With the consciousness of human beings to improve environmental protection with increasing consumption of traditional energy，the demand for renewable energy resource has been more concerned recently. Among all these renewable energy sources，solar energy is considered to be an ideal one as it is green，non-polluting，available everywhere and inexhaustible ^［1-2］. Solar radiation usually can be transformed into usable energy forms such as the electrical and thermal energies in which the photo-thermal conversion is the simplest and most efficient way. In order to achieve the best solar-to-heat conversion efficiency，the collectors should have an exceptional spectrally selective solar absorber surface with a high solar absorption in the solar spectrum region and very low emittance in the infrared region according to Kirchhoff’s law.

To meet the basic characteristics of the spectrally selective solar absorber mentioned above，six typical structures have been studied，including（a）intrinsic absorbers，（b）semiconductor-metal tandems，（c）multilayer absorbers，（d）cermets（Ceramic-Metal），（e）textured absorbers，and（f）photonic crystals^［3-4］. Among the various models applied for the selective solar absorber structures，most of the commercial selective solar absorber surfaces are based on metal-dielectric composites known as cermets. Several cermets based on various metals and dielectrics have been reported^［5-7］. One possible structure is a cermet film with a graded metal concentration profile，in which the metallic concentration is higher at the top than that at the bottom of the film ^［8］. The other possible structure is formed by two homogeneous cermet layers（double cermet layers）：one layer with a LMVF（low metal volume fraction）cermet is put on the top of layer with a HMVF（high metal volume fraction）cermet deposited onto an infrared reflector layer. The later one has higher photothermal conversion efficiency than the surface using only a single cermet layer or a graded film structure ^［9］. In double cermet layers，solar radiation is effectively absorbed internally by additional phase interference mechanism. Concerning the fabrication of such cermets，several techniques such as electroplating，metal pigmented anodized oxide，physical vapor deposition，chemical vapor deposition and so on，have been developed. However，among these various techniques，sputtering approach，one of the physical vapor depositions，is the most commonly and widely used ^［10-11］.

With respect to the dielectric components，the cermet absorber commonly consists of SiO₂，Al₂O₃，Cr₂O₃ and AlN matrix ^［12］. Various metal，such as Pt，Ni，Cr，V，Mo and so on has been embedded in the dielectric host matrix with the optical properties evaluated in advance. By taking the merit of the high melting point and high reflectivity of metals in the long wavelength range，W（tungsten）is usually considered to be a suitable metal material for photo-to-heat conversion and has been widely used in selective solar absorbers ^［13-15］. But as we know，W-SiO₂ double cermet selective solar absorber coating has not been studied so far. The research carried out in this work，therefore，not only will show the characteristics and optical properties of W-SiO₂ cermet layer with different metal volume fraction，but also will present the optimal feature of both solar absorption and thermal emittance for the selective solar absorber based on the double W-SiO_2-cermet-layers structure by using the sputtering method.

The solar selective absorbers based on the double W-SiO₂-cermet layers were deposited on the Si or K9 glass substrate by using an electron beam assisted sputtering system（INFOVION，Seoul，Korea）at room temperature with a background pressure of 4.5×10^-6 Torr. Before deposition，the substrates were ultrasonically cleaned and dried by nitrogen gas. The cleaned substrates were put on a rotating sample holder at a rate of 10 rpm to improve uniformity. The growth pressure was fixed at 1×10^-3 Torr by a throttle valve with the argon（Ar）gas flow rate of 10 sccm（standard cubic centimeter per minute）. As shown in Fig. 1，the multiple layers were deposited to be consisting of，in order from the bottom up，a W infrared reflection layer，a W-SiO₂ HMVF layer，a W-SiO₂ LMVF layer，and finally a SiO₂ antireflection coating on top. The direct current（DC）sputtering was used to deposit W，while SiO₂ was deposited by the radio frequency（RF）sputtering method. The complete experimental processing conditions for the multilayer structures are listed in Table 1.

W-SiO₂ cermets with different metal volume fraction were deposited by varying the deposition power of tungsten at 0 W，10 W，20 W，30 W，40 W，50 W and 60 W，respectively，while maintaining a constant deposition power of SiO₂ at 200 W. A step-profiler was used to calibrate the sputtering rate of each layer，and a variable-angle spectroscopic ellipsometer（J.A. Woollam VASE）was applied to precisely determine the optical constants and thickness of the nanocomposite solar absorber coatings. The reflectance spectrum studies were carried out with a UV–Vis–NIR spectrophotometer（UV 3600 plus，Shimadzu）in the wavelength range of 250∼2 500 nm and a Fourier-transform infrared spectrometer（Nicolet Nexus 470 FT-IR spectrometer）in the wavelength range of 2.5∼25.0 μm. The absorbance of the sample was calculated from the measured reflectance data according to the conditions of T = 0 and A = 1-R，since the transmittance T of the sample with the thick W reflection layer can be omitted.

For the high reflectance in the infrared region and high temperature resistance，noble and transition metals are often considered to be the appropriate materials for IR reflection layer. The infrared reflectance characteristics of pure Cu，W，Ni，Pt and Cr metals were studied by measuring the reflectance spectra of each thick metal layer deposited onto the Si substrate. The selection criteria will depend on the required reflectance spectra that the metal should reflect low in solar radiation region while high in the infrared region to achieve low emittance. The measured reflectance spectra of the various metals are shown in Fig. 2.

It can be seen in Fig. 2 that chromium exhibits a spectral feature of low reflectance of about 65% in the entire 400∼2 500 nm wavelength region，resulting in higher thermal emittance. On the contrary，copper shows a highly uniform reflectance of about 95% in the Vis-NIR range of 700∼2 500 nm，which could decrease the solar absorption in the visible region. Overall，tungsten has an average reflectivity lower than 50% in the UV-Vis region of 300∼800 nm and higher than 90% in the infrared region. This is supposed to be helpful to improve the solar absorbance and to reduce the thermal emittance at the same time. Additionally，tungsten has a high melting point，which can act as a diffusion barrier between the substrate and the cermet layer. Considering W-SiO₂ as the absorption layer，choosing tungsten as the base layer also has its advantage in the process of film growth. Thus，tungsten was chosen as an ideal IR reflection layer for the solar selective absorber consisting of double W-SiO₂ cermet layers.

Since the solar radiation is mainly absorbed in the two cermet layers，it is important to study the characteristics of single W-SiO₂ cermet layer with different W volume factor in the SiO₂ dielectric matrix. By changing the deposition power of W while keeping constant of the SiO₂ deposition power，the films with varying W concentration were prepared. Spectroscopic ellipsometry（SE）was used to investigate the optical constants of the sputtered W-SiO₂ nanocomposite thin films affected by the composition. The ellipsometric parameters（Ψ，Δ）were measured in the wavelength range of 300∼1 200 nm at three incident angles：65°，70° and 75°.

In data fitting，effective medium approximation（EMA）is usually used to depict the dispersing function of a macroscopic inhomogeneous medium ^［16］. The EMA model is suitable for many compositional materials，including the metal-dielectric composites. A widely used EMA theory for nanocomposite systems proposed by Bruggeman and Maxwell-garnet is expressed as ^［17-18］：

where εM-G is the effective dielectric function of the mixture，εa is the dielectric function of the guest material（inclusions），εb is the dielectric function of the matrix or host and fa is the volume fraction of the a component.

The measured and fitted ellipsometric parameters at the incident angle of 65° are presented in Fig. 3. The fitted ellipsometric data of the W-SiO₂ composites with different concentrations of W are in good agreement with the experimental ones over the entire measured spectral range，demonstrating the accuracy and reliability of the fitting procedure.

The optical constants deduced from the ellipsometric data using the EMA model are shown in Fig. 4. It can be seen clearly that with the increasing deposition power of W in the process，the values of both n and k in the spectral range of 300∼1 100 nm increase with deposition power of W，due to increasing fraction of metal W. The results indicate that by varying the concentration of W in the W-SiO₂ nanocomposites，both the refraction index n and the extinction coefficient k can be properly tuned to satisfy the required optical properties of materials for the photonic device design and applications in broad spectral regions.

The structure with double-cermet absorber layers has higher absorbance than that of using only single absorber layer because solar energy can be gradually absorbed layer by layer in the structure. To achieve the best efficiency of absorption，the most differentiated two composites with sputtering power of tungsten in 10 W and 60 W were chosen as the LMVF and HMVF absorption layer，whose metal volume fraction were 0.27 and 0.67 respectively.

In terms of the optical constants and thickness of each layer determined，the solar absorption of a multilayered film can be calculated by using the transfer matrix method（TMM）. Reversely，the film parameters of absorber containing double W-SiO₂ cermet layers were obtained by data fitting procedure assuming A=1 in the wavelength range of 250∼1 500 nm with the modified Levenberg-Marquardt optimization method，using the optical parameters of W-SiO₂ absorption layer obtained in advance. In terms of the schematic structure of the solar absorber as shown in Fig. 1，the optimal parameters are achieved as：SiO₂（47 nm）/W-SiO₂ LMVF（0.27VF，34 nm）/W-SiO₂ HMVF（0.67VF，94 nm）/W（~150 nm）. The reflectance spectra of the simulated and as-deposited solar selective absorber are shown in Fig. 5. It can be seen that the measured results agree well with the simulated ones，and the reflectance is very low with the average optical absorbance of about 95% in the wavelength range of 250∼1 500 nm，increases rapidly in the long wavelength region of 1 500∼5 000 nm，and keeps very high in the IR region of 5∼20 μm.

The experimental results for the optical absorbance，reflectance and transmittance spectra of the samples are shown in Fig. 6. The performance of solar selective absorber is characterized by solar absorbance αand thermal emittance . It can be presented as ^［19］：

where A（λ）and Lsun（λ）represent the optical absorbance and standard AM 1.5 solar radiation spectrum respectively. Based on the measured absorbance spectrum，the solar absorbance αwas determined to be approximately 95.3%，indicating the high and broad solar absorption properties of the double cermet film structure.

The thermal emittance ε，depending on the incident angle θ and temperature T can be given as follows ^［20］：

where ET,λ is Planck’s blackbody radiation and R(θ,λ) is the reflectance spectrum at incident angle θ. Then the thermal emittance ε0,T of the proposed double cermet sample can be figured out under the assumption that the optical constants of W and SiO₂ are not changed significantly when the working temperature is much lower than the melting point. The results are listed in Table 2. It can be seen at temperature of 600 K，the thermal emittance is about 0.124，implying the low thermal radiation of the proposed solar selective sample.

In this work，optimal structures of the solar selective absorber consisting of the double-W-SiO₂-cermet layers were designed and constructed. The factors which affect the spectral selectivity of the solar absorber，such as the IR reflector，volume fraction of metal，and thickness of individual layers，and so on have been studied，respectively. By changing the deposition power of metal W，films with varying metal volume fraction were prepared. The Bruggeman and Maxwell-garnet EMA model was applied to fit the complex refractive index of the samples. The optical constants deduced from the fitting procedure as well as those measured by ellipsometry were used as a database to optimize the absorption performance of the solar absorber. Using the transfer matrix method for the structure design，the optimal layer thickness and W volume were obtained as：W（~150 nm）/ W-SiO₂（94 nm，0.67HVF）/ W-SiO₂（34 nm，0.27LVF）/ SiO₂（47 nm）. The solar absorption device was fabricated using a combined DC-RF sputtering system. The experimental results were highly consistent with the simulated ones. A solar absorbance of 0.95 and thermal emittance of 0.12 at 600 K were achieved，implying the high absorption of the device in broad wavelength region and low thermal emittance at high working temperature. Since only two elemental compositions were used，the design will have its advantage in device fabrication and actual applications.