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

结果表明，具有合适结构的金属网在太赫兹波段具有较高的透过率。特别是，厚度远小于发射波长的滤光器不具有截止特性，从而即使在倾斜入射的情况下也会产生高透射率。根据模拟和实验结果，发现表面等离子体激元(SPP)对传输特性有贡献。然后，有可能通过在半导体表面附加一个存在SPP模的金属网格来修改THz发射的边界条件。实际上，我们制作了一种新的THz发射体的基本结构并进行了表征。当出射角度较大时，半导体内部的太赫兹辐射强度较大，但斯内尔定律所产生的临界角限制了向外的发射。在测试结构中，发现金属网改变了介电界面的边界条件，导致发射被提取到超过临界角的方向。因此，可以在优化的结构中增强太赫兹发射。在被测结构中，激光脉冲通过金属网照射半导体表面，导致激发功率降低。尽管如此，还是观察到了足够的排放。因此，在具有透明金属网或背面激发的结构中，可以进一步增强发射，从而获得高效率的发射。另一种选择是量子结构。然而，表面钝化是实际使用中的一个重要问题。本工作优化了一种适用于GaAs/AlGaAs量子结构的表面钝化方法，从而提高了器件的性能。开发和改进半导体THz发射体的一个重要问题是对半导体的表面和界面进行适当的控制。

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.