PHASE-MATCHED OPTICAL 2ND-HARMONIC GENERATION IN HELICALLY TWISTED SMECTIC-C-ASTERISK PHASE

Phase matching for optical second-harmonic generation in the helically twisted smectic-C liquid-crystalline phase is analyzed theoretically . Ten distinctive phase-matched combinations of the fundamental and th e second-harmonic eigenmodes exist. The effective second-order nonline ar optical susceptibility and the relation between the optical wave ve ctors and the wave vector of helical structure are determined for each of the combinations. The results obtained within the standard approxi mation of the slowly varying amplitude are compared with the exact num erical solutions of the nonlinear wave equation. The discrepancy betwe en the approximative and exact results is crucial only for the two pha se-matched combinations that are realized when the second-harmonic fre quency is close to the selective reflection band. In this case an addi tional enhancement of the second-harmonic beam can appear.