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The large third order susceptibility of Hg1_CdTe makes it a very promising material for many applications, including optical switching. There have been numerous studies' 4) ofthe optical nonlinearities of Hg1_CdTe and it is well known that for values ofx near 0.2 and for 10.6 ,tm radiation, both nonlinear absorption arising from two photon absorption and nonlinear refraction arising from the charge carriers generated by two photon absorption play a role in determining the overall nonlinearity. To optimize the performance of a device that utilizes the nonlinearity of Hg1 _CdTe it is sometimes necessary to separate the relative contributions of these two mechanisms. The recently developed "z-scan" technique provides a convenient way of achieving this5. We present here a study of the nonlinearity of Hg1_Cd7Te using the z-scan method. In the next section, we first describe a theory of z-scan results expected for the external self focusing case. Although there are many theoretical studies67 of the external seLf focusing case, it is hard to find them presented in a general form from which many cases of interest can be easily evaluated. Moreover, the refractive nonlinearity is usually assumed to be of the Kerr-type, which is not in general true. Also, the effects of nonlinear refraction and absorption are usually not considered together. We present our results here in terms of dimensionless parameters, and inclilde the near field and far-field effects. We also take into account the time dependences of the incident laser beam and the nonlinearity of the medium. The results are presented in forms of integrals which can be evaluated for arbitrary nonlinear phase changes. Next, we apply the theory to the case of Hg1 _Cd1Te in which the dominant nonlinear phase change arises from refraction due to generated free carriers and the dominant nonlinear absorption arises from two-photon absorption. The generation of the charge carriers via two-photon absorption and their decay by means of Auger recombination are taken into account. The temporal profile of the laser beam used in the experiment described in section 3 was close to Gaussian-this was taken into account in the theory.
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