We developed a differential transmission to study cyclotron resonance of GaAs/AlxGa1 xAs two-dimensional hole samples. The technique utilizes a modulated AuPd gate isolated by a Si3N4 dielectric from the sample, which is irradiated opposite the gate by millimeter waves ranging from 2 to 40GHz. This technique effectively removes the background signal and yields a hole effective mass of 0:41me with a cyclotron scattering time of 20 ps, consistent with the previous results using different techniques.