Frequency Shift of Terahertz Electromagnetic Waves Originating from Sub-Picosecond-Range Carrier Transport in Undoped GaAs/n-Type GaAs Epitaxial Layer Structures

Abstract

We have investigated the terahertz electromagnetic waves from undoped GaAs/n-type GaAs (i-GaAs/n-GaAs) structures with various i-GaAs-layer thicknesses, focusing on the relation between the sub-picosecond-range carrier-transport processes and terahertz-wave frequency. It is observed that the intense monocycle oscillation induced by the surge current of photogenerated carriers is followed by the signal of the coherent GaAs longitudinal optical (LO) phonon. The Fourier power spectra of the terahertz waveforms reveal that an increase in the built-in electric field of the i-GaAs layer, which is controlled by the i-GaAs layer thickness, causes a high frequency shift of the band due to the surge current. Consequently, we conclude that the photogenerated carriers are accelerated by the built-in electric field in the sub-picosecond range without being affected by the intervalley scattering. This demonstrates that the frequency tunable terahertz emitters are realized. Furthermore, we find that terahertz band of the coherent LO phonon is relatively intense, compared with those from bulk crystals. This phenomenon is discussed from the viewpoint of the effects of the built-in electric field on the terahertz radiation mechanism.