Nonlinear optical response of water dispersions of iron oxide nanoparticles

Abstract

Recently ferrofluids and magnetic nanofluids have attracted considerable attention as they are considered for a large variety of applications which require fluidic and magnetic properties simultaneously. Such liquid media are in principle easily controllable e.g. by a magnetic field and their flow and properties can be altered in a controllable way. In the case of iron nanoparticles, these can exhibit nonlinear optical properties as well, becoming therefore even more attractive for potential applications as they can combine the ferrofluid properties with nonlinear optical activity. In this talk I will report on some recent results obtained in our group concerning the third-order nonlinear optical properties of some water dispersions of iron nanoparticles. In fact, two types of iron nanoparticles are investigated here and will be presented, iron oxide nanoparticles (gamma-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> ) surrounded by polyelectrolyte brushes and similar size neat iron oxide nanoparticles. In both cases, the nonlinear optical properties of water dispersions of the iron nanoparticles have been assessed under the same experimental conditions. The measurements were performed using the Z-scan technique employing laser pulses from a 5 ns Q-switched Nd:YAG laser and a mode-locked 35 ps Nd:YAG laser both operated at 10 Hz, at 532 and 1064 nm. The advantage of the experimental technique used in this study is that it allows for the simultaneous determination of both the real and imaginary parts of the third-order susceptibility. The main results of our investigation can be summarised as follows: the magnitude of the third-order susceptibility chi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(3)</sup> was found to increase by almost 100% for visible excitation compared to the infrared excitation, for both coated and uncoated gamma-Fe <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> O <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</sub> nanoparticles. The third-order susceptibility of the coated nanoparticles was found to be an order of magnitude larger than that of uncoated ones in all cases. Moreover, it was found that both types of nanoparticles exhibited strong negative sign nonlinear refraction (i.e. defocusing) when excited at 532 nm, which was altered to positive sign nonlinear refraction (i.e. focusing) for visible excitation. The uncoated nanoparticles were found to exhibit saturable like absorption behavior both in the visible and in the infrared, while the coated ones were found to exhibit saturable like absorption at 532 nm and change it to reverse saturable like absorption under 1064 nm excitation. The corresponding third-order nonlinear susceptibility chi <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">(3)</sup> values were determined to be of the order of 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">-20</sup> m <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> /V <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> . For all samples under ps laser excitation no measurable nonlinear optical response has been observed. The present results will be discussed and compared with other existing literature reports.