By using the freespace terahertz （THz） electrooptic （EO） sampling technique, the THz electromagnetic wave waveforms emitted from intrinsic bulk GaAs photoexcited by femtosecond laser pulses under strong bias electric fields up to 300 kV/cm were recorded. From the experimental data, we can clearly see the THz electromagnetic wave emission waveforms, ETHz（t）, which are proportional to the acceleration/deceleration of electrons, have a bipolar feature. Power dissipation spectra of electrons for stepfunctionlike input electric fields have been obtained by calculating Fourier spectra of the measured THz traces. The cutoff frequency, νc, for negative power dissipation （i.e., gain） due to intervalley transfer is found to gradually increase with increasing bias electric fields, F0, for F0 < 50 kV/cm and saturate at 750 GHz above ～50 kV/cm at 10 K.