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.
KEYWORDS: Solar cells, Solar radiation, Solar energy, Sun, Optical filters, Diodes, Photovoltaics, Energy efficiency, Tandem solar cells, Multijunction solar cells
The photoelectrical responsibility of single photo-electronic devices makes it difficult to achieve high efficiency of photoelectric conversion in the full solar spectrum range. The key to overcome the physical limits is to develop the system consisting of a set of solar cells in which the photo-electronic conversion of each cell will match to the sub-spectrum of the solar radiation with high conversion efficiency. In this work, we have used the spectrum splitting method to divide the solar spectrum into four sub-ranges of 400-630nm, 630-800nm, 800-900nm and 900-1800nm, respectively. Four high performance single-junction photo diodes are used, and each of them has high quantum-efficiency of photo-electronic conversion matching to the sub-ranges of solar spectrum. Under the 0.5-6.0 SUN radiation condition, the photo-electrical conversion efficiency of the system with four solar cells has been measured with the result to show that the photo-electric conversion efficiency of 35% is achieved under the typical 2.8 SUN radiation condition. The results given in this work will provide a way to show the potencial to realize a high photo-electric conversion efficiency (>40%) of the solar system in application.
Facing great challenge in the optical and photonic fileds, the department of optical science and engineering at Fudan university has carried out unique research training and education programs with multiple levels to have students obtain broad knowledge and research training experience by attending the class studies and active research projects.
A thin SiN layer was coated upon GdFe/Si film in order to avoid the surface contamination and oxidation. From the SiN/GdFE interface, the genuine optical constants of GdFe layer were obtained using spectroscopic ellipsometry. The magneto-optical Kerr effect in the large energy range between 1.6 eV and 4.5 eV was also studied. Calculations show that the MOKE peak in the high energy range originates not only from its intrinsic magneto-optical characteristics of GdFe layer, but also from the interference of SiN layer. When the Fe content decreases from 79.3 to 75.8 at.%, the sign change of the complex Kerr rotation was observed and found being induced by the spin-flip.
A series of CoxAg100-x granular films was prepared by ion beam sputtering and annealed at 100 degree(s)C, 250 degree(s)C, 400 degree(s)C, 500 degree(s)C, respectively. The optical constants, the complex dielectric function and magneto- optical Kerr parameters were measured at room temperature. It was found that Kerr rotation and ellipticity with increasing of the Co content. For the samples Co17Ag83, the strong Kerr effect enhancement is observed around the Ag plasma edge. With the annealing temperature increasing, the enhancement peak shifted toward the low photon energy region. From the analysis of the numerical calculations, it is concluded that the magneto-optical Kerr effect enhancement is due to the presence of the steep plasma edge.
For prepared FexAg100-x granular films, the Kerr rotation and ellipticity change monotonically with the photon energy and after annealing have two peaks near 2.0 and 3.8 eV, respectively. The peak value increases with annealing temperature TA. Numerical calculations show that the peak at low energy is induced by the change in the off-diagonal element of dielectric tensor of Fe particles and the peak at high energy results from the changes in optical properties of Ag particles near its plasma absorption edge. These results clearly show the simultaneous existence of both the magneto-optical size effect of Fe particles and optical size effects of Ag particles in metallic granular films.
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.
In many optical experimental systems, an achromatic 1/4 wavelength retarder is needed to satisfy experiment condition. This can be fulfilled using a rhomb prism which has a nearly constant refractive index in the working wavelength range. To make the easy use of the prism in the optical system and to avoid the problem of central beam deviation, one type of the prism having three total internal reflection faces can be designed to have the total retarding angle equal to 90 degrees. Though the angle can be relatively easily calculated in principle, the size and optical opening window of the prism depend on the practical and optimum geometric design. In this paper, we give a few examples of geometric design by taking different size parameters into consideration. We have made the prisms and put them into the application of measuring the magneto- optical Kerr ellipticity with success. Detailed calculation and error reduction in the design are also given.
A new type of the normal incidence scanning reflectometer has been designed and constructed. In the system design, instead of using a reference reflective sample or adjusting the optical path as did in the traditional design, we use a fixed M-type fused quartz prism to split the incidence light source beam into two ones by a total internal reflection configuration. Therefore, these two light beam have the same spot sizes, intensities and spectral response. One beam goes directly to the detector and is used as the incidence reference beam. The second sampling beam goes to the sample first, and then is reflected by the sample with the beam intensity measured finally by the same detector. A metal disk with three holes is driven precisely by a stepping motor and is used to control the reference beam, reflection beam and background signals to be measured in sequence. Afterwards, the absolute reflectivity of the sample at the certain wavelength can be determined immediately by the computer through calculating those three signals. The system is controlled automatically by the computer and worked in a wavelength range from 400 to 800 nm under a 5-degree nearly normal incidence angle condition. In the paper, the details of the system design, optical element configuration and error reduction are given and discussed. The measured reflective spectral results for testing samples are also given and shown in good agreement with those measured by other optical method. The system designed in this work, however, is simpler and more reliable to be used in many optical measurements of the sample.
An improved type of scanning and analyzer rotating magneto-optic spectroscopy has been designed and constructed. By adding an achromatic quart-wavelength retarder to the system and using Fourier transformation, the complete magneto-optic parameters, both polar Kerr rotation angle 6kand ellipticity k' have been measured in the 1.5-5-eV photon energy range and at a near normal incident angle of less than 2 degrees. A fine step motor with 1000 steps per revolution and a hollow shaft, on which the analyzer is directly mounted, is used to control precisely the analyzer azimuthal angle. The magnetic field, spectral scanning, and retarder position, as well as data processing are totally controlled by a microcomputer. The magneto-optic spectral results of the system is illustrated for FeTeCo film samples.
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