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Sagnac Interferometry Project

Alexander Fried
Elizabeth Schemm
Hovnatan Karapetyan
Aharon Kapitulnik

The Sagnac interferometry project grew out of a collaboration between Aharon Kapitulnik and Prof. Marty Fejer (Applied Physics) to design an instrument that could detect signatures of anyon superconductivity in YBCO. The basic principle - the Sagnac effect- has been applied to many problems in measurement since its first use in 1913. Here we use it to measure polar Kerr effect (or Faraday effect, in transmission).

Polar Kerr effect is a magneto-optic phenomenon in which normally incident plane-polarized light, upon reflection from a ferromagnetic medium, has its major axis of polarization rotated relative to that of the incident beam. The angle between polarization axes of the incident and reflected beams, called the Kerr angle, is then proportional to the component of the sample′s magnetic moment parallel to the incident light.

Although Kerr rotation in materials commonly arises from an out-of-plane ferromagnetic moment, it is well worth noting that any effect that breaks time reversal symmetry with an out-of-plane moment can give rise to a Kerr signal. The deviation of Kerr angle from zero, in particular, is of interest for detection of such time reversal symmetry-breaking states, regardless of their origin.

The current generation of this project uses a modified interferometer design developed by Jing Xia (now a postdoc at Caltech) and Peter Beyersdorf (now at San José State University) [ref: Modified Sagnac interferometer for high-sensitivity magneto-optic measurements at cryogenic temperatures. Jing Xia, Peter T. Beyersdorf, M. M. Fejer, and Aharon Kapitulnik, Appl. Phys. Lett. 89, 062508 (2006) ]. By routing light along the fast and slow axes of polarization-maintaining (PM) fiber, we are able to effectively create a zero-area Sagnac loop that is sensitive (to one part in 10^6) to nonreciprocal effects arising only from interaction with the sample. As built, this instrument is shot noise limited above 10 uW incident optical power, with very low drifts with temperature (~50 nrad from 4K to room temperature) and in time (~20 nrad per day). The improved sensitivity allows us to search for signatures of broken time reversal symmetry, and to make careful studies of these transitions, in many systems of physical interest which were previously inaccessible by this technique.


Ongoing research: [titles/links to individual articles here]

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