Photoinduced Topological Transitions

• Photoinduced topological phase transitions are nonequilibrium phenomena where modulated electromagnetic fields change a material's topological character.
• Floquet engineering synthesizes an effective Hamiltonian through periodic driving, dynamically tuning key invariants like the Chern number.
• Experimental probes such as tr-ARPES and THz transport detect band inversions, gap closings, and chiral edge states that signal these transitions.

Photoinduced topological phase transitions are nonequilibrium phenomena in which temporally modulated electromagnetic fields induce abrupt changes in the topological character of quantum materials. These transitions can drive a system between distinct topological phases—including Chern insulators, topological semimetals, and higher-order topological states—by tuning photon parameters such as intensity, polarization, frequency, or temporal profile. The emergence of Floquet-engineered nonequilibrium Hamiltonians under periodic driving facilitates the engineering and control of topological indices (Chern number, Z₂ invariants, monopole charge, etc.) and associated protected boundary modes in both electronic and bosonic (e.g., magnonic) systems. Recent advances have established precise microscopic mechanisms, general classification schemes, experimental signatures, and dynamical universality for photoinduced topological phase transitions across a variety of material platforms.

1. Floquet Engineering Principles and Effective Hamiltonians

Photoinduced topological phase transitions are primarily driven by Floquet engineering: the synthesis of an effective static Hamiltonian $H_{\mathrm{eff}}$ describing the stroboscopic evolution of a periodically driven (typically time-dependent) quantum system. For a Hamiltonian $H(t+T) = H(t)$ subjected to optical driving, the Floquet–Magnus (or van Vleck) expansion is utilized in the high-frequency, off-resonant regime ($\hbar\omega \gg W$, $W$: bandwidth):

$$H_{\mathrm{eff}} = H_0 + \sum_{m\ge 1} \frac{[H_{-m}, H_{+m}]}{m\hbar\omega} + O(\omega^{-2}).$$

The photoirradiation modifies hopping amplitudes and introduces virtual photon processes resulting in synthetic mass terms, TRSB chiral hoppings (Haldane mass), and complex coupling structure, depending on polarization and pulse protocol. In the context of the Dirac/Weyl/Fermi surface Hamiltonians, circular or elliptic polarization is essential to break time-reversal symmetry and generate topological band inversions, whereas linear pumping preserves TRS and can only shift or split valley energies (Nobahari, 19 Nov 2025, Kitayama et al., 2020, Tang et al., 2023, Liu et al., 2023).

2. Representative Model Systems and Mechanisms

Photoinduced topological transitions have been realized in both electronic and magnonic lattices:

• Dirac systems (graphene family, organic salts): For monolayer graphene, silicene, and staggered Dirac materials, CPL generates a Haldane mass at each Dirac point, yielding a Floquet Chern insulator with a gap size $\Delta \sim (e v_F A)^2 / \hbar\omega$ and quantized Hall plateau (Ledwith et al., 2017). In the organic salt $\alpha$-(BEDT-TTF)$_2$I$_3$, a more exotic mechanism under elliptic or circular light can induce Dirac point collision and annihilation, resulting in a transition from a topological semimetal with separated gapped Dirac cones to a trivial insulator through the collapse of the Dirac pairs (Kitayama et al., 2022, Kitayama et al., 2021, Kitayama et al., 2020).
• Topological semimetals and Weyl systems: In crossing-line nodal semimetals, CPL gaps the line nodes and produces multiple high-charge Weyl points—e.g., an $N$-fold line-node semimetal transitions into a multiple-Weyl semimetal with Chern numbers $\pm N$ (Ezawa, 2017). In nodal-line semimetallic carbon allotropes, LPL splits the Dirac nodal ring into tunable pairs of Weyl points of type-I, type-II, or type-III, with their nature and position controlled by light intensity (Deng et al., 2020).
• Quantum spin Hall/valley Hall systems: In 1T′-MoS₂ and functionalized Xenes, CPL produces spin- and valley-dependent mass shifts, enabling transitions among QSH, quantum Hall, and quantum valley Hall states. The Chern numbers, Berry curvature, and edge transport can be manipulated via the light amplitude, frequency, and electric field, yielding a multi-phase Floquet topological diagram (Nobahari, 19 Nov 2025, Ledwith et al., 2017).
• Magnetic and magnonic systems: In topological magnon insulators (e.g., kagome, honeycomb, checkerboard ferromagnets), circularly polarized electric fields couple via the Aharonov–Casher effect, renormalizing exchange, Kitaev, and DMI couplings by Bessel functions of the field intensity. Photoexcitation drives Chern number reversals in magnon bands and controls the sign of the thermal Hall conductivity, with sharp transitions at zeros of Bessel functions (Owerre, 2018, Tang et al., 2023, Zhang et al., 2021, Tang et al., 2024).
• Higher-order topology and multiphase diagrams: Monolayer Ti$_2$SiCO$_2$ under CPL undergoes a transition from a second-order TI to a high-Chern-number QAH insulator ($C = \pm 2$), with field-tunable semi-Dirac and Dirac semimetallic phases emerging under LPL and gate control (Liu et al., 2023).

3. Universal Phenomenology and Topological Invariants

Transitions are universally characterized by:

• Band inversion or gap-closing at high-symmetry momentum: The effective masses induced by light cross zero at critical intensities/frequencies, closing the bulk gap at Dirac, quadratic, or multi-valley points and reopening with inverted eigenvalue order. The nature (nodal point, line, or Weyl) and symmetry of the band crossing determine the type of transition (Ezawa, 2017, Nobahari, 19 Nov 2025, Ledwith et al., 2017, Kitayama et al., 2020).
• Change in topological index: The Chern number, Z₂ invariant, or monopole charge jumps as the gap closes and reopens:
  • For a Dirac cone, $C$ jumps by $\pm 1$ per crossing.
  • For multiple-Weyl transitions, jumps by $\pm N$ (Ezawa, 2017).
  • For higher-order TIs, the quadrupole or corner-mode invariants change accordingly (Liu et al., 2023).
  • In magnonic systems, valley/spin/magnon-band Chern numbers reverse, observable directly in Berry curvature and edge spectra (Tang et al., 2023, Owerre, 2018, Tang et al., 2024).
• Floquet Fermi arcs and protected edge states: Nontrivial edge or surface modes appear across bulk gaps (e.g., Floquet Fermi arcs connecting Weyl projections, chiral magnon edge modes, or higher-order corner states), serving as direct indicators of nontrivial topology (Deng et al., 2020, Liu et al., 2023, Nobahari, 19 Nov 2025).

4. Experimental Realizations and Signatures

Ultrafast pump–probe and angle-resolved photoemission (tr-ARPES), transport, and optical probes have directly tracked photoinduced topological transitions:

• Direct band inversion and gap dynamics: tr-ARPES resolved bulk band closures and reversals of energy gap ($\sim$10–100 fs) in ZrTe$_5$ and Bi-doped (Pb,Sn)Se induced by strong ultrafast pulses. The phase switching is not attributable to heating or filling, but to dynamic lattice or screening effects that restore/excise band inversion, as evidenced by topological surface state gap opening and recovery (Huang et al., 2024, Mogi et al., 2024).
• Chiral edge/arc state observation: ARPES and THz transport reveal emergence/disappearance of Dirac, edge, and Fermi arc states (surface Hall plateau, quantized jumps in Hall response at Chern transition, appearance of chiral magnon edge modes) (Ezawa, 2017, Zhang et al., 2021, Liu et al., 2023, Tang et al., 2023).
• Thermal Hall effect reversals in magnons: The sign of magnon thermal Hall conductivity is a robust indicator of Chern number and switches exactly at the photoinduced topological transition, as demonstrated numerically and proposed for experimental detection (Tang et al., 2023, Tang et al., 2024, Owerre, 2018, Zhang et al., 2021).
• Scaling and defect dynamics: Ultrafast x-ray scattering with femtosecond resolution in LaTe$_3$ revealed non-equilibrium formation and slow coarsening of vortex-string (topological dislocation) defects, with anomalous subdiffusive dynamics (exponent $\alpha = 0.29 < 1/2$) and mesoscopic scaling laws (Orenstein et al., 2023).

5. Dynamical Scaling, Defects, and Nonequilibrium Frameworks

Beyond band structure, photoinduced transitions often generate spatially textured, inhomogeneous order with topological defects (e.g., vortex strings in CDW systems). Dynamical scaling hypotheses apply: structure factors $S(k,t)$ exhibit universal scaling form $S(k,t) = t^{\gamma} F(k t^\alpha)$, with exponents reflecting subdiffusive coarsening due to defect pinning and restricted motion ($\alpha < 1/2$) (Orenstein et al., 2023).

The methodology established—combining ultrafast scattering, scaling analyses, and numerical simulation—enables quantitative measurement and control of transient topological textures and provides rare access to nonequilibrium universality in real quantum materials.

6. Material Platforms and Design Principles

Photoinduced topological phase transitions have been observed across:

• 2D and 3D Dirac, Weyl, and nodal-line semimetals (graphene, black phosphorus, carbon allotropes, Pb(Sn)Se, $\alpha$-(BEDT-TTF)$_2$I$_3$) (Deng et al., 2020, Liu et al., 2017, Ezawa, 2017, Kitayama et al., 2020, Kitayama et al., 2022).
• Transition-metal dichalcogenides (1T$^\prime$-MoS$_2$) and functionalized Xenes with strong spin-orbit and valley couplings (Nobahari, 19 Nov 2025, Ledwith et al., 2017).
• Magnonic TIs (kagome, honeycomb, checkerboard lattices) with DMI and Kitaev-Heisenberg terms (Tang et al., 2023, Owerre, 2018, Zhang et al., 2021, Tang et al., 2024).
• Higher-order TIs and multilayer systems (Ti$_2$SiCO$_2$) (Liu et al., 2023).
• Van der Waals materials and organic conductors with tunable symmetry and lattice response.

Optimal design for control requires tuning resonance conditions, polarization (ellipticity drives richer transitions), symmetry breaking, and simultaneous electric field or strain controls (for valleytronics/multiphase diagrams) (Liu et al., 2023, Nobahari, 19 Nov 2025, Ledwith et al., 2017, Mogi et al., 2024).

7. Theoretical and Experimental Challenges, Outlook

Key open problems include quantifying Floquet heating and dissipation, managing occupation and lifetime of Floquet states, many-body correlations in nonequilibrium, topological defect kinetics, and integrating light-induced switching with practical devices. The ability to optically toggle, manipulate, and dynamically entangle topological orders (via controllable Chern invariant, Berry curvature, higher-order modes) places photoinduced transitions at the intersection of topological quantum matter, ultrafast optics, and materials-by-design—enabling advances in light-driven electronics, spintronics, magnonics, and optoelectronics (Tang et al., 2023, Ledwith et al., 2017, Nobahari, 19 Nov 2025, Mogi et al., 2024, Huang et al., 2024, Peng et al., 2023, Orenstein et al., 2023).

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Selected References:

• "Dynamical Scaling Reveals Topological Defects and Anomalous Evolution of a Photoinduced Phase Transition" (Orenstein et al., 2023)
• "Photoinduced topological phase transition from a crossing-line nodal semimetal to a multiple-Weyl semimetal" (Ezawa, 2017)
• "Photoinduced topological phase transitions in Kitaev-Heisenberg honeycomb ferromagnets with the Dzyaloshinskii-Moriya interaction" (Tang et al., 2023)
• "Photoinduced Floquet topological magnons in a ferromagnetic checkerboard lattice" (Zhang et al., 2021)
• "Photoinduced Topological Phase Transitions in Topological Magnon Insulators" (Owerre, 2018)
• "Photoinduced topological phase transition in monolayer Ti$_2$SiCO$_2$" (Liu et al., 2023)
• "Photoinduced Topological Phase Transitions in a Kitaev kagome magnet" (Tang et al., 2024)
• "Photoinduced Floquet Mixed-Weyl Semimetallic Phase in a Carbon Allotrope" (Deng et al., 2020)
• "Floquet theory of photoinduced topological phase transitions in the organic salt $α$-(BEDT-TTF)$_2$I$_3$ irradiated with elliptically polarized light" (Kitayama et al., 2021)
• "Photo-induced electronic and spin topological phase transitions in monolayer bismuth" (Peng et al., 2023)
• "Photo-induced topological phase transitions in strained black phosphorus" (Liu et al., 2017)
• "Optically Induced Topological Phase Transition in two dimensional Square Lattice Antiferromagnet" (Luo, 2020)
• "Direct observation of a photoinduced topological phase transition in Bi-doped (Pb,Sn)Se" (Mogi et al., 2024)
• "Optical manipulation of the topological phase in ZrTe5 revealed by time- and angle-resolved photoemission" (Huang et al., 2024)
• "Photoinduced topological phase transition in monolayer 1T$^\prime$-MoS$_2$" (Nobahari, 19 Nov 2025)