Abstract-Methodical inaccuracy of the Z-scan method for few-cycle terahertz pulses

Maksim Melnik, Irina Vorontsova, Sergey Putilin, Anton Tcypkin, Sergei Kozlov,

https://www.nature.com/articles/s41598-019-45735-6

Modern sources of THz radiation generate high-intensity pulses allowing to observe nonlinear effects in this spectral range. To describe many nonlinear effects theoretically, it is necessary to know the nonlinear refractive index coefficient of optical materials. The work studies the applicability of the Z-scan method to determine the nonlinear refractive index coefficient in the THz frequency range for few-cycle pulses. We have discussed the correctness of the known Z-scan method for calculating the nonlinear refractive index coefficient for broadband THz radiation regarding number of cycles pulses have. We have demonstrated that the error in determining the nonlinear refractive index coefficient is always greater than 70% for true single-cycle pulses. With the increase in the number of oscillations to the measurement error shows strong dependence on the sample thickness and can vary from 2% to 90% regarding the parameters chosen. The fact that such radiation dispersion length is commensurate with the nonlinear length or even less than the latter results in the discrepancy mentioned. It is demonstrated that the decrease in the sample thickness leads to the reduction of the nonlinear refractive index coefficient determination error, and this error is <2% when the ratio between the sample thickness and the pulse longitudinal spatial size is ≤1. This can relate to the fact that the nonlinear effects in such a thin sample occur faster than the dispersion ones.

New discovery about terahertz radiation benefits biomedicine

Authors of the research: Maxim Melnik and Maria Zhukova. Credit: ITMO University

https://phys.org/news/2019-05-discovery-terahertz-behefits-biomedicine.html

Scientists from ITMO University for the first time in the world managed to directly measure the nonlinear refractive index of matter in the terahertz range. The results of the experiments were compared with previous theoretical predictions to confirm the presence of nonlinear effects. The obtained data can be used to control light, as well as in fundamental and biomedical research. The results are published in Optics Express.

Over the past 40 years, terahertz radiation has found widespread use for quality control and safety systems, for substances determination, as well as for various biomedical research. However, high-power radiation sources in this range appeared relatively recently. Such powerful radiation changes the refractive index of the medium and affects how it transmits radiation from other ranges. Researchers call such effects nonlinearities and study them to create devices for controlling light.

For the first time in the world, the scientists from ITMO University directly measured the nonlinear refractive index of matter in the terahertz range. They adapted the previously known Z-scan method to obtain experimental data on how high-power radiation changes the refractive index of the sample. After that the researchers conducted a numerical simulation of the experiment and compared the results: they turned out to be similar.

"For the first time, we managed to reliably verify that terahertz radiation causes strong nonlinearity in the medium. So far, we conducted measurements only for water, but we plan to expand the range of media to perform experimental and theoretical studies of this kind. The data obtained will be useful for creating devices for light control, as well as for fundamental and biomedical research," commented Maria Zhukova, member of the Laboratory of Femtosecond Optics and Femtotechnologies at ITMO University.

"Our laboratory has long been engaged in the development of high-intensity terahertz radiation sources, and we have already achieved some outstanding results. But before going deep into the fundamental aspects of it, we decided to describe the nonlinearities it causes using rare equipment at ITMO University. We adapted the method for measuring nonlinearity in order to use existing experimental base as efficiently as possible," adds Maksim Melnik from the Laboratory of Femtosecond Optics and Femtotechnologies at ITMO University.