Theory of terahertz generation by optical rectification

Abstract

Intense pulses of light with wavelengths approximately ten times smaller than microwave sources and a hundred times larger than optical/near infra-red sources may be categorized as high-field Terahertz (THz) sources. By virtue of their large electromagnetic field amplitudes and relatively long wavelengths, they are uniquely amenable for electron acceleration, coherent X-ray generation and non-linear spectroscopy. Intra-pulse difference frequency generation or optical rectification of ultrafast optical pump pulses in nonlinear crystals has emerged as the most efficient approach for high-field THz generation. Earlier theoretical treatment of these systems had predicted conversion efficiencies as high as 10%, which opened up the possibility of generating THz pulses with energies on the order of 10 milli joules on a table-top. However, experimental demonstrations have achieved conversion efficiencies of only a few percent which motivates a re-examination of the existing theory. In this thesis, we re-formulate the problem by accounting for effects, previously not considered. These include: (i) the spatio-temporal distortions of ultrafast pulses, (ii) the nonlinear coupled interaction of optical and THz radiation in two spatial dimensions (2-D), (iii) self-phase modulation, (iv) stimulated Raman scattering and (v) crystal geometry. The key finding is that THz generation necessarily leads to broadening of the optical pump spectrum, resulting in a rapid spatio-temporal break-up of the pulse which limits further generation of THz radiation. Due to this self-limiting mechanism, the predicted conversion efficiencies reduce significantly in relation to earlier predictions, which is in line with experimental trends. Guidelines to optimize conversion efficiency and their ramifications on spatial and spectral properties of THz radiation are discussed. The predictions and analyses are supported by experiments. These findings direct future work towards careful engineering of such systems to achieve optimal THz pulse properties and the conception of approaches to circumvent the aforementioned self-limiting effects.