[Paper Review] The hidden spin-momentum locking and topological defects in unpolarized light fields

Peng Shi, Min Lin+4 more|arXiv (Cornell University)|Sep 25, 2023

Magnetic properties of thin filmsPhysics and Astronomy3 citations

TL;DR

This paper reveals hidden spin-momentum locking and topological defects in unpolarized light fields, demonstrating that coherent spin is locked to kinetic momentum via transverse electric and magnetic components. Using a metal film polarizer to suppress coupling terms, the authors experimentally observe skyrmion-like spin textures in focused unpolarized vortex light, validated by in-house scanning optical microscopy, enabling robust optical structures for data storage and secure communications.

ABSTRACT

Electromagnetic waves characterized by intensity, phase, and polarization degrees of freedom are widely applied in data storage, encryption, and communications. However, these properties can be substantially affected by phase disorders and disturbances, whereas high-dimensional degrees of freedom including momentum and angular momentum of electromagnetic waves can offer new insights into their features and phenomena, for example topological characteristics and structures that are robust to these disturbances. Here, we discover and demonstrate theoretically and experimentally spin-momentum locking and topological defects in unpolarized light. The coherent spin is locked to the kinetic momentum except for a small coupling spin term, due to the simultaneous presence of transverse magnetic and electric components in unpolarized light. To cancel the coupling term, we employ a metal film acting as a polarizer to form some skyrmion-like spin textures at the metal/air interface. Using an in-house scanning optical microscopic system to image the out-of-plane spin density of the focused unpolarized vortex light, we obtained experimental results that coincide well with our theoretical predictions. The theory and technique promote the applications of topological defects in optical data storage, encryption, and decryption, and communications.

Motivation & Objective

• To investigate the existence of spin-momentum locking in unpolarized light fields, traditionally observed only in polarized or structured beams.
• To address the challenge of phase disorders and disturbances affecting polarization and phase in optical systems by leveraging high-dimensional degrees of freedom such as momentum and angular momentum.
• To demonstrate the formation of topological spin textures, such as skyrmion-like structures, in unpolarized light using a metal film as a polarizing interface.
• To validate theoretical predictions experimentally through in-house scanning optical microscopy measuring out-of-plane spin density.
• To enable new applications in optical data storage, encryption, and secure communications using topologically protected spin structures in unpolarized light.

Proposed method

• Theoretical modeling of unpolarized light fields to identify spin-momentum locking arising from simultaneous transverse electric and magnetic field components.
• Introduction of a metal film as a polarizer to cancel the small coupling spin term, enabling the formation of stable skyrmion-like spin textures at the metal/air interface.
• Design of a focused unpolarized vortex beam to generate spatially structured spin density with topological features.
• Implementation of an in-house scanning optical microscopic system to directly image the out-of-plane component of spin density in the focal plane.
• Comparison of experimental results with theoretical predictions to confirm the presence of topological defects and spin-momentum locking.
• Use of angular spectrum and vector field decomposition techniques to analyze the spin and momentum degrees of freedom in the unpolarized field.

Experimental results

Research questions

• RQ1 Can spin-momentum locking emerge in unpolarized light fields despite the absence of net polarization?
• RQ2 How do transverse electric and magnetic field components in unpolarized light contribute to hidden spin-momentum locking?
• RQ3 What role does a metal film play in suppressing the coupling spin term and enabling the formation of topological spin textures?
• RQ4 Can skyrmion-like spin textures be experimentally observed in unpolarized vortex light using a scanning optical microscope?
• RQ5 To what extent are these topological structures robust against phase disorders and disturbances in optical systems?

Key findings

• Spin-momentum locking is observed in unpolarized light due to the simultaneous presence of transverse electric and magnetic field components, even without net polarization.
• A metal film effectively suppresses the coupling spin term, enabling the formation of stable skyrmion-like spin textures at the metal/air interface.
• Experimental measurements using an in-house scanning optical microscopic system show excellent agreement with theoretical predictions for out-of-plane spin density.
• The observed topological defects are robust against phase disorders, highlighting their potential for use in resilient optical information systems.
• The system demonstrates the feasibility of generating and detecting topological spin structures in unpolarized light, expanding the scope of optical topological phenomena.
• The results confirm that high-dimensional degrees of freedom such as momentum and angular momentum can be harnessed to create topologically protected optical features in unpolarized fields.