QPM

Quasi-Phase Matching (QPM) is a technique used in nonlinear optics to enable efficient frequency conversion by compensating the phase mismatch that arises when interacting waves propagate in a nonlinear medium.

Unlike conventional birefringent phase matching, QPM achieves phase matching by periodically modulating the material’s second-order nonlinear coefficient chi(2) along the propagation direction, effectively resetting the phase of the generated wave at regular intervals.

Implementation is commonly achieved by periodic poling, creating domains with alternating sign of chi(2). The period Λ is chosen so that the mismatch Δk equals 2πm/Λ, where m is the QPM order (often m=1).

Materials and devices include periodically poled lithium niobate (PPLN) and periodically poled potassium titanyl phosphate (PPKTP), fabricated by electric-field poling. This enables second-harmonic generation, sum- and difference-frequency generation, and various laser and spectroscopy applications.

Advantages include the ability to use noncritical phase matching in certain crystals, broader spectral acceptance, and the tailoring of interactions by engineering the poling pattern. Limitations involve domain accuracy, thermal sensitivity, and potential efficiency losses at higher QPM orders.

Other uses of the acronym exist in unrelated fields; for example, QPM may denote Quality Process Management or similar quality-assurance concepts, depending on context.