High harmonics with spatially varying ellipticity

We present a new method to produce ultrashort pulses of circularly polarized extreme ultraviolet (EUV) light. We combine two orthogonally polarized high-harmonic sources to produce a far-field beam with a uniform intensity distribution but with a spatially varying ellipticity—ranging from linearly to fully circularly polarized. This spatially varying ellipticity was characterized using EUV magnetic circular dichroism, which demonstrates that a high degree of circularity is achieved, reaching almost 100 % near the magnetic M-edge of cobalt. The spatial modulation of the polarization facilitates measurements of circular dichroism, enabling us to measure spectrally resolved magnetic circular dichroism without the use of an EUV spectrometer, thereby avoiding the associated losses in both flux and spatial resolution, which could enable hyperspectral imaging of chiral systems. Through numerical simulations, we also show the generality of this scheme, which can be applied with both the discrete harmonic orders generated by many-cycle pulses, or the high-harmonic supercontinua generated by few-cycle driving laser pulses. Therefore, this technique provides a promising route for the production of bright isolated attosecond pulses with circular polarization that can probe ultrafast spin dynamics in materials.