Extreme-ultraviolet frequency combs by surface-enhanced optical fields with diatomic molecules

An efficient and straightforward approach is proposed to produce extreme ultraviolet frequency combs by using high-order harmonic generation from diatomic molecules driven by surface-enhanced high-repetition-rate femtosecond pulses. For a nonpolar molecule, the maximum (minimum) energy conversion efficiency is observed as the molecule is oriented at an angle of 27° (108°) relative to the surface. For a polar molecule, the frequency components of the generated ultraviolet frequency combs are doubled as both odd- and even-order harmonics are generated, which shows different orientation dependence and shifts from the nonpolar molecule. The generated ultraviolet frequency combs are mainly emitted with angles smaller than 30° close to the surface, while there is a noticeable emission probability around the directions perpendicular to the surface when the molecules are oriented with large angles.