Effect of two-beam coupling in strong-field optical pump-probe experiments

TL;DR

This paper investigates how two-beam coupling affects phase shifts in strong-field pump-probe experiments, highlighting the role of nonlinear refractive index gratings and comparing theoretical models with experimental results in gases.

Contribution

It provides a comprehensive theoretical framework for two-beam coupling effects in pump-probe experiments, including delayed nonlinearities and experimental validation in noble gases.

Findings

Enhanced phase shift due to diffraction effects

Negative birefringence at high intensity explained by plasma grating

Theoretical results agree with recent experimental measurements

Abstract

Nonlinear optics experiments measuring phase shifts induced in a weak probe pulse by a strong pump pulse must account for coherent effects that only occur when the pump and probe pulses are temporally overlapped. It is well known that a weak probe beam experiences a greater phase shift from a strong pump beam than the pump beam induces on itself. The physical mechanism behind the enhanced phase shift is diffraction of pump light into the probe direction by a nonlinear refractive index grating produced by interference between the two beams. For an instantaneous third-order response, the effect of the grating is to simply double the probe phase shift, but when delayed nonlinearities are considered, the effect is more complex. A comprehensive treatment is given for both degenerate and nondegenerate pump-probe experiments in noble and diatomic gases. Results of numerical calculations are compared to a recent transient birefringence measurement [Loriot et al., Opt. Express 17, 13429 (2009)] and a recent spectral interferometry experiment [Wahlstrand et al., Phys. Rev. A 85, 043820 (2012)]. We also present results from two new experiments using spectrally-resolved transient birefringence with 800 nm pulses in Ar and air and degenerate chirped pulse spectral interferometry in Ar. Both experiments support the interpretation of the negative birefringence at high intensity as arising from a plasma grating.