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Researchers Achieve Phase Matching via Bandgap Widening in Infrared Nonlinear Optical Materials

Jul 10, 2020

Nonlinear optical (NLO) crystals have the frequency conversion capability that are important in the national defense and civil applications. A great number of Infrared (IR) NLO materials with high NLO coefficient (dij) have been explored, but many of them suffer from non-phase matching (PM), which dramatically decreases their ultimate NLO output efficiencies. 

In a study published in CCS Chem., a research group led by Prof. GUO Guocong from Fujian Institute of Research on the Structure of Matter (FJIRSM) of the Chinese Academy of Sciences has developed a new strategy involving modulating normal dispersion via bandgap (Eg) widening to achieve PM.  

Three new wide Eg (4.0 eV) IR NLO salt-inclusion sulfides [ABa3Cl2][Ga5S10] (A= K, Rb, and Cs) with PM behaviors were successfully synthesized. Typically, the wider the Eg, the flatter the normal dispersion curves in the mid- and far-IR regions.  

Compared with isomorphism selenides, dispersion curves of wider Eg sulfides are smoother in the IR regions, which make the PM cutoff wavelength blue-shift from 5.7 to 1.7 um, leading to the PM ability at the traditional fundamental frequency laser (2.0 um). These results provide a new design idea to obtain new PM IR NLO crystal materials.  

Moreover, [ABa3Cl2][Ga5S10] (A= K, Rb, and Cs) possessed large second-harmonic-generation (SHG) efficiency (1×AgGaS2) and hitherto the highest laser-induced damage thresholds (188–200 MW/cm2, at 1.06 μm, 10 ns pulse width) for known chalcogenides with second-harmonic generation (SHG) efficiencies comparable to that of commercial AgGaS2 crystal. These merits together with wide transparent window (0.4-12.3 μm) indicated that they are promising candidates as IR NLO materials. 

This study provides an effective way to explore new PM IR NLO materials based on the non-PM ones with good SHG efficiency and laser damage threshold (LDT). 

   

Schematic illustration of crystal structure and millimeter-level crystal wafer (Image by Prof. GUO’s group) 

Phase Matching Achieved by Band Gap Widening in Infrared Nonlinear Optical Materials [ABa3Cl2][Ga5S10] (A= K, Rb, Cs)

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