This paper presents an innovative technique to adjust the dielectric loss tangent of an oxide film on the surface of a GaAs wafer. In the experiments presented, a 300 nm thick film is annealed with extremely high power densities in a subterahertz resonator cavity to purify or spoil the surface film. Pulsed powerful radiation up to 80W of millisecond pulses up to 0.4s wide at 263 GHz is applied to a cavity resonator. Power densities up to 180 kW/mm^2 are achieved in the 0.408 mm diameter active zone. The loss tangent is adjustable in 250 steps or a range or more than two orders without significantly changing the real part of the permittivity of the surface film, and the subsequent insertion loss (IL) minimum of the GaAs wafer was adjusted over a range of about 6.5 dB from -8.914 dB to about -2.5 dB.