Abstract
This study is dedicated to investigate the effects of initial laser intensity on the nonlinear optical properties of the laser dye DQOCI dissolved in methanol with a concentration of 10 -5 M and doped with PMMA film. The properties were studied by using open and closed aperture Z-scan technique, with different levels of initial intensity (I0), excited by continuous diode solid-state laser at a wavelength of 532 nm. Three lenses of different focal lengths were employed to change the radius of the Gaussian laser beam and then change the initial intensity. For I0= 6.83 and 27.304 kWatt/cm2, the Z-scan curves show a saturation of absorption (SA) known as the negative type of nonlinearity, in which the absorption coefficient β 2 decreases and the transmittance increases with increasing the initial laser intensity. With I0 equal to 3.03 KWatt/cm2, the nonlinear absorption changes from SA to RSA, where the transmittances is reduced with the increase of intensity (z0) as analyzed by the theory of free carrier nonlinearities. The closed aperture z-scan shows a pre-focal transmittance minimum (valley) and a post focal transmittance maximum (peak) which reflects the z-scan signature of a positive nonlinearity (self-focusing) due to Kerr effect. Each of nonlinear refractive index (n2), nonlinear absorption coefficient (β 2), and third-order nonlinear optical susceptibility (χ3) are intensity-dependent.
Highlights
This study is dedicated to investigate the effects of initial laser intensity on the nonlinear optical properties of the laser dye DQOCI dissolved in methanol with a concentration of 10 -5 M and doped with PMMA film
Figure-5 shows the results of the open aperture z-scan experiment for the laser dye DQOCI dissolved in methanol (10 -5 M) with different I0 values, doped with PMMA, and excited by continuous solid-state laser at a wavelength of 532 nm
When I0 is 27.304 KWatt/cm2 with a lens of a focal length of f=5 cm, the nonlinear absorption changes from saturation of absorption (SA) to reverse saturable absorption (RSA) or two photon absorption (TPA), where the transmittances is reduced with the increase of intensity and the decrease of the focal length (Figure-6)
Summary
The second experiment is the open aperture z-scan, illustrated in Figure-(3 b), where the aperture in front of the detector was removed and a converging lens was used to collect the incident laser beam transmitted from the sample. With the open aperture z-scan, the nonlinear absorption coefficient (β2) and imaginary parts of the third-order nonlinear optical susceptibility (Imχ (3)) of the doped films were measured. An open-aperture Z-Scan measures the change in intensity of a beam, focused by lens l in Figure-3, in the far field at the detector PD, which captures the entire beam and gives an estimate of the absorptive nonlinearity of a sample [20]. With reverse saturable absorption (RSA) or two photon absorption, the absorption coefficient increases resulting in the transmittance’s decrease with the increase in the input laser intensity [10, 16]
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