Mercurous halides for long time-delay Bragg cells

The mercurous halides, Hg2CI2, Hg2Br2, and Hg2l2 are materials with highly anisotropic and unusual properties. They are transparent from the blue through the far infrared and they have high refractive indices and large birefringence. Most important, they support shear acoustic wave propagation along the crystallographic <1 1 0> directions whose velocity is extraordinarily low, slower even than the speed of sound in air. Because of this slow velocity Bragg cells fabricated from the mercurous halides, offer very long time delay signal processing in a compact package. What is more, the other properties combine to produce a very high acousto optic figure of merit which in turn implies very high efficiency at low drive power. We have fabricated cells from the mercurous halides with delays up to 180 micro seconds which require only a few hundred milliwatts drive1. Key to exploiting the mercurous halides for high performance Bragg cells is the availability of single crystals with the correct orientation and with low optical and acoustic loss. Considerable effort on crystal growth technology has produced remarkable i mprovem ent in these properties2.3. Crystals have been grown which permit fabrication of Bragg cells approaching 200jis delay time with reasonable optical quality. In this application, the extremely low acoustic velocity allows construction of a long delay device in less than 1/2 the volume required by other materials. Thus, the shorter acoustic path leads to a small optical aperture which, in turn, translates to a compact and practical signal processing element. Thus, development of Bragg cells of the mercurous halides will lead to compact, higher performance acousto-optic processors .