Time evolution of terahertz electromagnetic waves from undoped GaAs/n-type GaAs epitaxial layer structures clarified with use of a time-partitioning Fourier transform method

We have investigated the time evolution of terahertz electromagnetic waves caused by the surge current of photogenerated carriers, the so-called first burst, in two i-GaAs/n-GaAs epitaxial layer structures with different i-GaAs layer thicknesses of 500 and 1200 nm. The terahertz waveform of the first burst shows a narrowing with a decrease in the thickness of the i-GaAs layer. In accordance with the above phenomena, it is observed in the Fourier power spectra that the band of the first burst shows a high frequency shift with a decrease in the i-GaAs layer thickness. We elucidate the origin of the high frequency shift, focusing on the time-domain dynamics of the photogenerated carriers: we apply the time-partitioning Fourier transform, which is useful to investigate the time evolution of the frequency. From the time-partitioning Fourier power spectra, we obtain the evidence that the acceleration after the carrier generation dominates the time evolution of the frequency component leading to the high frequency shift.