Single-cycle undulator light

The past decade has witnessed a sharp rise of interest in coherent terahertz (THz) light sources for applications in condensed-matter physics. These sources are a powerful tool for studying collective excitations in solid-state systems: THz light can directly couple to low-energy excitations on the meV-scale such as the collective excitations of spins and phonons. Furthermore, coherent excitation of the material spin or phonon subsystem by a THz light pulse allows for tailoring the material’s macroscopic properties. This enables the creation of materials with new dynamic functionalities. To fully exploit the potential of the control of materials’ properties, a new generation of versatile sources of intense short-pulse THz light is needed. This thesis addresses the principles of generation of intense single-cycle THz pulses in an accelerator-based light source. The overarching principle is the phase-locked coherent emission of frequency-chirped waveforms from a specially prepared train of electron bunches inside a tapered undulator. The first part of the thesis (Ch. 1-2) motivates the THz light source development. It surveys the available light sources and scientific applications in the field of low-energy electrodynamics. Looking at a wide selection of THz-induced phenomena, the desired parameters of the proposed undulator-based THz source are determined. The second part (Ch. 3-4) focuses on the technicalities of the accelerator-based THz light source. It addresses the questions of electron beam requirements, photocathode gun performance, dynamics of electrons in an RF gun and in a superconducting linear accelerator. The beam dynamics simulations are carried out and the required characteristics of the electron bunch train are demonstrated. In what follows, the process of waveform-controlled single-cycle emission from an undulator is described: starting from the case of a single electron bunch and then proceeding to the single-cycle emission by the electron bunch train. The last part (Ch. 5) introduces a concept of tunable focusing of THz light. Specifically, it presents the model of an optical magnetic lens, based on a two-dimensional magneto-optical material immersed into a non-uniform magnetic field. To sum up, the formation of the electron bunch train with necessary spatiotemporal properties, single-cycle emission in a matching tapered undulator and tunable focusing of THz light are addressed in the thesis.