All-optical terahertz generation from semiconductors: materials and mechanisms

Terahertz frequency (1012 Hz) electromagnetic radiation possesses many exciting cross-disciplinary applications in the fields of astronomy, telecommunications, biology, physics, chemistry, and industry. Recent advancements in the field with the availability of high power sources, sensitive detectors, and efficient scanning systems, make it possible to use the terahertz (THz) radiation very effectively for applications such as biomedical imaging, security screening, and material property analysis. The real-world applications, however, are still limited by the lack of compact and cost effective systems, which can be achieved by having better sources and detectors working in this range. In this thesis, I will focus on the examination of semiconductor materials as potential sources of THz generation without having any need of external bias voltage. Coherent, pulsed THz radiation may be generated under excitation of ultrashort nearinfrared pulses from different semiconductor materials such as InAs, GaAs, InP, GaBiAs, InGaAsN, and ZnTe. The mechanisms behind THz generation from these materials will be discussed in detail. Terahertz time-domain spectroscopy (THz TDS) is used in order to measure both the amplitude and phase of THz generated from these materials. Information about different material parameters can be attained by investigating their response as THz emitters. In the absence of external bias, the THz radiation from a semiconductor sample may be generated due to mechanisms such as optical rectification (OR) and transient current (TC) effects depending on the material properties, incident excitation fluence, and experimental geometry. There are different ways to extract the information about the mechanism responsible for THz generation. These include effect of angular rotation of the crystal, the effect of magnetic field on the generated THz, and the effect of varying the optical fluence of the excitation radiation. Here I will present the general theory of optical rectification for zincblende 43m structures for any arbitrary crystallographic direction, along with experimental results for high-index GaAs (11N) crystal planes for different experimental geometries. Using this, it is possible to extract information about the intrinsic surface field of the material and the contribution of it to the THz generation. External magnetic fields enhance the THz radiation efficiency of transient current emitters. The theory of the effect of in-plane magnetic field rotation on THz generation for TC emitters is presented and the results for the (100) InAs emitter are compared with theory. Heavy noble gas ion irradiation can improve the THz emission efficiency of InP bulk and honeycomb nanoporous samples. Epilayer growth of GaBiAs layers on GaAs substrates increases the surface field contribution in THz emission which in turn increases the overall THz radiation from these samples. On the other hand, epilayer growth with dilute nitride doping in GaAs quenches the overall THz signal with an exponential decay with nitrogen content. The post-growth annealing of the semiconductors up to a critical temperature improves the THz emission efficiency.