Optical phonon effects on linewidth of several laser active ions in YAG

Abstract

An analysis of low temperature lineshapes (broadenings, asymmetries, splittings) for various absorption lines of several RE 3+ ions from the beginning Nd 3+ (4f3) and end of lanthanide series Er 3+ (4f 11 ), Tm 3+ (4f 12 ) embedded in the same crystal, (Y 3 Al 5 O 12 ) - YAG, known as important laser systems for 1-3μm emission is presented. New aspects of the dependence of electron - phonon coupling on RE 3+ ion and matrix are observed from the analysis of many transitions of these ions at low temperature. These data show that the electron - phonon coupling presents a symmetric behavior in the lanthanide series, larger toward beginning and end. With only very few exceptions, less than 2%, the broadenings or splittings of zero-phonon lines could be connected to optical Raman phonons of undoped lattice. That means that the parity of 4f n states could still be considered a good parent quantum number, even if the local symmetry has no inversion, and the near resonant coupling involves the even part of the electron-phonon coupling operator. Besides, the one phonon relaxation involve the undoped lattice phonons. For ions with small spin - orbit coupling, the vibronic contribution to the line intensity given by i.r. phonons through a Van Vleck mechanism could be important for spin forbidden transitions.