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  • Review Article
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Light-induced emergent phenomena in 2D materials and topological materials

Abstract

Light–matter interaction in 2D and topological materials provides a fascinating control knob for inducing emergent, non-equilibrium properties and achieving new functionalities in the ultrafast timescale (from femtosecond to picosecond). Over the past decade, intriguing light-induced phenomena, such as Bloch–Floquet states and photo-induced phase transitions, have been reported experimentally, but many still await experimental realization. In this Review, we discuss recent progress on the light-induced phenomena, in which the light field could act as a time-periodic field to drive Floquet states, induce structural and topological phase transitions in quantum materials, couple with spin and various pseudospins, and induce nonlinear optical responses that are affected by the geometric phase. Perspectives on the opportunities of proposed light-induced phenomena, as well as open experimental challenges, are also discussed.

Key points

  • Light–matter interaction plays critical roles in emerging exotic phenomena in 2D materials and topological materials not only as an experimental probe but also as a control knob for inducing emergent non-equilibrium properties that are otherwise not possible to be achieved in the equilibrium state.

  • Light, regarded as a time-periodic electric field, can induce the photo-dressing Floquet states, which can be further utilized to dynamically engineer the electronic properties of quantum materials, especially topological properties, dubbed Floquet engineering.

  • By resonantly exciting electrons or lattices, light–matter interaction can dynamically change the energy landscape of 2D and topological materials, leading to the light-induced phase transitions, such as the emergence of light-induced superconductivity or hidden states.

  • By coupling the angular momentum of light with spins and pseudospins, light–matter interaction can be used to detect and manipulate various quantum degrees of freedom for new concepts of device applications.

  • By coupling to geometric phase of the Bloch wavefunctions, light–matter interaction can be used as a powerful probe of geometric-phase-related properties and to manipulate the material’s response, leading to rich, nonlinear optical responses.

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Fig. 1: Coupling of light with 2D and/or topological materials and light-induced emerging phenomena.
Fig. 2: Floquet engineering and experimental evidence of Floquet states.
Fig. 3: Predicted light-induced topological phase transitions by Floquet engineering.
Fig. 4: Light-induced phase transitions.
Fig. 5: Emerging properties due to coupling to spin, sublattice pseudospin, valley and chirality.
Fig. 6: Geometric phase effect on nonlinear optical response.

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Acknowledgements

This work is supported by the National Natural Science Foundation of China (grant nos. 11725418, 11427903 and 12034001), National Key R&D Program of China (grant nos. 2020YFA0308800 and 2016YFA0301004), Tsinghua University Initiative Scientific Research Program and Tohoku-Tsinghua Collaborative Research Fund, Beijing Advanced Innovation Center for Future Chip (ICFC), Beijing Nature Science Foundation (JQ19001).

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Correspondence to Peizhe Tang, Dong Sun or Shuyun Zhou.

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Glossary

‘Multiphoton’ dressed states

The electronic states that are influenced by the optical field through virtual absorption or emission of multiple photons.

Volkov states

Free electron states in the field of an electromagnetic wave in vacuum. In solids, they refer to multiphonon dressed states of free-electron-like photoemission final states in the vacuum, in analogy to Floquet states, which are multiphonon dressed states of the electronic states inside the solids.

High harmonic generation

The emission of light at a higher harmonic (\(n\hbar \omega \)) of the fundamental laser (\(\hbar \omega \)) through a nonlinear optical process.

Stripe phase

1D modulation of charges. In cuprate superconductors, this refers to the concentration of doped charges along spontaneously generated domain walls between antiferromagnetic insulating regions.

1/8 Anomaly

The anomalous suppression of superconductivity in cuprate La2−xBaxCuO4 (and certain related compounds) near x = 1/8 doping.

Leggett mode

Collective excitations that can be ascribed to the relative phase fluctuations between two superconducting order parameters.

Stark effect

The shifting and splitting of spectral lines of atoms and molecules owing to the presence of an external electrical field. If an oscillating electric field (like a laser) is applied, it corresponds to an ac (optical) Stark effect.

Berry connection

An effective vector potential to describe the geometrical property of an energy band in the momentum space of crystalline solids, which is defined as \(A=\langle \psi | i{\nabla }_{k}| \psi \rangle \), where \(| \psi \rangle \) is the eigenstate of the system.

Berry curvature

A local gauge field to describe the geometrical property of an energy band, which is defined as Ω =  × A, where A is the Berry connection.

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Bao, C., Tang, P., Sun, D. et al. Light-induced emergent phenomena in 2D materials and topological materials. Nat Rev Phys 4, 33–48 (2022). https://doi.org/10.1038/s42254-021-00388-1

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