Terahertz non-linear optics in 2D materials and van der Waals topological metamaterials

Harnessing optical nonlinearities in two-dimensional materials in the terahertz (THz) regime provides a key pathway to access high-frequency spectral windows that are typically inaccessible with conventional solid-state laser technologies.
Over the past decade, the non-linear optical properties of graphene have been extensively investigated, and a wide range of related applications demonstrated, ranging from optical modulators [1] to saturable absorbers [2]. High harmonic generation (HHG) – the frequency up-conversion of an optical signal – in materials systems is governed by symmetries. This effect has been exploited in graphene [3], where HHG has been demonstrated, albeit only at odd multiples of the driving frequency owing to its inherent centro-symmetry. In topological insulators (TIs), unconventional HHG has been predicted [4], supported by the bulk and topological surface states, which are usually difficult to distinguish, relying on the ultrafast intraband dynamics, and the inversion symmetry-breaking even-order nonlinearity in the topological phase.
We recently exploited innovative technological approaches in large area topological insulator metametarials and heterostructures, multilayer graphene or twisted graphene to devise both high efficient non-linear micro-devices, optically pumped by quantum cascade lasers (QCLs) delivering 2W optical power, and electrically-driven integrated plasmonic lasers comprising integrated QCL heterostructures, to target HHG in the technologically relevant Reststrahlen gap (6-12 THz) at both
odd or even orders, via symmetry breaking [5], [6], [7].
This talk will provide future perspectives in the fascinating field of topological photonics and non-linear THz photonics.

References
[1]A. Di Gaspare et al., Electrically Tunable Nonlinearity at 3.2 Terahertz in Single-Layer Graphene, ACS Photonics, 10, 3171−3180 (2023);
[2]A. Di Gaspare et al., All in one-chip, electrolyte-gated graphene amplitude modulator, saturable absorber mirror and metrological frequency-tuner in the 2-5 THz range, Adv. Optical Mater., 10, 2200819 (2024);
[3]H. A. Hafez et al., Extremely efficient terahertz high-harmonic generation in graphene by hot Dirac fermions, Nature, vol. 561, no. 7724, pp. 507–511, (2018);
[4]J. Stensberg et al., Observation of terahertz second harmonic generation from Dirac surface states in the topological insulator Bi2Se3, Phys. Rev. B, vol. 109, no. 24, p. 245112, (2024);
[5]A. Di Gaspare et al., Compact terahertz harmonic generation in the Reststrahlenband using a graphene-embedded metallic split ring resonator array, Nature Commun., vol. 15, no. 1, p. 2312, (2024);
[6] A. Di Gaspare et al., Second and third harmonic generation in topological insulator-based van der Waals metamaterials, Nature Light Science&Applications in press (2025);
[7] A. Di Gaspare et al. Electrically-driven heterostructured wire lasers with integrated graphene plasmons, Nature Nanotechnology 20, 1611–1617 (2025).