ABSTRACT
The generation of elliptically polarized electromagnetic wave of an antiferromagnetic (AF)/dielectric sandwiched structure in the terahertz range is studied. The frequency and external magnetic field can change the AF optical response, resulting in the generation of elliptical polarization. An especially useful geometry with high levels of the generation of elliptical polarization is found in the case where an incident electromagnetic wave perpendicularly illuminates the sandwiched structure, the AF anisotropy axis is vertical to the wave-vector and the external magnetic field is pointed along the wave-vector. In numerical calculations, the AF layer is FeF2 and the dielectric layers are ZnF2. Although the effect originates from the AF layer, it can be also influenced by the sandwiched structure. We found that the ZnF2/FeF2/ZnF2 structure possesses optimal rotation of the principal axis and ellipticity, which can reach up to about thrice that of a single FeF2 layer.
ABSTRACT
Impure ZnO materials are of great interest in optic and electronic applications. In this work, the effects of Al-doping on the electronic structures of ZnO system are investigated in detail. We find that the crystal structure strains significantly due to the introduction of Al impurity. On the other hand, the electronic band structures show that the position of the Fermi level moves upwards and the bands split near the band gap due to the introduction of Al. This is attributed to the interaction between Al3p and Zn4s orbital, which tend to drive the system towards semimetal. Photoluminescence (PL) studies indicate that the Al-doped ZnO samples have a high density of defects. This can be explained qualitatively by the above analysis on electronic structure.
ABSTRACT
Y0.96 Er0.02 Yb0.02)O3 nanocrystals of 10 and 40 nm average particle size were prepared by combustion method. And bulk materials of the same components were obtained by annealing at 1 200 degrees C. X-ray diffraction (XRD), Fourier transform infrared (FTIR) spectra, transmission electron microscope (TEM), and scanning electron microscopy (SEM) were used to characterize the crystal structure and morphology of the samples. The upconversion emission spectra and NIR (near-infrared) emission spectra were measured, under 980 nm excitation. The research result indicates that as the particle size decreases, the upconversion red emission and NIR emission components increase in the emission spectra. This phenomenon is attributed to the large ratio of surface area to volume in nanocrystals. This characteristic makes the nanocrystals absorb more OH-, whose vibrational energy is 3 200-3 800 cm(-1). The increase in the OH- number enhances the rate of nonradiative relaxation from Er3+ 4I11/2 to 4I13/2 energy level (energy gap is 3 600 cm(-1)). This nonradiative relaxation process depopulates the 4I11/2 level and makes the green emission weaker. Meanwhile, this process populates the 4I3/2 level and makes the red and NIR emissions stronger. The intensity of 1.5 microm main peak is 1.6 times that of bulk materials. This result has great significance in actual applications of nanophosphors.
