Defect Solitons Research Articles

Defect solitons in two-dimensional optical lattices

We report on the existence and stability of solitons in a defect embedded in a square optical lattice based on a photorefractive crystal with focusing saturable nonlinearity. These solitons exist in different bandgaps due to the change of defect intensity. For a positive defect, the solitons only exist in the semi-infinite gap and can be stable in the low power region but not the high power region. For a negative defect, the solitons can exist not only in the semi-infinite gap, but also in the first gap. With increasing the defect depth, these solitons are stable within a moderate power region in the first gap while unstable in the entire semi-infinite gap.

Defect solitons in optically induced one-dimensional photonic lattices in LiNbO3:Fe crystal

We observe experimentally defect solitons with the odd and the even symmetry in one-dimensional optically induced nonlinear photonic lattices in self-defocusing LiNbO3:Fe crystal when a Gaussian probe beam is launched at normal incidence. We find that the odd symmetric defect soliton is stable while the even symmetric defect soliton is unstable.

Defect solitons in photonic lattices

Nonlinear defect modes (defect solitons) and their stability in one-dimensional photonic lattices with focusing saturable nonlinearity are investigated. It is shown that defect solitons bifurcate out from every infinitesimal linear defect mode. Low-power defect solitons are linearly stable in lower bandgaps but unstable in higher bandgaps. At higher powers, defect solitons become unstable in attractive defects, but can remain stable in repulsive defects. Furthermore, for high-power solitons in attractive defects, we found a type of Vakhitov-Kolokolov (VK) instability which is different from the usual VK instability based on the sign of the slope in the power curve. Lastly, we demonstrate that in each bandgap, in addition to defect solitons which bifurcate from linear defect modes, there is also an infinite family of other defect solitons which can be stable in certain parameter regimes.

ENERGY LEVELS AND PROPERTIES OF DEFECTS ON RECONSTRUCTED DISLOCATIONS IN SILICON

The results of calculations of the energy levels of anti-phase (soliton) defects and a vacancy-soliton complex on reconstructed 90 degree and 30 degree partials in silicon are given. A Phillips-Pandey Hamiltonian and the recursion method were used to evaluate these levels and explore the effect of relaxation of surrounding atoms. These calculations should have relevance to either well-annealed samples of plastically deformed silicon or to samples deformed sufficiently slowly that point defect clusters are not introduced. Such samples are diamagnetic. The diamagnetism of a defect with a single dangling bond can be explained by the Anderson negative-U mechanism and estimates are given which suggest the anti-phase defect (soliton) is indeed of this type. Some implications of this are included.

Solitons in a linear lattice with defects

Solitons are generated in an anharmonic linear lattice in which neighbouring atoms interact through a Morse potential by giving either a strong initial impulse or a large displacement to an end atom. Studies on the variation of the characteristic properties of the soliton with the strength of the initial pulse show that the velocity and the amplitude of the soliton increase with the strength of the initial impulse, but below a certain critical value for the initial impulse, only an oscillatory tail is generated. It is shown that when a defect is present in the lattice, a localised mode appears at the site of the defect and additional solitons travelling forward or even backwards, are generated. When two solitons collide at a defect region, they reemerge but leave a localised mode at the site of the defect. If an initial velocity is imparted to a particular particle, synchronously with the crossing of the particle by the soliton, a localised mode emerges at the site of the particle and additional solitons are also generated. When a soliton moves from a denser to a rarer medium, a strong localised pulse is created near the region of the density discontinuity and additional solitons appear; and further a weak oscillatory tail is left behind in the denser medium. On the other hand, if a soliton moves from a rarer to a denser medium, it is reflected back and a small localised mode is generated at the site of the density discontinuity. The variation of amplitude of the soliton with the velocity of propagation is also studied.