In high-energy physics, the structure of matter is explored by accelerating and colliding elementary particles like electrons and protons. In condensed matter physics, the fundamental excitations are called quasi-particles. The most familiar quasi-particles are electrons and holes in semiconductors, which can be created for example in a solar photovoltaic cell - by light with a sufficiently short wavelength. In this project, electrons and holes will be created by a weak near-infrared laser with a wavelength slightly longer than is visible to the human eye, and will be made to accelerate and then recollide with one another by a very strong electric field oscillating nearly 1 trillion times per second (1 Terahertz). The recollision process will be studied by analyzing the spectrum (which wavelengths are present) in the transmitted near-infrared light. This spectrum has been shown to contain up to 18 separate nearinfrared wavelengths, or sidebands, in addition to the wavelength of the near-infrared laser that creates electron-hole pairs. This research will elucidate how much quasiparticles can be accelerated without being disturbed by defects or the motion of atoms in their host material. The proposed research may lead to faster and more energy efficient optical communications and internet, and improved optical clocks that are necessary in the global positioning system. This project will support the training of two Ph. D. students, who will learn a variety of skills that are critical to preserving U. S. competitiveness in the high-technology sector.
High-order sideband generation, a new phenomenon in the interaction of light with matter, was recently discovered in the PI's research group. A relatively weak, continuous-wave near-infrared (NIR) laser at frequency ~350 THz, and an intense laser at frequency ~0.5 THz are incident on a thin film of semiconductor. A comb of equally-spaced sidebands is emitted, with sharp lines at sideband frequency = NIR frequency + 2n THz frequency, where n is an integer. Combs with up to 14 sidebands (order up to 2*14=28) above NIR frequency have been observed. The high-order sidebands can be understood in terms of a semiclassical model similar to one that was first introduced to explain high-order harmonic generation, an analogous phenomenon that occurs for atoms in intense laser fields. In high-order-sideband generation (HSG), the NIR laser creates excitons, bound electron-hole pairs. The strong THz field ionizes the excitons, and accelerates the resulting electron and hole into a large-amplitude oscillation. When the electron and hole recollide, the excess kinetic energy is carried off in sidebands above the NIR frequency. This project will explore the onset of high-order sideband generation, whether there is a fundamental limit on the number of observable sidebands, whether the shape of the sideband spectrum can be controlled, and whether, in the case of a circularly-polarized terahertz field, the polarization of the near-ir radiation is rotated. By exploring the limits of HSG, the proposed research will elucidate potential applications of HSG to electro-optic technologies ranging from optical communications to optical clocks. This project will support the training of two Ph.D. students, who will learn a variety of skills including near-ir and terahertz optics, cryogenics, electronics, computer programming, and mechanical and optomechanical design.
