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Dauphiné twins in quartz have been directly observed and photographed by using second-harmonic light. A theoretical analysis shows that a plane twin boundary in a plane parallel slab of quartz produces a system of interference fringes at the crystal output surface, corresponding to the existence of two second-harmonic beams with different directions. This was experimentally verified, and thus a topography of the twins has been obtained. Furthermore, the orientation of the twins relative to a twinless quartz crystal has been determined.
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Dolino et al. (1973) studied this question.
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