Self- and Cross-Phase-Modulation Coefficients in KDP Crystals Measured by a Z-Scan Technique

LLE Review, Volume 74 125 In the interaction of strong fields with matter, considerable interest has been shown in the development of high-efficiency frequency up-conversion of ultrashort laser pulses. One important area of interest is in ultrafast laser–solid interactions where up-conversion can lead to higher absorption due to higher-density interactions1 and to an enhancement of the pulse-intensity contrast by many orders of magnitude, allowing the high-intensity pulse to interact with the solid-density material.2 Efficient second-harmonic generation in KDP has been reported for ultrafast laser beams at intensities up to 400 GW/cm2.3 In this intensity region, nonlinear effects such as selfand cross-phase modulation4,5 (SPM, XPM) originating from third-order nonlinear susceptibility χ(3) may limit the efficiency of ultra-intense frequency-conversion processes that involve co-propagation of two beams with different wavelengths. SPM and XPM are responsible for spectral broadening in optical fibers and have been used in pulse compression to produce ultrashort laser pulses.6 XPM has been observed in fiber Raman soliton lasers7,8 and has proven to be important in optical parametric oscillators, optical parametric amplifiers,9 and the harmonic-generation process in bulk nonlinear crystals.10,11 Nonlinear phase changes can destroy the phase coherence required for efficient conversion. Z-scan,12,13 four-wave mixing,14,15 ellipse rotation,16 and nonlinear interferometer17,18 techniques have been used to measure the nonlinear refractive index n n n 2 2 3 3 8 = [ ] ( ) / χ associated with SPM. Nonlinear refractive index coefficients associated with SPM in KDP crystals, which are widely used in frequency conversion,19,3 were measured by degenerate three-wave mixing20 and timeresolved interferomery21 at 1 μm. In this article we report on the results of the single-beam Z-scan measurement12,13 of the nonlinear refractive index associated with SPM at wavelengths of 1.053 μm, 0.527 μm, and 0.351 μm and two-color Z-scan22 to measure the nonlinear coefficients of XPM between 1.053 μm and 0.527 μm in a KDP crystal. In the two-color Zscan measurement, two collinear beams with different wavelengths are used; a weak probe beam can be defocused by the action of the strong pump beam in a thin sample.22 The far-field intensity variation is used to determine the optical nonlinearity Selfand Cross-Phase-Modulation Coefficients in KDP Crystals Measured by a Z-Scan Technique