Nat. Phys. http://doi.org/bvg6 (2016)

High-harmonic generation (HHG) is an important nonlinear optical phenomenon occurring in a material on the attosecond timescale. However, the effect of electron–hole interactions on the generation process is still unknown. Now, Hanzhe Liu and co-workers from the SLAC National Accelerator Laboratory and Stanford University in the USA have analysed the role of electron–hole interactions by investigating HHG in a 2D material, an isolated monolayer MoS2 crystal. The sample was excited at normal incidence by 160 fs mid-infrared pulses at a photon energy of 0.3 eV, well below the direct bandgap of around 1.8 eV. Linearly polarized pump pulses with intensity up to 2.5 TW cm−2 at a 1 kHz repetition rate were sent to the sample. The output spectrum exhibited distinct peaks corresponding to the generation of harmonics. Both even and odd harmonics were observed from the isolated monolayer MoS2, while only odd harmonics were observed from an equivalent layer in the bulk MoS2. In the monolayer, the emergence of even harmonics is a consequence of the broken inversion symmetry, while the odd harmonics were enhanced due to the strong Coulomb interaction between electrons and holes.