Abstract

High harmonic generation (HHG) is a nonlinear process in which odd multiples of a fundamental driving field are produced when an intense laser pulse is focused into a low density gas. HHG is an attractive source of coherent, tuneable light with wavelengths in the XUV and soft X-ray range and has found a broad range of applications in ultrafast physics and imaging. However, due to the phase mismatch between the driving and generated fields, the intensity of the generated harmonics oscillates as a function of propagation distance, z, between 0 and some maximum value. The period of oscillation is 2L_c where L_c is the coherence length. One way of overcoming the problem of phase-mismatching is quasi-phase matching (QPM), in which harmonic generation is suppressed or modified in out-of phase regions. Here we propose a new class of QPM techniques by controlling the polarization in an optical waveguide by either: (i) rotating the polarization of the driving field in a circularly birefringent system [1]; or (ii) modulating the ellipticity of the driver polarization in a linearly birefringent system [2]. The key advantage of this new class of polarization-controlled QPM is its simplicity compared to other QPM techniques.

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