Extended Data Fig. 2: BoxCARS geometry.
From: Coherent nonlinear X-ray four-photon interaction with core-shell electrons
a, Coordinate system and arrangement of beams on the mask used for the phase-matching calculation. The holes of the spatial mask have diameters of 1 mm and therefore cover a range of angles on the order 2Θmax − 2Θmin ≈ 0.04°. b, Phase-matching efficiency (sinc2 function in equation (4)) calculated for the case of ω1 = ω2 = 867 eV, a fixed θ = θ3 = θS, as a function of a varying ω3. The refractive index value is approximated to 1 and the propagation length is set as large as the gas cell, 8 mm, thus showing an unrealistic ‘worst-case scenario’ for phase mismatch. A shorter length value would increase the phase-matching energy/angle tolerance.
