Figure shows a well-adjusted light source unit with all seven laser beams pointing toward the center. The illustration shows how the optical density of the solution affects the penetration and absorption of the laser beams. In our experiments, the concentration of azobenzene was high enough to absorb all light from the lasers. We estimated the optical density of the azobenzene solution at 56 mM as 10 in a 5 mm tube, which means that all light is absorbed in a thin layer of about 1 mm. Bubbling with hydrogen gas during irradiation agitates the solution, which ensures effective cis-trans conversion of all azobenzene molecules inside the NMR tube.