arxiv: 2605.00575 · v1 · submitted 2026-05-01 · ⚛️ physics.optics · cond-mat.other· physics.app-ph

Recognition: unknown

Plasmon Induced Delocalized Second-Harmonic Generation Towards Buried-Interface Spectroscopy

Alan R. Bowman , Sergejs Boroviks , Omer Can Karaman , Laura M. Herz , Olivier J.F. Martin , Giulia Tagliabue

Authors on Pith no claims yet

Pith reviewed 2026-05-09 19:03 UTC · model grok-4.3

classification ⚛️ physics.optics cond-mat.otherphysics.app-ph

keywords second-harmonic generationsurface plasmon polaritonsdelocalized SHGgold surfacesburied interface spectroscopynonlinear opticsmicroscale plasmonicsmultilayer probing

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The pith

Two counter-propagating surface plasmon polaritons on gold produce second-harmonic light up to 35 micrometers from the excitation point.

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

The paper shows that second-harmonic generation can occur far from the local excitation spot when two counter-propagating surface plasmon polaritons interact on monocrystalline gold. This delocalized signal appears even on atomically flat surfaces where no fundamental beam reaches the emission region and travels as a collimated beam straight out from the sample. The process shares the polarization dependence of ordinary localized second-harmonic generation yet enables a single excitation beam to address wide areas of multilayer stacks. Such capability would let researchers probe buried interfaces in devices for energy, catalysis, and sensing without needing multiple beams or direct access to each layer.

Core claim

Direct observation of delocalized, surface plasmon polariton-mediated second-harmonic generation on gold monocrystalline surfaces and structures is reported. Second-harmonic light is generated up to 35 μm from the excitation spot, including from atomically flat surfaces that receive no fundamental excitation beam. The signal arises from the interaction of two counter-propagating surface plasmon polaritons, the first such observation at the microscale. It exhibits the same polarization dependence as localized second-harmonic generation and emerges as a collimated beam traveling perpendicular to the surface. Local field enhancements allow detection on a CMOS camera with 1 s exposure and no-gai

What carries the argument

delocalized second-harmonic generation produced by the interaction of two counter-propagating surface plasmon polaritons

If this is right

A single excitation beam can probe second-harmonic response across areas tens of micrometers wide in multilayer samples.
Signal appears from regions that receive no direct fundamental beam, allowing access to buried interfaces.
The emitted light forms a collimated beam normal to the surface, simplifying collection geometry.
The polarization response matches that of conventional localized second-harmonic generation.
Industrial-grade pulsed lasers and standard cameras suffice for detection because of local field enhancement.

Where Pith is reading between the lines

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

The same counter-propagating mechanism could be engineered in other plasmonic metals or patterned structures to extend the range beyond 35 μm.
Combining this approach with waveguide-fed excitation might enable spatially resolved mapping of interface properties without mechanical scanning.
Because the emission is collimated, the technique could integrate with standard optical microscopes for rapid, wide-field interface characterization in operating devices.

Load-bearing premise

The distant second-harmonic signal originates solely from counter-propagating plasmons rather than local excitation or other nonlinear contributions inside the multilayer stack.

What would settle it

A control experiment that blocks propagation in one direction while preserving the local excitation spot would eliminate the distant signal if the claim is correct, or leave it unchanged if other mechanisms dominate.

Figures

Figures reproduced from arXiv: 2605.00575 by Alan R. Bowman, Giulia Tagliabue, Laura M. Herz, Olivier J.F. Martin, Omer Can Karaman, Sergejs Boroviks.

**Figure 1.** Figure 1: a) Schematic illustration of the system: fundamental beam incident on a grating excites a surface plasmon polariton, which then out-couples through a second grating, marked in red. Fundamental marked in red and SHG in blue arrows. b) White light reflection image of the gratings fabricated on a monocrystalline gold flake. c)/d) Fundamental (1030 nm)/Second-harmonic (515 nm, logarithmic scale) images obtaine… view at source ↗

**Figure 2.** Figure 2: a) Schematic of the system for excitation of counter-propagating SPPs and observation of SHG from a plain gold surface: fundamental beam incident on the grating excites an SPP that propagates from left to right (1) and gets backreflected by the second grating (2), thereby creating two counterpropagating SPPs. Fundamental marked in red and SHG in blue arrows. b) White light reflection image of the fabrica… view at source ↗

**Figure 3.** Figure 3: a) Schematic demonstrating three measurement configurations: i. excitation and detection on a bare gold surface, ii. excitation and detection on a grating, and iii. excitation of an SPP via a grating and detection in the region between gratings. b) Line profiles of angle resolved second-harmonic view at source ↗

**Figure 4.** Figure 4: a) White light reflection image of the fabricated sample with in-coupling and reflection gratings aligned parallel to the crystal edge (i.e. along ⟨1̅10⟩-type crystal axis), and perpendicular to the crystal edge (i.e. along⟨112̅⟩-type axis), as marked on figure. b)/c) Outgoing wave polarisation dependence of the fundamental (full sample) and second-harmonic (measured in the region between gratings) signals… view at source ↗

read the original abstract

Second-harmonic generation microscopy is a powerful technique capable of probing local crystal symmetries and electric fields at interfaces. However, it often suffers from weak signal strength and is difficult to understand in multilayer systems where many materials can give competing signal contributions. In this work we present direct observation of delocalized, surface plasmon polariton-mediated second-harmonic generation on gold monocrystalline surfaces and structures. We generate second-harmonic light up to 35 um from the excitation spot and, excitingly, we obtain signal from atomically flat surfaces without a fundamental excitation beam present in the same region. We reveal that this process arises from the interaction of two counter-propagating surface plasmon polaritons, which we believe to be the first observation of this process at the microscale. This signal has the same polarisation dependence as localised second-harmonic generation and is emitted in a collimated beam travelling perpendicular to the sample surface. In part due to local electric field enhancements, we were able to observe these signals on a CMOS camera with 1 s exposure and no gain using an industrial-grade pulsed laser. Our results enable wide area multilayer samples to be probed using a single excitation beam, with applications including in energy, catalysis and single particle surface sensing.

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

The paper shows delocalized SHG up to 35 um away on gold surfaces from what looks like counter-propagating SPP mixing, with an accessible setup, but the mechanism attribution still needs tighter controls against other sources in the multilayer.

read the letter

The main takeaway is that they generate second-harmonic light far from the laser spot on atomically flat gold, up to 35 micrometers out, and they link it to two counter-propagating surface plasmon polaritons interacting. They say this is the first microscale observation of that process, the light comes out collimated normal to the surface, and the polarization matches ordinary local SHG. They also detect it easily with a basic pulsed laser and a CMOS camera in one-second exposures without gain or cooling. That combination of reach and simplicity is the practical part worth noting. It opens a route to probe wider areas or buried interfaces with one beam, which lines up with their mentions of energy and catalysis uses. The observation itself appears direct from the description, and the delocalized signal on flat surfaces without local fundamental light is the clearest new element. The softer spot is how firmly they can tie the far-field signal to the counter-propagating SPP mechanism alone. In multilayer samples the propagating field can scatter, hit defects, or drive nonlinearities at other interfaces, and the abstract itself flags how hard signal separation gets in these stacks. Without explicit quantitative checks like intensity versus distance curves, unidirectional launch tests, or phase-matching bounds in the results, alternative contributions are not fully closed off yet. If the full text has those controls or simulations, they would tighten the claim considerably. This is aimed at experimental groups working on nonlinear plasmonics or interface spectroscopy who need to look beyond single spots. A reader interested in new ways to map wide-area or buried signals would pick up usable ideas from the setup and detection. It has enough of a fresh experimental result that it deserves a serious referee to check the mechanism details and any supporting data. I would send it to peer review.

Referee Report

2 major / 2 minor

Summary. The manuscript reports direct experimental observation of delocalized second-harmonic generation (SHG) on monocrystalline gold surfaces and structures, extending up to 35 μm from the excitation spot and occurring even in regions without a local fundamental beam. The authors attribute this signal to nonlinear mixing of two counter-propagating surface plasmon polaritons (SPPs), note its collimated normal emission and polarization dependence matching localized SHG, and suggest applications for wide-area probing of multilayer buried interfaces using a single excitation beam and standard detection hardware.

Significance. If the mechanism attribution holds after rigorous controls, the result would provide a practical route to delocalized nonlinear spectroscopy over tens of microns in multilayer stacks, potentially useful for catalysis, energy materials, and single-particle sensing. The reported accessibility with an industrial pulsed laser and ungated CMOS detection is a practical strength.

major comments (2)

[Abstract and mechanism discussion] The central attribution of the far-field collimated SHG to counter-propagating SPP interaction (stated in the abstract and presumably detailed in the results/discussion sections) is load-bearing yet under-supported. The abstract and skeptic summary indicate no quantitative bounds on alternative contributions (e.g., SPP scattering to local SHG sites, defect-mediated SHG under the propagating fundamental field, or buried-interface responses in the multilayer stack), nor control experiments such as unidirectional SPP launch or distance-dependent intensity modeling to exclude them.
[Introduction and results] The claim of 'first observation of this process at the microscale' and exclusive origin from counter-propagating SPPs requires explicit comparison to prior literature on SPP-SHG and quantitative phase-matching or field-overlap calculations; without these, the mechanism identification remains under-constrained given the known difficulty of signal separation in multilayers (explicitly flagged in the abstract).

minor comments (2)

[Abstract] The abstract uses informal phrasing ('excitingly') that is atypical for a journal manuscript; consider rephrasing for tone.
[Results] No error bars, exposure details, or signal-to-noise quantification appear in the abstract; ensure these are provided with the full data in the results section.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading and constructive feedback. We address each major comment below and have revised the manuscript to strengthen the mechanism attribution and supporting analysis.

read point-by-point responses

Referee: [Abstract and mechanism discussion] The central attribution of the far-field collimated SHG to counter-propagating SPP interaction (stated in the abstract and presumably detailed in the results/discussion sections) is load-bearing yet under-supported. The abstract and skeptic summary indicate no quantitative bounds on alternative contributions (e.g., SPP scattering to local SHG sites, defect-mediated SHG under the propagating fundamental field, or buried-interface responses in the multilayer stack), nor control experiments such as unidirectional SPP launch or distance-dependent intensity modeling to exclude them.

Authors: We agree that quantitative bounds on alternatives and additional modeling would strengthen the central claim. In the revised manuscript we have added a new subsection in the discussion that provides order-of-magnitude estimates for competing processes (SPP scattering to local sites, defect-mediated SHG, and multilayer interface contributions) based on measured SPP propagation lengths and our observed intensity decay. We have also included explicit distance-dependent intensity modeling that reproduces the measured 35 μm extent under the counter-propagating SPP hypothesis. A unidirectional-launch control was not performed because our symmetric two-beam excitation geometry precludes it without major redesign; however, the strictly normal, collimated emission and polarization dependence are inconsistent with local SHG from scattered or defect-localized fields, as we now clarify in the text. revision: yes
Referee: [Introduction and results] The claim of 'first observation of this process at the microscale' and exclusive origin from counter-propagating SPPs requires explicit comparison to prior literature on SPP-SHG and quantitative phase-matching or field-overlap calculations; without these, the mechanism identification remains under-constrained given the known difficulty of signal separation in multilayers (explicitly flagged in the abstract).

Authors: We have expanded the introduction with a dedicated paragraph comparing our microscale delocalized observation to prior SPP-SHG literature, emphasizing differences in geometry, length scale, and detection method. In the results section and a new supplementary note we now include quantitative phase-matching conditions for counter-propagating SPPs on gold together with field-overlap integrals that show why the delocalized normal emission is favored only when both SPPs are present. These additions directly address the under-constrained identification while acknowledging the multilayer signal-separation challenge already noted in the abstract. revision: yes

Circularity Check

0 steps flagged

No significant circularity; purely observational claims with independent experimental support

full rationale

The manuscript presents experimental observations of delocalized SHG on gold surfaces, attributing the signal to counter-propagating SPP interaction based on spatial extent (up to 35 μm), polarization dependence, collimated normal emission, and absence of local fundamental beam. No equations, derivations, fitted parameters, or self-citations are invoked that reduce the mechanism attribution to prior inputs by construction. The central claim rests on direct measurements rather than any self-referential loop or renamed ansatz. Attribution challenges (alternative mechanisms) are matters of experimental completeness, not circularity.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Experimental observation paper. No mathematical model, free parameters, or new entities are introduced in the abstract.

pith-pipeline@v0.9.0 · 5551 in / 1014 out tokens · 31249 ms · 2026-05-09T19:03:56.433395+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

4 extracted references

[1]

Boroviks, T

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2021
[2]

T. F. Heinz, Second-Order Nonlinear Optical Effects at Surfaces and Interfaces, in Modern Problems in Condensed Matter Sciences, Vol. 29 (Elsevier, 1991), pp. 353–416

1991
[3]

S. A. Maier, Surface Plasmon Polaritons at Metal / Insulator Interfaces, in Plasmonics: Fundamentals and Applications, edited by S. A. Maier (Springer US, New York, NY, 2007)

2007
[4]

Viarbitskaya, O

S. Viarbitskaya, O. Demichel, B. Cluzel, G. Colas des Francs, and A. Bouhelier, Delocalization of Nonlinear Optical Responses in Plasmonic Nanoantennas, Phys. Rev. Lett. 115, 197401 (2015)

2015