Nonlinear Refractive Index Measurement in Semiconductor-Doped Glasses

Abstract

There are several techniques in use for non-linear refractive index measurement, namely, interferometric techniques, in which conventional inter-ferometers are used, degenerate for wave mixing (DFWM), and z-scan, Each of these techniques suffers from some shortcmings. For example conventional interferometers like Fabry-Perot and Twyman-Green need high quality optical components, unwanted reflections on these components produce noise, and the device limits the probe-pump anglc, or in z-scan technique one needs very sensitive detectors and since the intensity is monitored by the nonlinear absorption, which is usually present, reduces the measurement accuracy. In the techniqucs introduced here, in principle, only a plate of the sample is required, and even parallelism of the plate surfaces is not curcial. Experiments can be carried out successfully if the angle between the plate surface is less than few minutes. In the first technique, the probe beam strikes the surface at an arbitray angle of incidence. The reflected beam from the two surfaces of the sample interfere on a photo-sensitive screen like CCD, and more or less linear interference fringes are produced. When the pump beam is switched on, the interference pattern deforms. The amount and the direction of the deformation give the value and the sign of the non-linear refractive index. In this technique the probe-pump angle can be varied from 00 to 1900. In the second technique, interference between the reflected probe beam from the sample and the diffracted pump beam from the grating induced by the interference of the probe and the pump beams, leads to a series of circular fringes. When the non-linear sample is replaced by a linear material like fuse silica glass, the above mentioned circular fringes are formed, but the number of fringes in a specified angular interval remains fixed as the pump beam intensity increases. But, in the case of a non-linear sample the number changes due to self focusing or defocusing induced by pump beam. The curvature of diffracted wave is deduced from the measurement of the radii of circular fringes for different pump intensities and this leads to the evaluation of non-linear refractive index. It is shown that an accuracy of π/10 in measuring the phase of the diffracted spherical wave front, leads to an accuracy of 5% in measurement of nonlinear retractive index. Bout techniques have been carried out using the second harmonic of a Nd,YAG laser with 8ns pulse duration, and the samples were Schotts OG550 filters of 1mm thickness. The exeperimental results of both techniques are in agreement with each other and with the results of the other reports.