  Spin pumping converting sub-terahertz electromagnetic waves to DC spin
currents has recently been demonstrated in antiferromagnets (AFMs) with
easy-axis magnetic anisotropy. However, easy-plane AFMs such as NiO, which are
easier to prepare experimentally, are considered to be bad candidates for spin
pumping because the N\'{e}el vector oscillation is linearly polarized, placing
a major restriction on the material choice for practical applications. Through
a case study of NiO, we show that an applied magnetic field below the spin-flop
transition can substantially modify the polarization of the resonance
eigenmodes, which enables coherent sub-terahertz spin pumping as strong as that
in easy-axis AFMs. In addition, we find that an applied magnetic field can
significantly reduce the threshold of N\'{e}el vector auto-oscillation
triggered by spin-transfer torques. These prominent field-assisted effects can
greatly facilitate spintronic device engineering in the sub-terahertz frequency
regime.
