Monochromatic high-order harmonic generation by a Bessel-Gauss beam in periodically modulated media

High-harmonic generation (HHG) has become a multipurpose source of coherent XUV radiation used in various applications. One of the notable aspects of HHG is its wide spectrum consisting of many harmonic orders. This might represent a bottleneck in HHG utility for applications requiring a single wavelength. We propose a method to generate radiation consisting of a single high-order harmonic frequency employing a Bessel-Gauss driving beam and a periodically modulated gaseous medium. We validate it by analytical calculations and numerical simulations. Our method provides a way to generate monochromatic harmonic radiation directly from the source without the need for additional monochromatizing optics. Thus it represents a substantial enhancement of the flux and simplification of the setup for numerous applications requiring monochromatic short-wavelength radiation.