Simple strategy for enhancing terahertz emission from coherent longitudinal optical phonons using undoped GaAs/n ‐type GaAs epitaxial layer structures

Abstract

AbstractWe have explored the feasibility of enhancing terahertz (THz) emission from coherent longitudinal optical (LO) phonons using simple epitaxial layer structures: undoped GaAs/n ‐type GaAs (i ‐GaAs/n ‐GaAs) epitaxial layer structures. Initially, using a numerical simulation we have confirmed that a decrease in the i ‐GaAs layer thickness d increases the built‐in electric field in the i ‐GaAs layer. From the THz wave measurement, it is observed in the THz waveforms of the samples that the monocycle oscillation, the so‐called first burst, resulting from the surge current of the photogenerated carriers, accompanies with a clear oscillatory profile with a period of 113 fs corresponding to the GaAs LO phonon frequency (8.8 THz). The Fourier transform spectra show that the intensity of the LO phonon band remarkably increases with a decrease in d that causes the enhancement of the built‐in electric field in the i ‐GaAs layer. Thus, we conclude that the utilization of the i ‐GaAs/n ‐GaAs structure is a useful way to enhance the THz emission from the coherent LO phonon. We also performed the time‐partitioning Fourier transform in order to investigate the decay time of the THz wave from the coherent LO phonon. The decay time of each sample is almost the same: 2.0 ps. (© 2011 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)