Influence of infrared optical properties by transformation of the crystal structure in Al-doped vanadium dioxide films

Some infrared-active phonons in VO2 films suppress their modulation performance in the infrared region. Al-doped VO2 films, due to transforming VO2 crystal into the M2 phase, promptly eliminate absorption peaks in far-IR/THz bands and present widely modulating properties. Furthermore, we found high-frequency shifts of phonon vibration modes in Raman spectra by Al doping, indicating the stronger V-O bonds as the evidence of VO2 crystalline modification. However, although the high-frequency shifts and peak broadening were observed in V-O-V bending modes, mid-infrared spectra as the other phonon characterization show that its resonances are less involved, which is different from the remarkable variation of THz phonons. We attribute the difference to the distinct origins of phonon vibrations. As Al doped into films, the group-rotational peaks were rapidly erased with crystalline deformation whereas the high-frequency bending modes only slightly changed. (C) 2016 Optical Society of America