Radiative cooling is a passive cooling strategy that can radiate heat to outer space through the 8 to 13 μm waveband (atmospheric window) and is now widely used for buildings, wearable fabric, solar cells, and electronic devices. Daytime radiative cooling requires both high reflection in the solar spectrum and high absorption/emission in the 8 to 13 μm range. Previous multilayered structures were optimized by changing the thickness ratio of the layers, but the optical properties of multilayer thin films, such as absorptivity, transmissivity, and reflectivity, are determined by complex factors. In this work, several initial multilayer structures were selected and then the thickness of each layer was globally optimized; the theoretically smallest thickness with the best absorption performance was achieved in the 8 to 13 μm range, which significantly improved the cooling performance and reduced costs. We developed an optimized SiO2-Ta2O5 alternating multilayer photonic radiative cooling thin film and fabricated it using ion source assisted electron beam evaporation with an average emissivity of 0.876 within the 8 to 13 μm range and an average reflectivity of 0.963 in the 0.3 to 2.5 μm waveband; it achieved an average temperature reduction of 20.1°C lower than the uncoated substrate and 3.2°C lower than the ambient temperature under direct sunlight with an average solar power of 859.3 W/m2.
The properties of coatings deposited by electron-beam (e-beam) technique can be easily influenced by environmental humidity, causing spectrum shift, residual stress evolution, and wave front errors. HfO2 / SiO2 multilayer coatings with different overcoat layer deposition parameters have been prepared. The optical spectrum shifts induced by atmosphere-vacuum effect are investigated by a spectrometer. The laser resistance is studied and their damage morphologies are characterized by a scanning electron microscope. The surface morphologies and the global mechanical stresses of the films are analyzed by an atomic force microscope and zygo interferometer, respectively. The experimental results demonstrate that by introducing a dense capping SiO2 layer employed with plasma ion assisted deposition, considerable environmental stability of e-beam coatings can be improved due to retarded water vapor transport. A relatively smaller grain size can be obtained as well. Moreover, the laser-induced damage threshold shows no significant difference.
The properties of coatings deposited by electronic beam (e-beam) technique can be easily influenced by environmental humidity, causing spectrum shift, residual stress evolution, and wave front errors. In this work, HfO2/SiO2 multilayer coatings with different overcoat layer deposition process were prepared. The optical spectrum shift caused by atmosphere-vacuum effect of the prepared samples was investigated by spectrometer. The laser-induced damage resistance was studied and the damage morphologies were characterized by Scanning Electron Microscope (SEM). The surface morphology and global coating stress of the films were analyzed by Atomic Force Microscope (AFM) and Zygo interferometer, respectively. The experimental results demonstrate that by a capping SiO2 layer employed by plasma ion assisted deposition (PIAD), considerable stability concerning the environmental stability of e-beam coatings can be improved due to delayed water vapor transport rate. A relatively smaller grain size can be obtained as well. Moreover, the laser- induced damage threshold (LIDT) shows no significant differences.
It has been well-known that subwavelength metal slits grating structures function as extraordinary optical transmission filters due to excitation of surface plasmon-polaritons. In this work, we show that complex metal-dielectric subwavelength grating structures can also be designed to function as reflection-type optical filters with narrow spectral line-width in the infrared regime. Unlike extraordinary transmission filters, the new reflection optical filters with sharp spectral profiles can be designed within the entire infrared range by adjusting the height and period of the subwavelength grating structure. Detailed analysis has shown that the contrast of refractive indices of two dielectric materials embedded in the metal-dielectric structure is critical for giving sharp reflection spectral profiles.
The free-electron relaxation time is a crucial property to be considered in the design of optical devices, because it determines the dielectric function. Thus, an accurate understanding of this relaxation time is essential for design optimization. Some simulations showed that the relaxation times of Au thin films with thicknesses below 30 nm are different from those of the bulk material. Therefore, we deposited films with four different thicknesses below this value and used near-infrared spectroscopic ellipsometry to show that the relaxation time is dependent on the film thickness. We fitted the ellipsometry spectrum of Au thin films with a thickness <30 nm and found the imaginary part of the dielectric function of the thin films to vary with the film thickness in the near-infrared region. Furthermore, different relaxation times were used to simulate the reflectance of a Fabry–Pérot absorber and a plasmonic metamaterial absorber. The simulation results indicated that the obtained relaxation time enables a more reliable evaluation of optical device design.
A series of Fe(1-x)CoxSi2 thin films with variation of x was prepared by reactive deposition epitaxy (RDE) method. The optical properties of the samples are reported in this paper. The dielectric function of the samples was measured by spectroscopic ellipsometer in the photon energy range of 0.26 - 4.8 eV at room temperature. It's interesting to find that the dielectric function of Fe(1- x)CoxSi2 films is strongly dependent on the phase of the films: (1) The dielectric function spectra show interference peaks in the low photon energy range for the beta phase Fe(1-x)CoxSi2 samples. (2) The dielectric function spectra show a feature between the semiconductor and metal feature for the samples containing both beta and sigma phase Fe(1-x)CoxSi2. (3) The dielectric function spectra show metal feature for the sigma phase Fe(1-x)CoxSi2 samples. According to the x-ray diffraction results, the variation of the dielectric spectra is arisen form the change of the Fe-Si phase in the samples, rather than that from the variation of x.
This paper reports the temperature dependence of the magneto-optical properties of Zn1-xMnxSe samples prepared by molecular beam epitaxy method. In the magneto- optical spectra, there exist several Faraday rotation peaks. The peaks located at approximately 2.18, approximately 2.36 and 2.45 - 2.57 eV are attributed to Mn2+ d yields d* transitions. The peak located at approximately 2.7 eV is attributed to the interband transitions and higher order Mn2+ d yields d* transitions, which are blue-shifted with decreasing temperature. The positions of the rotation peaks induced by Mn2+ d yields d* transitions show weak temperature dependence.
For the granular type of Co-Ag (1.8 nm)/Ag multilayers with the Co contents of 22, 45, and 65 at%, the magneto-resistance ratio ((Delta) R/R) has a maximum found with respect to the AG spacer layer thickness. It is found that with increasing Co content the (Delta) R/R peak shifts towards thicker AG layers. For the Co-Ag/Ag multilayers with low Co contents, (Delta) R/R increases monotonically with increasing magnetic layer thickness, but has a maximum near the magnetic layer thickness of 2.2 nm for the sample having a higher Co content. The peak is thought as a result of the antiferromagnetic (AFM) coupling between the nearest Co clusters in neighboring magnetic layers. In the high field region, the magnetization and the square root of (Delta) R/R vary as a linear function of the reciprocal applied field for all multilayers. In the low field range, they are proportional to the applied field for samples with thin magnetic layers.
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