Room Temperature Integrated Terahertz Emitters based on Three-Wave Mixing in Semiconductor Microcylinders

In the last years, the Terahertz (THz) domain has attracted an increasing interest in the scientific community due to the large number of applications that have been identified (Tanouchi, 2007). Even if many different Terahertz sources like photomixers, quantum cascade lasers, and photoconductive antennas (Mittleman, 2003) have been investigated in the past, the fabrication of a compact device operating at room temperature and with an output power at least in the ┤W range still constitutes a challenge. A very promising approach to this problem relies on the nonlinear optical process called Difference Frequency Generation (DFG) in materials like III-V semiconductors (Boyd, 2003). In this chapter, we will propose an efficient, compact, and room-temperature THz emitter based on DFG in semiconductor microcylinders. These are whispering gallery mode (WGM) resonators capable to provide both strong spatial confinement and ultra-high quality factors. Nonlinear optics applications benefit from an ultra-high-Q cavity, since the fields involved in the nonlinear mixing interact for a long time, giving rise to an efficient conversion. The structure we investigate is based on the technology of GaAs, owing to its wide transparency range (between about 0.9 and 17 ┤m), large refractive index for strong field confinement, and a huge nonlinear coefficient. Moreover, it offers attracting possibilities in terms of optoelectronic integration and electrical pumping. After an introductory part about whispering gallery modes, we will present the study of the DFG inside GaAs microcylinders. The evanescent coupling with an external waveguide allows a selective excitation of the pump cavity modes. At the end, on the theoretical premises of the first part, we will show that an appealingly simple structure can be used to confine both infrared and THz modes. Moreover, embedding self-assembled quantum dots in the cavity allows the integration of the pump sources into the device. With an appropriate choice of the cylinder radius, it is possible to phase match two WGMs with a THz mode, and have a compact, room-temperature THz emitter suitable for electrical pumping.