Magneto-optics

Abstract

The magneto-optic properties of interest are the Faraday Effect, Kerr Rotation, and the Cotton–Mouton Effect. In 1846, Michael Faraday discovered that when linearly polarized light passes through glass in the presence of a magnetic field, the plane of polarization is rotated. The Faraday Effect is now used in a variety of microwave and optical devices. Normally the Faraday experiment is carried out in transmission, but rotation also occurs in reflection, the so-called Kerr Rotation that is used in magneto-optic disks with Mbit storage capability. Other magneto-optic phenomena of less practical interest include the Cotton– Mouton Effect, a quadratic relationship between birefringence and magnetic field, and magnetic circular dichroism that is closely related to the Faraday Effect. A number of nonlinear optical effects of magnetic or magnetoelectric origin are also under study. Almost all these magnetooptical effects are caused by the splitting of electronic energy levels by a magnetic field. This splitting was first discovered by the Dutch physicist Zeeman in 1896, and is referred to as the Zeeman Effect. When linearly polarized light travels parallel to a magnetic field, the plane of polarization is rotated through an angle ψ. It is found that the angle of rotation is given by . . . ψ(ω) = V(ω)Ht, . . . where H is the applied magnetic field, t is the sample thickness, ω is the angular frequency of the electromagnetic wave, and V(ω) is the Verdet coefficient. Faraday rotation is observed in nonmagnetic materials as well as in ferromagnets. The Verdet coefficient of a commercial one-way glass is plotted as a function of wavelength in Fig. 31.1(a). Corning 8363 is a rare earth borate glass developed to remove reflections from optical systems. A polarized laser beam is transmitted through the glass parallel to the applied magnetic field. The plane of polarization is rotated 45◦ by the Faraday Effect. The transmitted beam passes through the analyzer that is set at 45◦ to the polarizer. But the reflected waves coming from the surface of the glass and from the analyzer are rotated another 45◦ as they return toward the laser.