The generation of high-order harmonics of an intense infrared laser field upon its nonlinear interaction with atomic gases grants access to coherent light with wavelengths approaching atomic dimensions in the Angstrom scale (0.1 nm) in a lab-scale arrangement. The highest frequencies achievable through this process increase with the pump wavelength, reaching the X-ray limit for mid-infrared driving fields. Within its common theoretical description, the individual atomic emitters are considered as point-like dipoles whose properties dictate both the spectral and directional characteristics of the emitted high-frequency radiation.

In this work, Khairulin and coauthors report a theoretical analysis of the high-harmonic emission process when the wavelengths generated approach atomic dimensions. Under those circumstances, the point-like approximation breaks down and a new framework is required to correctly describe the features of the emission pattern. Fortunately, this does not mean that previous studies on the subject are unworthy –including the correction factor proposed here suffices to account for the distributed nature of the excited dipoles and therefore predict the directionality and spectrum of the atomic-scale radiation. These results may help to design future experiments aimed at generating and harnessing coherent X-rays.

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