In this work, we experimentally observe orientation-dependent high-harmonic spectra in rocksalt crystal MgO driven by a linearly polarized 790 nm laser, featuring distinct peaks at $0^{\circ }, {45}^{\circ }$, and ${90}^{\circ }$ that correspond to orientation angles between the laser polarization and [100] crystal axis of MgO. A localized orbital framework is developed to understand these spectral features from an atomic orbital perspective. Decomposition using atomiclike Wannier orbitals shows that low-order harmonics are primarily dominated by couplings among $\mathrm{Mg}\text{−}s, \mathrm{O}\text{−}s$, and $\mathrm{O}\text{−}p$ orbitals. Conversely, the response at higher orders is governed by $\mathrm{Mg}\text{−}p$ and $\mathrm{Mg}\text{−}d$ orbitals, with their relative contributions varying systematically with harmonic order and laser intensity. In the minimal model where only $\mathrm{O}\text{−}p–\mathrm{Mg}\text{−}s$ couplings are retained, the $\mathrm{O}\text{−}{p}_x–\mathrm{Mg}\text{−}s$ and $\mathrm{O}\text{−}{p}_y–\mathrm{Mg}\text{−}s$ channels produce yield maxima at $0^{\circ }$ and ${90}^{\circ }$, respectively, and each generates symmetric features near ${45}^{\circ }$. Their constructive superposition at ${45}^{\circ }$ results in a local maximum of the total yield at this angle. This work establishes a localized orbital picture of high-harmonic generation in crystals, bridging atomic orbital interactions and macroscopic polarization dynamics in periodic systems.

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• Atomic orbital
• Electronic structure of atoms & molecules
• Photoemission
• Strong electromagnetic field effects
• Strong-field-induced spectra
• Ultrafast phenomena
• XUV generation