MBE-Komponenten, Germany). In addition to, a Reflection high-energy electron diffraction (RHEED) system (RH 20 SS, Stable INSTRUMENTE, Germany) for monitoring surface morphology and crystalline quality during crystal growth. Quadrupole mass spectrometer was exploited to monitor the antimony flux. The MBE base pressure was retained under $2 \times {10^{ - 10}}\; Torr.$
(Bruker, VERTEX 70) Fourier-transform infrared spectroscopy (FTIR) was utilized to measure the normal- and oblique-incidence reflectance spectra. By fitting the data of the normal-incidence reflectance spectrum with the Drude-Lorentz dispersion model, we extracted the complex permittivity for each Sb-doped Ge film [49–51].
Another double-side polished Si substrate was used to fabricate the device. It went through the following steps, in order. It was RCA cleaned, loaded into the MBE system, degassed, and then gotten its 50 nm homo-epitaxial silicon layer. Under the same mentioned above growth conditions (pressure, Ge deposition rate, Sb flux), 100 nm Ge layer at 600$^{\circ}{C}$, 100 nm Sb-doped Ge layer at 250$^{\circ}{C}$ (${\lambda _{ENZ}} = 5.37\; \mu m$), 60 nm Ge layer at 250$^{\circ}{C}$ (the spacer), 80 nm Sb-doped Ge layer at 150$^{\circ}{C}$ (${\lambda _{ENZ}} = 4.17\; \mu m$), and then 80 nm Ge layer at 150$^{\circ}{C}$ were successively epitaxially grown. After that, the wafer was cut into two halves. One half was kept as it was, and the other had a thick gold layer (300 nm) deposited on its backside using magnetron sputtering equipment. The sputtering system is mainly composed of (Kurt J. Lesker) Magnetron Sputtering Sources, (SQM-160, Inficon) Thin Film Deposition Monitor, and (CESAR Power Generator, Advanced Energy) RF power supply. (200 KV, Tecnai G2) Cross-section transmission electron microscopy (XTEM) was used to investigate the device structure.
3. Device structure and simulation results
To qualify valid simulations, Sb-doped Ge of different growth temperatures (150$^{\circ}{C}$, 250$^{\circ}{C}$) complex electric permittivity data were extracted by fitting the data of the normal-incidence reflectance spectrum with the Drude-Lorentz dispersion model over a spectral range of 2.5-15 $\mu m$. The film thicknesses were simultaneously determined by fitting reflectance and scanning electron microscopy (SEM). In doped semiconductor films, the ENZ wavelength has a small variation with thickness for thin films ≥ 20 nm [40]. In that thickness range, the ENZ wavelength of Sb-doped Ge films mainly depends on carrier concentration and crystallinity [53]. Therefore, we intended to design each Sb-doped Ge layer in our device with a thickness between 80-100 nm with the keeping of high crystal quality. The permittivity of two Sb-doped Ge thin films of 1 $\mu m$ thickness are presented in Fig. 1(a). The films show different ENZ wavelengths of $4.17\; \mu m$ and $5.37\; \mu m$ according to different carrier concentrations of $1.49\; \times {10^{20}}\; c{m^{ - 3}}$ and $1.00\; \times {10^{20}}\; c{m^{ - 3}}$ due to different growth temperatures of 150$^{\circ}{C}$ and 250$^{\circ}{C}$, respectively. The complex permittivities of films have been seen to differ somewhat across deposited films. However, this impact has no discernible influence on the simulated model or the measured reflectance spectra.
Fig. 1. a) Real (black) and imaginary (red) parts of the electric permittivity extracted from the measured normal-incidence reflectance spectra of two Sb-doped Ge films of different growth conditions (solid and dashed). b) Schematic of the device consisting of a metallic thick gold film, a silicon substrate, a bilayer of Ge and Sb-doped Ge of ${\mathrm{\lambda }_{\textrm{ENZ}}} = 5.37\mathrm{\;\ \mu m}$, Ge-spacer, and a bilayer of Sb-doped Ge of ${\mathrm{\lambda }_{\textrm{ENZ}}} = 4.17\mathrm{\;\ \mu m}$ and Ge.
Figure 1(b) shows the device structure of two (germanium)-(hyperdoped germanium) bilayers separated by 60 nm Ge-spacer. The 1st bilayer consists of a pair of Ge and Sb-doped Ge $({{\mathrm{\lambda }_{\textrm{ENZ}}} = 4.17\mathrm{\;\ \mu m}} )$ thin films of equal thickness of 80 nm. But the 2nd bilayer made of a pair of Ge and Sb-doped Ge $({{\mathrm{\lambda }_{\textrm{ENZ}}} = 5.37\mathrm{\;\ \mu m}} )$ thin films of equal thickness of 100 nm. The thickness of each ENZ layer is chosen to be roughly equal to $({{\raise0.7ex\hbox{${{\mathrm{\lambda }_{\textrm{ENZ}}}}$} \!\mathord{\left/ {\vphantom {{{\mathrm{\lambda }_{\textrm{ENZ}}}} {50}}} \right.}\!\lower0.7ex\hbox{${50}$}}} )$. The order of layers in each bilayer is designed to keep the whole device consisting of alternating semiconductor-hyperdoped semiconductor thin films. Moreover, the Ge layer of the 2nd bilayer acts as a virtual substrate for the upper films to enable high-quality crystallinity. An optically thick layer (300 nm) of gold is deposited on the opposite side of the Si substrate. This layer's role is to prevent light transmission from the substrate. Undoped silicon substrate represents a perfect choice because it has no absorption band in the mid-IR range [54]. Our device with such structure guarantees a zero light transmission and single-crystal quality of Ge and doped Ge layers, which are two contradictory factors if the Ge films were grown directly on a metallic layer.
First, we Simulated the reflectance spectra of thin-film structures for different thicknesses of Ge-spacer at constant bilayers thicknesses. The materials’ extracted electric permittivities were employed in the simulations. The simulated optical reflectance versus wavelength for different thicknesses of Ge-spacer is shown in Fig. 2(a). The Ge-spacer thickness heavily influences the absorption depth, central wavelength, and bandwidth. For spacer thicknesses less than $0.21\; \mu m$, the absorption occurs around the Sb-doped Ge ENZ wavelength of $5.37\; \mu m$. That enables the interaction between these two modes of absorption. However, the $4.17\mathrm{\;\ \mu m}$ Berreman mode is almost not influenced by the Ge-spacer thickness because its surface plasmon polariton occurs at a wavelength far from this of the resonance absorption mode caused by the spacer. The optimal thickness of the Ge-spacer layer is found to be 70 nm. This thickness enables the strongest and broadest available absorption close to the middle wave infrared (MWIR) atmospheric window [55].
Fig. 2. a) Simulated reflectance of P-polarized light of $50^\circ $ incidence angle versus wavelength and Ge-spacer layer thickness for the device. b,c) Simulated normal incidence reflectance (black), transmittance (blue), and calculated absorptance (red) for the device without gold film (b) and with gold film (c). d,e) Simulated reflectance versus wavelength and incidence angle for S-polarized light (d) and P-polarized light (e).
Then, the normal incidence reflectance and transmittance spectra were simulated for the optimized film structure of 70 nm Ge-spacer without (Fig. 2(b)) and with (Fig. 2(c)) the existence of the bottom thick gold film. The absorptance spectra in both cases were calculated according to the equation $A + R + T = 1$ where A is the absorptance, R is the reflectance, and T is the transmittance [41]. In the case of gold film absence, most of the light power is transmitted, and the central absorption wavelength is located at $6.3\; \mu m$ with a strength of 0.24. However, adding the gold thick ground plane vanishes the transmittance, enables an additional light trip through the film structure, locates the central absorption wavelength at $5.1\; \mu m$, and enhances the absorptance strength to be 0.74.
Furthermore, the oblique incidence reflectance spectra for both S-polarized (Fig. 2(d)) and P-polarized (Fig. 2(e)) light were simulated at different incidence angles. The S-polarization shows a uniform spectral profile, a constant central absorption wavelength at $5.1\; \mu m$, and a changing absorption strength with incidence angle variance. It displays an absorption strength of 0.75 at an incidence angle of $10^\circ $, and its absorptance increases with the incidence angle increasing up to 0.88 at $50^\circ $. Afterward, it decreases to reach 0.7 at $70^\circ $. In contrast, the P-polarization shows entirely different behavior, particularly at large incidence angles. This could be attributed to the ability of P-polarized light to excite Berreman modes in each ENZ layer. Increasing the incidence angle supports the $4.17\; \mu m$ Berreman mode confinement at the expense of both $5.37\; \mu m$ Berreman mode and $5.1\; \mu m$ absorption resonance mode, broadening the absorption band and decreasing the absorption strength. The simulation results of the absorptive thin film metamaterial structure composed of alternating semiconductor-hyperdoped semiconductor layers integrated with two different ENZ layers reveal a promising ultra-wide P-polarization absorption on the condition of adequately manipulating the incidence angle.
4. Experimental results and discussions
Figure 3(a) shows the measured normal incidence optical reflectance and transmittance besides the calculated absorptance of the fabricated device without the bottom thick gold film. It has an absorption strength of 0.4 at $5.0\; \mu m$ superior to the simulated one, likely due to the polished bottom side of the silicon substrate, which works as a partial mirror. As well, Fig. 3(b) displays the same measured and calculated spectra as in Fig. 3(a), but for the fabricated device with bottom gold film. In this case, the device has an absorption strength of 0.62 at $5.1\; \mu m,$ in apparent agreement with simulation results.
Fig. 3. a,b) Measured normal incidence reflectance (black), transmittance (blue), and calculated absorptance (red) for the device without gold film (a) and with gold film (b). c,d) Measured reflectance versus wavelength and incidence angle for S-polarized light (c) and P-polarized light (d). e) Calculated absorptance of P-polarized light for the device at different incidence angles: $40^\circ $ (black) and $50^\circ $ (red).
The measured oblique incidence reflectance spectra for S- and P-polarized light at different incidence angles are presented in Figs. 3(c) and 3(d), respectively. The S-polarization exhibits the same behavior as the simulation predicted, with maximum absorption of 0.93 at $5.1\; \mu m$ for $50^\circ $ incidence angle. Although the P-polarization likewise follows the expected behavior, it demonstrates absorption strengths far superior to those given by simulation. As shown in Fig. 3(e), a flat-top wideband ($1.74\; \mu m$) absorption ($> 75\%$) is enabled at an incidence angle of $50^\circ $ centered at $5.13\; \mu m$ wavelength. Moreover, the incidence angle of $40^\circ $ allows wideband near-perfect absorption ($> 93\%$) over a wavelength range of $1.8\; \mu m$ centered at $5.3\; \mu m$. In comparison to the simulation results, the measured absorption spectra have broader and stronger absorption spectral bands. The experiment and simulation results have shown Berreman modes confinement at large angles and band broadening, but the simulation predicts these confinements will not be as strong as observed from the fabricated sample. This could be attributed to Fabry-Perot cavities formed by the multiple interfaces. That makes the whole reasons of absorption arising from the $5.1\; \mu m$ absorption resonance interference, $4.17\mathrm{\;\ \mu m}$ Berreman mode, and $5.37\mathrm{\;\ \mu m}$ Berreman mode work together and support each other to achieve that high absorption rather than working at the expense of each other. However, the simulation could not predict such an effect because the formed boundaries at the interfaces between layers (as shown in Fig. 4) could not be introduced to the simulation inputs. These boundaries could be attributed to the roughness of the interfaces due to the different substrate growth temperatures of successive layers. This could also be the reason for the observed broadening in the measured spectra in Fig. 3(b) compared to the corresponding simulated ones in Fig. 2(c).
Fig. 4. A high-resolution cross-section transmission electron microscope (HR-XTEM) bright-field (BF) images show the uniform planar layers of the device (Taking into account that HR-XTEM barely distinguishes between them by a little color contrast). The scale bar is 100 nm (left) and 10 nm (top). The top image shows the boundary between two successive layers. The topmost Ge layer's corresponding selective area electron diffraction (SAED) pattern (top right) shows its single-crystal quality. The high-energy electron diffraction (RHEED) pattern corresponding to each layer is shown at its right, emphasizing the entire Sb-doped Ge and Ge layers’ single crystallinity.
Our results represent a considerable achievement compared to the recently published attempts in the same spectral region or for similar but different metamaterial structures. That, Zhai et al. achieved around 0.7 absorption in the middle wave infrared range by designing a metamaterial made of a polymeric matrix embedded with randomly distributed resonant polar dielectric microspheres [45]. While Smith et al. realized 0.9 $\mathrm{\mu m}$ wideband absorption over a spectral range: 1.5-2.4 $\mathrm{\mu m}$ using a metamaterial structure composed of 10-bilayers of indium tin oxide (ITO) and SiO2 [43].
5. Conclusion
In conclusion, we have designed and fabricated a novel class of absorptive metamaterials for broadband near-perfect light absorption in the medium wave infrared (MWIR) region utilizing multiple Ge and Sb-doped Ge bilayer thin-film structures. Two bilayers of different ENZ materials separated by a semiconductor spacer structure backed by substrate and gold thick ground plane device was experimentally proven to realize more than 93% P-polarized light absorption over a wavelength range of 1.8 µm centered at 5.3 µm for an angle of incidence of 40°. Besides, enabling a flat-top 75% P-polarized light absorption band over 1.74 µm range centered at 5.13 µm for an angle of incidence of 50°.
Funding
State Key Laboratory of Modern Optical Instrumentation; Zhejiang University; National Natural Science Foundation of China (91950205).
Acknowledgments
Author contributions. H.Y. supervised the project. H.A. Conceived the idea, performed the doped-Ge and Ge deposition (MBE work), characterized the samples, performed the simulations, and wrote the manuscript. S.L. assisted part of the characterization. J.Z. performed the deposition of Au layer. All authors discussed the results and contributed to the final version of the manuscript.
Disclosures
The authors declare no conflicts of interest.
Data availability
The data that support the findings of this study are available from the corresponding author upon reasonable request.
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