pith. sign in

arxiv: 2604.27294 · v1 · submitted 2026-04-30 · ❄️ cond-mat.mes-hall · cond-mat.mtrl-sci

Electrically Tunable Terahertz Chirality from Quantum Geometry

Sobhan Subhra Mishra , Thomas CaiWei Tan , Faxian Xiu , Ranjan Singh This is my paper

Pith reviewed 2026-05-07 09:24 UTC · model grok-4.3

classification ❄️ cond-mat.mes-hall cond-mat.mtrl-sci
keywords terahertz chiralityquantum geometryBerry curvatureDirac semimetalFloquet Weyl nodeselectrical gatingpolarization controlCd3As2
0
0 comments X

The pith

Gate voltage reshapes Fermi pockets in a Dirac semimetal to electrically tune the chirality of emitted terahertz radiation.

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

The paper shows that applying an electrostatic gate to the three-dimensional Dirac semimetal Cd3As2 lets researchers adjust the polarization state of terahertz waves the material emits under optical excitation. Light creates temporary Floquet Weyl nodes; the gate then changes the size and shape of the surrounding Fermi pockets, which alters how strongly the electronic wavefunction geometry contributes to one linear polarization component of the emitted field. The orthogonal component driven by photon drag stays fixed, and the two components are locked at right angles by the excitation geometry, so changing their relative strength moves the total polarization across all possible states. A sympathetic reader would care because this converts an otherwise fixed material response into a voltage-controlled source of chiral terahertz light without mechanical parts or external optics.

Core claim

In the 3D Dirac semimetal Cd3As2, photoexcitation creates Floquet Weyl nodes whose surrounding Fermi pockets can be reshaped by electrostatic gating. This reshaping selectively modulates the amplitude of the linearly polarized THz field component driven by Berry curvature, up to 60% at positive bias and 49% at negative bias, while the photon-drag component in the orthogonal direction remains constant. Because the excitation geometry locks the two components in phase quadrature, the resulting vector field can be steered across the Poincaré sphere, reaching a circular polarization degree of approximately -42 degrees at +10 volts. The work therefore demonstrates that Fermi-surface engineering,

What carries the argument

Gate-induced reshaping of Fermi pockets around photoinduced Floquet Weyl nodes that selectively tunes the amplitude of the Berry-curvature-driven THz emission component.

If this is right

  • The two orthogonal fields remain phase-locked at π/2, enabling continuous polarization steering across the Poincaré sphere.
  • Near-circular polarization (χ ≈ −42°) becomes reachable at modest positive gate bias of +10 V.
  • Fermi-surface tuning extends beyond DC transport to control optical responses in topological materials.
  • The unchanged photon-drag component provides a stable reference against which the tunable Berry-curvature component can be compared.

Where Pith is reading between the lines

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

  • Voltage-tunable chiral THz emitters could replace mechanical polarizers in compact time-domain spectroscopy setups.
  • The same gating principle may apply to other topological semimetals for engineering additional quantum-geometric optical effects.
  • On-chip integration with existing semiconductor processes could produce electrically programmable THz sources for sensing or communications.

Load-bearing premise

The observed selective modulation of one THz polarization component is produced by changes in Berry curvature distribution from Fermi-pocket reshaping rather than by unrelated voltage effects such as carrier-density shifts or heating.

What would settle it

Repeating the experiment on a conventional non-topological semiconductor under identical optical excitation and gating should eliminate the selective amplitude modulation between the two orthogonal components.

Figures

Figures reproduced from arXiv: 2604.27294 by Faxian Xiu, Ranjan Singh, Sobhan Subhra Mishra, Thomas CaiWei Tan.

Figure 1
Figure 1. Figure 1: All-electrical tuning of chiral THz emission from Cd3As2; (a) Floquet band engineering in a 3D Dirac semimetal. Circularly polarized (CP) photoexcitation breaks time-reversal symmetry, splitting the doubly degenerate Dirac node into a pair of Weyl nodes separated by ∆𝑘 in momentum space; (b) Device schematic for electrically tunable chiral THz emission. A 40 nm Cd3As2 thin film grown on a c￾cut sapphire su… view at source ↗
Figure 2
Figure 2. Figure 2: Selective electrical tuning of the Floquet THz component. view at source ↗
Figure 3
Figure 3. Figure 3: Voltage-dependent polarization ellipses of the emitted THz field. (a) Parametric plot of EY(t) versus EXZ(t) at 0 V under left circularly polarized pump, showing a slightly vertically elongated ellipse (EY > EXZ). (b) Polarization ellipse at +10 V, where EY≈EXZ and the THz field is closest to circular. (c) Polarization ellipse at +50 V, with reduced EY and a clearly horizontally elongated ellipse. (d) Pola… view at source ↗
Figure 4
Figure 4. Figure 4: Quantitative polarization analysis of electrically tuned chiral THz. view at source ↗
read the original abstract

Quantum geometry encoded in the momentum space structure of electronic wavefunctions, governs charge dynamics through Berry curvature, enabling unconventional transport and optical responses. In topological semimetals, this geometry is sampled over Fermi pockets, suggesting electrical control by Fermi surface tuning, yet such control has remained largely limited to DC transport. Here we show that electrostatic gating of the 3D Dirac semimetal Cd3As2 reshapes Fermi pockets surrounding photoinduced Floquet Weyl nodes, enabling electrical control of terahertz (THz) emission chirality. Gate tuning selectively modulates the Berry curvature driven linearly polarized THz component by up to 60% and 49% at positive and negative bias, respectively, while the orthogonal linearly polarized photon-drag component remains unchanged. With the two orthogonal fields intrinsically phase-locked at \sfrac{\pi}{2} by the excitation geometry, the selective gate-tuned amplitude control enables the polarization tuning across the Poincar\'e sphere, achieving near-circular polarization (\chi\approx-42{\deg}) at +10 V. These results establish Fermi surface tuning as a general route to programmable quantum geometric control of chiral terahertz emission.

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

2 major / 2 minor

Summary. The manuscript presents an experimental study on the 3D Dirac semimetal Cd3As2, demonstrating that electrostatic gating can electrically tune the chirality of terahertz (THz) emission. This is achieved by reshaping Fermi pockets surrounding photoinduced Floquet Weyl nodes, which modulates the Berry curvature-driven linearly polarized THz component by up to 60% at positive bias and 49% at negative bias. The orthogonal linearly polarized photon-drag component remains unchanged. Due to the intrinsic phase-locking at π/2 between these components from the excitation geometry, this selective control allows tuning of the polarization state across the Poincaré sphere, reaching near-circular polarization with χ ≈ -42° at +10 V. The work positions Fermi surface tuning as a route to programmable quantum geometric control of chiral THz emission.

Significance. Should the attribution to quantum geometry be robustly supported, this paper offers a significant advance by showing electrical control over quantum geometric effects in the THz regime, extending beyond DC transport. The quantitative modulation values and the achievement of near-circular polarization highlight the potential for applications in chiral light generation and polarization control. The approach leverages the material's topological properties and photoinduced states in a novel way. Strengths include the clear separation of the two polarization components and the use of gating to achieve tunability.

major comments (2)
  1. Discussion of mechanism: The central claim that gate-induced changes selectively affect the Berry curvature component due to Floquet Weyl nodes lacks supporting quantitative modeling. For instance, there is no calculation showing how the Berry curvature integral over the reshaped Fermi pockets predicts the observed 60% and 49% modulations, as opposed to generic effects from carrier density variation.
  2. Experimental section on polarization analysis: The use of the unchanged photon-drag amplitude as proof of selectivity assumes that this component has no sensitivity to gating-induced changes in the electronic structure; however, without explicit checks or simulations of its expected behavior under gating, this does not fully exclude alternative explanations.
minor comments (2)
  1. In the abstract, the notation for the phase lock 'at π/2' should be rendered consistently, and the degree symbol in χ≈−42° should be clarified as the ellipticity angle.
  2. Ensure that all quantitative claims in the main text are accompanied by error bars or statistical analysis to support the modulation percentages.

Simulated Author's Rebuttal

2 responses · 0 unresolved

We thank the referee for their careful reading of our manuscript and for the constructive comments, which help clarify the presentation of our experimental results on gate-tunable THz chirality in Cd3As2. We address each major comment below and indicate the revisions we will incorporate.

read point-by-point responses

  1. Referee: Discussion of mechanism: The central claim that gate-induced changes selectively affect the Berry curvature component due to Floquet Weyl nodes lacks supporting quantitative modeling. For instance, there is no calculation showing how the Berry curvature integral over the reshaped Fermi pockets predicts the observed 60% and 49% modulations, as opposed to generic effects from carrier density variation.

    Authors: We agree that a direct quantitative evaluation of the Berry curvature integral over the gated Fermi pockets would strengthen the mechanistic attribution. However, such a calculation requires a full Floquet-engineered band structure under electrostatic gating, including self-consistent treatment of the photoinduced Weyl nodes and scattering, which lies beyond the scope of the present experimental work and available computational resources. The experimental evidence for selectivity rests on the differential response: the Berry curvature-driven component modulates by up to 60% (positive bias) and 49% (negative bias), while the orthogonal photon-drag component remains constant within experimental uncertainty. Generic carrier-density effects would be expected to influence both components comparably, yet the data show otherwise. In the revised manuscript we will add a dedicated paragraph in the discussion section that (i) outlines the expected scaling of the Berry curvature contribution with Fermi-pocket volume around the Floquet nodes and (ii) contrasts this with the density dependence of the photon-drag term, supported by references to existing theoretical studies of Floquet Weyl states in Cd3As2. revision: partial

  2. Referee: Experimental section on polarization analysis: The use of the unchanged photon-drag amplitude as proof of selectivity assumes that this component has no sensitivity to gating-induced changes in the electronic structure; however, without explicit checks or simulations of its expected behavior under gating, this does not fully exclude alternative explanations.

    Authors: We appreciate the need for explicit verification. The photon-drag term originates from the asymmetric momentum distribution of photoexcited carriers and depends primarily on overall carrier density and relaxation times rather than on Berry curvature. Our gating data already show that the total emitted THz field amplitude varies modestly while the photon-drag projection stays constant, indicating that density changes alone do not drive the observed selectivity. To address the concern directly, we will include in the revised experimental section (i) the full gating dependence of the extracted photon-drag amplitude together with simultaneously measured DC conductivity and (ii) a brief comparison to literature values for photon-drag sensitivity in similar Dirac systems. These additions will make the assumption explicit and rule out simple density-driven alternatives. revision: yes

Circularity Check

0 steps flagged

No significant circularity; central claim is experimental observation of gate-tuned THz emission

full rationale

The paper presents an experimental demonstration of electrostatic gating modulating THz emission chirality in Cd3As2, with selective amplitude control of one polarization component while the orthogonal component remains unchanged. No derivation chain, equations, or fitted parameters are shown that reduce any 'prediction' or first-principles result to the inputs by construction. The quantum-geometry interpretation (Berry curvature from Floquet Weyl nodes) is offered as an explanatory framework for the observed selectivity rather than a self-contained mathematical derivation that loops back to its own assumptions or self-citations. The result is therefore self-contained as an empirical finding against external benchmarks (measured gate-dependent emission spectra), with no load-bearing steps matching the enumerated circularity patterns.

Axiom & Free-Parameter Ledger

0 free parameters · 2 axioms · 0 invented entities

The claim rests on standard condensed-matter assumptions about Berry curvature in topological semimetals and the existence of photoinduced Floquet Weyl nodes; no new free parameters or invented entities are introduced in the abstract.

axioms (2)
  • domain assumption Quantum geometry encoded in the momentum space structure of electronic wavefunctions governs charge dynamics through Berry curvature.
    Invoked in the opening sentence as the governing principle for the observed optical response.
  • domain assumption In topological semimetals this geometry is sampled over Fermi pockets, allowing electrical control by Fermi surface tuning.
    Used to link gating to selective modulation of the linearly polarized THz component.

pith-pipeline@v0.9.0 · 5509 in / 1352 out tokens · 59388 ms · 2026-05-07T09:24:12.446101+00:00 · methodology

discussion (0)

Sign in with ORCID, Apple, or X to comment. Anyone can read and Pith papers without signing in.

Reference graph

Works this paper leans on

5 extracted references

  1. [1]

    Jay-Gerin, J.-P., Aubin, M. J. & Caron, L. G. The electron mobility and the static dielectric constant of Cd3As2 at 4.2 K. Solid State Commun. 21, 771–774 (1977)

    1977

  2. [2]

    Cheng, L. et al. Giant photon momentum locked THz emission in a centrosymmetric Dirac semimetal. Sci. Adv. 9, eadd7856 (2023)

    2023

  3. [3]

    W., Hsieh, D., Steinberg, H., Jarillo -Herrero, P

    McIver, J. W., Hsieh, D., Steinberg, H., Jarillo -Herrero, P. & Gedik, N. Control over topological insulator photocurrents with light polarization. Nat. Nanotechnol. 7, 96 –100 (2012)

    2012

  4. [4]

    Tu, C.-M. et al. Helicity-dependent terahertz emission spectroscopy of topological insulator S b 2 T e 3 thin films. Phys. Rev. B 96, 195407 (2017)

    2017

  5. [5]

    Lu, W. et al. Ultrafast photothermoelectric effect in Dirac semimetallic Cd3As2 revealed by terahertz emission. Nat. Commun. 13, 1623 (2022)

    2022