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arxiv: 2605.13707 · v1 · submitted 2026-05-13 · ❄️ cond-mat.mtrl-sci · physics.optics

Recognition: unknown

Giant optical spin-orbit interactions in ferroelectric van der Waals waveguides

Ding Xu , Saeed Rahmanian Koshkaki , Vicente Galicia , Chun-Ying Huang , Victoria Quir\'os-Cordero , Jakhangirkhodja A. Tulyagankhodjaev , Andr\'e Koch Liston , Daniel G. Chica
show 9 more authors
Emma Lian Amirhosein Amini Yongseok Hong Taketo Handa P. James Schuck Xiaoyang Zhu Xavier Roy Arkajit Mandal Milan Delor
Authors on Pith no claims yet

Pith reviewed 2026-05-14 17:42 UTC · model grok-4.3

classification ❄️ cond-mat.mtrl-sci physics.optics
keywords van der Waals waveguidesoptical spin Hall effectspin-orbit interactionNbOI2ferroelectricbirefringencenonlinear opticsspin-momentum locking
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The pith

Van der Waals ferroelectric waveguides produce giant optical spin-splitting on sub-micrometer scales

A machine-rendered reading of the paper's core claim, the machinery that carries it, and where it could break.

The paper shows that highly birefringent van der Waals materials such as the ferroelectric NbOI2 form waveguides that generate strong optical spin-orbit interactions. Experiments with femtosecond microscopy reveal giant spin-splitting through the optical spin Hall effect, which separates optical spin currents spatially over sub-micrometer distances. These results match predictions from a microscopic light-matter interaction model and include polarization-controlled steering of the guided light. The same behavior appears across other van der Waals waveguides and follows an empirical scaling law that ties spin-splitting strength directly to dielectric anisotropy.

Core claim

In NbOI2 slab waveguides, light propagation and harmonic conversion beyond the total internal reflection barrier produce giant optical spin-splitting via the optical spin Hall effect. This splitting separates optical spin currents on sub-micrometer scales in quantitative agreement with a microscopic light-matter interaction model, enables polarization-controlled waveguide steering, and generalizes across various van der Waals waveguides through a scaling law that links dielectric anisotropy to geometric spin-splitting.

What carries the argument

Optical spin Hall effect in highly birefringent ferroelectric van der Waals waveguides, arising from dielectric anisotropy and spin-momentum locking

If this is right

  • Optical spin currents separate spatially on sub-micrometer scales inside the waveguides
  • Polarization control produces active steering of guided light beams
  • The spin-splitting effect and its scaling law extend to multiple van der Waals waveguide materials
  • The platform supports densely integrated opto-spintronic devices on a chip

Where Pith is reading between the lines

These are editorial extensions of the paper, not claims the author makes directly.

  • These waveguides could combine with other two-dimensional layers to form hybrid devices that route both light and spin information without external magnets
  • The anisotropy scaling suggests a design rule for choosing or engineering vdW materials to reach even smaller device footprints
  • Harmonic generation observed alongside the spin-splitting opens routes to nonlinear spin-orbit devices in the same compact geometry

Load-bearing premise

The microscopic light-matter interaction model fully accounts for the observed spin-splitting without significant unaccounted contributions from material defects, surface effects, or fabrication variations.

What would settle it

A set of measurements on additional NbOI2 samples or other birefringent vdW slabs that show spin-splitting values deviating substantially from the model's quantitative predictions or that break the reported scaling with dielectric anisotropy would falsify the central claim.

Figures

Figures reproduced from arXiv: 2605.13707 by Amirhosein Amini, Andr\'e Koch Liston, Arkajit Mandal, Chun-Ying Huang, Daniel G. Chica, Ding Xu, Emma Lian, Jakhangirkhodja A. Tulyagankhodjaev, Milan Delor, P. James Schuck, Saeed Rahmanian Koshkaki, Taketo Handa, Vicente Galicia, Victoria Quir\'os-Cordero, Xavier Roy, Xiaoyang Zhu, Yongseok Hong.

Figure 1
Figure 1. Figure 1: Optical spin-splitting in NbOI2 slab waveguides. a, Schematic of optical spin splitting in a NbOI2 planar waveguide. The incident light is focused on the NbOI2 edge to couple into waveguide modes beyond the far-field light cone. Inset: side view of the NbOI2 crystal structure along the non-polar c-axis and polar b-axis. b, Measured dielectric function of NbOI2 along all crystal axes. Subscripts r, i indica… view at source ↗
read the original abstract

Optical spin-orbit interactions (SOI) link photonic spin to momentum, offering a route toward on-chip polarization control and beam steering. Nevertheless, achieving sufficient optical SOI and nonlinearities on sub-micrometer scales - a prerequisite for dense photonic integration - remains an outstanding challenge. Here, we show that highly birefringent van der Waals (vdW) waveguides provide an ideal, chip-compatible platform to address this limitation. We focus on the ferroelectric semiconductor NbOI2, which exhibits record optical nonlinearities and dielectric anisotropy. Using femtosecond optical microscopy, we image light propagation and harmonic conversion beyond the total internal reflection barrier over tens of micrometers in NbOI2 slab waveguides. We report giant optical spin-splitting through the optical spin Hall effect, which facilitates spatial separation of optical spin currents on sub-micrometer scales, in quantitative agreement with a microscopic light-matter interaction model. We further leverage optical spin-momentum locking to realize polarization-controlled waveguide steering. We generalize these observations across various vdW waveguides and empirically confirm a scaling law linking dielectric anisotropy to geometric spin-splitting. Our results establish highly anisotropic vdW waveguides as an ideal platform for densely integrated opto-spintronic technologies.

Editorial analysis

A structured set of objections, weighed in public.

Desk editor's note, referee report, simulated authors' rebuttal, and a circularity audit. Tearing a paper down is the easy half of reading it; the pith above is the substance, this is the friction.

Referee Report

3 major / 3 minor

Summary. The manuscript reports giant optical spin-orbit interactions in ferroelectric van der Waals waveguides using NbOI2 slabs. It demonstrates sub-micrometer spatial separation of optical spin currents via the optical spin Hall effect, imaged with femtosecond microscopy showing propagation and harmonic conversion beyond total internal reflection over tens of micrometers. Results are claimed in quantitative agreement with a microscopic light-matter interaction model, with applications to polarization-controlled steering and a generalized scaling law for anisotropic vdW materials.

Significance. Should the quantitative agreement hold after addressing controls, this establishes highly birefringent vdW waveguides as a promising platform for dense opto-spintronic integration, leveraging strong dielectric anisotropy and nonlinearities for on-chip spin-momentum control. The empirical scaling law provides a useful design guideline for material selection in photonic devices.

major comments (3)
  1. [Abstract] Abstract: The central claim of 'quantitative agreement' with the microscopic light-matter interaction model is load-bearing but unsupported by details on error bars, data exclusion criteria, or statistical comparison metrics. This makes it impossible to assess whether the sub-micron spin-splitting is robustly captured by the model or influenced by unmodeled effects.
  2. [Model derivation and experimental comparison] Model derivation and experimental comparison: The model assumes ideal uniform slab geometry and uniform dielectric anisotropy. However, mechanical exfoliation of NbOI2 typically produces local thickness fluctuations of several nm, which alter effective TE/TM birefringence and can induce apparent transverse shifts via modified Goos-Hänchen or Imbert-Fedorov effects at interfaces. The manuscript must include AFM thickness maps and simulations incorporating measured variations to confirm the splitting arises purely from optical SOI.
  3. [Scaling law results] Scaling law results: The empirical confirmation of the scaling law linking dielectric anisotropy to geometric spin-splitting requires explicit reporting of the number of vdW materials tested, the range of anisotropy values, and fit quality (e.g., R² or residuals) to substantiate the generalization claim.
minor comments (3)
  1. [Abstract] Abstract: Quantify the term 'giant' spin-splitting with a specific numerical value (e.g., shift magnitude in nm) or direct comparison to prior SOI platforms for clarity.
  2. [Figures] Figures: All microscopy images and propagation plots should include scale bars; quantitative data panels must display error bars and indicate the number of independent measurements.
  3. [General] General: Ensure consistent use of terminology such as 'optical spin currents' and 'spin-momentum locking' to avoid potential ambiguity with electronic spin concepts.

Simulated Author's Rebuttal

3 responses · 0 unresolved

We thank the referee for their thorough review and constructive comments. We address each major point below and have revised the manuscript to incorporate additional details and data where appropriate.

read point-by-point responses

  1. Referee: [Abstract] Abstract: The central claim of 'quantitative agreement' with the microscopic light-matter interaction model is load-bearing but unsupported by details on error bars, data exclusion criteria, or statistical comparison metrics. This makes it impossible to assess whether the sub-micron spin-splitting is robustly captured by the model or influenced by unmodeled effects.

    Authors: We agree that additional statistical details would strengthen the presentation of the quantitative agreement. In the revised manuscript, we have added error bars to the experimental spin-splitting data in the relevant figures. We have also expanded the methods section to describe the data analysis procedure, including signal-to-noise thresholds used for data inclusion. A goodness-of-fit metric (mean squared deviation between model predictions and measured shifts) is now reported to allow readers to evaluate the robustness of the agreement. revision: yes

  2. Referee: [Model derivation and experimental comparison] Model derivation and experimental comparison: The model assumes ideal uniform slab geometry and uniform dielectric anisotropy. However, mechanical exfoliation of NbOI2 typically produces local thickness fluctuations of several nm, which alter effective TE/TM birefringence and can induce apparent transverse shifts via modified Goos-Hänchen or Imbert-Fedorov effects at interfaces. The manuscript must include AFM thickness maps and simulations incorporating measured variations to confirm the splitting arises purely from optical SOI.

    Authors: This is a valid concern. We have added AFM thickness maps for the NbOI2 waveguides used in the primary experiments to the supplementary information; these show that thickness variations are limited in the imaged regions. We have also performed new simulations that incorporate the measured thickness profiles, confirming that any additional shifts from modified interface effects remain small relative to the observed optical spin-orbit splitting. These results are now discussed in the revised main text and support that the dominant mechanism is the modeled SOI. revision: yes

  3. Referee: [Scaling law results] Scaling law results: The empirical confirmation of the scaling law linking dielectric anisotropy to geometric spin-splitting requires explicit reporting of the number of vdW materials tested, the range of anisotropy values, and fit quality (e.g., R² or residuals) to substantiate the generalization claim.

    Authors: We have revised the scaling-law section and supplementary material to explicitly state the number of vdW materials examined, the range of dielectric anisotropy values covered, and quantitative fit metrics (including R² and residual analysis). These additions substantiate the empirical generalization without altering the original conclusions. revision: yes

Circularity Check

0 steps flagged

No significant circularity; derivation self-contained

full rationale

The paper reports experimental femtosecond microscopy of propagation and harmonic conversion in NbOI2 vdW waveguides, followed by observation of giant spin-splitting via optical spin Hall effect. It states quantitative agreement with an independent microscopic light-matter interaction model and an empirically confirmed scaling law linking dielectric anisotropy to geometric spin-splitting across multiple vdW materials. No quoted equations or sections reduce the central claim to fitted parameters renamed as predictions, self-definitional loops, or load-bearing self-citations whose content is unverified outside the present work. The model is presented as microscopic and external to the data; the scaling is described as empirical generalization rather than derivation by construction. This satisfies the default expectation of a non-circular experimental paper with theoretical comparison.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on the material's reported dielectric anisotropy and nonlinearities as inputs from prior work, plus a microscopic model whose parameters are not detailed here; no new free parameters or invented entities are introduced in the abstract.

axioms (1)
  • domain assumption Dielectric anisotropy in vdW materials produces birefringence sufficient for geometric spin-splitting via the optical spin Hall effect
    Invoked to explain the observed sub-micron separation and scaling law.

pith-pipeline@v0.9.0 · 5597 in / 1289 out tokens · 60177 ms · 2026-05-14T17:42:13.436516+00:00 · methodology

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Reference graph

Works this paper leans on

1 extracted references · 1 canonical work pages

  1. [1]

    (1) Schaefer, B. et al. Measuring the Stokes polarization parameters. Am. J. Phys. 75, 163– 168 (2007). (2) Abdelwahab, I. et al. Giant second-harmonic generation in ferroelectric NbOI2. Nat. Photonics 16, 644–650 (2022). (3) Koshkaki, S. R., Manjalingal, A., Blackham, L. & Mandal, A. Exciton-Polariton Dynamics in Multilayered Materials. Preprint at https...

    work page doi:10.48550/arxiv.2502.12 2007