IR Bandwidth and Crystal Thickness Effects on THG Efficiency and Temporal Shaping of Quasi-Rectangular UV Pulses: Part II - Incident IR Ripple

Abstract

We have investigated the effect of incident ir spectral bandwidth and crystal thickness, on uv pulses produced by third harmonic generation (THG) in a crystal pair. Our focus is on the third harmonic generation efficiency and longitudinal uv intensity profile parameters of SNLO predictions that are evaluated for three incident ir spectral bandwidths and a range of crystal thicknesses. These results represent a continuation of earlier work in which the effects of the same selected ir bandwidths and range of crystal thicknesses were investigated using a pair of BBO Type I crystals in a simplistic geometry for which the longitudinal intensity plateau has a zero slope, 'flattop' profile. The current work is distinguished from the previous work by an imposed ripple on the ir intensity longitudinal profile and constitutes a portion of a Part II effort to which we have made reference. As with preceding work, all third harmonic data are net results at the exit of the second BBO crystal. Predictions are obtained with the modified SNLO code developed by Arlee Smith at the Sandia National Laboratories. This modification has allowed us to pursue the 'coupled' case in which the output of the first BBO crystal is used asmore » input to the second one. This includes both the fundamental and second harmonic light. Defined parameters are consistent with previous work. The presented cases are best results. The criteria for selection of these reported cases are highest THG efficiency combined with minimum intensity ripple in the plateau. The incident ir pulse is quasi-rectangular with an imposed 5.2 % (rms) intensity ripple added to the plateau. The ir pulse bandwidth is centered at 800 nm. Second harmonic generation occurs in the first BBO crystal and THG occurs in the second crystal as a consequence of sum frequency generation. Type I phase matching is used throughout, so that for a negative uniaxial crystal: n{sub 2}{sup e}({theta}) = n{sub 1}{sup o};(SHG) (1.1) 3n{sub 3}{sup 3}({theta}) = 2n{sub 2}{sup o} + n{sub 1}{sup o};(THG) where n{sub 2}{sup e}({theta}) and n{sub 3}{sup e}({theta}) are the angle dependent extraordinary refractive indices for the second and third harmonics respectively, and n{sub 1}{sup 1} and n{sub 2}{sup o} are the ordinary refractive indices for the fundamental and second harmonic respectively. Although our goal at this stage has not been to comply with all the LCLS injector laser specifications, the results provided here represent a parameter study that can be used to determine candidate bandwidth dependent, crystal thickness combinations for the detailed design of compliant THG subsystems. This simplistic geometry better elucidates acceptance bandwidth limitations that are intrinsic to the crystal material.« less