Low dimensionality in CDW systems leads to anisotropic optical properties, in both equilibrium and non-equilibrium conditions. Here we perform polarized two-color pump probe measurements on a quasi-1D material ZrTe$_3$, in order to study the anisotropic transient optical response in the CDW state. Profound in-plane anisotropy is observed with respect to polarization of probe photons. Below $T_\mathrm{ CDW}$ both the quasi-particle relaxation signal and amplitude mode (AM) oscillation signal are much larger with $\mathbf{ E}_\mathrm{ pr}$ nearly parallel to $a$ axis ($\mathbf{ E}_\mathrm{ pr} \parallel a$) than for $\mathbf{ E}_\mathrm{ pr}$ parallel to $b$ axis ($\mathbf{ E}_\mathrm{ pr} \parallel b$). This reveals that $\mathbf{ E}_\mathrm{ pr} \parallel a$ signal is much more sensitive to the variation of the CDW gap. Interestingly, the lifetime of the AM oscillations observed with $\mathbf{ E}_\mathrm{ pr} \parallel b$ is longer than $\mathbf{ E}_\mathrm{ pr} \parallel a$. Moreover, at high pump fluence where the electronic order melts and the AM oscillations vanish for $\mathbf{ E}_\mathrm{ pr} \parallel a$ , the AM oscillatory response still persists for $\mathbf{ E}_\mathrm{ pr} \parallel b$. We discuss possible origins that lead to such unusual discrepancy between the two polarizations.
Categories: cond-mat.str-el