Magneto-optical deflection of light
We have recently developed a new optical technique to measure magnetic fields. Since it is sensitive to minute movements of a laser beam, it is a fast method with very high axial resolution.
Generally magneto-optical activity is observed in transmission through Faraday rotation, which is a function of the light path through the medium. However, we have shown that if the propagation direction of the light beam, rather than its polarization state, is observed, then this permits magnetic fields to be mapped with high axial resolution. The essence is that the physics that determines any deflection arises within a few wavelengths at an interface and is therefore a convenient sub-micron probe of a magnetic structure.
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Figure: Deflection of a circularly polarized beam as detected on a position sensitive detector, and recorded on an oscilloscope (raw data trace, in black). The blue line is the measured current. The inset shows the opposite deflections for the two circular polarization states (red and blue) and the linearly polarized laser beam (trace at center). |
Another useful property of deflection measurements is that the sign and magnitude of a magnetic field pulse may be determined in a “single-shot”, since the direction of the deflection may be positive or negative depending on the polarity of the magnetic field pulse. Furthermore, deflections do not suffer from n-π ambiguities, which can plague Faraday rotation. We have used single interface refraction to measure the magnetic field inside a solenoid. A custom-built pulsed 1.8 MW power supply was used to generate 100 μs single-cycle magnetic field pulses. The position of the laser beam was observed on a position sensitive detector and was shown to closely follow the calculated magnetic field pulse, as can be seen in the image.
[1] A. Ghosh, W. Hill, and P. Fischer, “Observation of the Faraday effect via beam
deflection in a longitudinal magnetic field”, Phys. Rev. A, 76,
(2007), 055402.
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