High harmonic spectroscopy reveals anisotropy of the charge-density-wave phase transition in TiSe.

Charge density waves (CDW) appear as periodic lattice deformations which arise from electron-phonon and excitonic correlations and provide a path towards the study of condensate phases at high temperatures. While characterization of this correlated phase is well established via real or reciprocal space techniques, for systems where the mechanisms interplay, a macroscopic approach becomes necessary. Here, we demonstrate the application of polarization-resolved high-harmonic generation (HHG) spectroscopy to investigate the correlated CDW phase and transitions in TiSe₂. Unlike previous studies focusing on static crystallographic properties, the research examines the dynamic reordering that occurs within the CDW as the material is cooled from room temperature to 14 K. By linking ultrafast field-driven dynamics to the material's potential landscape, the study demonstrates HHG's unique sensitivity to highly correlated phases and their strength. The findings reveal an anisotropic component below the CDW transition temperature, providing insights into the nature of this phase. The investigation highlights the interplay between linear and nonlinear optical responses and their departure from simple perturbative dynamics, offering a fresh perspective on correlated quantum phases in condensed matter systems.