A. M. SUHAIL ET AL.

68

The sold curve in Figure 9 is the best fit for Equation

(5). The Equation (5) shows clearly that the depth of the

absorption dip is linearly proportional to the 2PA coeffi-

cient β, but the shape of the trace is primarily determined

by the Rayleigh range of the focused Gaussian beam.

The fitted value of ß is on the order of 50 cm/GW. This

value is ten times of magnitudes higher than the value

observed with bulk CdS sample. This results is in a good

agreement with values mentioned in [23]. The natural

logarithm of the (1-T) values are plotted as a function of

the natural logarithm of the incident intensity Io in Fig-

ure 10. The curve can be reasonably fitted with a straight

line with a slope of 0.97. This indicates that the 2PA was

occur in CdS pump by 800 nm laser source of 51 fs pulse

duration as shown in Figure 10.

The formation of surface defects may contribute to the

absorption mechanism of the prepared film due to small

increase in the linear absorption cross section [24]. The

formation of the surface defects in CdS and in the other

sulfur compound increase the nonlinear scattering lead-

ing to decreasing in the nonlinear absorption coefficient.

This was observed by viewing the transmittance light

through IR camera.

4. Conclusions

A CdS nanocrystalline film was prepared by the chemi-

cal spray pyrolysis technique. The nonlinear absorption

coefficient was measured by fully computerized the z-

scan technique. The measurements show that the nonlin-

ear absorption coefficient for the nanocrystallites is one

order of magnitude higher than that of the bulk CdS ma-

terial. This increase in the nonlinear responsivity when

the crystalline size approaches nano-scale dimensions

may be attributed to collimating of the incident intensity

of the pumped laser which led to improve the nonlinear

-6

-5

-4

-3

-2

-1

0

012345

Ln(1-T)

Ln(I

0

)

Figure 10. Plot of Ln(1-T) vs. Ln(Io) at 800 nm wavelength,

the solid line is the example of the linear fit at 800 nm with

slope s = 0.97.

dynamic of the CdS nanocrystallite.

5. Acknowledgements

This work has been carried out in the physics Department,

School of Engineering and Applied Sciences, Harvard

University. The authors would like to thanks Mazur Re-

search Group in Harvard University for their help

through this work. Thank also to Christopher C. Evans,

Jonthan D. B. Bradley, and Eric Mazur for their interest,

guide and useful discussion. We thanks also the Ministry

of Higher Education in the Republic of Iraq for support

this work.

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