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Real Time Imaging Polarimeter using Pancharatnam Retarder N. Glazar * , C. Culbreath, and H. Yokoyama Liquid Crystal Institute, Kent State University In 1956, S. Pancharatnam discovered an optical phase encountered via cyclic loop of polarization state that is distinct from the ordinary phase shift associated with optical path length [1]. Twenty-seven years later this phenomenon was rediscovered by M.V. Berry [2]. We have been investigating liquid crystal devices that employ Pancharatnam-Berry phase to manipulate the wavefront of light. Using our automated maskless photoalignment device [3], we construct a Pancharatnam retarder by patterning an azobenzene alignment layer [4] in a nematic liquid crystal cell. Fig. 1(a) shows the alignment pattern for the Pancharatnam retarder, a continuously winding periodic structure. The effect of this retarder on an optical wave is to impose a phase shift Δφ= ±2ψ ,where ψ is the azimuthal angle of the optic axis. A salient feature of the Pancharatnam phase, in contrast to the ordinary phase, is that the magnitude of the phase shift is determined only by the azimuthal angle ψ, and is therefore achromatic. In addition, the phase shift has an inverse relationship for right and left hand circular polarization modes. Light passing through the Pancharatnam retarder will emerge with two orthogonal circularly polarized light beams that are deflected at opposing angles. The maximum intensity of the deflected beams is achieved when anisotropic layer is voltage-tuned to provide half-wave ordinary phase retardation. If we detune from this condition, part of the incident beam emerges from the retarder without deflection. In biological and materials sciences, there is a growing demand for time-resolved high-spatial resolution spectroscopic imaging polarimeter. To meet these stringent demands, we introduce a novel video-rate imaging polarimeter that employs the previously discussed Pancharatnam retarder as an achromatic polarizing beam splitter, seen in Fig 2(a). An arbitrary light beam passes from a sample through a Pancharatnam retarder that is detuned from the half-wave condition, and is split into three beams: two circular polarized beams, and a central unaffected beam. The central beam is split into two linearly polarized beams with a Wollaston prism. Each of the resulting beams (R,L,P,S) is imaged onto a CCD sensor. As depicted on the Poincaré sphere in Fig. 2(b), the four polarization modes are sufficient to recreate the original Stokes parameters. The CCD sensors provide a 2D map of the Stokes parameters of the light emanating from the sample. A 4x4 calibration matrix for the system is determined via measurement of known polarization states [5]. The Pancharatnam retarder as well as the other optics are static, allowing for video-rate polarimetry. [1] S. Pancharatnam, Proc. Indian Acad. Sci. - Sect. A 44, 247 (1956). [2] M. V. Berry, Proc. R. Soc. A Math. Phys. Eng. Sci. 392, 45 (1984). [3] C. Culbreath, N. Glazar, and H. Yokoyama, Rev. Sci. Instrum. 82, 126107 (2011). [4] M. Schadt, K. Schmitt, V. Kozinkov, and V. Chigrinov, Jpn. J. Appl. Phys. 31, 2155 (1992). [5] R.M.A. Azzam, Opt. Acta Int. J. Opt. 29, 685 (1982). * presenting author; E-mail: [email protected] S 3 R L S P S 1 S 2 Sample Objective Pancharatnam Retarder Wollaston Prism CCD Sensors L P S R (a) (b) 200 μm (a) (b) Fig. 1 (a) Typical alignment pattern (b) Micrograph of Pancharatnam retarder Fig. 2 (a) Schematic diagram of device (b) Poncaré sphere representation of operation
