Modified Sagnac interferometer for high-sensitivity magneto-optic measurements at cryogenic temperatures

Abstract

A method to measure the two-dimensional image of magneto-optical Kerr effect Rev. Sci. Instrum. 74, 4718 (2003); 10.1063/1.1618012 Measurement of the scattered light magneto-optical Kerr effect from plasmon-resonant Ag particles near a magnetic filmThe authors describe a geometry for a Sagnac interferometer with a zero-area Sagnac loop for measuring the magneto-optic Kerr effect at cryogenic temperatures. The apparatus is capable of measuring absolute Kerr rotation without any modulation of the magnetic state of the sample and is intrinsically immune to reciprocal effects such as linear birefringence and thermal fluctuations. A single strand of polarization-maintaining fiber is fed into a He-3 cryostat, eliminating the need for optical viewports. With an optical power of only 10 W, they demonstrate static Kerr measurements with a shot-noise-limited sensitivity of 1 ϫ 10 −7 rad/ ͱ Hz down to 0.5 K.

Zero loop-area Sagnac interferometer at oblique-incidence for detecting in-plane magneto-optic Kerr effect

“…Up to the sample, the arrangement is similar to normal-incidence zero loop-area Sagnac interferometers as reported by Xia et al and more recently by Fried et al [7][8][9] Briefly, a linearly polarized, collimated broad-band light source centered at 780 nm with a full bandwidth of 30 nm and a power of 6 mW (QPhotonics, Ann Arbor, MI) passes through a 50-50 beam splitter (BS) and a linear polarizer (PL) before being focused into a 1-m polarization-maintaining (PM) fiber. The transmission axis (TA) of PL is aligned to the initial linear polarization of the optical beam and to the slow-axis FIG. 1.…”

“…Compared to finite loop-area Sagnac interferometers at oblique incidence, the zero loop-area Sagnac interferometer at oblique incidence avoids separating and recombining optical beams and thus has much fewer optical elements. It is thus inherently more stable and promises higher sensitivity to time-reversal breaking effects in a sample, just as normal incidence Sagnac interferometers as reported by Xia et al and Fried et al [7][8][9] …”

“…As a result, it has shown by Xia et al and Fried et al that such a zero-area Sagnac interferometer can detect Kerr rotation as small as 10 -7 radians without modulating the magnetization. [7][8][9] So far though only the normal-incidence geometry has been demonstrated for zero loop-area Sagnac interferometers and in this geometry only the effect from a magnetization perpendicular to the surface can be measured.…”

“…3 When modulating the magnetization in a sample is not feasible, Kapitulnik, Fejer and coworkers showed that the effect of linear birefringence can be removed efficiently if the magneto-optic effect is measured with a Sagnac interferometer. [4][5][6][7][8][9][10] In this case an optical beam and its time-reversed counterpart traverse an identical loop-wise path including reflection from a magnetized sample but in the opposite direction. One then measures the difference of the phases acquired by these two beams.…”

“…loopless). [7][8][9][10] For a finite loop-area Sagnac interferometer, optical beams a are produced through beam-splitting optical elements and configured to being either normally incident or obliquely incident on a sample surface so that effects of magnetization along the surface and perpendicular to the surface can both be measured. Because the two beams are steered separately and eventually recombined before detection, more optical elements are needed, making finite loop-area Sagnac interferometers more subject to imperfection and drift in the optical system in addition to the rotational effect.…”