[zz] Spontaneous Polarization

Introduction to Spontaneous Polarization and Hysteresis

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    Spontaneous polarization is a novel mechanism for spin polarizing atoms using linearly polarized light. The effect was first predicted in 1987 [1], and first observed in 1995 in our lab [2]. In contrast to standard optical pumping techniques using circularly polarized light to induce spin polarization, spontaneous polarization can produce spin polarization with linearly polarized or even unpolarized light. The effect amplifies any initial bias in spin polarization, so that the atoms spontaneously "jump" into either of two stable spin polarization states (parallel or antiparallel to an applied magnetic field). If the pump light has a degree of circular polarization, the atomic spin polarization exhibits marked hysteresis in switching between the bistable states as the circular polarization of the pump is varied. Possible applications of spontaneous polarization might include the search for a permanent atomic electric dipole moment (our main research interest) and spin exchange polarization of noble gases.

1. E.N. Fortson and B.R. Heckel, Phys. Rev. Lett. 59, 1281 (1987). ()
2. W.M. Klipstein, S.K. Lamoreaux, and E.N. Fortson, Phys. Rev. Lett. 76, 2266 (1996). ()

		Hysteresis:	a retardation of an effect when the forces acting upon a body are changed (as if from viscosity or internal friction); especially : a lagging in the values of resulting magnetization in a magnetic material (as iron) due to a changing magnetizing force.
		The graphic at left is an artistic representation of spontaneous polarization. Atoms of cesium contained in the cylindrical cell are spin polarized (the vertical arrows represent the atomic spins) by absorbing some of the laser light incident upon them (the large arrow passing through the cell). Superimposed on this is a schematic diagram of a hysteresis curve, one of the most distinct and striking experimental signatures of spontaneous polarization.

At left is a typical hysteresis curve from our experimental data. The vertical axis is the transmitted light from a probe laser beam, which is initially linearly polarized. As it passes through the cesium vapor in the cell, it undergoes a Faraday rotation of its plane of polarization which is proportional to the degree of atomic spin polarization (i.e. alignment) along its propagation direction. This rotation is detected by next passing the probe beam though an analyzing polarizer at 45 degrees to its initial polarization, producing a light intensity on a photdiode detector which is roughly proportional to the atomic polarization (for small rotations).     The horizontal axis shows the degree of circular polarization of the pump beam (the light which is causing the atomic spin polarization). A quarter-wave retardation plate (QWP) varies the light's polarization as its angle changes, from full left circular polarization at -45 degrees to pure linear at 0 degrees to full right circular at +45 degrees. The curve shown is generated by scanning the QWP from -45 to +45 and then back to -45. The atoms spin polarize along an applied magnetic field and remain so even as the QWP passes 0 degrees, where the light changes from left to right cirular polarization. When the light has enough of the opposite sense of circular polarization, at a QWP angle of +10 degrees in this case, the atoms spontaneously flip and align in the opposite direction along the magnetic field. When the QWP scans back down, the atoms remain in this new state until the QWP reaches past -10 degrees, and then spontaneously flip back to the original polarization direction. The hysteresis curve thus generated is qualitatively similar to that of the "magnetization in a magnetic material (as iron) due to a changing magnetizing force" mentioned in the definition above.