Birefringence Induced by Ultrasonic Waves in the Isotropic Phase of PCB

Abstract

We have studied the birefringence induced by ultrasound Δn in the isotropic phase of pentylcyanobiphenyl (PCB) as a function of temperature, frequency and acoustic intensity. We have found that Δn is proportional to the square root of the acoustic intensity P and that the ratio Δn/√P decreases with increasing frequency indicating that our data are within the wr ≳ 1 regime, where r denotes the orientational relaxation time. We have also measured in the same experiment the ultrasonic absorption coefficient α. From the comparison between the behavior of the induced birefringence and that of the ultrasonic absorption we have shown that the optical relaxation frequency (T _op ⁻¹) is smaller by a factor 6 to 8 than the acoustical relaxation frequency (T _ac ⁻¹). This result is explained by the fact that the absorption coefficient contains the contribution of all the fluctuation modes of the order parameter, while the induced birefringence is governed by the modes of longest wavelengths.