Intense TeraHertz Time Domain Spectroscopy of Water-based Liquid Crystals

Intense terahertz time-domain spectroscopy enables the characterization of highly absorbing and optically thick media by combining bright terahertz sources with off-focus probing conditions that suppress nonlinearities. Here, we apply it to cromolyn sodium salts dissolved in pure water, across the isotropic and nematic phases. Transmission measurements performed on half-millimetre thick liquid samples kept at 20 °C and 40 °C reveal systematic reductions in the terahertz absorption when compared to pure water, which is consistent with effective-medium predictions for isotropic solutions. A distinct excess absorption, however, emerges exclusively for the nematic phase, indicating a novel spectroscopic signature of supramolecular ordering. Polarized optical microscopy confirms the presence of domains with sizes comparable to the terahertz wavelengths used, suggesting that scattering could contribute to the anomalous terahertz response recorded in the nematic phase. Our results demonstrate that intense terahertz time-domain spectroscopy provides a non-contact probe of mesophase formation in lyotropic chromonics without the need to apply external bias. In turn, this offers a route to studying opaque systems wherein the microstructural organization lacks the strong optical signatures of liquid crystals.