Polarized second-harmonic generation with broadband femtosecond pulses

We computationally investigate polarized second-harmonic generation (SHG) of a spectrally broad femtosecond pulse following transmission through traditional quarter wave plates (QWPs). Because the sideband modes of a broadband pulse can interact through sum-frequency generation processes, the SHG responses for several experimentally relevant cases exhibit asymmetries between individual sideband modes, spectral peak shifts, and, critically, artificial chiral signatures. Remarkably, errors in the various sum-frequency sidebands are found to compensate for each other so that the total SHG response approaches the ideal narrowband response. This occurs in the absence of significant axis misalignment in a compound QWP. Hence, our results suggest that polarized femtosecond SHG can be remarkably tolerant against the broad bandwidth of ultrashort pulses. (C) 2007 Optical Society of America.