Measurement of the thermo-optic coefficient in liquids by laser-induced conical diffraction and thermal lens techniques

Abstract

We report on a method to determine the temperature coefficient of the refractive index (dn/dT) for non-fluorescent liquids which is based on the experimental observation of conical diffraction induced by self-phase modulation (SPM). Light absorbing impurities were added to allow for heat deposition by the excitation laser beam. We studied samples of tetrahydrofuran (THF, C4H8O) and toluene (C6H5CH3) with suspended cadmium selenide/zinc sulfide (CdSe/ZnS) core–shell quantum dots as absorbing impurities and samples of water (H2O) and THF with a small addition of blue-black commercial ink. The well-known thermal lens (TL) technique was also employed in this work for the sake of comparison with the proposed conical diffraction pattern. Thermo-optical parameters obtained by the mode-mismatched thermal lens (TL) technique validated the SPM measurements of dn/dT. In addition, we applied in this work the Fresnel–Kirchhoff diffraction formalism for the theoretical prediction of the rings that are observed in the far-field region. The technique proposed herein to determine dn/dT may serve as a fast, cost-effective optical method for evaluating adulterated or impure chemicals.