Observation of Polarization of the Soft X-Ray Laser Line in Neonlike Germanium Ions

A 3-cm-long curved Ge slab target was irradiated by 1.053 μm glass laser light with irradiance of 2.9 × ${10}^3$ W/${\mathrm{cm}}^2$. The 19.6 nm laser line due to the $3p\to 3s$, $J=0\mathrm{}\to 1$ transition in Ne-like Ge ions was found to be polarized parallel to the target surface, with a degree of polarization of -53%. This polarization is accounted for by the anisotropy in the radiation trapping of the resonance line in an expanding plasma, which has a velocity gradient in the direction normal to the target surface.