Fig. 1: HHG spectra from layered materials assisted with phonons.

a HHG spectrum for hBN with the excitation of the TO (E_1u mode at Γ point, red curve) and with the frozen ions (blue curve). We employ a laser pulse with a wavelength of λ = 800 nm (i.e., ħω = 1.55 eV) and I₀ = 1 × 10¹²W cm^–2. The adjacent hBN layers stack laterally with the AA’ configuration of a hexagonal structure. b HHG spectrum for graphite with the excitation of the TO (E_1u mode at Γ point, red curve) and with the frozen ions (blue curve). The laser wavelength is λ = 1.6 μm (ħω = 0.78 eV) and the intensity is I₀ = 1 × 10¹¹W cm⁻². The adjacent carbon layers stack laterally with the AB configuration of a hexagonal structure. c HHG spectrum for 2H-MoS₂ with the excitation of the TO (\({E}_{2g}^{1}\) mode at Γ point, red curve) and with the frozen ions (blue curve). The laser wavelength is λ = 4.133 μm (ħω = 0.3 eV) and the intensity is I₀ = 1 × 10¹¹W cm⁻². The insets show the atomic displacements of the corresponding modes. The black arrows mark the corresponding cutoff energies of the phonon-mediated spectra. The harmonic intensity is probed along the direction parallel with the laser polarization of the corresponding applied laser pulse.