Fig. 3

a Schematic diagram of the experimental setup. Here, the mid-IR pump beam, being at a horizontal polarization state (namely, extraordinary light state) is incident upon the xz surface of the z-cut CPPLN crystal. b Spectral broadening for the pump laser pulse with central wavelength 3601 nm passing through a bare LN crystal. c Photograph of a dazzling output white-laser spot emitted from the designed CCPLN sample (with a dimension in length, width, and thickness of 20 mm × 6 mm × 2 mm) under the pump of the mid-IR femtosecond laser. d Normalized spectrum of the output HHG signals generated by mid-IR femtosecond laser via synergic HHG and SPM effects as estimated by a criterion of − 25 dB. Note that the spectrum has been normalized against the maximum value. e-f The numerical simulations of SPM spectral broadening of the mid-IR pump laser and 2nd-10th HHG pulses in the NLSE model. The light-colors filled areas represent the SPM broadened curves corresponding to their original laser signals (marked by darker colors)