Observation of Nonlinear Optical Coupling in the Kiloelectronvolt X-Ray Regime

Experimental findings with Xe(M) radiation in the ~1 keV X-ray region have confirmed the presence of a predicted zone of anomalously strengthened radiative coupling operative at sufficiently high intensity (I >; 10
¹⁵
W/cm
²
) and frequency (hω >; 5 eV). These new results herald the general existence of a strongly enhanced modality of radiative interaction that is based on ordered-driven electron motions in the attosecond regime.