Characterization of Magneto-Optical Rare Earth-Doped Ingaasp Thin Films on InP
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B.J. STADLER, K. VACCARO, A. DAVIS, E.A. MARTIN, G.O. RAMSEYER,* AND J.P. LORENZO USAF Rome Laboratory, Hanscom Air Force Base, MA 01731 USA *GriffissAir Force Base, NY 13441 USA ABSTRACT
We have investigated the properties of rare earth-doped InGaAsP thin films with special interest in magneto-optical device applications. Magneto-optical properties have been used in optical systems as isolators, waveguides, and switches. These materials and devices can be used to expand the functionality of InP opto-electronic integrated circuits (OEICs). Thin films of InP, InGaAs, and InGaAsP, grown by liquid phase epitaxy, were lattice matched to the (100) InP substrates. The films were n-type, with the carrier concentration decreasing by an order of magnitude in the doped films due to gettering by the rare earth elements. The doped films contained 2.6x10 8 - 1.5x10 20 cm"3 rare earth elements, which were observed to segregate toward the film/melt interface in the more highly doped films. A broad photoluminescence was observed at 1.52 pm in the Er-doped films. The Verdet constant was measured through the sample thickness, and the substrate signal dominated the measurements. However, the measured values were in agreement with published values for InP, which gives an indication of the films' host value. The Verdet constants increased from 4 to 7 deg/T/mm as the wavelength decreased toward the band edge. The band edges of our samples were 0.93, 1.62, and 1.30 pm, respectively. Rare earth dopants were observed to raise the refractive index of the InP films, and waveguiding at 1.3 pm was achieved in the rare earth-doped InP films and in the InGaAsP films. INTRODUCTION
This study focuses on the Faraday effect in rare-earth-doped InGaAsP materials. The Faraday effect involves the rotation of the plane of polarization of light propagating through a material parallel to an applied magnetic field. Along with the applications14 mentioned above,"-31 Faraday 51 as magnetic and electric field sensors, optical data storage media,[ rotators have also been used 1 61 modulators. and spatial light Faraday rotation occurs in a medium when the refractive indices of left- and right-circularly polarized light are unequal. This inequality is caused by an applied magnetic field through a splitting of the ground or excited state of a resonant excitation. Faraday rotation, 0(k), is therefore strongest near resonant absorption peaks, but it is useful at other wavelengths, as well. The material property used to compare Faraday rotators is the Verdet constant, V(?,), which is defined as follows: 0(,) =V(,) B (1) where k is the wavelength of the light propagating through the sample, ( is the path length, and B is the applied magnetic field tensor. It is important to note that Faraday rotation is nonreciprocal. That is, light traveling back through the material will continue to rotate in the same direction. This property is used in isolators since reflected light traveling back through the rotator will be rotated further from the initial polarization. Theref
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