Spatiotemporal Electron Microscopy of Phonon Polaritons in MoO3

Frank H.L. Koppens; Hanan Herzig Shenfux; Harel Nahari; Ido Kaminer; Michael Yannai; Raphael Dahan; Yaniv Kurman; Yuval Adiv

arxiv: 2605.29433 · v1 · pith:Y6EAHDSM · submitted 2026-05-28 · physics.optics

Spatiotemporal Electron Microscopy of Phonon Polaritons in MoO3

Harel Nahari , Yaniv Kurman , Raphael Dahan , Yuval Adiv , Michael Yannai , Hanan Herzig Shenfux , Frank H.L. Koppens , Ido Kaminer This is my paper

Reviewed by Pith2026-06-29 06:11 UTCgrok-4.3pith:Y6EAHDSM open to challenge →

classification physics.optics

keywords phonon polaritonsMoO3PINEMspatiotemporal dynamicsvan der Waals materialsmid-infrared imaginganisotropic crystals

0 comments

The pith

Extending PINEM imaging to 12 micrometers maps the spatiotemporal dynamics of phonon polaritons inside anisotropic MoO3 flakes.

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

The paper extends photon-induced near-field electron microscopy from visible and near-IR wavelengths out to 12 micrometers. This bandwidth increase is then used to track phonon polaritons in alpha-MoO3, a van der Waals crystal with strong in-plane anisotropy. Imaging inside a cavity-like flake produces maps of how the polaritons are distributed in space, how they evolve in time, and how their lifetimes change across different wavelengths. A reader would care because the method supplies nanometer-scale and sub-picosecond data on polaritonic excitations that are otherwise difficult to access in the mid-infrared.

Core claim

By nearly doubling the operational bandwidth of PINEM to reach 12 um, the authors visualize the spatiotemporal dynamics of phonon polaritons in alpha-MoO3, revealing their spatial distribution, time evolution, and wavelength-dependent lifetime within a cavity-like flake.

What carries the argument

Photon-induced near-field electron microscopy (PINEM) extended to mid-infrared wavelengths, applied to phonon polaritons in an anisotropic van der Waals flake.

If this is right

Phonon polaritons exhibit wavelength-dependent lifetimes inside the cavity-like flake.
Their spatial distribution can be directly imaged at nanometer resolution.
Sub-picosecond temporal dynamics become accessible in the mid-infrared.
The same approach can be applied to other van der Waals materials whose polaritonic properties have been hard to probe.

Where Pith is reading between the lines

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

The technique could be tested on other in-plane anisotropic crystals to check whether cavity-induced lifetime variations are generic.
If the wavelength dependence of lifetime holds across samples, it may constrain models of phonon-polariton damping in layered materials.
Extending the method still further in wavelength could open access to lower-energy excitations such as plasmons in similar flakes.

Load-bearing premise

The extended PINEM setup at 12 micrometers still generates clean signals whose contrast and timing directly correspond to the actual phonon polariton fields inside the MoO3 flake.

What would settle it

Independent mid-infrared optical measurements on the same MoO3 flake that show lifetimes or spatial wavelengths differing from those extracted from the PINEM images would falsify the claim that the electron-microscopy data faithfully report the polariton properties.

read the original abstract

Photon-induced near-field electron microscopy (PINEM) has emerged as a powerful technique for imaging optical excitations with nanometer spatial and sub-picosecond temporal resolution. Recent years have extended the bandwidth of operation of PINEM experiments from the visible range to the mid-infrared, revealing the spatiotemporal dynamics of polaritons and their exotic phenomena. In this study, we nearly double the bandwidth of PINEM, going deeper into the infrared up to 12 um. Leveraging this advancement, we investigate the spatiotemporal dynamics of phonon polaritons (PhPs) in {\alpha}-MoO3, a material of growing interest thanks to its in-plane anisotropy. Visualizing PhPs in a cavity-like flake reveals their spatial distribution, dynamics, and wavelength-dependent lifetime. Our work pushes the frontiers of PINEM imaging and highlights its potential for probing hard-to-access polaritonic properties of novel van der Waals materials.

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

They extended PINEM to 12 μm and mapped phonon polariton dynamics in MoO3, which is a concrete incremental advance if the images and controls hold up.

read the letter

The one or two things to know about this paper are that it reports extending PINEM to 12 μm wavelength and applies the technique to map phonon polariton behavior in MoO3.

They nearly double the operational bandwidth from prior mid-IR work and then visualize the polaritons in a flake that acts like a cavity. This lets them look at how the modes are distributed spatially, how they evolve in time, and how lifetime changes with wavelength. The in-plane anisotropy of α-MoO3 is highlighted as a feature that makes the dynamics more complex and worth studying.

The work does well in identifying a gap in accessible wavelengths for PINEM and targeting a material that's popular in nanophotonics right now. If the experimental realization is clean, it provides a new tool for probing these excitations with the high spatial and temporal resolution that electron microscopy offers.

Where it could be softer is in the validation of the extension itself. Getting the IR source and the electron interaction to work reliably at 12 μm is nontrivial, and the abstract doesn't detail the challenges or the mitigations. I'd want to see the raw data and any comparisons to simulations or other measurements to confirm that the visualized features are indeed the PhPs and not something else. The low soundness score in the report reflects this lack of visible supporting evidence.

Overall, this is for researchers in polaritonics and ultrafast electron microscopy. Someone in that niche would get concrete value from the demonstration if the results hold.

It should go to peer review. The claim is specific enough and the application relevant enough that referees can assess the technical details properly.

Referee Report

1 major / 0 minor

Summary. The paper claims to nearly double the operational bandwidth of photon-induced near-field electron microscopy (PINEM) from the mid-infrared into the deeper infrared up to 12 μm. It applies this extension to map the spatiotemporal dynamics of phonon polaritons (PhPs) in anisotropic α-MoO3, specifically visualizing their spatial distribution, dynamics, and wavelength-dependent lifetime within a cavity-like flake.

Significance. If the experimental claims hold, the work would represent a meaningful technical advance by extending PINEM into a wavelength range that accesses phonon polaritons in van der Waals materials, potentially enabling new studies of their anisotropic properties with combined nanometer spatial and sub-picosecond temporal resolution.

major comments (1)

[Abstract] The provided manuscript text consists only of the abstract and contains no data, figures, methods details, error analysis, or controls. This prevents verification of whether the claimed bandwidth extension to 12 μm was achieved or whether the visualized features correspond to the stated PhP properties in the MoO3 flake.

Simulated Author's Rebuttal

1 responses · 0 unresolved

We thank the referee for their review and for highlighting the need for full manuscript details to assess the claims. We address the comment below.

read point-by-point responses

Referee: [Abstract] The provided manuscript text consists only of the abstract and contains no data, figures, methods details, error analysis, or controls. This prevents verification of whether the claimed bandwidth extension to 12 μm was achieved or whether the visualized features correspond to the stated PhP properties in the MoO3 flake.

Authors: The full manuscript (arXiv:2605.29433) contains the complete set of figures, methods, error analysis, and controls, including spatiotemporal maps of PhPs, wavelength-dependent lifetime data, and supporting measurements that establish the PINEM bandwidth extension to 12 μm. The abstract was provided as a summary excerpt; the full text directly addresses verification of the claimed extension and the PhP properties in the cavity-like α-MoO3 flake. revision: no

Circularity Check

0 steps flagged

No circularity: experimental demonstration with no derivations or fitted predictions

full rationale

The manuscript is an experimental report on extending PINEM bandwidth to 12 μm and applying it to image phonon polaritons in α-MoO3 flakes. No equations, theoretical derivations, parameter fits, or predictions appear in the abstract or described structure. Claims rest on direct imaging results and extracted quantities (spatial distribution, dynamics, lifetime), not on any reduction to self-defined inputs or self-citations. The work is incremental hardware extension plus materials application; no load-bearing step reduces by construction to prior results or fits.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

No mathematical model, free parameters, axioms, or invented entities are described in the abstract.

pith-pipeline@v0.9.1-grok · 5711 in / 1066 out tokens · 23484 ms · 2026-06-29T06:11:57.535438+00:00 · methodology

discussion (0)

Reference graph

Works this paper leans on

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2024

[1] [1]

& García de Abajo, F.J

Polman, A., Kociak, M. & García de Abajo, F.J. Electron-beam spectroscopy for nanophotonics. Nat. Mater. 18, 1158–1171 (2019)

2019

[2] [2]

Roques-Carmes et al., Free-electron–light interactions in nanophotonics

C. Roques-Carmes et al., Free-electron–light interactions in nanophotonics. Appl. Phys. Rev. 10, 011303 (2023)

2023

[3] [3]

& Zewail, A

Barwick, B., Flannigan, D. & Zewail, A. Photon-induced near-field electron microscopy. Nature 462, 902–906 (2009)

2009

[4] [4]

García de Abajo, Asenjo-Garcia A

F.J. García de Abajo, Asenjo-Garcia A. & Kociak, M. Multiphoton absorption and emission by interaction of swift electrons with evanescent light fields. Nano Letters, 10(5), 1859–1863 (2010)

2010

[5] [5]

T., Zewail, A

Park, S. T., Zewail, A. H. Photon-induced near-field electron microscopy (PINEM): theoretical and experimental. New Journal of Physics, 12, 123028 (2010)

2010

[6] [6]

Piazza, L., Lummen, T., Quiñonez, E. et al. Simultaneous observation of the quantization and the interference pattern of a plasmonic near-field. Nat Commun 6, 6407 (2015)

2015

[7] [7]

Lummen, T., Lamb, R., Berruto, G. et al. Imaging and controlling plasmonic interference fields at buried interfaces. Nat Commun 7, 13156 (2016)

2016

[8] [8]

Wang, K., Dahan, R., Shentcis, M. et al. Coherent interaction between free electrons and a photonic cavity. Nature 582, 50–54 (2020)

2020

[9] [9]

Kfir, O., Lourenço-Martins, H., Storeck, G. et al. Controlling free electrons with optical whispering-gallery modes. Nature 582, 46–49 (2020)

2020

[10] [10]

Dahan, R., Nehemia, S., Shentcis, M. et al. Resonant phase-matching between a light wave and a free-electron wavefunction. Nat. Phys. 16, 1123–1131 (2020)

2020

[11] [11]

Henke, JW., Raja, A.S., Feist, A. et al. Integrated photonics enables continuous-beam electron phase modulation. Nature 600, 653–658 (2021)

2021

[12] [12]

Fishman, T., Haeusler, U., Dahan, R. et al. Imaging the field inside nanophotonic accelerators. Nat Commun 14, 3687 (2023)

2023

[13] [13]

Free-electron interaction with nonlinear optical states in microresonators

Yujia Yang et al. Free-electron interaction with nonlinear optical states in microresonators. Science 383, 168–173 (2024)

2024

[14] [14]

Gaida, J.H., Lourenço-Martins, H., Yalunin, S.V. et al. Lorentz microscopy of optical fields. Nat Commun 14, 6545 (2023)

2023

[15] [15]

Nabben, D., Kuttruff, J., Stolz, L. et al. Attosecond electron microscopy of sub-cycle optical dynamics. Nature 619, 63–67 (2023)

2023

[16] [16]

Bucher T., Nahari H., Herzig Sheinfux H. et al. Coherently amplified ultrafast imaging using a free-electron interferometer. Nat. Photon. 18, 809–815 (2024)

2024

[17] [17]

Gaida, J.H., Lourenço-Martins, H., Sivis, M. et al. Attosecond electron microscopy by free-electron homodyne detection. Nat. Photon. 18, 509–515 (2024)

2024

[18] [18]

Bucher, T., Gorlach, A., Niedermayr, A. et al. Superluminal correlations in ensembles of optical phase singularities. Nature 651, 920–926 (2026)

2026

[19] [19]

N., Fogler, M

Basov, D. N., Fogler, M. M. & García de Abajo, F. J. Polaritons in van der waals materials. Science 354, aag1992 (2016)

2016

[20] [20]

Zhang, Q., Hu, G., Ma, W. et al. Interface nano-optics with van der Waals polaritons. Nature 597, 187–195 (2021)

2021

[21] [21]

de Abajo, F. J. G. et al. Roadmap for photonics with 2D materials. ACS Photonics 12, 3961–4095 (2025)

2025

[22] [22]

Henry, C. H. & Hopfield, J. J. Raman Scattering by Polaritons. Physical Review Letters 15(25), 964–966 (1965)

1965

[23] [23]

Kurman Y. et al. Spatiotemporal imaging of 2D polariton wave packet dynamics using free electrons. Science 372, 1181-1186 (2021)

2021

[24] [24]

Kurman Y. et al. Dynamics of optical vortices in van der Waals materials. Optica, 10, 612–618 (2023)

2023

[25] [25]

Zheng Z. et al. A mid-infrared biaxial hyperbolic van der Waals crystal. Sci. Adv. 5, eaav8690 (2019)

2019

[26] [26]

Ma, W., Alonso-González, P., Li, S. et al. In-plane anisotropic and ultra-low-loss polaritons in a natural van der Waals crystal. Nature 562, 557–562 (2018)

2018

[27] [27]

Guo, X., Wu, C., Zhang, S. et al. Mid-infrared analogue polaritonic reversed Cherenkov radiation in natural anisotropic crystals. Nat Commun 14, 2532 (2023)

2023

[28] [28]

Hu, G., Ou, Q., Si, G. et al. Topological polaritons and photonic magic angles in twisted α-MoO3 bilayers. Nature 582, 209–213 (2020)

2020

[29] [29]

Duan, J., Álvarez-Pérez, G., Lanza, C. et al. Multiple and spectrally robust photonic magic angles in reconfigurable α-MoO3 trilayers. Nat. Mater. 22, 867–872 (2023)

2023

[30] [30]

Enders, M.T., Sarkar, M., Giteau, M. et al. Deeply subwavelength mid-infrared phase retardation with α-MoO3 flakes. Commun Mater 5, 16 (2024)

2024

[31] [31]

Qu, Y. et al. Tunable planar focusing based on hyperbolic phonon polaritons in α-MoO3. Adv. Mater. 34, e2105590 (2022)

2022

[32] [32]

Unveiling the Mechanism of Phonon-Polariton Damping in α-MoO3

Javier, T., Zhou, Y. Unveiling the Mechanism of Phonon-Polariton Damping in α-MoO3. ACS Photonics 2024, 11, 9, 3570–3577

2024

[33] [33]

Caldwell, J., Kretinin, A., Chen, Y. et al. Sub-diffractional volume-confined polaritons in the natural hyperbolic material hexagonal boron nitride. Nat Commun 5, 5221 (2014)

2014

[34] [34]

Guo, X., Li, N., Yang, X. et al. Hyperbolic whispering-gallery phonon polaritons in boron nitride nanotubes. Nat. Nanotechnol. 18, 529–534 (2023)

2023

[35] [35]

Herzig Sheinfux, H., Orsini, L., Jung, M. et al. High-quality nanocavities through multimodal confinement of hyperbolic polaritons in hexagonal boron nitride. Nat. Mater. 23, 499–505 (2024)

2024