Tracking multiscale liquid crystal order with high harmonic spectroscopy
Can high-harmonic generation track how microscopic intermolecular interactions give rise to macroscopic order? We address this question in the thermotropic liquid crystal 4-cyano-4'-n-pentylbiphenyl, a model system with a well-defined nematic-to-isotropic transition. Using a temperature-controlled flat jet, we show that polarization-resolved high-harmonic generation driven by mid-infrared strong-field pulses is highly sensitive to liquid-crystal order. In the nematic phase, the harmonic yield forms a four-lobed angular pattern with twofold rotational symmetry and is strongest when the driving-field polarization is perpendicular to the molecular director; this anisotropy vanishes in the isotropic liquid. A tight-binding model parameterized by first-principles calculations attributes this response to intermolecular orbital hybridization in short-range antiparallel molecular arrangements. Temperature-dependent measurements across multiple harmonic orders resolve the same first-order phase transition, with each harmonic order exhibiting a distinct optical sensitivity. A multiscale model combining the microscopic response, Landau-de Gennes orientational correlations and phase matching explains this behaviour through the interplay between orientational correlation lengths and harmonic coherence lengths. High-harmonic generation therefore links local electronic coupling to mesoscopic order and macroscopic propagation, providing an all-optical probe of multiscale organization in soft matter.