Abstract
We report on the observation of coherent terahertz (THz) emission from the quasi-one-dimensional charge-density wave (CDW) system, blue bronze (K0.3MoO3), upon photo-excitation with ultrashort near-infrared optical pulses. The emission contains a broadband, low-frequency component due to the photo-Dember effect, which is present over the whole temperature range studied (30–300 K), as well as a narrow-band doublet centered at 1.5 THz, which is only observed in the CDW state and results from the generation of coherent transverse-optical phonons polarized perpendicular to the incommensurate CDW b-axis. As K0.3MoO3 is centrosymmetric, the lowest-order generation mechanism which can account for the polarization dependence of the phonon emission involves either a static surface field or quadrupolar terms due to the optical field gradients at the surface. This phonon signature is also present in the ground-state conductivity, and decays in strength with increasing temperature to vanish above , i.e. significantly below the CDW transition temperature. The temporal behavior of the phonon emission can be well described by a simple model with two coupled modes, which initially oscillate with opposite polarity.
Highlights
The non-equilibrium dynamics of complex solids, following ultrafast excitation, continues to be an important avenue to elucidate the coupling between the various degrees of freedom and their resultant ordered phases, as it allows one to track the evolution following selective excitation, and capture signatures of transient non-thermalized states and quasi-particle populations [1,2,3]
The photo-Dember effect, which is present over the whole temperature range studied (30–300 K), as well as a narrow-band doublet centered at 1.5 THz, which is only observed in the CDW state and results from the generation of coherent transverse-optical phonons polarized perpendicular to the incommensurate CDW b-axis
The main findings of the paper are summarized in the scheme in figure 1(a), i.e. we observe both a lowfrequency broadband emission due to the PD effect, as well as a long-lived coherent phonon emission, which is composed of the temporal interference of two near-degenerate modes, whose field polarization remains perpendicular to the baxis
Summary
Coherent photo-induced phonon emission in the charge-densitywave state of K0.3MoO3 K Rabia1,2,5 , F Meng1,5, M D Thomson1 , M Bykov3 , R Merlin4 , S van Smaalen3 and H G Roskos1 Original content from this Keywords: charge density wave, phonon, terahertz, coherent emission, time-resolved, blue bronze work may be used under the terms of the Creative Commons Attribution 3.0 licence.
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