Muddasir Naeem, Muhammad Tuoqeer Anwar
The effect of temperature and strain on acoustic phonons in bulk materials (polymethyl methacrylate, Scotch tape, and Kapton tape) is investigated using backward Brillouin light scattering (BLS) spectroscopy. The acoustic phonon mode frequency decreases with increasing temperature because of the alteration of the phonon velocity and refractive index. Acquired spectra enable extraction of key elastic parameters, including phase velocity, acoustic attenuation, longitudinal modulus, and phonon mean free path, and facilitate a systematic analysis of their evolution under varying temperature and strain. Low-frequency phonon modes occur in Scotch tape due to the presence of an excess amount of impurities, which dominate at high temperatures. We demonstrate a compact strain applicator device that enables controlled application of uniform strain. We observe that uniaxial strain effect lowers the phonon frequency as the system transitions from compression to tension, thereby reducing phonon velocity, whereas biaxial strain produces a more drastic change in phonon frequency and consequently the elastic properties. An additional low-frequency mode appears under high-strain outside the elastic regime. Beyond the fundamental understanding, these findings emphasize Kapton’s role as a robust and stable substrate and Scotch tape’s function as a model soft polymer, offering a broader perspective on strain–phonon interactions in flexible materials.