(a) Vectorial magnetic oscillation control and the resultant THz radiation by double-pulse excitation. The precession motions of the spins are induced in a multidomain single crystal of NiO via a stimulated Raman-type nonlinear optical process, as shown in the inset. Spin motions result in a macroscopic magnetization vector (M⁽²⁾) that behaves as an isotropic harmonic oscillator. By changing the interval and polarization of the incident laser pulses, an arbitrary trajectory of the magnetic oscillations is obtained. (b) Two orthogonal magnetic oscillation modes are labelled x and y in this NiO crystal. They are selectively kicked by tuning the polarization azimuth of the excitation light pulse.

(a) Schematic illustrations of the experiment concerning coherent control with double-pulse excitation, wherein both pulses are linearly polarized along the x-axis. The interval τ between the two pulses can be tuned. (b) Two-dimensional plot of the time-domain THz signal. The signs (+/0/−) and amplitudes of the electric fields are expressed by colours (red/white/blue) and their shades. The periodically aligned horizontal white lines correspond to the suppression of the spin motion by the destructive interference between two two-excitation pulse-induced waves. Temporal profiles at fixed values of τ are shown in c and d. The two waves constructively (c) or destructively (d) interfere with one another.

(a) Schematic illustrations of the vectorial control of the magnetization vector. Both pulses are linearly polarized. The first is polarized along the x-axis, whereas the second has an azimuthal angle of ψψ. By properly tuning τ and ψψ, we can manipulate the motion of the magnetization vector to follow an arbitrarily designed direction and amplitude of polarization. (b) Ellipticity of the measured THz radiation at Ω_mag as a function of the interval τ between the two linearly polarized excitation pulses. The first pulse is x-polarized, whereas the polarization azimuth of the second is 45° with respect to the x-axis. Any ellipticity between the purely right and left circular polarizations is obtained by tuning τ. The solid curve is a fit of the data by a sine curve. The three-dimensional trajectories of the electric field vectors with fixed values of τ are shown in (c, d). The radiation is purely circularly polarized (c) or linearly polarized (d). The dots correspond to the projections of the experimental data onto the x- and y-axes. The solid curves correspond to the fit of exponentially decaying sine functions; the phase differences between the x- and y-components are determined by Ω_magτ/π.