Nonlinear Effects in Photonic Materials - Abstract
Babushkin, Ihar
Coherent ultrabroadband radiation (supercontinuum) is required in many physical applications. Self-phase modulation in noble-gas-filled hollow waveguides with an instantaneous Kerr-nonlinearity is the key physical mechanism for spectral broadening and compression of mJ-pulses to a duration below 5 fs or two optical cycles (see e.g. [1]). Recently an enormous spectral broadening of more than two octaves [2] with a much lower threshold has ben achieved using nJ-pulses in microstructure fibers with an instantaneous Kerr-type nonlinearity in the anomalous dispersion range which is caused by the emission of dispersive (non-solitonic) radiation by solitons [3]. On the other hand, materials with a slow nonlinearity, such as photorefractive materials, were up to now not considered as media for femtosecond nonlinear processes and spectral broadening. In the present talk we theoretically consider a planar rib waveguide with a guiding layer formed from the photorefractive-photovoltaic material (LiNbO3 doped with Cu), which possess a non-instantaneous (slow) nonlinearity with a response time in the ms range. Our numerical simulations predict that the propagation of a short optical pulse through such slow nonlinear waveguide results in self-steepening and in the generation of a supercontinuum with a spectral width of more than one octave. The spectral broadening, achieved during the propagation is strongly asymmetric, with new spectral components being formed only on the red side, in contrast to the symmetric broadening due to self-phase modulation in Kerr media. With further propagation, the self-steepening results in an increase of the peak pulse intensity and the higher peak intensity enhances the spectral red-shift which in turn yields a stronger self-steepening. Therefore at some critical propagation distance a sharp shock-like peak is formed. Thus the mechanism of spectral broadening is in this case related to shock formation and strongly differs from the both abovementioned mechanisms in instantaneous nonlinear media. One can obtain a sufficiently broad supercontinuum both in normal and anomalous dispersion regime. In the region of normal dispersion the shock arises on the leading edge of the pulse, whereas for anomalous dispersion region the shock is formed on the trailing edge. The phase of the achieved supercontinuum is smooth with low noise and suggests the possibility of pulse compression. [1] T. Brabec and F. Krausz, Rev. Mod. Phys. 72, 545 (2000). [2] J.K.Ranka et al., Opt. Lett. 25,25 (2000). [3] A. Husakou and J. Herrmann, Phys.Rev. Lett. 87, 203901 (2001).