Terahertz fingerprints of BNA crystal: Anisotropic absorption of low-frequency modes and its impact on terahertz generation efficiency.

Low-frequency vibrational modes of organic terahertz (THz) generator crystals significantly impact their THz emission efficiencies. In this work, experimental THz time-domain spectroscopy (THz-TDS) and solid-state density functional theory (DFT) calculation are combined to investigate anisotropic THz absorption and characteristics of low-frequency vibrations of N-benzyl-2-methyl-4-nitroaniline (BNA). Experimental spectral results reveal that BNA has strong polarization-dependent absorption peaks in the range of 0.5-3.5 THz. Theoretical calculations have identified the low-frequency vibrational modes responsible for strong absorption, such as the 2.46-THz mode along the z-axis and 3.16-THz mode along the x-axis. Quantitative analysis has further revealed their motional features. Numerical simulations demonstrate anisotropic phonon absorption reduces THz yield at 1.18-2.17 THz range, but enables thickness scaling, while strongly suppressing 2.65-3.48 THz generation requiring optimal thickness. These findings provide critical insights for designing high-efficiency organic THz sources by mitigating phonon losses.