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The polarized light transmittance of magnetic fluids under longitudinal magnetic field and the influence of magnetic fluids concentration,dielectric constants of the remanent liquid phase within the magnetic fluids,the ratio of dipole moment to the thermal energy of single magnetic nanoparticle and the number of magnetic nanoparticles per agglomeration are investigated numerically according to the analytical expression when considering both the geometric shadowing effect and the Faraday rotation effect.Theoretical results indicate that the magnetic fluids concentration,dielectric constants of the remanent liquid phase within the magnetic fluids and the ratio of dipole moment to the thermal energy of the single magnetic nanoparticle affect the polarized light transmittance apparently.The polarized light transmittance of magnetic fluids increases linearly or oscillates with the strength of the longitudinal magnetic field for low or high concentration samples,respectively.In certain ranges,the polarized light transmittance of magnetic fluids increases with dielectric constants of the remanent liquid phase and the ratio of dipole moment to the thermal energy of single magnetic nanoparticle.While the polarized light transmittance is almost independent of the number of magnetic nanoparticles per agglomeration.The distinct difference of the parameter-dependent polarized light transmittance at low and high magnetic field regions is obtained.The applications to several photonic devices based on the polarized light transmittance of magnetic fluids under longitudinal magnetic field are proposed.

Composite magnetic fluids were obtained by doping silicon dioxide and polystyrene nonmagnetic microspheres into the pure ferrite magnetic fluids.The relationship between the magnetic-field-induced birefringence of composite magnetic fluids and the species,concentrations of nonmagnetic microspheres and pure ferrite magnetic fluids were investigated qualitatively.The results indicate that pure magnetic fluids with different concentrations doped with equal quantity of polystyrene microspheres have different variation trends for the birefringence with respect to the externally magnetic field.Different kinds of nonmagnetic microspheres have different influences on the change of birefringence after doping into the same pure magnetic fluids.When doping with two-kind-mixed nonmagnetic microspheres(equal quantity but different proportion between the two kinds of microspheres),one kind of the nonmagnetic microspheres will play a leading role in the variation of birefringence with externally magnetic field.Then,the variation trend of the birefringence with externally magnetic field for the as-prepared composite magnetic fluids is similar to that only doped with the leading nonmagnetic microspheres.

The relationship between the birefringence of Fe3O4 magnetic fluids and the ambient temperature(in the range of 20~80 ℃)at several different magnetic field strengths(lies in 0~200 mT)are investigated.The results show that similar dependence of birefringence on magnetic field strength and temperature happens for magnetic fluids with different concentrations.Moreover,the birefringence of the magnetic fluid is inversely proportional to the ambient temperature at fixed magnetic strength and proportional to the magnetic field strength at fixed ambient temperature.In a given temperature and magnetic field strength,the birefringence of the magnetic fluid with high concentration is larger than that with low concentration.The phenomena and results are analyzed carefully and the physical mechanism is clarified in detail.

We experimentally demonstrate a passively Q-switched erbium-doped fiber laser (EDFL) operation by using a saturable absorber based on Fe3O4 nanoparticles (FONP) in magnetic fluid (MF). As a kind of transition metal oxide, the FONP has a large nonlinear optical response with a fast response time for saturable absorber. By depositing MF at the end of optical fiber ferrule, we fabricated a FONP-based saturable absorber, which enables a strong light-matter interaction owing to the confined transmitted optical field within the single mode fiber. Because of large third-order optical nonlinearities of FONP-based saturable absorber, large modulation depth of 8.2% and non saturable absorption of 56.6% are demonstrated. As a result, stable passively Q-switched EDFL pulses with maximum output pulse energy of 23.76 nJ, repetition rate of 33.3 kHz, and pulse width of 3.2 {\mu}s are achieved when the input pump power is 110 mW at the wavelength of 980 nm. The laser features a low threshold pump power of ~15 mW., Comment: 4 pages,6 figures