Extended search for sub-eV axion-like resonances via four-wave mixing with a quasi-parallel laser collider in a high-quality vacuum system

Resonance states of axion-like particles were searched for via four-wave mixing by focusing two-color pulsed lasers into a quasi-vacuum. A quasi-parallel collision system that allows probing of the sub-eV mass range was realized by focusing the combined laser fields with an off-axis parabolic mirror. A 0.10 mJ/34 fs Ti:sapphire laser pulse and a 0.14 mJ/9 ns Nd:YAG laser pulse were spatiotemporally synchronized by sharing a common optical axis and focused into the vacuum system. No significant four-wave mixing signal was observed at the vacuum pressure of $3.7 \times 10^{-5}$ Pa, thereby providing upper bounds on the coupling-mass relation by assuming exchanges of scalar and pseudoscalar fields at a 95% confidence level in the mass range below 0.21 eV. For this search, the experimental setup was substantially upgraded so that the optical components were compatible with the requirements of the high-quality vacuum system, hence enabling the pulse power to be increased. With the increased pulse power, a new kind of pressure-dependent background photon emerged in addition to the known atomic four-wave mixing process. This paper shows the pressure dependence of these background photons and how to handle them in the search.