When the light beam emitted by a laser pointer hits a glass window a few millimeters thick, some of the light is refracted, which means that the speed of the light wave changed when it entered the glass. This change can be easily quantified: the refractive index for air and glass is approximately 1.00029 and 1.5. The refraction phenomenon is well known; it is usually taught in High School Physics and fits in the theory of Linear Optics.
The light emitted by the laser pointer has low power, a few thousandths of a watt. However, when the laser power is very high (tens or thousands of watts), other phenomena occur in the interaction between the light and the material, which are called “non-linear.” One makes the glass act like a convex or concave lens. The magnitude of this effect depends not only on the characteristics of the light beam, but also on the material, and can be quantified by the non-linear refractive index.
A multidisciplinary team that included researchers from the Brazilian Federal University of Pernambuco (UFPE), the Brazilian Mackenzie Presbyterian University and the United States Air Force Research Laboratory (AFRL) was able to determine, for the first time, the non-linear refractive coefficient of ZrTe2 (zirconium telluride) – a two-dimensional material, still little studied, from the family of transition metal dichalcogenides (TMDs). The study was recently reported in an article published in Applied Physics Letters.
TMDs have unique electrical and optical properties that depend on their thickness, which can reach the subnanometer scale. In fact, extremely thin, transparent and flexible layers can be produced with these metals, which, of course, have aroused much interest for several applications. Important detail: for several of these applications, such as those related to medical imaging, the most important properties are those of the Non-Linear Optics universe.
In this context, Professor Anderson Stevens Leonidas Gomes (UFPE) has been working on, for about three years, a research line focused on the study of the nonlinear properties of TMDs, carried out in collaboration with researchers from AFRL. “They prepare the material and we perform the nonlinear optical characterization,” says the scientist. “The role of Melissa Maldonado, who completed a doctorate and is now a postdoctoral fellow under my supervision, was and is critical, since she has mastered the characterization techniques and was the main researcher responsible for the work in this line of research,” he adds.
Several materials from the family of two-dimensional TMDs have been studied by the group. In the work published in Applied Physics Letters, AFRL researchers prepared zirconium telluride nanoflakes of about 1 nm thick and 50 to 100 nm long, using a method based on laminating the surface of thicker nanoparticles. The samples traveled from the state of Ohio to São Paulo and arrived at Mackenzie, where part of the characterization was carried out. Finally, the samples were sent to UFPE, where their nonlinear optical properties were probed using a laser emitting extremely short and strong pulses, which interacted with the zirconium telluride nanoflakes. After performing a series of mathematical calculations, Professor Gomes and his group were able to determine the material’s nonlinear refractive coefficient. Through computer simulations, the authors were also able to understand the origin of the phenomenon and, therefore, create possibilities to control it.
“The main scientific contribution of this study is to measure, unequivocally and appropriately, one of the optical coefficients that indicate the magnitude of the material’s optical non-linearity,” says Professor Anderson Gomes, who is the corresponding author of the article. “This coefficient is important to define which photonic applications can be explored in this material,” he adds.
The study received financial support from the Brazilian federal agencies CNPq and Capes, the INCT-Nanocarbono, the Foundation for the Support of Science and Technology of the State of Pernambuco (FACEPE) and the US agency AFOSR. The authors used computational resources from the Advanced High Performance Computing Center (NACAD) at COPPE/UFRJ (Brazil).
Paper: Femtosecond nonlinear refraction of 2D semi-metallic redox exfoliated ZrTe2 at 800nm. Melissa Maldonado, Manoel L. da Silva Neto, Pilar G. Vianna, Henrique B. Ribeiro, Cid B. de Araujo, Christiano J. S. de Matos, Leandro Seixas, Ali M. Jawaid, Robert Busch, Allyson J. Ritter, Richard A. Vaia, and Anderson S. L. Gomes. Appl. Phys. Lett. 118, 011101 (2021); doi: 10.1063/5.0031649
Contact: Prof Anderson Gomes (UFPE) – anderson.lgomes@ufpe.br