Abstract
A model for terahertz (THz) generation by optical rectification using tilted-pulse-fronts is developed. It simultaneously accounts for in two spatial dimensions (2-D) (i) the spatio-temporal variations of the optical pump pulse imparted by the tilted-pulse-front setup, (ii) the nonlinear coupled interaction of THz and optical radiation, (iii) self-phase modulation and (iv) stimulated Raman scattering. The model is validated by quantitative agreement with experiments and analytic calculations. We show that the optical pump beam is significantly broadened in the transverse-momentum (kx) domain as a consequence of its spectral broadening due to THz generation. In the presence of this large frequency and transverse-momentum (or angular) spread, group velocity dispersion causes a spatio-temporal break-up of the optical pump pulse which inhibits further THz generation. The implications of these effects on energy scaling and optimization of optical-to-THz conversion efficiency are discussed. This suggests the use of optical pump pulses with elliptical beam profiles for large optical pump energies. Furthermore, it is seen that optimization of the setup is highly dependent on optical pump conditions. Trade-offs in optimizing the optical-to-THz conversion efficiency on the spatial and spectral properties of THz radiation are discussed to guide the development of such sources.
Highlights
Terahertz (THz) sources are characterized by wavelengths roughly hundred times larger than optical and ten times smaller than radio frequency sources
We present the formulation of a 2-D model which simultaneously considers the spatio-temporal variations of the optical pump pulse, cascading effects, SPM and stimulated Raman scattering (SRS), angular and material dispersion, THz absorption as well as geometry of the nonlinear crystal
There is an optimal h and win for each optical pump condition. (a) For h=1.5 mm and win=2.5 mm there is minimal absorption and conversion efficiency is 0.7% (b) for h=4.5 mm absorption increases and conversion efficiency drops to 0.3 % (c) for large win=10 mm, only small portions of the beam are involved in THz generation due to disruption of phasematching by enhanced dispersive effects in the presence of cascading effects, leading to an overall drop in conversion efficiency to 0.4 %
Summary
Terahertz (THz) sources are characterized by wavelengths roughly hundred times larger than optical and ten times smaller than radio frequency sources. We present the formulation of a 2-D model which simultaneously considers the spatio-temporal variations of the optical pump pulse, cascading effects, SPM and stimulated Raman scattering (SRS), angular and material dispersion, THz absorption as well as geometry of the nonlinear crystal. It can account for the effects of finite beam size, spatial walk-off, spatial frequency variations and beam propagation which is not possible with our previous 1-D formulation [22]. It makes it convenient to include the transmission of THz radiation at the crystal boundary
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