Research on a highly efficient semiconductor THz emitting source using a metal thin-film mesh

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It was confirmed that a metal mesh having an appropriate structure exhibits a high transmittance as a filter in the THz region. Especially a filter with the thickness much smaller than the transmitted wavelength does not have the cutoff property resulting in a high transmittance even for the tilted incidence. According to the results of simulation and experiment, it was found that the surface plasmon-polariton (SPP) contributes the transmission property. Then there is possibility that the boundary condition at the semiconductor surface for the THz emission is modified by attaching a metal mesh where the SPP modes exists.Actually a fundamental structure of the new THz emitter was fabricated and characterized. Though the THz generation inside the semiconductor is intense for large exit angle, the critical angle resulting from Snell's law limits the emission to the outside. In the tested structure, it was found that the metal mesh changed the boundary condition at the dielectric interface, resulting in an extraction of the emission to the direction beyond the critical angle. Therefore THz emission can be enhanced in a optimized structure. In the tested structure, the semiconductor surface was irradiated with the laser pulse through the metal mesh resulting in a decreased excitation power. Still a sufficient emission was observed. Thus, in the structures with a transparent metal mesh or with the excitation at the back surface, further enhancement of the emission can be achieved resulting in a highly efficient emission.Another candidate for a THz emitter is a quantum structure. However, the surface passivation is an important issue in an actual use. In this work, a surface passivation method was optimized for GaAs/AlGaAs quantum structures, resulting in an improvement of the device performance.One of the important issues for developing and improving the semiconductor THz emitter is to control the surfaces and interfaces of semiconductors appropriately.