Enhanced ultrafast nonlinear absorption and optical limiting of indolium squaraine for laser protection

The ultrafast optical nonlinear properties of an indolium squarine 1,3-bis[(3,3-dimethylindolin-2-ylidene) methyl]squaraine (ISQ) are systematically investigated via multiple pulse-width Z-scan technique and femtosecond transient absorption spectrum. The results indicate that the sample exhibits superior excited-state absorption which mainly originates from the singlet excited-state. The relatively long lifetime measured in ISQ provides efficient excitons accumulation on excited-state, making it suitable for optical limiting (OL) under various pulse duration. The nonlinear absorption coefficient of ISQ ranges from similar to 10(-8) m/W with nanosecond (ns) pulse to similar to 10(-12) m/W with femtosecond (fs) pulse. ISQ film is also prepared via doping ISQ into polymethyl methacrylate, the OL thresholds of ISQ solution and ISQ film are measured with 532 nm ns pulse. The OL threshold of ISQ/toluene is 25.5 mJ/cm(2), and that of the film is 30.6 mJ/cm(2), which basically retain the OL performance of solution. Our results indicate that indolium squaraine could be a potential candidate for future application in laser protection.

Automated summary

The ultrafast optical nonlinear properties of an indolium squarine have been investigated, and it shows excellent excited-state absorption suitable for optical limiting under different pulse durations. The ISQ film also exhibits similar optical limiting performance to the solution, suggesting it could be a potential candidate for laser protection.