Barrier control in tunneling e^+ e^- photoproduction

Tunneling electron-positron pair production is studied in a new setup in which a strong low-frequency and a weak high-frequency laser field propagate in the same direction and collide head-on with a relativistic nucleus. The electron-positron pair production rate is calculated analytically in the limit in which in the nucleus rest frame the strong field is undercritical and the frequency of the weak field is below and close to the pair production threshold. By changing the frequency of the weak field one can reduce the tunneling barrier substantially. As a result tunneling pair production is shown to be observable with presently available technology.