Theory of terahertz generation by optical rectification

Ravi, Koustuban

Master Thesis

PUBDB-2015-01424

Theory of terahertz generation by optical rectification

Ravi, K.

2014
Cambridge, MA, USA

Cambridge, MA, USA (2014) = Massachusetts Institute of Technology, Masterarbeit, 2014

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 non-linear 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. 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 present a formulation which for the first time simultaneously considers effects of: (i) the spatio-temporal distortions of ultrafast pulses, (ii) the non-linear coupled interaction of optical and THz radiation in two spatial dimensions (2-D), (iii) self-phase modulation and (iv) stimulated Raman scattering. The key finding is that THz generation necessarily leads to broadening of the optical pump spectrum, which ultimately limits further generation of THz radiation. Due to this self-limiting mechanism, it is shown that 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 other THz properties 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.

Keyword(s): Unveröffentlichte Hochschulschrift

Note: Copyright 2014, Massachusetts Institute of Technology. All rights reserved.
Note: Massachusetts Institute of Technology, Masterarbeit, 2014

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