4 -[4 - (Dimethylamino) styryl) - 1 - docosyl pyridinum bromide ](abbreviated as DASPB) is a nonlinear dye which shows strong two-photon absorption (TPA) and subsequent frequency upconversion fluorescence behavior when excited with near infrared radiation. The dye possesses a much larger TPA cross section and much stronger upconversion fluorescence emission than those of common organic dyes (such as rhodamine). In order to increase the concentration of absorptive or fluorescent centers as well as the optochemical and optophysical stability and effective use of this highly nonlinear dye, it has doped in methyl methacrylate - methacrylic acid co-polymer (PMMA-MA). Linear absorption spectrum, single photon induced fluorescence spectrum and two photon induced fluorescence at different wavelength are studied. The nonlinear optical transmission at both linear absorption region and linear transmission region are studied using nano and pico second laser pulses. The optical limiting behavior of the dye is also studied in cw, pico and femto second region. Using Z-scan studies it is found that in linear absorption region, the DASPB doped in PMMA-PA shows saturation absorption at lower intensity and reverse saturated absorption at higher intensity. At transmission region above 800 nm, it has shown the absorption due to two photon induced fluorescence as well as reverse saturation absorption. With experimental results, the molecular TPA cross-sections and TPF cross-section of the sample are calculated.
With the use of high power lasers in varied applications, search for optical limiting materials has become very important to protect the eyes and other sensors. These materials are based on the principle of reverse saturable absorption (RSA) when absorption cross-section for excited state becomes more than the ground state. In such cases transmittance goes down as we increase the intensity. In this paper, we report the experimental results on two dye doped polymer films - Disperse Orange-25 and Disperse Yellow-7 prepared using hot-press technique. For both the dyes Polymehtylmethacrylate-Methacrylic acid (PMMA-MA) has been chosen as a matrix because of its easy availability and easiness of processing. We have used Z-scan technique with CW laser @ 532 nm to study nonlinear optical properties of these materials. The closed aperture Z-scan shows positive nonlinear refraction for both the materials. While the open aperture Z-scan gives RSA at higher intensities for both the samples, making them candidates for optical limiting.
Nonlinear absorption, 1D self-focusing and 1D self-trapping of optical beams are studied in 150 ppm Fe-doped Bi12SiO20 crystal using single beam Z-scan technique in nano second regime using coherent and incoherent laser pluses with out applying any external electric field across the crystal. When the crystal is placed in expanding beam 1D self-trapping is observed due to 1D self-focusing. The nonlinear absorption is also observed at high intensities due to trap assisted excited state absorption as well as two photon absorption. The ground state absorption and increased nonlinear absorption at 532 nm and 595 nm compared to pure BSO indicates the increased impurity traps in the crystal lattice due to iron incorporation. The studies reveal that incorporation of iron in BSO making it an excellent optical limiter.
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