Control of generation of terahertz electromagnetic waves in semiconductor epitaxial structures by internal electric field

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We have investigated the control mechanisms of the terahertz (THz) electromagnetic waves from coherent longitudinal optical phonons in semiconductor epitaxial structures with use of internal electric fields.It was found that the THz electromagnetic waves are markedly enhanced by a surface electric field due to surface Fermi level pinning in i-GaAs/n-GaAs epitaxial structures and by a piezoelectric field due to lattice strain in (11n)-oriented GaAs/In0.1Al0.9As strained multiple quantum wells via an increase in phonon polarization. Furthermore, it was demonstrated that the THz electromagnetic waves are continuously and drastically enhanced by an applied bias voltage, which controls the internal electric field, in a p-i-n diode structure of GaAs: This provides us a novel concept of a THz device.