Nonlinear all-optical control and detection of ultrafast bandgap engineering

by Prof. Giancarlo Soavi (Institute of Solid State Physics, Friedrich Schiller University Jena)

Thursday 17 Jul 2025, 16:15 → 17:30 Europe/Berlin

ENC-D114 (ENC)

Layered materials, such as graphene and transition metal dichalcogenides (TMDs), possess unique nonlinear optical (NLO) properties [1], which allow a full control over the intensity [2,3], polarization [4] and orbital angular momentum [5] of the light emitted via perturbative harmonic generation processes. Furthermore, TMDs have emerged as the ideal platform to study the interplay between space-inversion and time-reversal symmetries, which can be independently engineered in by tuning the number of layers (space-inversion) and via excitation with circularly polarized light (time-reversal). In this talk, I will discuss our recently developed approaches to modulate the bandgap in TMDs using ultrafast and off-resonant excitation, and the possibility to subsequently detect broken time-reversal symmetry using second [6,7] and third harmonic generation. In particular, all-optical bandgap modulation can be achieved via the coherent optical Stark and Bloch-Siegert effects, with the additional degree of freedom to create either a symmetric or asymmetric bandgap opening in the ±K valleys, depending on the number of layers of the sample and the polarization state of the excitation beam. In monolayer TMDs, a valley asymmetric bandgap modulation effectively corresponds to breaking of time-reversal symmetry. In bilayer TMDs, broken time-reversal symmetry manifests itself as a valley symmetric bandgap modulation, which lifts the spin degeneracy. Subsequently, both symmetric and asymmetric all-optical bandgap modulation can be probed via polarization and intensity dependent measurements of perturbative nonlinear optics.

References

1. Dogadov, O. et al., “Parametric Nonlinear Optics with Layered Materials and Related Heterostructures”, Laser & Photon. Rev., Vol. 16, No. 9, 2100726, 2022

2. Soavi, G. et al., “Broadband, electrically tunable third-harmonic generation in graphene”, Nat. Nanotechnol., Vol. 13, 583-588, 2018

3. Ghaebi, O. et al., “Ultrafast Opto-Electronic and Thermal Tuning of Third-Harmonic Generation in a Graphene Field Effect Transistor”, Adv. Science, Vol. 11, No. 31, 2401840, 2024

4. Klimmer, S. et al., “All-optical polarization and amplitude modulation of second-harmonic generation in atomically thin semiconductors”, Nat. Photon., Vol. 15, 837-842, 2021

5. Sinelnik, A. et al., “Ultrafast all-optical second harmonic wavefront shaping”, Nat. Comms., Vol. 15, 2507, 2024

6. Herrmann, P. et al., “Nonlinear valley selection rules and all-optical probe of broken time-reversal symmetry in monolayer WSe₂”, Nat. Photon., Vol. 19, 300-306, 2025

7. Herrmann, P. et al., “Nonlinear All-Optical Coherent Generation and Read-Out of Valleys in Atomically Thin Semiconductors”, Small, Vol. 19, No.37, 2301126, 2023