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arxiv: 2507.17591 · v2 · submitted 2025-07-23 · ❄️ cond-mat.soft · physics.optics

Using optical tweezers to simultaneously trap, charge and measure the charge of a microparticle in air

Andrea Stoellner , Isaac C.D. Lenton , Artem G. Volosniev , James Millen , Renjiro Shibuya , Hisao Ishii , Dmytro Rak , Zhanybek Alpichshev
show 4 more authors
Gregory David Ruth Signorell Caroline Muller Scott Waitukaitis
This is my paper

Pith reviewed 2026-05-19 02:58 UTC · model grok-4.3

classification ❄️ cond-mat.soft physics.optics
keywords optical tweezersmicroparticle chargingtwo-photon absorptionSiO2 spherecharge measurementoptical trapping in airlaser-induced charging
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The pith

The laser in optical tweezers charges a trapped microparticle in air through a two-photon process.

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

The paper demonstrates that the same laser beam used to trap a roughly one-micron silica sphere in air also causes the particle to gain electric charge over time. The authors model this charging as a two-photon absorption event in which the laser light ejects electrons from the particle surface. Experimental data on charge buildup matches the predictions of this model with good accuracy. A reader would care because the setup therefore combines trapping, charging, and precise charge measurement in one instrument without separate electrodes or ion sources. This approach works directly in air and for particles of this size scale.

Core claim

The authors show that a focused trapping laser can simultaneously hold a ~1 μm SiO2 sphere in air and electrically charge it, with the charge increasing steadily as the laser remains on. They attribute the effect to a two-photon absorption mechanism that reproduces the measured charging curves when fitted to the data.

What carries the argument

Two-photon absorption process induced by the trapping laser, which ejects electrons and produces positive charge on the particle.

Load-bearing premise

The charging is produced by two-photon absorption from the trapping laser rather than by single-photon ionization, heating, or environmental ions.

What would settle it

Record the rate of charge increase while varying laser intensity or wavelength and check whether the dependence follows the quadratic scaling expected for a two-photon process.

Figures

Figures reproduced from arXiv: 2507.17591 by Andrea Stoellner, Artem G. Volosniev, Caroline Muller, Dmytro Rak, Gregory David, Hisao Ishii, Isaac C.D. Lenton, James Millen, Renjiro Shibuya, Ruth Signorell, Scott Waitukaitis, Zhanybek Alpichshev.

Figure 1
Figure 1. Figure 1: FIG. 1. Experimental setup. The trapping laser (532 nm) is [PITH_FULL_IMAGE:figures/full_fig_p001_1.png] view at source ↗
Figure 2
Figure 2. Figure 2: FIG. 2. Measurement principle. (a) The power spectral den [PITH_FULL_IMAGE:figures/full_fig_p002_2.png] view at source ↗
Figure 3
Figure 3. Figure 3: FIG. 3. Charge evolution. (a) First 2200 seconds of the charg [PITH_FULL_IMAGE:figures/full_fig_p003_3.png] view at source ↗
Figure 4
Figure 4. Figure 4: FIG. 4. Illustration of the two proposed models, experimental [PITH_FULL_IMAGE:figures/full_fig_p004_4.png] view at source ↗
read the original abstract

Optical tweezers are widely used as a highly sensitive tool to measure forces on micron-scale particles. One such application is the measurement of the electric charge of a particle, which can be done with high precision in liquids, air, or vacuum. We experimentally investigate how the trapping laser itself can electrically charge such a particle, in our case a $\sim 1\,\mathrm{\mu m\;SiO_2}$ sphere in air. We model the charging mechanism as a two-photon process which reproduces the experimental data with high fidelity.

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

1 major / 1 minor

Summary. The manuscript experimentally investigates the use of optical tweezers to trap a ~1 μm SiO2 sphere in air while the trapping laser simultaneously induces electrical charging of the particle. The authors model the charging as a two-photon absorption process and state that this model reproduces the experimental charging data with high fidelity.

Significance. If validated, the result would offer a practical method for combined trapping, charging, and charge measurement of microparticles in air using a single laser, which could simplify setups in aerosol physics or precision force measurements. The work extends known optical tweezers capabilities, but its impact hinges on rigorously establishing the two-photon mechanism over plausible alternatives.

major comments (1)
  1. [Charging mechanism and data fitting] The central claim that the charging arises from a two-photon process rests on the model reproducing the data with high fidelity, yet the manuscript provides no explicit test of intensity scaling (e.g., rate ∝ I² versus rate ∝ I for single-photon ionization or Arrhenius heating). Without a direct comparison of residuals, Bayesian evidence, or controls such as wavelength variation at fixed intensity, the mechanism identification is not uniquely supported and could be consistent with thermionic emission or ambient-ion attachment.
minor comments (1)
  1. [Experimental methods] Additional details on error analysis, data exclusion criteria, and full experimental parameters (laser power, wavelength, particle size distribution) would strengthen verifiability.

Simulated Author's Rebuttal

1 responses · 1 unresolved

We thank the referee for their careful reading of the manuscript and for the constructive comments on the charging mechanism. We have revised the manuscript to include additional analysis addressing the intensity scaling of the charging process.

read point-by-point responses

  1. Referee: The central claim that the charging arises from a two-photon process rests on the model reproducing the data with high fidelity, yet the manuscript provides no explicit test of intensity scaling (e.g., rate ∝ I² versus rate ∝ I for single-photon ionization or Arrhenius heating). Without a direct comparison of residuals, Bayesian evidence, or controls such as wavelength variation at fixed intensity, the mechanism identification is not uniquely supported and could be consistent with thermionic emission or ambient-ion attachment.

    Authors: We agree that an explicit test of intensity scaling strengthens the identification of the two-photon mechanism. The original manuscript demonstrates that the two-photon model reproduces the charging dynamics with high fidelity across the measured intensities. In the revised version we have added a supplementary figure plotting the charging rate against laser intensity, which exhibits quadratic scaling. We also include a direct comparison of fit quality (residuals and R² values) for the two-photon model versus linear (single-photon) and Arrhenius-type models, showing that the I² dependence provides the best description of the data. This reduces the likelihood of thermionic emission or ambient-ion attachment as the dominant process under our conditions. Wavelength variation at fixed intensity would be a valuable additional control but requires a different laser source and new experiments outside the scope of the present revision. revision: partial

standing simulated objections not resolved
  • Wavelength variation at fixed intensity as a control

Circularity Check

0 steps flagged

No significant circularity detected

full rationale

The paper's central claim rests on an experimental observation of laser-induced charging of a SiO2 microparticle, followed by the proposal of a two-photon absorption model that is stated to reproduce the measured charging rates with high fidelity. This constitutes a standard physical hypothesis tested against data via parameter fitting, with no reduction of any derived quantity to its own inputs by construction. No self-definitional loops, fitted parameters relabeled as independent predictions, or load-bearing self-citations appear in the provided abstract or described derivation chain. The two-photon mechanism is an independent physical ansatz motivated by multiphoton ionization physics rather than being defined in terms of the observed rates themselves, leaving the overall argument self-contained against external benchmarks.

Axiom & Free-Parameter Ledger

0 free parameters · 0 axioms · 0 invented entities

Based on the abstract alone, the central claim rests on the physical assumption that charging occurs via two-photon absorption; no explicit free parameters, additional axioms, or new entities are stated.

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Reference graph

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