Strong polaritonic interaction between flux-flow and phonon resonances in Bi2Sr2CaCu2O8+x intrinsic Josephson junctions: Angular dependence and the alignment procedure

► The interplay between geometric and velocity matching electromagnetic resonances in the flux-flow state is studied. ► A very strong interaction is reported, which leads to formation of phonon-polaritons. ► A special focus is made on the analysis of the angular dependence of the resonances. ► We describe an accurate sample alignment procedure that prevents intrusion of Abrikosov vortices. Bi2Sr2CaCu2O8+x single crystals represent natural stacks of atomic scale intrinsic Josephson junctions, formed between metallic CuO2–Ca–CuO2 and ionic insulating SrO–2BiO–SrO layers. Electrostriction effect in the insulating layers leads to excitation of c-axis phonons by the ac-Josephson effect. Here we study experimentally the interplay between and velocity matching (Eck) electromagnetic resonances in the flux-flow state of small mesa structures with c-axis optical phonons. A very strong interaction is reported, which leads to formation of phonon-polaritons with infrared and Raman-active transverse optical phonons. A special focus in this work is made on analysis of the angular dependence of the resonances. We describe an accurate sample alignment procedure that prevents intrusion of Abrikosov vortices in fields up to 17T, which is essential for achieving high-quality resonances at record high frequencies up to 13THz.