Efficient collinear fourth harmonic generation by two-channel multistep cascading in a two-dimensional nonlinear photonic crystal

Summary form only given. Quasi-phase matching (QPM) (Fejer et al, 1992) is an established technique for efficient frequency conversion. QPM structures have a uniform refractive index, and a periodic quadratic nonlinearity. The nonlinearity is usually periodic in one direction, and is piecewise constant in strips. A two-dimensional generalization was recently proposed by Berger (Phys. Rev. Lett. vol. 81, p. 4136, 1998) and studied experimentally by Broderick et al (Phys. Rev. Lett. vol. 84, p. 4345, 2000). In Broderick´s samples, the refractive index is again uniform, but the periodicity derives from a hexagonal lattice, with a single hexagon in each unit cell in which the nonlinearity is inverted. Using a fundamental wavelength of /spl lambda/=1533 nm, Broderick et al demonstrated efficient second harmonic (SH) generation, and also third and fourth harmonic (FH) generation, the latter of which is of importance here. FH generation occurred stepwise, but was not efficient since not all steps were phase matched. Saltiel et al (2000) proposed a scheme for efficient FH generation, in which, however, the fundamental and FH are not collinear. Other FH generation schemes either involve nonphase matched processes, two separate crystals, or rely on some property of the host´s refractive index (Hooper et al, 1994; Pfister et al, 1997). Here, we discuss a scheme that makes use of a single nonlinear crystal, and in which the FH is collinear with the fundamental. The efficiency is also larger than in comparable schemes.