Abstract
Near- and far-field beam profiles were measured for THz pulses generated in LiNbO3 by optical rectification of 200 fs pulses with a tilted pulse front. The variation of the THz beam size and a dramatically increasing divergence angle with increasing pump fluence were observed in the (horizontal) plane of the pulse front tilt. No significant variation was observed in the vertical direction. The reason for the observed nonlinear beam distortion is the shortening of the effective interaction length for THz generation caused by the combined effect of pump spectral broadening and angular dispersion in the tilted pulse front geometry. Our results indicate that nonlinear THz beam distortion effects have to be taken into account when designing intense THz sources and related experiments.
Highlights
The development of intense THz sources has gained breathtaking speed in the last few years
Near-field and far-field THz beam profiles and THz divergence angles were measured for a tilted pulse front pumping (TPFP) LN source pumped by 200 fs laser pulses
One-dimensional model calculations revealed the strong decrease of the pump dispersion length and the interaction length for THz generation with increasing pump fluence
Summary
The development of intense THz sources has gained breathtaking speed in the last few years. Primary attention was paid to the generation of ever higher THz pulse energies and field strengths. In the lower frequency part of the THz spectral range (0.1–2 THz), it was optical rectification pumped by femtosecond laser pulses with a tilted pulse front in LiNbO3 (LN) [1] which had the leading role both in THz pulse energy and field strength. More than 0.4 mJ of energy [2] and 1.2 MV cm−1 peak electric field [3] have been reported. Efficiency as high as 3.8% has been reported from a cryogenically cooled LN source [4]. At higher (1–5 THz) frequencies, up to 0.9 mJ energy and 40 MV cm−1 have been reported from organic crystals [5]
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