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Nonlinear refraction from χ(2)(2) in KTP

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Abstract

We describe the cascading of two second-order nonlinearities [χ(2)(2)] to produce an effective third-order nonlinearity in KTP.1 This nonlinearity appears as either an irradiance dependent loss or phase distortion on the incident fundamental beam. The loss mechanism is simply the depletion of the fundamental due to second-harmonic generation. For low conversion efficiencies the loss of the fundamental is proportional to irradiance and appears as an effective Im[χ(3)] (i.e., as two-photon absorption). The refractive part of the nonlinearity [i.e., Reχ(3)] is less well known and usually ignored since it occurs off phase matching where ΔkL ≠ 0. This effect gives rise to either a phase advance or phase delay depending on the sign of ΔkL. In the experiment, we use a hydrothermally grown KTP sample of length L = 1 mm cut for type II SHG of 1064 nm and monitor the induced phase change of the incident fundamental beam using the Z-scan technique.2 The sign of the nonlinear refractive index changes sign as the angle tuned KTP sample is rotated through phase matching in accordance with theory. The experiment gives a maximum effective nonlinear refractive index n2≃±1 × 10-11 esu (±2 × 10-14 cm2/W) for an angle detuning of ±5°. This index scales with L and (deff)2 where we measure deff(KTP) = 3.1 pm/V.

© 1991 Optical Society of America

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