A general description is given of a long-neglected use for the piezo-optic or photoelastic effect (stress-induced birefringence). An acoustic vibration, such as a fundamental extensional mode in a bar or long thin plate, is set up in a block of isotropic transparent material, such as glass or fused silica; the vibration is sustained by a transducer. The resulting modulated birefringence can be used in a variety of ways, notably to produce a beam of alternately left- and right-circularly polarized light for circular-dichroism measurements. Strains of the order 10−5 are required, considerably below the breakage point for most materials. Because advantage is taken of the high Q of the vibrational modes, typically 103 to 104, very small transducer power is needed, usually less than 1 W. The literally enormous useful angular aperture, of the order 50° total cone angle, makes the device far superior to Pockels or Kerr cells for many applications. Reference is made to current practical realizations and to present and future uses of the device.
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