All optically biaxial crystals exhibit orientations in which they behave as thermally insensitive (TI) wave plates. Given sufficient electro-optic coupling at these orientations, biaxial, ferroelectric crystals may be advantageously employed as single-crystal, temperature-insensitive electro-optic modulators. I have experimentally confirmed the TI orientation in the principal dielectric plane of the biaxial crystal KTiOPO4 (KTP). KTP behaves as a TI wave plate at 33.7 deg from the Z axis in the X–Z principal plane. The effective electro-optic coupling, Reff, at this orientation is 19 pm/V. The electro-optic coupling and utility of these TI orientations in KTiOAsO4 and KNbO3 are also examined. The calculated Reff of KNbO3 at the TI orientation is expected to be >2000 pm/V.
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In KTP α, β, and γ correspond to the X, Y, and Z axes, respectively. nα′ < nβ′ < nγ′.6–11,14
In KNbO3α, β, and γ correspond to the Z, X, and Y axes, respectively (the Z, X, and Y axes have unit cell lengths of 0.5722, 0.5697, and 0.3971 nm, respectively). In both crystals Z is coincident with the twofold or polar axis. Note that in KNbO3, nγ′ < nβ′ < nα′.5,12,13
Table 2
Estimated Location of the Principal-Plane TI Orientation in Degrees Measured from Z axis
Crystal
λ = 1.064 μm
λ = 632.8 nm
KTP
30.4
32.0
KTA
29.7
30.4
KNbO3
53.9
54.3
Table 3
Propagation Direction, Field Direction, and Effective Electro-optic Coefficients
In KTP α, β, and γ correspond to the X, Y, and Z axes, respectively. nα′ < nβ′ < nγ′.6–11,14
In KNbO3α, β, and γ correspond to the Z, X, and Y axes, respectively (the Z, X, and Y axes have unit cell lengths of 0.5722, 0.5697, and 0.3971 nm, respectively). In both crystals Z is coincident with the twofold or polar axis. Note that in KNbO3, nγ′ < nβ′ < nα′.5,12,13
Table 2
Estimated Location of the Principal-Plane TI Orientation in Degrees Measured from Z axis
Crystal
λ = 1.064 μm
λ = 632.8 nm
KTP
30.4
32.0
KTA
29.7
30.4
KNbO3
53.9
54.3
Table 3
Propagation Direction, Field Direction, and Effective Electro-optic Coefficients