Dynamical Symmetry Breaking Induced by Ultrashort Laser Pulses in KTaO_3(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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Raman selection rules were observed to be violated dynamically under resonant and coherent pumping of the Raman-active two-phonon mode in the cubic perovskite-type oxide KTaO_3. In this study, two intense femtosecond laser pulses having near infrared frequencies were used to excite the crystal with a relative time delay within ±200fs. The frequency difference of the pulses was set to be almost equal to the frequency of one of the Raman-active two-phonon modes, with both phonons being at the Brillouin-zone (BZ) edge point. As a result, not only the conventional multistep coherent anti-Stokes Raman scattering (CARS) signals, but a new series of multistep signals with the frequency spacing of originally Raman-inactive single phonons were observed in the wavelength range from near infrared to the whole visible region. Importantly, the observed frequency spacing of the multistep signals does not correspond to the single-phonon frequency at the Г-point but that at the BZ-edge point. In addition, the angle spacing of the observed signals is almost constant; it is also quite different from the angle distribution of the conventional CARS signals. A model mechanism of BZ folding from the BZ edge to the Г-point due to coherent accumulation of two phonons at the BZ edge is presented to explain all aspects of this new phenomenon. Two phonons at the BZ edge and those at the Г-point are anharmonically coupled within a decay time of coherent phonons, and those originally at the BZ edge become Raman-active. This novel phenomenon should provide a new tool to observe phonon modes at the BZ edge other than inelastic neutron scattering.
社団法人日本物理学会の論文
2006-02-15

Dynamical Symmetry Breaking Induced by Ultrashort Laser Pulses in KTaO_3(Condensed Matter : Electronic Structure, Electrical, Magnetic and Optical Properties)

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