Millimeter wave and far‐infrared investigation on KDP with asymmetric interferometers

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Abstract
The complex dielectric constant in KDP is determined at ω = 4.62 cm−1 (Δ = 2.16mm) for E ‖ c and E ⊥ c between 100 and 295 K by measurement of the complex reflection coefficient using a Michelson interferometer. At room temperature the complex dielectric function (ω) is determined for E ‖ c and E ⊥ c in the far infrared using an asymmetric Fourier spectrometer. The data for E ‖ c, T > TC are discussed with the model of modemode coupling. The soft mode frequency of this model is found to vary as ω = A(T‐117.7)/(T + 155) with temperature. For E ⊥ c a heavily damped mode is found below 80 cm−1 which might be due to a proton E‐mode.