Hyper-Rayleigh scattering measurements of magnetite nanoparticles: determination of the first order hyperpolarizability anisotropy

The hyper-Rayleigh scattering technique was used to determine the first order hyperpolarizability β of magnetic nanoparticles dispersed on colloidal solutions. Pulse trains of mode-locked pulses of 100 ps on an a Q-switcher envelope of 150 ns emitted by a Nd:YAG laser, centered on 1064 nm, were used since this method allows measurements as a function of the incident beam intensity without the need of external elements. In order to determine the procedure to measure second-order optical nonlinearities on magnetic nanoparticles and avoid cumulative effects during the measurements, that lasts between to consecutive pulse trains, the results were studied for different values of the Q-switcher repetition rate, from 5 Hz to 800 Hz. Since cumulative effects were verified for higher values of repetition rates, all measurements were performed at the rate of 30 Hz. Therefore, the first-order hyperpolarizability β was measured in the presence and absence of external magnetic field of magnitude H = 800 G. The linear attenuation spectrum was determined and didn't change with the appliance of magnetic field since large aggregates of nanoparticles were not formed. Nonlinear scattering measurements were performed in the case were the laser light polarization was parallel and perpendicular to the external field lines, employing a half-wave plate to change the light polarization state. In the absence of magnetic field, βH=0 = 8:5(1)×10-28 cm5/esu, while in their presence of magnetic field, β = 9:8(2)×10-28 cm5/esu and β⊥ = 8:1(1)×10-28 cm5/esu, showing an anisotropy β-β⊥/β of about 17%.