Measurement of the magnetic properties of thin films based on the spin Hall effect of light

Characterization of Aligned Polymers Using the Spin Hall Effect of Light

In this paper, we propose a scheme based on the spin Hall effect of light (SHEL) for characterizing aligned polymers. Poly (3-hexylthiophene) (P3HT) films were prepared via the solution shear coating method. The experimental results obtained by using SHEL indicated that the alignment of polymer chains could be enhanced by UV irradiation and was positively correlated with the irradiation time, which was consistent with the measurements of the UV-Vis spectrometer and the polarizing optical microscope (POM). Compared with other optical methods, such as POM that characterizes aligned samples using light intensity, the amplified shift in SHEL can significantly reduce technical noise and exhibit high sensitivity. Even for the weak alignment film, this method can still recognize its alignment and achieve a signal-to-noise ratio (SNR) of 30 dB. This renders SHEL a highly precise technique for studying aligned polymers, which is valuable for the development of organic electronics.

Observation of the spin Hall effect of light by a single-photon detector

We use a single-photon detector to detect the spin Hall effect of light (SHEL) of a quasi-single-photon beam obtained in this Letter. The physics of the spin Hall effect and its quantum weak measurement method with a dimensionless pointer are elucidated through particle number representation. Our weak measurement scheme obviates the necessity of high-resolution single-photon array detectors. Consequently, we have successfully observed the spin Hall effect within a 20 ns temporal window using a position-resolution-independent single-photon detector with remarkably low-noise levels. The weak measurement of the dimensionless pointer presented in this Letter boosts both the detection accuracy and the response speed of the photonics spin Hall effect, thereby contributing significantly to fundamental theoretical research in spin photonics and precise measurements of physical property parameters.

Observation of the mixed magneto-optical Kerr effects using weak measurement

In this paper, we use weak measurement to study the mixed magneto-optical Kerr effects (MOKEs) in a magnetic single layer of Co and in organic/Co bilayer films. The relationship between the amplified shift and the MOKE parameters is theoretically established and then experimentally observed as a function of both the magnetization intensity and the magnetization direction in magnetic thin films with an arbitrary magnetization. Furthermore, we experimentally observe a magnetic coupling at the organic/ferromagnetic interface. Due to this being a robust coupling, there is a significant dependence of the amplified shift with the thickness of the organic layer. This work has value for its application in the magnetic measurement technique for the more general case based on weak measurement.

Thickness-dependent in-plane shift of photonic spin Hall effect in an anisotropic medium

As the in-plane spin splitting (IPSS) has a broad application for the precision measurement and sensing, it is extremely important to explore its enhancement mechanism via the photonic spin Hall effect (PSHE). However, for a multilayer structure, the thickness in most of previous works is generally set as a fixed value, lacking the deeply exploration of the influence of thickness on the IPSS. By contrast, here we demonstrate the comprehensive understanding of thickness-dependent IPSS in a three layered anisotropic structure. As thickness increases, near the Brewster angle, the enhanced in-plane shift exhibits a thickness-dependently periodical modulation, besides with much wider incident angle than that in an isotropic medium. While near the critical angle, it becomes thickness-dependently periodical or linear modulation under different dielectric tensors of the anisotropic medium, no longer keeps almost constant in an isotropic medium. In addition, as exploring the asymmetric in-plane shift with arbitrary linear polarization incidence, the anisotropic medium could bring more obvious and wider range of thickness-dependently periodical asymmetric splitting. Our results deepen the understanding of enhanced IPSS, which is expected to promise a pathway in an anisotropic medium for the spin control and integrated device based on PSHE.

Estimation of Kerr angle based on weak measurement with two pointers

In this paper, we propose a weak measurement method using two pointers to estimate the magneto-optical Kerr angle, which is robust to ellipticity. The double pointers are the amplified displacement shift and intensity of the post-selected light beam, which are the conventional information carried by the light beam and can be directly output by a detector (such as a charge-coupled device). We demonstrate that the product of the double pointers is only related to the phase variation between two basic vectors and independent of the amplitude errors. In the measurement process, when there is an amplitude change or additional amplitude noise between two eigenstates, the product of two pointers is very useful in extracting phase information and shielding amplitude noise. In addition, the product of two pointers has a good linear relationship with the phase variation and a larger dynamic measurement range. This method is applied to measure the magneto-optical Kerr angle of NiFe film. The Kerr angle can be directly obtained using the product of the light intensity and the amplified displacement shift. This scheme is of great significance for the measurement of the Kerr angle of magnetic films.

Effect of the incident polarization on in-plane and out-of-plane spin splitting near the critical angle

To reveal the effect of the incident polarization on the spin splitting of the photonic spin Hall effect (that is, the spatial and angular in-plane and out-of-plane spin splitting), we systematically study the phenomena and characteristics of these four spin splitting generated when the beam with arbitrary linear polarization is reflected from the non-absorbing medium interface and the absorbing medium interface. Several features of the relationship between the incident polarization and the four kinds of spin splitting are found. In addition, It is also found that the in-plane angular and spatial shifts are significantly enhanced near the critical angle, even reaching their theoretical upper limit. However, the out-of-plane shifts are not enhanced. The research in this paper will contribute to a deeper understanding of PSHE. These findings can also provide new ideas and methods for precision metrology, photonic manipulation, and photonic device fabrication.

Quadratic spin Hall effect of light due to phase change

The spin Hall effect (SHE) of light has brought important applications, but the involved spin states only split in one direction. Here we employ an accurate three-dimensional model of light to show that the SHE generally exhibits quadratic spin splitting, i.e., both vertical and horizontal splitting, in the presence of a fast phase change of reflection. Further, we disclose that the two splittings are actually different from each other, and that they originate from the vertical and horizontal spin momentum flows, respectively, owing to the spatial gradient of polarization in the individual direction. Finally, it is found that by tuning the incident angle and polarization of light, one can manipulate the quadratic SHE so as to realize a variety of spin splittings, such as unbalanced quadratic splitting and off-center splitting of spin states.

Topological spatial differentiators upon reflection of the normally incident light

We theoretically propose topological spatial differentiators by the normal-incidence reflection of light. Firstly, a three-dimensional propagation model is established for the light normally incident on the interface between two media. It is found that due to the spin-orbit interaction of light, a given circularly polarized light always induces oppositely polarized light carrying a topological charge, so the two intrinsic spin components are separated radially or azimuthally. Moreover, the normally reflected fields are approximately proportional to two kinds of second-order spatial differentiations of the input circularly and linearly polarized fields. Further results applying to the two-dimensional image processing for edge detection validate the two topological spatial differentiators.

Multi-angle excited MOI and image processing strategies specified for detection of orthogonal weld defects

This paper develops an integrative scheme combining new image acquisition, filtering and enhancement methods specified for orthogonal weld defect detection based on magneto-optical imaging (MOI) technique. For image acquisition, a controllable magnetic system enabling rotation of magnetic angles is invented to accurately collect MO images. Multiple images are acquired, yet few are utilized for further processing in the conventional method based on human subjective preferences, bearing chances that images containing defects are discarded. Therefore, we turn to an automated-filtering system to scrutinize MO images and filter effective images through Bhattacharyya coefficient screening method. This not only elevates efficiency and objectivity but also eliminates missed inspection. For image enhancement, normalization method is used to balance the image intensity, followed by image fusion and edge extraction by a two-dimensional gradient method. Our pre- and post-processing approaches significantly improve accuracy in defect recognition and precision in MO images.