Ultrafast Spin Dynamics Resolved with High-Harmonic Generation Microscopy

Compact radiation sources based on high harmonic generation (HHG) enable access to ultrafast phenomena with femtosecond and even attosecond temporal resolution [1]. While both ultrafast spectroscopy and real-space imaging with high-harmonic radiation are rather established techniques [2,3], the combination of nano-imaging with femtosecond temporal resolution has remained challenging. Here, we demonstrate the first imaging of femtosecond dynamics using ultrafast high-harmonic generation microscopy, harnessing both the femtosecond duration and the nanometric wavelength of HHG. We quantitatively map demagnetization dynamics in self-organized networks of nanoscale magnetic domains upon excitation with femtosecond laser pulses [see Fig. 1(a)]. In the experiment, scattering patterns from the sample are recorded as a function of pump-probe delay using the 38
^th
harmonic order (wavelength 21 nm) with both left- and right-handed circular polarization [4]. Each diffraction pattern is reconstructed by holographically-enhanced coherent diffractive imaging [3]. The ratio of the reconstructed exit wave amplitudes forms a dichroic image of the magnetic pattern [c.f. dichroic phase maps in Fig. 1(c)]. An overall measure of the magnetization in the worm-like domain pattern is obtained from the standard deviation of the magnetization across all image pixels, which is displayed in Fig. 1(b) as a function of delay. The temporal trace exhibits the well-known features of ultrafast demagnetization curve, with a rapid drop within a few hundreds of femtoseconds and a subsequent partial re-magnetization on a picosecond time scale [5]. The images in Fig. 1 represent snap-shots from a magnetization-dynamics movie with 30 nm spatial- and 50 fs temporal resolution. Generally, for magnetic dichroic imaging using HHG we demonstrate a spatial resolution of 19 nm at the illumination wavelength of 21 nm.