arxiv: 2605.05999 · v1 · submitted 2026-05-07 · ⚛️ physics.optics

Recognition: unknown

Near-unity efficiency optical vortex generation in van der Waals materials

Sujeong Byun , Munseong Bae , Hangsung Cho , Haejun Chung , Kostya S. Novoselov , James Bullock , Sejeong Kim

Authors on Pith no claims yet

Pith reviewed 2026-05-08 06:25 UTC · model grok-4.3

classification ⚛️ physics.optics

keywords optical vortexvan der WaalshBNBessel beamspin-orbit couplingbirefringenceconversion efficiency

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The pith

Bessel beams through van der Waals crystals generate optical vortices at up to 82 percent efficiency.

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

The paper establishes that using Bessel beams as input to van der Waals crystals enables near-unity efficiency in generating optical vortices through spin-orbit coupling. This is shown by achieving 0.82 efficiency in 27.4 micrometer thick hBN, compared to the 0.5 limit for standard Gaussian beams. The reason is the single transverse wave vector in Bessel beams aligning with the large birefringence of the material. Readers would care because this offers a simple way to make compact light-twisting devices without complex fabrication, potentially advancing fields like optical communications and microscopy.

Core claim

By sending a Bessel beam through a thin van der Waals crystal, the large birefringence and the beam's uniform transverse wave vector allow spin-orbit coupling to produce an optical vortex with efficiency reaching 0.82 and a topological charge shift of plus two.

What carries the argument

The single transverse wave vector of the Bessel beam combined with vdW crystal birefringence for enhanced spin-orbit conversion.

If this is right

Efficiency is higher for Bessel beams than Gaussian due to their plane wave distribution.
Conversion efficiency varies with the numerical aperture of the focusing lens.
The output vortex has topological charge increased by 2.
This approach works in microscale vdW materials without fabrication.

Where Pith is reading between the lines

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

This method could be extended to other vdW materials for different wavelengths or efficiencies.
Compact vortex generators might integrate into photonic circuits for quantum information processing.
Testing with varying crystal thicknesses could optimize for even higher efficiencies close to 1.

Load-bearing premise

The single transverse wave vector of Bessel beams with vdW birefringence leads to near-unity conversion without major losses from scattering or absorption.

What would settle it

Measuring the power and polarization of input and output beams in a 27.4 micrometer hBN sample under Bessel beam illumination and finding efficiency significantly below 0.82.

Figures

Figures reproduced from arXiv: 2605.05999 by Haejun Chung, Hangsung Cho, James Bullock, Kostya S. Novoselov, Munseong Bae, Sejeong Kim, Sujeong Byun.

**Figure 1.** Figure 1: Optical vortex generation within a hBN crystal with a Bessel incident beam. (a) Schematic illustration of view at source ↗

**Figure 2.** Figure 2: Demonstration of optical vortex generation using a hBN crystal. (a) The transmission setup used for view at source ↗

**Figure 3.** Figure 3: Thickness-dependent optical vortex generation. (a) Thickness characterisation of hBN crystals. (left) view at source ↗

**Figure 4.** Figure 4: Numerical aperture-dependent conversion efficiency. (a) Analytically calculated conversion efficiencies view at source ↗

read the original abstract

Optical spin-orbit coupling provides a promising, fabrication-free route for developing ultra-compact optical vortex generators. However, the conversion efficiency has been theoretically limited to 0.5. Here, we demonstrate enhanced vortex generation efficiency by employing a Bessel beam as the input and propagating it through van der Waals (vdW) crystals. The large birefringence of vdW crystals and the single transverse wave vector of a Bessel beam allow a near unity spin-orbit conversion efficiency and a topological charge transition of $\ell \rightarrow \ell + 2$. Through combined analytical and experimental investigations, we demonstrate a conversion efficiency of up to 0.82 in hexagonal boron nitride (hBN) crystals with a thickness of $27.4\,\mu\mathrm{m}$. The higher efficiency of Bessel input beams over Gaussian beams is attributed to their distinct transverse wave vector distribution of constituent plane wave components. Furthermore, we demonstrate the dependence of conversion efficiency on the numerical aperture (NA) of the objective lens, which is in good alignment with theoretical predictions. These demonstrations provide a fabrication-free route to highly efficient optical vortex generation via microscale vdW materials platforms.

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.

Desk Editor's Note private letter to a colleague

Bessel beams let them hit 0.82 efficiency for vortex conversion in 27 µm hBN by matching the crystal birefringence better than Gaussian inputs, but the power budget still needs explicit checking.

read the letter

The main takeaway is that switching to a Bessel input beam in van der Waals crystals like hBN lifts the spin-orbit vortex conversion efficiency from the usual 0.5 ceiling to a measured 0.82. The paper attributes this to the Bessel beam's single transverse wave-vector component, which avoids the averaging that hurts Gaussian beams across the crystal's birefringence. They combine a simple analytical model with experiments on 27.4 µm thick hBN flakes and show that efficiency falls with increasing objective NA exactly as the theory predicts. That match is the strongest part of the work; it gives some independent support for the mechanism. The result is new in the sense that no prior spin-orbit paper cited in the abstract used this input beam shape to clear the 0.5 limit in a microscale vdW platform. It is useful for anyone who needs compact, fabrication-free vortex sources for tweezers, communications, or sensing. The soft spot is the missing loss accounting. Reaching 0.82 in a 27 µm sample means reflection, absorption, scattering, and any unconverted light must all be small, yet the abstract gives no breakdown of total transmission versus the desired ℓ+2 component. If even modest scattering or higher-order modes are present, the quoted number could be optimistic. The NA dependence helps, but without the raw power measurements or error bars it is hard to judge how tight the claim really is. This is the kind of paper that belongs in a reading group focused on applied optics or 2D materials photonics. I would cite it if the full methods and data hold up under review. It deserves a serious referee because the experimental direction is concrete and the efficiency gain is large enough to matter for devices, even if the current write-up leaves the loss budget underspecified.

Referee Report

2 major / 2 minor

Summary. The paper claims that propagating a Bessel beam through van der Waals crystals (e.g., 27.4 μm hBN) enables optical vortex generation via spin-orbit coupling with conversion efficiency up to 0.82 and a topological charge shift ℓ → ℓ + 2. This exceeds the 0.5 theoretical limit for Gaussian beams because the Bessel beam's single transverse wave vector, combined with the crystal birefringence, produces near-unity conversion. The work combines analytical modeling with experiments showing higher efficiency for Bessel versus Gaussian inputs and NA dependence that matches theory.

Significance. If the efficiency result is robustly verified, the approach would offer a fabrication-free route to compact, high-efficiency vortex generators using microscale vdW platforms. This could advance applications in singular optics, quantum information, and beam shaping, with the NA-matching data providing a testable signature of the proposed mechanism.

major comments (2)

[Abstract and experimental results] Abstract and experimental results section: the central claim of 0.82 efficiency in 27.4 μm hBN requires an explicit power-budget accounting (measured total transmission, reflection, absorption, scattering, and residual unconverted power) to confirm that cumulative losses over the propagation length remain low enough to support the reported value; without this, the assumption that higher-order effects are negligible cannot be assessed.
[Theoretical model] Theoretical model section: the analytical expression for Bessel-beam conversion efficiency should be derived explicitly as a function of the transverse wave vector and birefringence, then compared quantitatively to the Gaussian case with the same parameters to demonstrate why the efficiency increase occurs and to allow direct fitting to the NA-dependence data.

minor comments (2)

[Experimental methods] The manuscript should include error bars on all efficiency measurements and specify the operating wavelength, input power, and detection method used to extract the ℓ+2 component.
[Results figures] Figure captions and text should clarify how the output vortex mode is isolated from residual input polarization components.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their thorough review and positive evaluation of our manuscript. We have addressed each of the major comments in detail below and revised the manuscript accordingly.

read point-by-point responses

Referee: [Abstract and experimental results] Abstract and experimental results section: the central claim of 0.82 efficiency in 27.4 μm hBN requires an explicit power-budget accounting (measured total transmission, reflection, absorption, scattering, and residual unconverted power) to confirm that cumulative losses over the propagation length remain low enough to support the reported value; without this, the assumption that higher-order effects are negligible cannot be assessed.

Authors: We agree that providing an explicit power budget is essential to substantiate the efficiency measurement. In the revised manuscript, we have added a detailed power-budget analysis in the experimental results section. This includes the measured total transmission through the 27.4 μm hBN sample, calculated reflection losses using Fresnel equations, estimated absorption from the material's extinction coefficient, assessment of scattering losses, and the measured residual power in the unconverted Gaussian component. These data confirm that losses are minimal and do not undermine the reported 0.82 efficiency, which is calculated as the power in the vortex mode relative to the input after propagation. revision: yes
Referee: [Theoretical model] Theoretical model section: the analytical expression for Bessel-beam conversion efficiency should be derived explicitly as a function of the transverse wave vector and birefringence, then compared quantitatively to the Gaussian case with the same parameters to demonstrate why the efficiency increase occurs and to allow direct fitting to the NA-dependence data.

Authors: We thank the referee for highlighting the need for a more explicit derivation. We have revised the theoretical model section to include the full analytical derivation of the spin-orbit conversion efficiency for a Bessel beam input. The efficiency is expressed as a function of the transverse wave vector k_perp and the crystal birefringence Δn, derived from the propagation phase difference accumulated by the ordinary and extraordinary components. We quantitatively compare this to the Gaussian beam case under identical parameters, showing that the Bessel beam's delta-like k_perp spectrum enables near-unity conversion by satisfying the optimal condition uniformly, whereas the Gaussian beam's broad spectrum leads to an average efficiency capped at 0.5. The derived expression is directly fitted to the experimental NA-dependence data, with the fit parameters matching the known birefringence of hBN. revision: yes

Circularity Check

0 steps flagged

No circularity; central result is direct experimental measurement

full rationale

The paper reports conversion efficiency as an experimental measurement performed on physical hBN samples of specified thickness, with the value 0.82 obtained from direct observation rather than any derivation that reduces by the paper's equations to a fitted parameter or self-referential input. Analytical modeling is used only to interpret the measured dependence on NA and to attribute the difference between Bessel and Gaussian inputs to transverse wave-vector distributions; these models do not define or constrain the reported efficiency number itself. No self-citation, ansatz smuggling, or uniqueness theorem is invoked as load-bearing justification for the efficiency claim. The result therefore remains self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 1 axioms · 0 invented entities

The central claim rests on standard material birefringence of hBN and the wave-vector properties of Bessel beams; no free parameters are explicitly fitted in the abstract, and no new entities are postulated.

axioms (1)

domain assumption Large birefringence of vdW crystals enables efficient spin-orbit coupling
Invoked to explain near-unity conversion when paired with Bessel beam's single transverse wave vector.

pith-pipeline@v0.9.0 · 5520 in / 1200 out tokens · 17139 ms · 2026-05-08T06:25:36.769861+00:00 · methodology

discussion (0)

Reference graph

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