Quantum Coherence and Dynamical Instability in Quantum Wells Driven by Intense Terahertz Fields.

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****NON-TECHNICAL ABSTRACT****Beginning when early humans harnessed fire for heat and light, the control of electromagnetic radiation has been central to the development of our species. The notion of electromagnetic radiation is nearly 150 years old, proposed by Maxwell in 1865 and demonstrated with the discovery of radio waves in 1866. Radio waves remained largely a laboratory curiosity for nearly 50 years. It is difficult to imagine modern life without radio waves, microwaves, heat, light, and X-rays, which are now all understood to be manifestations of electromagnetic radiation, listed in order of increasing frequency. However, lying between the frequencies of microwaves and heat, stretching from 0.1 to 10 trillion cycles per second (0.1-10 terahertz) is the so-called 'terahertz gap.' Electromagnetic waves exist in this frequency range, but they are extremely difficult to generate and control. This individual investigator award supports a project that will use the world's brightest pulses of terahertz waves, generated by accelerator-driven 'free-electron lasers', to search for new quantum-mechanical phenomena predicted to occur in nanometers-thick semiconductor devices. The semiconductor devices under study are similar to those used to modulate light in fiber-optic communications, and as ultrafast transistors in cellular telephones. This project will support the education of two PhD students, as well as undergraduate and high-school interns. The students will learn the most advanced techniques to generate and manipulate electromagnetic radiation across the electromagnetic spectrum, preparing them for leadership in the nation's scientific and technological workforce, and bringing mankind closer to harnessing terahertz radiation for future technologies. ****TECHNICAL ABSTRACT****Strong oscillating electric fields with frequencies between 1 and 5 THz are increasingly available in laboratories and even inside chip-scale devices like terahertz quantum cascade lasers. A growing body of theory predicts that fascinating and potentially useful phenomena will occur in semiconductor quantum wells driven by strong THz fields. For example, calculations based on commonly used approximations for many-electron systems predict that chaotic dynamics can occur in doped quantum wells for experimentally-achievable conditions. The observation of chaotic dynamics in an experiment on a manifestly quantum system would be very surprising, since chaos arises from nonlinearities while quantum mechanics is a linear theory. This project will support experimental searches for precursors of chaotic dynamics in doped quantum wells. The intense terahertz radiation necessary for these experiments will be generated by free-electron lasers at UC Santa Barbara and in Dresden, Germany. This project will also support studies of how strong terahertz radiation changes the quantum states and band structure of semiconductors and semiconductor quantum wells, as measured by terahertz-induced changes in the near-infrared absorption and emission. Two PhD students will receive deep and broad training in semiconductor physics, nanofabrication, optics at terahertz, near-IR and visible frequencies, mechanical design, cryogenics, electronics, and computer control of laboratory instrumentation, as well as in international collaboration, preparing them for leadership in the nation's science and technology workforce. The supported PhD students will also mentor a diverse group of undergraduate researchers and high-school student interns, sparking their interest in pursuing careers in science or engineering.