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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.

The operation of Ultrafast Electron Diffractometers (UEDs) and Free-Electron Lasers (FELs) relies on high-brightness electron beams produced by radio-frequency (RF) photocathode guns. The next generation of high-repetition rate UEDs and FELs requires electron beams with a high average brightness. To this end, we introduce a continuous wave RF photocathode gun at 325 MHz with an APEX-like geometry. The gun allows for the production of electron beams with very high both peak and average 5D brightness while having moderate RF power consumption. The gun is operated in blowout regime with an energy gain of 0.4 MeV and a peak cathode field of 35 MV/m. Via massive numerical simulations, we exemplify three regimes of the gun operation: (i) 160 fC electron beams with a 5-nm-scale emittance for UEDs, (ii) 1.6 pC beams with a 20-nm-scale emittance for table-top FELs and dielectric-based accelerators, and (iii) 16 pC beams with a 50-nm-scale emittance for inverse Compton sources and other accelerator-based photon sources. We introduce a simple analytical model for the formation of the virtual cathode - the onset of the suppression of photoemission current due to space-charge forces. The model accounts for the laser pulse duration. Furthermore, our extensive numerical simulations indicate a well-pronounced maximum in the 5D beam brightness for the laser spot radius approximately 150% of that corresponding to the onset of the virtual cathode. The finding does not support the common approach in the literature that in the blowout regime the laser spot radius must be much larger than the critical radius corresponding to the virtual cathode onset., 9 pages, 5 figures