Frequency conversion in aperiodic quasi-phase-matched structures

Abstract

The influence of disordering of one-dimensional quasi-phase-matched structures in nonlinear-optical crystals on the spectral characteristics of parametric processes is studied in the general form, taking into consideration arbitrary spatial distribution of the second-order susceptibility, sum- and difference-frequency interactions in the forward and backward directions, and continuous-wave and pulsed pumping. It is shown that, at the initial low-gain stages of all processes, the impact of the medium spatial structure can be described in terms of a single functional parameter, nonlinear transfer function (T-function). Expressed in terms of dimensionless phase mismatches, the T-function is considered as a universal characteristic of the structured medium. The method of its direct measurement is proposed, based on spontaneous parametric down-conversion. Examples of the T-function approach are demonstrated for short-pulse second harmonic generation, as well as for generation and detection of spectral-shaped terahertz signals.