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 optical components are 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 photons 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.