THz pulse compression

Abstract

Compact THz-based particle accelerators require Terahertz (THz) pulses of tens of mJ of energy to achieve high acceleration fields of hundreds of MV/m. Despite the promising prospects of scaling THz generation to these energy ranges by non-linear optical processes, their conversion efficiency for high-energy (HE) THz applications is currently too low. Aiming to provide a new way of producing HE THz pulses, this thesis presents the development of a THz pulse compression system by which the signal emitted by a continuous-wave (CW) THz source is enhanced in intensity and compressed in duration into short pulses. For this purpose, a 900 mm long bow-tie enhancement cavity resonant with the Gaussian mode of a 100 GHz frequency CW incident electro-magnetic wave was implemented in a quasi-optical way. Inside the cavity, the intensity of the incident 100 GHz radiation was enhanced a certain number of times expressed through the parameter E. Subsequently, the cavity circulating THz radiation was extracted out in the form of a short pulse by using a laser-driven semiconductor switch. In particular, for a 145-μW cavity input light and a 532-nm, 7-ns, 50-mJ excitation laser pulse, the compression system was performed under two specific scenarios. First, for a E≈17.5 and by using an intrinsic silicon wafer, a maximum power of 940 μW was extracted, corresponding to ~38% of the cavity circulating power. Second, for a E≈6.5 and by utilizing an intrinsic gallium arsenide wafer, a maximum power of 635 μW was extracted, corresponding to ~67% of the cavity circulating power. In both cases, the duration of the extracted pulse was about ~28 ns, distributed in four main oscillations, each of them with a period of around 6~7 ns and different amplitude. These results proved that the magnitude, shape and duration of the extracted pulse were dependent on four factors: the cavity length, the parameter E, the energy of the excitation laser pulse and the switching dynamics of the semiconductor wafer. Based on observations made from the current compression system, two conclusions can be drawn: (i) The length of the resonator needs to be matched to the semiconductor response and the laser driver pulse length used. (ii) The cavity and semiconductor switch size as well as laser energy needed needs to be scaled to a high power THz source such as gyrotrons. Such gyrotron sources have been developed to the multi-MW scale for electron-cyclotron resonance heating of fusion plasmas.