| Abstract [eng] |
Nonlinear light-matter interactions via random-quasi phase matching in polycrystalline materials featuring quadratic nonlinearity offer unique opportunities for ultrabroadband frequency conversion without a need of any temperature or angle tuning. Random quasi-phase matching, stemming from the orientation disorder of tiny crystallites, enables efficient broadband frequency conversion through second-order nonlinear interactions. The latter frequency conversion is limited only by the transparency window of material. The efficiency of broadband frequency conversion could be significantly increased in the beam filamentation regime, where the interplay between self-focusing, multiphoton absorption and diffraction results in the intensity clamping of the ultrashort pulse to a high value. Moreover, in the spectral domain, beam filamentation results in a large-scale spectral broadening, termed supercontinuum generation. A particularly interesting case of filamentation-assisted frequency conversion refers to filamentation in polycrystalline zinc blende semiconductors, such as zinc selenide (ZnSe) and zinc sulfide (ZnS), which exhibit wide transparency range, extending from the visible to the far infrared. The aim of this work is to study the femtosecond mid-IR pulse filamentation-assisted even and odd harmonics generation in polycrystalline ZnSe. Experiments were performed using femtosecond optical parametric amplifier (Orpheus, Light Conversion, Ltd.), operating at 10 kHz pulse repetition rate. First, it was confirmed that efficient generation of even and odd harmonics in ZnSe stems from polycrystalline structure of this material, which provides random quasi phase-matching for multiple three-wave mixing processes. Second, angle-resolved harmonics spectra, exhibit distinct fine-structure consisting of multiple quasi-periodic peaks, which is an indication of incoherent light nature and carries information on crystallite grain size and its distribution. Third, polarization analysis revealed that fundamental harmonics at the output of the sample has linear (original) polarization, attesting no apparent back conversion from harmonic field due to temporal pulse separation which is governed by the group velocity mismatch. |
