Infinite-mode double-grating interferometer for investigating thermal-lens-acting fluid dynamics

Abstract

In this paper, we introduce the use of an infinite-mode double-grating interferometer in investigating the fluid dynamics induced by a thermal lens when an intense laser beam—which we call the pump beam—passes through a liquid sample containing strongly absorbing nanoparticles. As the pump beam passes through the sample in normal conditions, since absorption occurs over the beam’s trace, temperature increases and density decreases. The density and temperature changes in the sample can be translated to a phase shift in the passing of a plane probe beam through the sample. By causing two parts of the probe beam to interfere, one part passing from the thermal lens area and the other from another area of the sample spatially separate from the first, it is possible to determine the phase shift value. When the propagation directions of the interfering beams are parallel, infinite-mode double-grating interference fringes are formed. In the infinite mode, the formation and evolution of interference fringes directly reflects any change of density or temperature in the sample. The thermal convection is detected visually and quantitatively determined by chasing the first infinite-mode interference fringe produced. In this work, the measurement of the thermal convection velocity is reported in detail. The measured value is comparable with values obtained with other methods under the same conditions. The proposed method is reliable and very simple; further, the convection process can be visualized in real-time.