Study of nonlinear absorption in a dye doped polymer film due to frequency up-converted fluorescence

Nonlinear absorption study is performed in 4-[4-(Dimethylamino)styryl]-1- docosyl pyridinium bromide (DASPB) doped in PAMA-PA polymer film at linear absorbing region using cw, nanosecond and picosecond 532 nm laser beams and in two photon absorbing region using pico-second 800 nm, 900 nm and 1,000 nm laser beams. The linear transmission of dye in PMMA-PA film was about 10 %. The nonlinear absorption properties leading to optical limiting using 532 nm pico second and nano second pulses are studied. From the results, it can be noted that at very low input intensity, linear transmission is preserved, with increase in input intensity DASPB starts to contribute to saturation absorption which increases the transmission above linear level, and at higher intensities the excited state frequency up-converted fluorescence (nonlinear absorption) starts to contribute and it dominates with further increase in intensity. This kind of behavior has interesting applications in optical limiting and optical switching