Evolution of coherent collective modes through consecutive CDW transitions in (PO$_{2}$)$_{4}$(WO$_{3}$)$_{12}$ mono-phosphate tungsten bronze

摘要

All optical femtosecond relaxation dynamics in a single crystal ofmono-phosphate tungsten bronze (PO$_{2}$)$_{4}$(WO$_{3}$)$_{2m}$ with alternatestacking m=6 of WO$_{3}$ layers was studied through the three consequent chargedensity wave (CDW) transitions. Several transient coherent collective modesassociated to the different CDW transitions were observed and analyzed in theframework of the time dependent Ginzburg-Landau theory. Remarkably, theinterference of the modes leads to an apparent rectification effect in thetransient reflectivity response. A saturation of the coherent-mode amplitudeswith increasing pump fluence well below the CDWs destruction threshold fluenceindicates a decoupling of the electronic and lattice parts of the orderparameter under strong optical drive.