Broadband Carrier-Envelope Phase-Controlled Stimulated Ultraviolet Emission from Carbon Dioxide Ions.

A coherent broadband ultraviolet light source is essential for both fundamental research and industrial applications, yet its generation remains challenging. Here, we demonstrate microjoule-level ultraviolet emission spanning 300-450 nm from carbon dioxide ions (CO_{2}^{+}), driven by carrier-envelope-phase (CEP)-stabilized, near-single-cycle light pulses. The radiation arises from transitions between the first excited and ground cationic states, created by strong-field ionization with near-infrared pulses, and self-seeded by either self-phase modulation (SPM) or third-harmonic (TH) generation in the ultraviolet range. Interestingly, while TH-seeded stimulated emission is unaffected by CEP, SPM-seeded ultraviolet radiation shows strong CEP dependence. This arises from a Ramsey-type interference, where the relative phase between the SPM and the strong-field-prepared ionic polarization controls the constructive or destructive interference of the superposition between two ionic states, directly modulating the ionic dipole strength. Leveraging this CEP-sensitive lasing, we demonstrate a single-shot CEP tagging technique with remarkable resolution and simplicity. Our Letter establishes a powerful method for generating and controlling broadband ultraviolet stimulated emission in molecular ions using strong, waveform-stabilized ultrashort pulses, highlighting new possibilities in strong-field ionic quantum optics.