Harmonic generation predominantly from a single spin channel in a half metal

TL;DR

This paper shows that in half-metallic chromium dioxide, high harmonic generation mainly occurs from a single spin channel due to the energy gap in the minority channel, which suppresses its contribution.

Contribution

It demonstrates that harmonic generation in a half metal can be dominated by one spin channel when photon energy is below the minority band gap, a novel insight into spin-dependent nonlinear optics.

Findings

Harmonics mainly originate from the majority spin channel.

Energy gap in the minority channel suppresses its harmonic contribution.

Harmonic generation depends on photon energy being below the minority band gap.

Abstract

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 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 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 O-$2p$ and Cr-$3d$ states. Harmonics {mainly} from a single spin channel can be detected, regardless of laser field strength, as far as the photon energy is below the minority band energy gap. This prediction should be tested experimentally.