Control of high harmonic generation in mixed gaseous media

High harmonic generation (HHG) in gaseous media provides a robust method for producing coherent extreme-ultraviolet (EUV) radiation and attosecond pulses. However, the spectral and temporal properties of these pulses – such as bandwidth and chirp – are fundamentally limited by the underlying generation mechanisms. Typically, tailoring the EUV emission involves modifying the properties of the intense infrared femtosecond driving pulse, and/or the macroscopic laser-matter configuration. Here, we focus on controlling the HHG process through the gas specie, introducing mixed-gas targets as a practical approach to enhance control over the EUV harmonic radiation. Through advanced simulations assisted by artificial intelligence that take into account both the quantum microscopic and macroscopic aspects of HHG, we demonstrate how mixtures of argon and helium modulate the emitted EUV harmonics. A simple model reveals that these modulations arise from coherent interference between harmonics emitted by different species at the single-atom level, and that they can be tuned by adjusting the macroscopic relative concentrations. Furthermore, by spatially separating the gas species into two distinct jets in a symmetric configuration, we gain additional control over the whole harmonic bandwidth. This strategy provides a realistic and versatile pathway to tailor EUV light and attosecond sources via HHG, while also enabling the identification of species-specific contributions to the process. In particular, it holds potential for application in high harmonic spectroscopy, where unknown features of a gas target can be identified through the HHG spectrum generated from a mixture with a known reference gas.