Studying the Linear and Nonlinear Optical Properties by Using Z-Scan Technique for CdS Thin Films Prepared by CBD Technique

CdS films prepared by using CBD technique, linear optical properties tests measured by UV-3000 Nano from OPTIMA, nonlinear properties contained the nonlinear refractive index and nonlinear absorption coefficient by using single light beam source green semiconductor developed laser (SZ303 LASER) with material Nd:YVO4+KTP or Aluminum Alloy, and Wavelength Range is (532 nm), Beam Dimension 6 Meter distance Output Laser spot 18 mm ± 2.0 mm.


Introduction
CdS films prepared by using Chemical Bath Deposition as a preparation technique deposit at different durations started with (30 minutes) and ending in (120 minutes) by adding 30 minutes on every sample. CBD technique is the simplest method (Khallaf, 2009) and doesn't costs, also it gives a diversified thicknesses at every times that deposition time increased. Samples testes to calculate linear and nonlinear properties and that by using (UV-3000 Nano) to linear optical properties and (Z-scan method) for calculate the nonlinear optical properties. Z-scan technique is amongst the simplest and most sensitive of these techniques. The basic Z-scan technique has been described by Sheik-Bahae, Said, and Van Stryland (1989) and , and a brief summary of the theory of the technique is presented here. The most important aspects to be considered for an experimental setup, along with some of the constraints that need to be placed on the design of the setup, will be highlighted (Sheik-Bahae & Hasselbeck, 2000).

Theory of Z-Scan Method
The z-scan technique operates on the principle of moving the sample by focusing the Gaussian laser beam focused on it (Sheik-Bahae, Said, & Van Stryland, 1989;Sheik-Bahae & Hasselbeck, 2000). The intensity of the laser beam changes through the detector whenever the sample is moved and moved, due to the interaction between the medium and the laser light. A change in the mean means that the sample is experiencing a different laser beam intensity during the sample resulting from the interaction between the material and the intensity of the laser, dependent on the sample position (z) relative to the focus (z = 0). By measuring the transmitted power (the transmittance) through the sample as a function of z-position of the sample, information about the light-matter interaction can be extracted. The two nonlinear interactions that can be determined in this fashion are the sample's nonlinear index of refraction and nonlinear absorption coefficient (Sheik-Bahae & Hasselbeck, 2000). To detect the nonlinear properties as the absorption coefficient and the nonlinear refractive index, there are two methods, each of which is calculated as one of them depending on the other : (i) The geometry in which a finite aperture is kept before the detector is known as a closed-aperture (CA) Z-scan; (ii) the geometry in which the aperture is removed to focus all the transmitted light into the detector is referred to as an open-aperture (OA) Z-scan (Costela et al., 1996;Hutchings, Sheik-Bahae, Hagan, & Van Stryland, 1992;Chari et al., 1996). The main principle of zscan technique is based on transforming the phase distortion to amplitude distortions during beam propagation (Sheik-Bahae, Said, & Van Stryland, 1989).

Closed -Aperture Z-Scan
As shown in Figure (1), the sample is moved from the position Z to -Z and +Z, which is the propagation direction, with no open aperture before the detector where the energy is fixed to a certain extent while retaining the input energy pulse (Sheik-Bahae, Said, & Van Stryland, 1989). The measured normalized transmittance of the sample is monitored through the aperture in the far field as a function of the position z. All this experimental process depend on the beam parameters and the samples thickness L. The size of the aperture is signified by its transmittance (S) is about 0.1<S<0.5 in all type of z-scan that done in all experimental for determining nonlinear refraction (Hutchings, Sheik-Bahae, Hagan, & Van Stryland, 1992;. The change in permeability between the top and the bottom or the peak and the valley can be defined in this technique as : (1) where T p and T v are the normalized peak and valley transmittances. The empirically determined relation between the induced on axis phase shift, ΔΦ 0 , and ΔT pv for a third-order nonlinear refractive process is, (2) If the Z-scan aperture is closed to allow linear transmission of less than 10 percent or 0.1 < < 0.5, (Hutchings, Sheik-Bahae, Hagan, & Van Stryland, 1992;Hutchings, Sheik-Bahae, Hagan, & Van Stryland, 1992;Van Stryland & Sheik-Bahae, 1998) then equation (1) be: As for the nonlinear refractive index, it is calculated by the difference between peak and valley in permeability measured by the following equation: where k is the wave vector the irradiance on axis 0 is : : The peak power given by (Sheik-Bahae, Said, Wei, Hagan, & Van Stryland, 1990): Where E: the energy of the pulsed laser, ∆ : the time duration, o ω : the beam radius at the focal point. : The effective length of the sample can be determined from the following formula (Hutchings, Sheik-Bahae, Hagan, & Van Stryland, 1992;Chari et al., 1996;Van Stryland & Sheik-Bahae, 1998): where, L : the sample length, ° : is the linear absorption coefficient given by : where T is the linear transmittance.

Open Aperture Z-Scan
Change in the intensity of the laser beam A concentration by the lens is measured in the second geometry of the z-scan system and as in Figure (1), where the closed aperture is raised and the beam is fully controlled on the detector which measures the change of laser beam intensity according to the sample movement on the propagation direction to calculate Nonlinear absorption. Each change in the intensity of the laser beam in the sample is only the result of the multi-photon absorption process during the process of moving the sample on the waist of the laser beam ). The intensity is greatest in the focal plane, the largest nonlinear absorption is observed. At the "tails" of the Z-scan signature, where | | >> , the beam intensity is too weak to elicit nonlinear effects. The higher order of multi-photon absorption present in the measurement depends on the wavelength of light and the energy levels of the sample (Selvan et al., 2002). The normalized change in transmitted intensity can be approximated by the following equation (Hutchings, Sheik-Bahae, Hagan, & Van Stryland, 1992;Lu et al., 1997;Van Stryland & Sheik-Bahae, 1998;Selvan et al., 2002): where, : is the sample position at the minimum transmittance, m: integer, ( ) ∶ the minimum transmittance.

Linear Optical properties
The visual properties involved were calculated the absorbance, transmittance, absorption coefficient α (cm -1 ), energy gap (E g ), refractive index (n), and extinction coefficient (k) by using UV-SP-3000 Nano. Transmittance

Nonlinear Optical Properties
Nonlinear optical properties contained nonlinear refractive index (n 2 ) in (cm 2 /GW) units, and nonlinear absorption coefficient (β). Nonlinear refractive index measured through closed aperture z-scan and found that it is decreasing with increasing of thicknesses (time deposition increase). The nonlinear refractive index for 30 minute time deposit is more bigger (8.83 10 -5 cm 2 /GW), for 60 minute (7.68 10 -5 cm 2 /GW), for 90 minute (5.45 10 -5 cm 2 /GW) and for last sample for 120 minute (2.60 10 -5 cm 2 /GW), this results detects decreasing in nonlinear properties with increasing of thick films or thick media. The Figure (9) shows the normalized transmittance for CdS films.  The nonlinear absorption or nonlinear refractive index depending at laser wavelength but this dependence isn't strongly clear because there is much more affects at Nonlinear properties.

Conclusions
The increasing in time deposition accompanied by increasing in thicknesses of films. And that lead to decreasing in absorbance and increasing in transmittance. The energy gap was in the general appropriate with grains size after reading the structural properties of CdS films. Nonlinear optical properties done by using (SZ-303 LASER) with material Nd:YVO4+KTP or Aluminum Alloy and that usage considered the first using of this kind of lasers.