Thermal nonlinear optical response of meso-tetraphenylporphyrin under aggregation conditions versus that in the absence of aggregation

Abstract

In this work, measurement of thermally induced nonlinear refractive index of meso-tetraphenylporphyrin (H₂TPP) at different concentrations in 1,2-dicoloroethane using a double-grating interferometer set-up in a pump–probe configuration is reported. The formation of aggregates of H₂TPP at concentrations greater than ca. 5 × 10⁻⁵ M was evident by deviation from Beer’s law. An almost focused pump beam passes through the solution. A part of the pump beam energy is absorbed by the sample and therefore a thermal lens is generated in the sample. An expanded probe beam propagates through the sample and indicates the sample refractive index changes. Just after the sample a band-pass filter cuts off the pump beam from the path but the distorted probe beam passes through a double-grating interferometer consisting of two similar diffraction gratings with a few centimetres distance. A CCD camera is installed after the interferometer in which on its sensitive area two diffraction orders of the gratings are overlying and producing interference pattern. The refractive index changes of the sample are obtained from the phase distribution of the successive interference patterns recorded at different times after turning on of the pump beam using Fourier transform method. In this study, for different concentrations of H₂TPP in 1,2-dichloroethane solution the thermal nonlinear refractive index is determined. Also, we present the measurement of the temperature changes induced by the pump beam in the solution. We found that value of nonlinear refractive index increased by increasing the concentration up to a concentration of 5 × 10⁻⁴ M and then decreased at higher concentrations. In addition, we have investigated the stability of the observed thermal nonlinearity after a period of two weeks from the sample preparation.

Keywords:

• Meso-tetraphenylporphyrin (H₂TPP)
• self-aggregation
• thermal lens
• nonlinear refractive index
• double gratings interferometer
• pump–probe arrangement

Acknowledgments

We would like to thank Dr. Mohammad Yeganeh for his kind help.