Abstract

When an intense laser beam is tuned in the transparency region of a generic nonlinear crystal, three components of second (SH) and third (TH) harmonic beams are generated: one backward, in reflection and two forward, in transmission. The first transmitted SH component comes from the homogeneous solution of the nonlinear equations and travels with the group velocity and the absorption coefficient according to material dispersion at the harmonic wavelength. The second component has its origin in the inhomogeneous solution of the nonlinear equation and it propagates into the material with the same phase velocity and absorption coefficient as the fundamental beam, regardless of the dispersion at the harmonic frequency. It is trapped and dragged along in the same direction as the pump and it is known as “phase locked” (PL) harmonic component. These two different components of the SH can only be distinguished when they are induced with short pulses and far from the usual phase-matching condition [1]. These theoretical predictions were experimentally validated first by the measurement of the two forward SH components in transparent materials [2].

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