Harmonic generation in atoms and molecules has reshaped our understanding of ultrafast phenomena beyond the traditional nonlinear optics and has launched attosecond physics. Harmonics from solids represent a new frontier, where both majority and minority spin channels contribute to the harmonics. This is true even in a ferromagnet whose electronic states are equally available to optical excitation. Here, we demonstrate that harmonics can be generated mostly from a single spin channel in half-metallic chromium dioxide. An energy gap in the minority channel greatly reduces the harmonic generation, so the harmonics predominantly emit from the majority channel, with a small contribution from the minority channel. However, this is only possible when the incident photon energy is well below the energy gap in the minority channel, so all the transitions in the minority channel are virtual. The onset of the photon energy is determined by the transition energy between the dipole-allowed transition between the $\mathrm{O}\text{−}2p$ and $\mathrm{Cr}\text{−}3d$ states. The harmonics mainly from a single spin channel can be detected, regardless of laser field strength, as long as the photon energy is below the minority band energy gap. This prediction should be tested experimentally.

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• High-order harmonic generation
• Magnetization dynamics
• Ultrafast phenomena
• Femtosecond laser spectroscopy