Sensitive measurement of the magneto-optic effect in the near infrared wavelength region with weak alternating magnetic fields

In this paper, we present a sensitive measurement method of the Verdet constant of magneto-optic materials with an auto-balanced photodetector and a lock-in amplifier under weak alternating magnetic fields in the near infrared wavelength region. We describe the theories and formulas for the measurement method by accounting for the characteristics of the auto-balanced photoreceiver and lock-in amplifier. The measurement method was verified by measuring the Verdet constant of the well-known BK7 samples at 1,310 and 1,550 nm wavelengths. This method was used to measure the Verdet constant of the regioregular poly(3-hexylthiophene-2,5-diyl) thiophenes (P3HT) materials prepared at various baking and cooling conditions for the wavelengths of 1310 and 1550 nm.