Compact and Tunable Room Temperature THz Source from Quantum Dot Based Ultrafast Photoconductive Antennae

Abstract

Novel materials, notably quantum-dot (QD) semiconductor structures, offer the unique possibility of combining exploitable spectral broadening of both gain and absorption with ultrafast carrier dynamic properties. Thanks to these characteristics, QD-based devices have enhanced the properties of CW devices as well as the development of compact ultrashort pulse lasers and opened up new possibilities in ultrafast science and technology. In this paper we review recent progress in generation of CW and pulsed THz radiation from QD based photoconductive antennae (PCA) pumped by ultrafast and dual wavelength semiconductor lasers. By engineering the design of the QD structure, effective pump wavelengths can be tuned in the range between 0.9-1.3 μm ZKLFK LV ZHOO beyond the GaAs energies, hence compact and relatively cheap ultrafast and narrow line double-wavelength semiconductor and fibre pump lasers can be used for pumping such antennae for both pulsed and CW THz generation. However, antennae possess a low coefficient of optical-to-terahertz conversion due to the carrier screening effect and low quantum efficiency. To overcome these limitations, an optical nano-antennae technique can be employed. Such nano-antennae can be used to enhance the electric field and increase the absorption cross section in the active layers of the photoconductive antenna. We present our recent results on enhancement of THz generation in QD based log-periodic PCA with silver nano-antennae embedded in the antenna gap. Our first results demonstrated that using silver spheroid nano-antennae fabricated by a relatively simple method, can increase the coefficient of optical-to-terahertz conversion up to 4 times. In conclusion the development of an ultra-compact, efficient, room temperature THz source is possible. The inclusion of multiple bandgap-engineered semiconductor materials and quantum-confined structures enables additional pump absorption energy ranges and ultrafast charge carrier dynamics, crucial in the efficient generation of THz radiation.