Boson Peak and Fracton of Polymethyl Methacrylate Detected by Terahertz Time-Domain and Low-Frequency Raman Spectroscopies

Terahertz time-domain spectroscopy and lowfrequency Raman scattering were performed on polymethyl methacrylate to detect the boson peak (BP) and fracton, which are universal excitations of glass. In infrared spectrum, BP is observed in the alpha(nu)/nu(2) plot, where alpha(nu) is absorption coefficient. Log-log plot of alpha(nu), nu chi ''(nu) and the vibrational density of states show linear frequency dependence above the BP frequency, where chi ''(nu) is the imaginary part of Raman susceptibility. We determined the infrared light vibration coupling constant C-IR(nu) and Raman coupling constant C-Raman(nu) to investigate how the fractal dimension and the fracton dimension appear in C-IR(nu) and C-Raman(nu).