Optical Properties of Quantum-Confined Semiconductor Structures Driven by Strong Terahertz Fields

Abstract

The development of femtosecond pulse lasers is one of the key breakthroughs in the field of terahertz (THz) technology. THz radiation (also called T-rays) lies in the frequency gap between the infrared and microwaves, loosely referred to the frequencies from 300 GHz to 30 THz. THz radiation has long been used in astronomy and analytical science. Despite great scientific interest, however, the THz frequency range remains one of the least developed regions of the electromagnetic spectrum, due to relative lack of convenient radiation sources, detectors and transmission technology. Thanks to the development of femtosecond pulse lasers, THz research has come into the center stage during the last quarter century. Most milestones in the development of THz technology, such as THz time-domain spectroscopy and THz pulse imaging, rely on the generation of THz pulse by employing high-power femtosecond laser pulse. In parallel to the development of THz pulsed sources, there has been a rapid expansion in developing continuous or quasi-continuous THz wave sources, such as THz free-electron lasers, photomixers, and quantum cascade lasers. High-power and frequency-tunable THz free-electron lasers have been successfully applied in scientific research into optical and transport properties in different semiconductor structures. The strong THz ac fields can coherently modify the optical properties of semiconductors. This modulation of optical properties is an essential ingredient for advanced optoelectronic devices. This chapter reviews two topics about THz radiation and is organized as follows. In section 2, the generation of THz pulses by exploiting femtosecond laser pulses to excite photoconductive antennas, nonlinear optical crystals, and quantum-confined structures is introduced, with emphasis on the large-aperture photoconductive antennas. In section 3, the fundamental theory about optical absorption and formulations of semiconductor Bloch equations (SBEs) are presented, which have been used to investigate the optical response of semiconductor structures pumped by intense THz radiation and probed by an infrared pulse. The modulated optical properties in quantum-confined semiconductor structures driven by strong THz fields are discussed in section 4. Finally, a brief summary is given in the last section. The contents of the chapter are based on the work by the authors’ research group at Xi’an Institute of Optics and Precision Mechanics, Chinese Academy of Sciences, but some 14