quasiphase

Quasiphase, usually encountered as quasi-phase matching (QPM), is a technique in nonlinear optics used to enable efficient frequency conversion by periodically modulating a material’s nonlinear response to compensate for phase mismatch between interacting waves.

In a second-order nonlinear process, the generated wave at frequency ω3 arises from the nonlinear polarization P(2) that couples waves at ω1 and ω2. If the phase mismatch Δk = k3 − k1 − k2 is nonzero, the converted intensity oscillates and growth is limited. QPM mitigates this by reversing or modulating the sign or strength of the nonlinear coefficient χ(2) with a spatial period Λ. This periodic structure introduces a reciprocal lattice vector 2π/Λ into the interaction, so that Δk − 2πm/Λ ≈ 0 for some order m (often m = 1). The first-order QPM is the most common, giving a nonzero effective nonlinearity despite the inherent phase mismatch.

Practically, QPM is realized by periodically poling ferroelectric crystals, most notably lithium niobate (periodically poled lithium niobate, PPLN) and lithium tantalate (PPLT). The poling period Λ is designed for the target wavelengths and temperature. Higher-order QPM uses larger Λ but reduces the effective nonlinearity, while aperiodic or chirped poling can broaden the operational bandwidth.

Applications include second-harmonic generation, sum- and difference-frequency generation, optical parametric amplification, and terahertz generation, as well as the production of entangled photon pairs. QPM provides a flexible alternative to traditional birefringent phase matching, enabling efficient conversion across a wide range of wavelengths and materials, with design trade-offs related to fabrication and thermal sensitivity.