hull@hao.UUCP (Howard Hull) (11/29/84)
> I tried this a few weeks ago, no response...how about you physics types. > My understanding is that it rotates the angle of polarization of reflected > light. > Thanks ../Dave > -- Your perception is exactly correct. Shine monochromatic linear polarized light directly on the pole of an iron electromagnet with a polished end, and use an analyzer to extinguish the reflected light. Turn off the electromagnet and the polarization of the reflected light immediately changes to allow it to pass through the analyzer. The effect takes place within 0.050 wavelengths of the surface and is a magneto-optical effect. Supplemental and other reference information follows: Interaction between light, atomic or molecular mass and charge, and applied electric and/or magnetic fields produces transformations on the polarization of the incident beam that may be analyzed in the reflected or transmitted beams. The distribution of electric fields and magnetic dipole moment in atoms or molecules resulting from quantum atomic properties, or, in the case of large molecules, from spiral or crystalline structure are responsible for response of the medium to light and/or externally applied magnetic fields. Remember that a linearly polarized beam can be resolved into two opposite circularly polarized beams, and that a circularly polarized beam may be resolved into two orthagonal (w.r.t. to E or H vector) linear beams, one of which is time-phase retarded by 90 degrees with respect to the other. Longitudinal field effects are rooted in differential light propagation speeds for two orthagonally referenced incident beams (one orientation has to move some electrons around and thus slows down while the other doesn't). Transverse field effects are rooted in differential absorbtion (one orientation loses energy to electrons by exciting them to thermal valence level or coherent radio or light emission levels, the other doesn't). Birefringence is the property of differential light propagation velocity, and Dichroism is the property of differential absorbtion. The property of Optical Activity is one of having different propagation velocity or absorbtion for two opposite incident circular polarizations. Molecules with spiral structure (i.e. sugar) are capable of this sort of influence on the incident beam. Optical Activity may either be inherent with the atomic or molecular structure, or it may be induced while an electric field is applied, or while and after a magnetic field is applied. Various of the above properties can be observed in some cases for white light, but they are more usually observed at specific wavelengths for metallic or organic liquid or vapor. Birefringence and Dichroism are more common excited or unexcited properties for solids, although liquids or gasses may exhibit some of these properties under an applied electric or magnetic field due to the responsive alignment of molecules under stress. For solids the situation is more rigidly classifiable, even though where crystals are not specified below, criteria may apply to a liquid or gas as well: Substance: Transparent Transparent Doped Metallic Dielectric Crystal Semiconductor Reflector Mode: Electro-optic (Ext. E Vector) Effect Name: Stark (very high voltage gradients, E equivalent to Zeeman H) for formulae, see reference [1] Kerr (t) Pockles (l) Delta = Delta = Ck*l*(Ex^2)*Lambda Cp*(n^3)*Ez/2 affects: Linear Pol Linear Pol Prop Vel Differential Differential Absorbtion no no manifestation: Lin <-> Circ Lin <-> Circ Magneto-optic (Ext. H Vector) Effect Name: Cotton-Mouton Ferromagnetic Voigt (t) Kerr (t) Double Refraction Porter, Spencer Delta Phi = Delta Phi = & LeCraw (YIG) see note (i) see note (i) affects: Linear Pol [Unknown but Linear Pol Linear Pol Prop Vel Differential probably a Differential Differential Absorbtion no laser polar yes yes manifestation: Lin <-> Cir effect -fast! Lin <-> Cir Lin <-> Cir see ref. [3] Optically Active (Ext. E, H, or Int. Molecular Structure) Str or Appl H Str or Appl H plus free carriers Effect Name: Faraday (l) Faraday (l) Theta = Theta = Cv*Hz*l see note (ii) affects: Circular Pol Circular Pol Prop Vel Differential Differential Absorbtion no yes manifistation: Lin Rotation Lin Rotation Notes: (i) Delta Phi = (q^4)*N*(Lambda^3)*(Hx^2)*l/4*pi*(Cm^6)*n*(Mv^3) (ii) Delta Phi = (q^3)*N*(Lambda^2)*(Hz)*l/2*pi*(Cm^4)*n*(Meff^2) Definitions: (t) transverse. Rooted in transverse Zeeman effect (l) longitudinal. Rooted in longitudinal Zeeman effect l = Path length through portion of substance subject to applied field n = Index of refraction for substance in absence of applied field pi = 3.14159 The next two are described w.r.t. a specific crystal axis (Fast Eigen) Delta = Differential Phase for orthagonal incident E orientations Delta Phi = Differential Phase for orthagonal incident H orientations Lambda = Wavelength of incident light Theta = Angle between incident and transmitted E vector Lin Linear Cir Circular Pol Polarization Prop Propagation Vel Velocity <-> Transformed Str Structure Appl Applied (to or from) YIG Yttrium Iron Garnet Ck = Electro-optic Kerr Constant for the medium Cp = Electro-optic Pockels Constant for the medium Cm = Voight or Kerr Magneto-optic constant Cv = Verdet Constant for Optically Active or Activated Material Ex = Transverse applied electric field voltage Ez = Longitudinal applied electric field voltage Hx = Transverse applied magnetic field strength Hz = Longitudinal applied magnetic field strength For some reason, older books are more likely to mention many of these effects than modern books on Solid State Physics (within my grasp, anyway). Modern references will usually describe only one at a time, although the detail of the description is greater. [1] Optical Processes in Semiconductors, Jacques I. Pankove (Dover 1971) [2] Polarized Light, William A. Shurcliff (Harvard 1966) [3] Porter, C.S., E.G. Spencer, and R. LeCraw, "Transparent ferro- magnetic light modulator using yttrium iron garnet," J. Appl. Phys. 29,495(1958). [4] Fundamentals of Optics, Jenkins and White (McGraw-Hill 1957) Regards, Howard Hull, HAO {ucbvax!hplabs | allegra!nbires | harpo!seismo } !hao!hull