Optical Observation of Elastic Resonances in NH4H2PO4

Abstract

Elastic resonances in NH₄H₂PO₄ (ADP) have been studied by an optical method which depends on the elasto‐optic effect. Displacement models of the overtone resonance modes have been inferred from the optical patterns. For a (zxt) 45° plate, the transmission patterns for the stronger resonances are consistent with the picture of two longitudinal standing waves traveling at right angles; the reduced frequency constants are independent of the mode number m, and the sample resonates as m² Lamé plates. For a (zxt) 0° plate, the greater part of the transmission patterns can be explained in terms of two shear standing waves traveling at right angles; the reduced frequency constants depend on the mode number n, but they tend to become constant as n increases. For both types of plate, the configuration of the electrodes is important in determining which modes are excited and to what extent.