BNA Organic THz Generation Crystal, CA 17 mm on 1" Sapphire

Overview

BNA is one of the highest-performance organic crystals available for broadband, high-field terahertz generation by optical rectification. Its second-order nonlinear coefficient is far larger than that of inorganic electro-optic crystals such as ZnTe or GaP, which allows it to deliver both wide spectral coverage and high peak electric field from the same element. This particular configuration is grown on a 1-inch sapphire substrate with a clear aperture of 17 mm and a crystal thickness of 350 um, giving a mechanically robust part that mounts directly into a standard 1-inch optic holder on the optical table.

Why BNA for Yb-driven THz

Unlike DAST, DSTMS and OH1, which need pump wavelengths in the 1.3 - 1.5 um range and therefore an optical parametric amplifier in front of them, BNA is efficiently pumped near 0.8 um and around 1.03 um. That makes it directly compatible with an Yb femtosecond platform without a wavelength-conversion stage, which removes cost, alignment complexity and one source of instability from the beamline. Published work has demonstrated broadband, high-average-power terahertz generation from BNA at repetition rates up to 540 kHz, confirming that the crystal tolerates the high average powers typical of modern Yb amplifiers.

Typical setup

The femtosecond pump is focused onto the BNA crystal; the generated terahertz pulse is collimated by a gold-coated off-axis parabolic mirror, relayed through the sample, and refocused onto an electro-optic sampling crystal. A quarter-wave plate, Wollaston prism and balanced photodetector then read out the terahertz field waveform, while a motorised delay line scans the probe timing. Because the pump beam and the terahertz beam are collinear at the crystal exit, a pump-blocking filter such as black polyethylene or PTFE is normally placed immediately after the crystal.

Handling and storage

Organic terahertz crystals are sensitive to humidity, mechanical shock and optical damage. Store the element in a dry, sealed enclosure, keep the mounted crystal inside a nitrogen or dry-air purge box during operation, and increase pump fluence gradually while monitoring the terahertz signal. Do not clean the crystal face with solvents. The sapphire substrate carries the mechanical load, so always handle the part by the substrate edge.