High-harmonic generation in metalic titanium nitride

High-harmonic generation is the cornerstone of nonlinear optics.  Having been demonstrated in a wide range of crystalline systems, including dielectrics, semiconductors, and semi-metals, as well as in gases, it left metals out due to their low damage threshold. We report on the high-harmonic generation in metallic titanium nitride (TiN) films, a refractory plasmonic metal, known for its high melting temperature and laser damage threshold. We show that TiN can withstand few-cycle infrared laser pulses with peak intensities as high as 13 TW/cm²,  enabling the emission of intraband harmonics up to photon energies of 11 eV. We study the intensity and angle dependence of the harmonic yield and compare them with the numerical calculations. We show that the intensity scaling and angular anisotropy of the emitted VUV radiation is consistent with the the anisotropic conduction band structure of TiN. The metal harmonics will allow establish the link between the fields of solid and plasma HHG. They also pave the way for compact and efficient plasmonic devices producing vacuum ultraviolet (VUV) frequency combs.