Fig. 1. Schematic of one-dimensional aperiodic materials. (a) Chirped crystal; (b) quasi-periodic crystal

Fig. 2. SHG in two-dimensional random SBN material. (a) Schematic; (b) experimental photograph (arrow indicates direction of fundamental beam)^[20]

Fig. 3. Spectral broadening of incident femtosecond pulse at 2.4 μm central wavelength in 5-mm-long polycrystalline ZnSe via RQPM frequency mixing^[26]

Fig. 4. Harmonic and supercontinuum generation in polycrystalline Cr∶ZnS femtosecond laser. (a) 1^st to 4^th harmonic of fundamental beam; (b) supercontinuum spanning from 1.8 μm to 4.5 μm^[28]

Fig. 5. OPO based on polycrystalline ZnSe. (a)Schematic of device with photo of ZnSe placed at the Brewster angle shown in bottom right; (b) broadband output spectrum spanning from 3 μm to 7.5 μm; (c) cavity length detuning^[10]

Fig. 6. Schematic of laser beam with certain cross section passing through polycrystalline material. (a) Main view; (b) side view

Fig. 7. Schematic of random rotation transformation between reference coordinate and crystal coordinate^[38]

Fig. 8. Effective nonlinear coefficient distribution of crystal grains after random rotation of one million times. (a) SHG with polarization parallel to that of fundamental beam; (b) SHG with polarization perpendicular to that of fundamental beam^[10]

Fig. 9. Statistical result of SHG power of polycrystalline nSe with different lengths^[10]

Fig. 10. Grain-size distributions of ZnSe under different treatment methods. (a) Original CVD ZnSe; (b) treatment in 850 ℃ Se vapor for 12 h; (c) treatment in 850 ℃ Zn vapor for 168 h; (c) treatment in 850 ℃ vacuum for 168 h^[42]

Fig. 11. (Left) Polycrystalline material generated by Neper and (right) its meshed morphology

Fig. 12. Statistical properties of grains generated by Neper software. (a) Grain size; (b) sphericity