arXiv subjects

Zhen-Lai Wang

Publications and source records attributed to Zhen-Lai Wang.

9 recordsLinked to original sources

arXiv2025

A decomposition for transverse spins in structured vector fields

Classical vector waves can possess intricate spin angular momenta (SAM), which are \emph{perpendicular} to the propagation direction, as revealed by the recent recognition of surprisingly transverse SAM in electromagnetic (EM) fields. In this paper, we employ the Hertz potential method to define structured vector fields and analytically decompose the SAM of the wave fields in two parts. Our novel approach of decomposition not only confirms that transverse SAM may originate from the first-order spatial inhomogeneity of the Poynting momentum, but also points out that for \emph{non-planar vector waves with near fields}, an extraordinary spin appears as a distinct part out of transverse spin. By four examples of vector beams, we further demonstrate that the proposed transverse spins prevail universally in both propagating and evanescent waves. This work renews our fundamental understanding of the decomposition of SAM for classical vector waves.

physics.optics↗

arXiv2024

Transverse spin and orbital angular momenta in rotating-wave structured light

Within monochromatic optical fields, we demonstrate the rotating-wave structured light with wave vortex carrying an intrinsic transverse orbital angular momentum orthogonal to the propagation direction of light. Remarkably, we find that such a rotating-wave structured light reveals highly nontrivial features of transverse spin and orbital angular momentum densities. The normalized total angular momentum density is conserved universally, suggesting a mutual conversion of the intrinsic transverse spin and orbital angular momentum in free space. Despite such mutual conversion at local level, the integral intrinsic orbital angular momentum can be well defined with the topological charge of the vortex per photon. Moreover, the orientation of transverse spin density is governed by the direction of the Poynting momentum density, manifesting a spin-momentum-locking effect in free space.

physics.optics↗

arXiv2023

Gravitational Spin Hall Effect of Dirac Particle and the Weak Equivalence Principle

We present a spin-induced none-geodesic effect of Dirac wave packets in a static uniform gravitational field. Our approach is based on the Foldy-Wouthuysen transformation of Dirac equation in a curved spacetime, which predicts the gravitational spin-orbit coupling. Due to this coupling, we find that the dynamics of the free-fall Dirac wave packets with opposite spin polarization will yield the transverse splitting in the direction perpendicular to spin orientation and gravity, which is known as the gravitational spin Hall effect. Even in a static uniform gravitational field, such effect suggests that the weak equivalence principle is violated for quantum particles.

gr-qc↗

arXiv2020

Gravity-induced geometric spin Hall effect as a probe of universality of free fall of quantum particle

We present a novel fundamental effect that for the matter waves the space-averaging free-fall point of quantum particles undergoes a spin-dependent transverse shift in the gravitational field of Earth. This effect is similar to the geometric spin Hall effect (GSHE) [Aiello et al., Phys. Rev. Lett. 103, 100401 (2009)] and can be called gravity-induced GSHE. This effect suggests that there might be violations of the universality of free fall (UFF) or weak equivalence principle (WEP) in the quantum domain.

gr-qc↗

arXiv2019

A New Improved Energy-Momentum Tensor and Its Possible Role in Gravity

Motivated by a special consideration in quantum measurement, we present a new improved energy-momentum tensor. The new tensor differs from the traditional canonical and symmetric ones, and can be derived as Nother current from a Lagrangian with second derivative. We also attempt to construct a gravitational coupling in such a way that the new energy-momentum tensor becomes the source of the gravitational field. The theory we obtain is of an Einstein-Cartan type, but derived from a minimal coupling of a Lagrangian with second-derivative, and leads to additional interaction between torsion and matter, including the scalar field. For the scalar field, the theory can also be derived in the Riemann space-time by a non-minimal coupling. Our study gives hint on more general tests of general relativistic effects.

gr-qc↗

arXiv2018

Extraordinary spin density and energy back-flow under interference

A novel phenomenon was reported recently that the "local optical spin density" based on Poynting vector might be counter-intuitively opposite to the integrated spin orientation while the one related to the canonical expression might not [Leader, Phys. Lett. B 779, 385-387 (2018)]. However, the "local optical spin density" of the canonical expression can also be counter-intuitively opposite to the integrated spin orientation under the interference of plane waves, even if all of the plane waves possess the same polarization handedness. Moreover, the interference fields might acquire a transverse spin density (perpendicular to the propagation plane), which can have more well-controlled relations with the polarization. Meantime, in such a case its energy flux exhibits counter-intuitive back-flow and a circular motion (vortex) in the propagation plane locally, which implies a transverse local orbital angular momentum density.

physics.optics↗

arXiv2018

Anomalous Geometric Spin Hall Effect of Light?

The geometric spin Hall effect of light (GSHEL), similar to the spin Hall effect of light, is also a spin-dependent shift of the centroid of light beam's intensity (energy flux), but it is a purely geometric effect that does not depend on a particular light-matter interaction. In this paper, we discuss the GSHEL with respect to momentum instead of energy flux, and find out that in the case of the symmetric energy-momentum tensor, the shift of the centroid of momentum flux is double that of energy flux. Interestingly, for the canonical energy-momentum tensor, the centroid shift of momentum flux agrees with that of energy flux. If we consider the effect of orbital angular momentum, however, the centroid displacement of momentum flux is twice that of energy flux for both energy-momentum tensors. To tell which energy-momentum tensor of light field would be more "correct", we propose a experimental scheme to test the GSHEL of momentum flux through the mechanical effect of light.

physics.optics↗

arXiv2017

Gravitational Coupling from Active Gravity

We attempt to construct a gravitational coupling by pre-selecting an energy-momentum tensor as the source for gravitational field. The energy-momentum tensor we take is a recently derived new expression motivated by joint localization of energy and momentum in quantum measurement. This energy-momentum tensor differs from the traditional canonical and symmetric ones, and the theory we obtain is of an Einstein-Cartan type, but derived from a minimal coupling of a Lagrangian with second-derivative, and leads to additional interaction between torsion and matter, including the scalar field. For the scalar field, the theory can also be derived in the Riemann space-time by a non-minimal coupling. Our study gives hint on more general tests of general relativistic effects.

gr-qc↗

arXiv2017

New Energy-Momentum and Angular Momentum Tensors with Applications to Nucleon Structure

We present a new type of energy-momentum tensor and angular momentum tensor. They are motivated by a special consideration in quantum measurement: Given a wave in mutual eigen-state of more than one physical observables, the corresponding physical currents should be proportional to each other. Interestingly, this criterion denies the traditional canonical and symmetric expressions of energy-momentum tensor and their associated expressions of angular momentum tensor. The new tensors we propose can be derived as Noether currents from a Lagrangian with second derivative, and shed new light on the study of nucleon structures.

physics.gen-ph↗