The linear electro-optic coefficients of KH2PO4 and a number of its isomorphs have been measured at room temperature using dc voltages. The results at 546 nm are as follows: r63=10.3±0.1×10-12m/V for KH2PO4, 10.9±0.1×10-12 m/V for KH2AsO4, and 13.0±0.2×10-12 m/V for RbH2AsO4; r41=8.77±0.14×10-12 m/V for KH2PO4, 24.5±0.4×10-12 m/V for NH4H2PO4, 12.5±0.4×10-12 m/V for KH2AsO4, and 8.8±0.4×10-12 m/V for KD2PO4 which was about 90% deuterated. A transverse modulator based on a single 45° Y-cut crystal of NH4H2PO4 is proposed which requires only 45 V rms for 50% amplitude modulation of 632.8 nm light.
JAMES H. OTT and T. R. SLIKER, "Linear Electro-Optic Effects in KH2PO4 and Its Isomorphs," J. Opt. Soc. Am. 54, 1442-1444 (1964)