Page 1 of 7 Rotation of polarization plane of a laser beam by fluid vorticity Ratan Joarder 1 *, Abhishek Kumar 1 , Manas Jain 1 , Sandeep Pandey 1 , Hari Chand 1 , Nagendra Singh 2 , and Samir Kumar Biswas 2 1 Aerospace Engineering Department, Indian Institute of Technology Kharagpur, West Bengal- 721302, India 2 Indian Institute of Science Education and Research Mohali, Manauli PO 140306, India *E-mail: jratan@aero.iitkgp.ac.in Abstract: The polarization plane of a beam of light experiences angular drag when it traverses a medium rotating about an axis parallel to the direction of the beam. Here, fluid vorticity in a horizontal Rijke tube is used to cause rotation in the polarization plane of a continuous laser beam. The vorticity distribution, and acoustic pressure history in the tube have been predicted numerically. The thermo-acoustic phenomenon in the tube has been found to generate the vorticity. A suitable optical arrangement has been used to capture the light intensity variation due to rotation of the plane of polarization of the beam, and up to 4 degrees of rotation has been observed. It is a known fact that light travelling through matter is affected by the motion of the medium. Of special interest in the context of the present study is the rotation of the plane of polarization of a continuous laser beam when it passes through a medium rotating about an axis parallel to the direction of the beam. The rotation of the polarization plane of a beam when it passes through a rotating dielectric is termed as “polarization drag” 1 in the literature. The theoretical concept was demonstrated experimentally by R. V. Jones 2 for a solid medium. Later, Steinitz and Averbukh 3 proposed to use an optical centrifuge pulse to excite unidirectional rotation of microscopic gas particles and it was demonstrated experimentally by Milner et al. 4 The specific optical rotary power (i.e., polarization rotation angle per unit propagation length per unit density) of the gas molecules was found to be nine order of magnitude higher than that obtained in Jones’s experiment 2 . Thermo-acoustic or heat driven oscillations in gas turbine combustors/rocket motors are characterized by low amplitude acoustic oscillations initially. The amplitude can grow under certain conditions, and can result in large amplitude oscillations of acoustic pressure. During this period, the laminar velocity structure breaks down, and macroscopic gas vortices are generated 5 . Vorticity signifies bulk rotation of the medium, and can be calculated by taking curl of the velocity vector. Rijke tube 1, 6-7 is a device where thermo-acoustic instability is easily reproducible. A brief description of the operational mechanism of a Rijke tube is provided in the Sec. 2 of the Supplementary Material. The horizontal Rijke tube was introduced by Matveev et al. 8-9 . Further work on the horizontal Rijke tube had been carried out by Song et al. 10 . In a horizontal Rikje tube, the longitudinal acoustic modes of the tube are excited by heating the air at a suitable antinode. It is observed in the present numerical simulations on Rijke tube that the bulk rotation of the gas or vorticity is generated during the instability, and this is accompanied by production of sound. Hence, the vorticity or the bulk rotation of a gaseous medium in the Rijke tube can be utilized to see its effects on the rotation of the polarization plane of a laser beam. Experiments are conducted under similar conditions as the numerical simulations. A continuous wave (CW) polarized laser beam is passed through the vortical region of the Rijke tube during the experiments. It is observed that the plane of polarization of the laser beam rotates when it passes through air which is undergoing bulk rotation about an axis parallel to the direction of beam propagation. The dynamic behavior of the polarization rotation angle has This is the author’s peer reviewed, accepted manuscript. However, the online version of record will be different from this version once it has been copyedited and typeset. PLEASE CITE THIS ARTICLE AS DOI: 10.1063/5.0252849