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Wang Group
Fakultät Physik
Student sits at workstation and looks at laptop. © Aliona Kardash​/​TU Dortmund
Publication in Nano Letters

Cavity engineering boosts terahertz high-harmonic generation in a Dirac semimetal

Illustration of giant enhancement of semimetallic terahertz high-harmonic generation by a cavity © Siyu Duan​/​TU Dortmund
In collaboration with researchers from Basel, Pisa and Shanghai, our group reports giant enhancement of terahertz high-harmonic generation in the three-dimensional Dirac semimetal Cd₃As₂. By fabricating microcavity structure to enhance near-field strength of terahertz pulses, we drives Dirac fermions far from equilibrium deeply into the nonperturbative regime, leading to unprecedentedly strong high-harmonic emission. The results have recently been published in the renowned journal Nano Letters.

To the publication

Matter driven far from thermal equilibrium may exhibit properties on transient time scales that are very different from the responses of its thermal equilibrium state. A prominent example is high-order harmonic generation. While in the weak-field regime the nonlinear response is perturbative, a strong driving field can push the system into nonperturbative regime and eventually toward saturation. Experimentally accessing near-saturation high-harmonic emission at terahertz frequencies remains challenging.

To access the nonperturbative regime of the extremely nonlinear terahertz response in Dirac system, we fabricated a designed structure of metasurface microcavities on a nanometer Cd₃As₂ film. The microcavities resonantly enhance the near-field strength of the picosecond terahertz excitation pulse, with a maximum electric-field enhancement of approximately 49 at the cavity slit. Compared with the bare Cd₃As₂ film, the cavity-engineered Cd₃As₂ exhibits an enhancement of the third-harmonic intensity by more than three orders of magnitude. With increasing fluence, we observe nearly zero power-law dependence indicates that the cavity engineering enables us to enter deeply into the extreme nonperturbative regime, close to saturation. The microcavities also enhance the fifth-harmonic emission based on a less powerful table top laser source, which was previously only resolvable by a powerful accelerator-based source.

The experimental results confirm the predictions of Boltzmann transport theory and substantiate a field-driven kinetic description of the strong nonthermal nonlinearity at terahertz frequencies. The observed room-temperature and highly efficient high-harmonic generation demonstrates the potential of cavity engineering for controlling nonlinear optical responses in quantum materials and for future terahertz optoelectronic applications.