This highly interesting paper provides excellent experimental agreement with the authors’ accompanying theoretical modelling of the extensive spectral coverage achieved in short, bulk samples of transition metal (Cr²⁺) doped polycrystalline zinc sulphide, effectively covering the spectral range from about 400 nm to 4 µm which was also accompanied by a lower level continuum that extended beyond 15 µm. Pumped by a continuous-wave Er-fiber laser (~17 W) and seeded by the few cycle, ~0.5 MW pulses from a Cr:ZnS mode-locked laser around 2.4 µm, in addition to spectral broadening due to χ(3) processes, intra-pulse three-wave-mixing and cascaded quadratic nonlinearities also contribute to the overall broadening. The broad bandwidth of the random quasi-phase matched process in ZnS (or ZnSe) also enabled the conversion of the mid-IR spectrum to the second and fourth harmonics, while the long-wavelength, lower level MIR signal was attributed to intra-pulse difference frequency generation. Thermally induced optical waveguiding was shown to be vital to the supercontinuum process, and it was concluded that without it, supercontinuum generation could not proceed over the short (~9 mm) crystal lengths deployed. The authors also interestingly note that the long wavelength component may exhibit inherited coherence characteristics from the fundamental that could possibly allow pulse compression.

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