Inhibition of light tunneling for multichannel excitations in longitudinally modulated waveguide arrays

We consider evolution of multichannel excitations in longitudinally modulated waveguide arrays where refractive index either oscillates out-of-phase in all neighboring waveguides or when it is modulated in phase in several central waveguides surrounded by out-of-phase oscillating neighbors. Both types of modulations allow resonant inhibition of light tunneling, but only the modulation of latter type conserves the internal structure of multichannel excitations. We show that parameter regions where light tunneling inhibition is possible depend on the symmetry and structure of multichannel excitations. Antisymmetric multichannel excitations are more robust than their symmetric counterparts and experience nonlinearity-induced delocalization at higher amplitudes.Comment: 17 pages, 6 figures, to appear in Physical Review

Lifetime, collapse and escape paths for hopfions in bulk magnets with competing exchange interactions

The lifetimes of magnetic hopfions on a discrete lattice with competing exchange interactions are calculated within the framework of the transition state theory for magnetic degrees of freedom. Three sets of discrete model parameters corresponding to the same continuous micromagnetic model are considered. Minimal energy paths for hopfion collapses were found on the multidimensional energy surface of the system. The activation energies of the collapse processes have been calculated. It turned out that the activation energy differs significantly for the three considered values of the parameters, which indicates the importance of lattice effects, when the hopfion radius equals several lattice constants. Along with the collapse, the hopfion escape process through the sample boundary is studied. It is shown that this process does not require an activation energy. The lifetimes of hopfions are found and it is shown that they can exist only at temperatures of a few kelvins and practically cannot be generated due to thermal fluctuations

Asymmetric solitons and domain walls supported by inhomogeneous defocusing nonlinearity

We show that an inhomogeneous defocusing nonlinearity that grows toward the periphery in the positive and negative transverse directions at different rates can support strongly asymmetric fundamental and multipole bright solitons, which are stable in wide parameter regions. In the limiting case when nonlinearity is uniform in one direction, solitons transform into stable domain walls (fronts), with constant or oscillating intensity in the homogeneous region, attached to a tail rapidly decaying in the direction of growing nonlinearity.Comment: 3 pages, 5 figures, to appear in Optics Letter

The effect of confinement and defects on the thermal stability of skyrmions

The stability of magnetic skyrmions against thermal fluctuations and external perturbations is investigated within the framework of harmonic transition state theory for magnetic degrees of freedom. The influence of confined geometry and atomic scale non-magnetic defects on the skyrmion lifetime is estimated. It is shown that a skyrmion on a track has lower activation energy for annihilation and higher energy for nucleation if the size of the skyrmion is comparable with the width of the track. Two mechanisms of skyrmion annihilation are considered: inside the track and escape through the boundary. For both mechanisms, the dependence of activation energy on the track width is calculated. Non-magnetic defects are found to localize skyrmions in their neighborhood and strongly decrease the activation energy for creation and annihilation. This is in agreement with experimental measurements that have found nucleation of skyrmions in presence of spin-polarized current preferably occurring near structural defects

Dynamic versus Anderson wavepacket localization

We address the interplay between two fundamentally different wavepacket localization mechanisms, namely resonant dynamic localization due to collapse of quasi-energy bands in periodic media and disorder-induced Anderson localization. Specifically, we consider light propagation in periodically curved waveguide arrays on-resonance and off-resonance, and show that inclusion of disorder leads to a gradual transition from dynamic localization to Anderson localization, which eventually is found to strongly dominate. While in the absence of disorder, the degree of localization depends critically on the bending amplitude of the waveguide array, when the Anderson regime takes over the impact of resonant effects becomes negligible.Comment: 13 pages, 5 figures, to appear in Physical Review

Light bullets by synthetic diffraction-dispersion matching

We put forward a new approach to generate stable, fully three-dimensional light bullets, which is based on the matching of the intrinsic material dispersion with a suitable effective diffraction. The matching is achieved in adequate waveguide arrays whose refractive index is periodically modulated along the direction of light propagation. We show that by using non-conventional, out-of-phase longitudinal modulation of the refractive index of neighboring channels, it is possible to tune the effective diffraction to match the intrinsic material group velocity dispersion. Three-dimensional light bullets are shown to form at reduced energy levels, in settings where the dispersion would be far too weak to generate bullets in the absence of array.Comment: 13 pages, 4 figures, to appear in Physical Review Letter

Generation of platicons and frequency combs in optical microresonators with normal GVD by modulated pump

We demonstrate that flat-topped dissipative solitonic pulses, platicons, and corresponding frequency combs can be excited in optical microresonators with normal group velocity dispersion using either amplitude modulation of the pump or bichromatic pump. Soft excitation may occur in particular frequency range if modulation depth is large enough and modulation frequency is close to the free spectral range of the microresonator.Comment: 10 pages, 4 figures, to appear in EP