Synthesis by size focusing of lithium tantalate nanoparticles with a tunable second harmonic optical activity†

Abstract

Nonlinear optics at the nanoscale has emerged as a sought-after platform for sensing and imaging applications. The development of these materials is having an impact on fields that include advanced information technology, signal processing circuits, and cryptography. Lithium tantalate (LiTaO₃) is an attractive nonlinear optical material due to its high optical damage threshold (e.g., tolerance to >500 MW cm⁻² from a nanosecond pulsed laser) and broad range of ultraviolet-visible (UV-vis) transparency (i.e., 0.28–5.5 μm) relative to many other nonlinear optical materials (e.g., niobates, titanates). Despite many synthetic reports on metal oxides, very little is known about the preparation of uniform, crystalline LiTaO₃ nanoparticles (NPs) of a pure phase, as well as details on their mechanism of nucleation and growth. In this article, we introduce a solution-phase method for the preparation of LiTaO₃ NPs with tunable dimensions. This solution-phase process results in the formation of crystalline, uniform NPs of LiTaO₃ of a pure phase when carried out at 220 °C. This method can prepare crystalline LiTaO₃ NPs without the need for further heat treatment or the use of an inert atmosphere. Results presented herein also provide insights into the growth mechanism of these NPs. The reaction included the processes of oriented attachment and Ostwald ripening. The results of our study also indicate that the growth of the LiTaO₃ NPs was a result of a “size focusing” effect, which enables the ability to tune their diameters from 200 to 500 nm. The crystalline NPs were optically active towards second harmonic generation (SHG). These studies deepen our understanding of the methods by which NPs can be prepared from metal oxides. These studies specifically demonstrate the preparation of optically active LiTaO₃ NPs of uniform and controllable dimensions that could be used in a broad range of fundamental studies and applications in nanophotonics.