Coherent photo-induced phonon emission in the charge-density-wave state of K0.3MoO3

• Published in: New journal of physics Vol. 21; no. 1; pp. 013013 - 13024
• Main Authors: Rabia, K, Meng, F, Thomson, M D, Bykov, M, Merlin, R, van Smaalen, S, Roskos, H G
• Format: Journal Article
• Language: English
• Published: Bristol IOP Publishing 18-01-2019
• Subjects: blue bronze; Broadband; charge density wave; Charge density waves; Coherence; coherent emission; Coupled modes; Emission; Near infrared radiation; Optical pulses; phonon; Phonons; Physics; Polarity; terahertz; time-resolved; Transition temperature
• ISSN: 1367-2630
• DOI: 10.1088/1367-2630/aaf81f

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 T ∼ 100 K , 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.