Tunning Nonlinear Optical Properties of Titanium Nitride Nanocrystals

Titanium Nitride (TiN) nanomaterials have become a research interest since their present interesting photothermal and absorption/scattering properties. This novel material has a localized surface plasmon resonance response (LSPR) in the NIR window, in addition to having a higher thermal stability-resistivity compared to the commonly used plasmonic Gold (Au) or Silver (Ag) nanoparticles. Furthermore, several studies have confirmed the biocompatibility and low cost of fabrication of TiN nanoparticles with the intention of replacing Au for biomedical applications. [1] Currently, TiN nanoparticles are studied for implant coatings, photothermal therapy and bio-sensing, among other.<br/><br/>The search of plasmonic nanomaterials with tunable nonlinear optical properties has become crucial for laser-based applications in integrated electronics, biomedical and imaging, to mention a few [2]. Particularly, saturable absorption and self-focusing effect, are desirable third order non linearities for the design of non-reciprocal light propagation optoelectronic devices, high resolution spectroscopy, thermal lenses, and optical switches. Previous studies demonstrated non-linear absorption coefficients for TiN higher than Au, under the same laser radiation conditions. [3] Furthermore, the agglomeration of these nanoparticles can be utilized to enhance scattering properties, making these colloidal solutions ideal for tunable absorption. Therefore, it is of high value to investigate non-linear optical properties of TiN nanoparticles and TiN clusters in the NIR regime for the above-mentioned applications, particularly in the biomedical field.<br/><br/>In this work, the optical nonlinearities of TiN as a function of nanoparticle concentration in DI water solutions were investigated. Additionally, the effect of nanoparticle clustering, fabricated by 1 µm size clusters of 50 nm TiN nanoparticles, on nonlinearities was studied. It was found that the nonlinear absorption coefficient increases linearly with concentration for both models, however clusters required higher concentrations to exhibit similar values in absorption. Similarly, the optical threshold of reverse saturable absorption for cluster solutions was higher compared to the freestanding nanoparticles due to the collective scattering phenomenon. Finally, the self-focusing effect on TiN nanoparticles was explored on the continuous regime. Kerr lens with focal length as a function of solution concentration were obtained. The refractive index changes, due to photothermal effects and diffusive heat transfer, produced changes in the beam diameter and consequently on the laser fluence. This work provides a comprehensive study of the tunability of nonlinear properties by the parameters of concentration and agglomeration. <br/> <br/><b>References. </b><br/>[1] G. V. Naik, V. M. Shalaev, A. Boltasseva, Adv. Mater.2013, 25, 3264.<br/>[2] Capretti, A., Wang, Y., Engheta, N., & Dal Negro, L. (2015). <i>Acs Photonics</i>, <i>2</i>(11), 1584-1591.<br/>[3] A. N. Sabzeghabae, C. Berrospe, L. Mangolini, A. Aguilar, J. Biomed. Mater. Res., Part A2021, 109, 2483