Highly anisotropic transient optical response of charge density wave order in ZrTe${}_3$

Abstract

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.

6 pages, 4 figures

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