Optical setup and data acquisition details

We use an inverted microscope for sensing experiments in reflection, with only a few modifications necessary for phase-sensitive operation. These modifications include using a coherent source, and implementing a Wollaston prism and analyzer. The collimated output of a VCSEL-based laser diode module (Thorlabs Inc., CPS850V) is directed toward a lens and focused in the back-focal plane of a 5X objective. The objective collimates the light, and after being resonantly reflected from the GMR sensor, the beam is directed toward the Wollaston prism (Thorlabs Inc., WPQ10) by a beam splitter. The Wollaston prism is aligned and fixed, such that the orthogonally polarized TE and TM modes are split into two diverging beams with approximately equal intensity. The analyzer then enables the common components of these beams to interfere, and the interferogram is recorded with a CMOS camera (Thorlabs Inc., Compact USB 2.0 CMOS Camera). The contrast of the interferogram fringes can be adjusted in the beginning of a measurement by rotating the analyzer. For sensing experiments, we take images at a rate of 0.2 fps. Fast Fourier transform-based post processing is performed with a custom MATLAB script.