A candidate for replacing industry standards in terahertz generation

Terahertz (THz) spectroscopy leverages the non-ionizing characteristics of THz frequency light to enable sensing and imaging applications, such as material characterization. In the exploration of nonlinear stress-strain behavior, nonlinear optical (NLO) crystals are preferred due to the high field, broadband pulses they can generate.

Out of the currently available NLO options, N-Benzyl-2-methyl-4-nitroaniline (BNA) has emerged as an industry standard for THz generation at 800 nm, a wavelength accessible in many labs. Furthermore, it is an organic crystal, meaning that its molecular components can be modified to fine-tune THz generation properties.

To enhance the THz generation and stability of BNA, Biggs et al. synthesized four BNA derivatives by introducing electron-withdrawing and donating groups to its appended benzyl group. This modification aims to increase BNA molecular hyperpolarizability, a determining factor of THz generation capability, and strengthen noncentrosymmetric packing geometries required for stability.

“The weakness of BNA is that at high pump powers, BNA begins to melt,” said author Megan Biggs. “We hoped that because BNA is already a good THz generator, a crystal with a similar structure may improve the THz output, while potentially increasing the damage threshold.”

The researchers used density functional theory, phase matching experimental results, and crystal quality to explain THz generation performance. They found that while all derivatives were crystallized in the same underlying structure, F-BNA significantly outperformed BNA in both the Fourier amplitudes and damage threshold, making it an ideal candidate for replacing BNA as the industry standard.

“By switching to using F-BNA, researchers can safely generate strong field strengths with less concern for damage to the THz generator,” said Biggs.

Source: “Terahertz generation of BNA-derivatives,” by Megan F. Biggs, Paige K. Petersen-Barlow, (Enoch) Sin-Hang Ho, Caitlin C. Chartrand, Abigail J. Lattin, Halle M. Gebhardt, Matthew J. Lutz, William J. Hom, Connor D. Barlow, Gus H. Phillips, Kailyn M. Sorensen, Megan E. Moody, Brendon Jentzsch, Yu-Cheng Hong, Olivia N. Rollans, Brigham Richards, Elisha Jones, Ashton Roma, Meredith Shull, David J. Michaelis, and Jeremy A. Johnson, Journal of Applied Physics (2025). The article can be accessed at https://doi.org/10.1063/5.0300701 .