J. Wouters

Precise measurements of Faraday rotation using ac magnetic fields

We discuss several important issues concerning the use of a lock-in amplifier and the experimental procedure that need to be addressed to obtain accurate measurements of the Faraday rotation using ac magnetic fields. Our study was conducted on BK7 glass. We show that if electronic interference is avoided and the dependence of the signal on the average light intensity is taken into account, an accurate value of the Verdet constant for thin samples can be determined.

Differential detection for measurements of Faraday rotation by means of ac magnetic fields

We demonstrate that by using a combination of a Wollaston prism and two photodiodes the accuracy in the measurements of Faraday rotation with ac magnetic fields can be greatly improved. Our experiments were performed on microscope cover glass plates with thicknesses between 0.13 and 0.16 mm. We show that our setup is capable of distinguishing between the Faraday rotation signals of glass plates having a difference in thickness of a few micrometers, corresponding to Faraday rotations of hundreds of microdegrees per Tesla only.

Preparing polymer films doped with magnetic nanoparticles by spin-coating and melt-processing can induce an in-plane magnetic anisotropy

Faraday rotation has been used to investigate a series of polymer films doped with magnetic iron oxide nanoparticles. The films have been prepared by spin-coating and melt-processing. In each case, upon varying the angle of optical incidence on the films, an in-plane magnetic anisotropy is observed. The effect of such an anisotropy on the Faraday rotation as a function of the angle of optical incidence is verified by comparison with magnetically poled films. These results demonstrate that care should be taken upon analyzing the magnetic behavior of such films on account of the sample preparation techniques themselves being able to affect the magnetization.

Plasmon-assisted enhancement of third-order nonlinear optical effects in core (shell) nanoparticles

We study resonant self-action effects of light in plasmonic γ-Fe2O3 (Au) core (shell) nanoparticles (NPs) in a polymer matrix by the spectroscopic Z-scan technique. Plasmon-assisted enhancement of the local optical field in NPs results in the appearance of saturable absorption, as well as in the amplification of nonlinear refraction by more than an order of magnitude compared with nonresonant conditions.

Second Harmonic Generation in Core (Shell) γ-Fe2O3 (Au) Nanoparticles

The films containing core (shell) γ-Fe2O3 (Au) nanoparticles that possess the plasmon resonance at the wavelength 550 nm are prepared. The second harmonic generation in these structures is shown to be diffuse and depolarized, that is typical for the hyper-Rayleigh scattering.. The studies of the magnetization-induced second harmonic generation shows that the ratio of the magnetic to nonmagnetic components of the hyperpolarizability of a single nanoparticle, γM/γ0 ~ 0.4 in the field of about 3 kOe.

Faraday rotation in magnetic colloidal photonic crystals

Faraday rotation for magnetic field sensing can find applications in satellite altitude monitoring. Enhancing and tuning Faraday rotation is demonstrated in hybrid magnetic photonic crystals, based on an independent nanoscale engineering of two different materials (silica and iron oxide) at different length scales (< 20 and > 200 nm). An engineering approach towards combined photonic band gap properties and magnetic functionalities, based on independent nanoscale engineering of two different materials at different length scales, is conceptually presented, backed by simulations, and experimentally confirmed. Large (> 200 nm) monodisperse nanospheres of transparent silica self-assemble into a photonic crystal with a visible band gap, which is retained upon infiltration of small (< 20 nm) nanoparticles of magnetic iron oxide. Enhancing and tuning Faraday rotation in photonic crystals is demonstrated.

Magnetic field sensing based on Faraday rotation in inorganic/polymer hybrid materials

A new type of ferrofluid was synthesized and characterized by Faraday rotation. The ferrofluid is composed of ironoxide nanoparticles that are stabilized by covalently attached polyethylene glycol chains. This material shows great promise as magnetic sensing material.

Calorimetric single particle detection in high magnetic fields at low temperatures

High resolution calorimetric detection of single particles in magnetic fields up to 8 T at temperatures <0.1 K is reported. A careful study of heavily doped Si and Ge thermistors reveals an extremely large positive magnetoresistance in the variable range hopping conductivity regime. This phenomenon is used to optimize the impedance and the temperature sensitivity of a calorimetric detector. Initial experiments show an energy resolution full width at half maximum of 800 eV for 62.5 keV conversion electrons and an energy threshold of <400 eV.

Nonlinear optics of magnetic nanoparticles

Nonlinear magneto-optical properties of magnetic core(shell) gamma-Fe₂O₃(Au) and Fe₃O₄ (magnetite) nanoparticles are studied. Nonlinear magneto-optical Kerr effect in hyper-Rayleigh scattering at the second harmonic wavelength is about 10-20% for both types of nanoparticles.

Magnetic behaviour of Co impurity in dilute PtFe alloys: a nuclear orientation study

Behaviour of the Co magnetic moments in dilute PtFe alloys was studied in the temperature range of 5-50 mK and the external magnetic field range of 0-8.5 T using low-temperature nuclear orientation. Kondo behaviour of Co, known to exist in pure Pt, was found in the sample of lowest concentration only (Pt+0.22 at.% Fe). Large misalignment of the Co magnetic moments was seen, especially for alloys having Fe concentration slightly above the percolation limit 0.76 at.%. An anomaly in the hyperfine field value on the Co nucleus was also found in these alloys. >