Bright Spatial Solitons Research Articles

Oscillatory Neck Instability of Spatial Bright Solitons in Hyperbolic Systems

The breakup of spatial bright optical solitons due to oscillatory neck instability is experimentally studied by propagating a laser beam in normally dispersive and self-focusing Kerr media. This intriguing and unusual phenomenon, recently predicted for solitons of the (2+1)-dimensional hyperbolic nonlinear Schrödinger (NLS) equation, is observed in the spatially resolved temporal spectrum. The snake instability that is known to occur in hyperbolic systems is also demonstrated to validate our experimental approach. Our results not only apply to photonics but also to other fields of physics, such as hydrodynamics or plasma physics, in which the hyperbolic NLS equation is used as a canonical model.

Modulation instability of copropagating light beams in nonlinear metamaterials

Linear stability analysis predicts that transverse modulation instability (MI) can occur for two optical beams copropagating in a Kerr nonlinear metamaterial (MM) interacting with each other through cross-phase modulation. A detailed discussion on the role of cross-phase modulation-induced coupling between beams on MI is presented, and it is found that MMs will enrich the propagating characteristics of beams. MI that occurs when beams are in different refractive-index regions is particularly interesting since it is a precursor to the formation of bright and dark spatial solitons. Furthermore, transverse MI can develop for different combinations of the signs of refractive index decided by the two beams, respectively, which makes the temporal MI in optical fibers find its transverse MI counterpart in MMs. Based on the Drude model, numerical simulations are performed to confirm theoretical predictions.

Screening-photovoltaic spatial solitons in biased two-photon photovoltaic photorefractive crystals

Steady-state bright (or dark) spatial solitons are predicted for biased two-photon photovoltaic-photorefractive materials due to two-photon photorefractive effects. These solitons differ from those solitons recently predicted by Hou et al. [Physical Review A71, 053817 (2005)] in their properties and experimental conditions. These solitons are known as two-photon screening-photovoltaic solitons. We present the one-dimensional dynamical evolution equation and find the soliton solutions under appropriate conditions in a steady state. By use of perturbation techniques, we also investigate the self-deflection process of these bright solitons with taking diffusion effect into account.

Self-deflection suppression of bright spatial solitons in periodically poled photovoltaic photorefractive crystals

The propagation behavior of bright spatial solitons in periodically poled photovoltaic (PV) photorefractive (PR) crystals is investigated by considering the diffusion effect. Our analysis indicates that the diffusion effect is modulated by the periodic domain inversion structure (PDIS) of PV PR crystal and therefore can be managed by designing PDIS. By employing numerical integration and perturbation analysis, we find that the self-deflection of the bright soliton beam arising from diffusion effect can be suppressed in PV PR crystals with an appropriate PDIS.

Exact bright and dark spatial soliton solutions in saturable nonlinear media

We present exact analytical bright and dark (black and grey) solitary wave solutions of a nonlinear Schrödinger-type equation describing the propagation of spatial beams in media exhibiting a saturable nonlinearity (such as centrosymmetric photorefractive materials). A qualitative study of the stationary equation is carried out together with a discussion of the stability of the solutions.

Temporal development of open-circuit bright photovoltaic solitons

This paper investigates the temporal behaviour of open-circuit bright photovoltaic spatial solitons by using numerical techniques. It shows that when the intensity ratio of the soliton, the ratio between the soliton peak intensity and the dark irradiance, is small, the quasi-steady-state soliton width decreases monotonically with the increase of τ, where τ is the parameter correlated with the time, that when the intensity ratio of the soliton is big, the quasi-steady-state soliton width decreases with the increase of τ and then increases with τ, and that the formation time of the steady-state solitons is not correlated with the intensity ratio of the soliton. It finds that the local nonlinear effect increases with the photovoltaic field, which behaves as that the width of soliton beams is small and the self-focusing quasi-period is short. On the other hand, we also discuss that both the time and the temperature have an effect on the beam bending.

NEW CLASS OF BRIGHT SPATIAL SOLITONS OBTAINED BY HIROTA'S METHOD FROM GENERALIZED COUPLED MODE EQUATIONS OF NONLINEAR OPTICAL BRAGG GRATING

We have demonstrated by Hirota's bilinear method the existence of a new class of bright spatial soliton solutions from the same model of nonlinear optical Bragg grating considered previously by another group of researchers. The explicit expressions obtained from these soliton profiles are distinctly different from the previous results and offer a much more flexible choice of physical parameters for device design. It was further shown that the present formulation provides a classification scheme incorporating previous results as special cases of different parameter sets. Finally, due to the diffraction effect, these solitons were shown to exhibit a certain degree of instability in their perturbed profiles as they propagate along the grating.

Experimental study of bright photovoltaic solitons in lithium niobate crystal

The effect of background beam with different Glass constant on the relative spatial refractive perturbation in photovoltaic crystal was discussed, in which was introduced the concrete conditions for the formation of bright or dark photovoltaic spatial solitons in crystals with negative refractivity change. Based on the results, for what is to our knowledge the first time, the bright photovoltaic solitons was experimentally observed in LiNbO3:Fe by use of e-ray signal of 532nm and o-ray background illumination of 488nm.

Large self-deflection of bright and dark screening photovoltaic solitons in LiNbO3

We demonstrate large self-deflection of both bright and dark screening photovoltaic solitons existing in LiNbO3 crystal under an external applied field with numerical method, and we also demonstrate that the self-deflection is not only connected with the acceptor concentration NA but also with the external applied field E0. Under the same E0, when NA is lower, the self-deflection is more obvious. Under the same NA, when E0 is higher, the self-deflection is more obvious. We also find that self-deflection of the bright screening spatial soliton and that of the dark one are different: the bright screening photovoltaic soliton deflects obviously; while only one side of the dark soliton deflects, its extreme value point and the other side almost do not deflect.

Observation of two-dimensional holographic photovoltaic bright solitons in a photorefractive-photovoltaic crystal

Two-dimensional holographic photovoltaic bright spatial solitons are observed in a Cu:K0.25Na0.75Sr1.5Ba0.5Nb5O15 crystal in which two coherent laser beams, a signal beam, as well as a strong and uniform pump beam at 532nm are coupled to each other via two-wave mixing. Such solitons originate from the self-trapping of the signal beam due to both photovoltaic effects and holographic focusing effects.

Amplitude and phase reconstruction of photorefractive spatial bright-soliton in LiNbO_3 during its dynamic formation by digital holography

The time behaviour of bright spatial solitons in congruent undoped lithium niobate crystal is experimentally investigated. Full field characterization of the optical wavefront emerging from the crystal during the soliton formation process is performed by digital holographic method. Experimental results of the amplitude and phase maps of the field distribution at the exit face of the crystal allow the real-time monitoring of the evolution of the soliton beam from the application of the external field to the end of the process when the generation of the channel waveguide appears to be stable. The features of the dynamics of the soliton formation are visualized, analyzed and compared to a time-dependent numerical model.

A semi-classical approach to two-frequency solitons in a three-level cascade atomic system

A semi-classical theory of two intense optical fields interacting with a third-order non-linear medium composed of a three-level cascade atomic system is presented. It is predicted that non-linear atom-field interactions allow the formation of two-frequency bright, dark and grey spatial solitons. We demonstrate through numerical simulations and analytic stability analysis that the bright and grey solitons are stable.

Photovoltaic solitons in two-photon photorefractive materials under open-circuit conditions

We present the evolution equation of one-dimensional spatial soliton in two-photon photorefractive media under open-circuit conditions. In the steady state regime, our solutions show that the dark and bright photovoltaic spatial solitons can be supported in two-photon photorefractive media under open-circuit conditions.

Optical spatial soliton supported by photoisomerization nonlinearity in a polymer with a background beam

We present detailed theoretical studies of optical spatial solitons (SSs) supported by photoisomerization nonlinearity in a polymer sample. One-dimensional dark and bright SSs and their existence curves are presented. Several combinations of polarizations of the signal and background beams can be used to form the SSs.

Formation of photovoltaic bright spatial soliton in photorefractive LiNbO3 crystal by a defocused laser beam induced by a background laser beam

We have shown, for the first time to our knowledge, both theoretically and experimentally that a bright photovoltaic (PV) spatial soliton (SS) can be formed in a photorefractive crystal with large Glass constant and negative refractive index perturbation, provided that the Glass constant of the background beam is larger than that of the self-trapped (signal) beam. The ratio of the effective Glass constants of the self-trapped beam and the background beam is the key parameter that determines whether the PV SS is bright or dark. We have demonstrated experimentally that bright SSs can be formed in LiNbO3 crystals using all four combinations of polarizations of the signal beam and the background beam.

Experimental investigation of phase-dependent interactions of photovoltaic bright spatial solitons in photorefractive Fe:LiNbO3

Phase-dependent interactions of two photovoltaic bright spatial solitons in photorefractive Fe:LiNbO3 have been studied experimentally. We have observed that in-phase solitons attract each other and fuse together for a small enough crossing angle, whereas out-of-phase solitons repel each other. These results are similar to interactions of bright screening spatial solitons found in saturable photorefractive media such as strontium barium niobate.

Do Solitonlike Self-Similar Waves Exist in Nonlinear Optical Media?

We show analytically that bright and dark spatial self-similar waves can propagate in graded-index amplifiers exhibiting self-focusing or self-defocusing Kerr nonlinearities. The intensity profiles of the novel waves are identical with those of fundamental bright or dark spatial solitons supported by homogeneous passive waveguides with the same type of nonlinearity. Thus, we reveal a previously unnoticed connection between spatial solitons and self-similar waves. We also suggest that the discovered self-similar waves can be used in a promising scheme for the amplification and focusing of spatial solitons in future all-optical networks.

3D-soliton waveguides in lithium niobate for femtosecond light pulses

We show that efficient waveguides can be written by bright spatial solitons in the volume of lithium niobate photorefractive crystals by cw and pulsed laser beams. Using high-repetition-rate femtosecond laser pulses, an efficient formation of soliton waveguides (SWGs) is possible, after accumulating a large number of pulses, because the characteristic photorefractive build-up time is much longer than the pulse period and the efficient two-photon absorption may contribute to the solitonic confinement. These results open the possibility of writing reconfigurable single SWGs and SWG arrays (with any spatial orientation and large range of periods) and optimally guiding the femtosecond pulsed laser beams through them, creating a graded refractive-index profile matched to the spatial beam profile. Our experiments also show a small increase in pulse duration (small dispersion) in these waveguides.

Electrically switched photoinduced waveguide in unpoled strontium barium niobate

The unpoled photorefractive strontium barium niobate crystal exhibiting quadratic electro-optic properties is characterized. It is used to form one-dimensional and two-dimensional bright photorefractive spatial solitons for either positive or negative low applied electric field. Solitons induce long-lived index changes in the medium which forms waveguides that can be tuned using an external voltage.

The self-deflection of photovoltaic bright spatial solitons on the basis of higher-order space-charge field

We investigate the effects of the higher-order space-charge field on the self-deflection of photovoltaic bright solitons in a photovoltaic photorefractive crystal by both numerical and perturbation methods under steady-state conditions. Our results indicate that there exits a characteristic value of photovoltaic fields for photovoltaic bright solitons. When the photovoltaic field is less than the characteristic value, these solitons always bend in the direction opposite to the crystal's c axis, and the absolute value of the spatial shift that is due to the first-order diffusion term alone is always larger than that which is due to both the first-order diffusion term and the higher-order terms acting together. If the strength of the photovoltaic field is larger than the characteristic value, soliton bending in both the same direction as and in the direction opposite to the crystal's c axis is possible. Whether the direction is in the same or in the opposite direction will depend on the strength of the photovoltaic field and on the input intensity. Specifically, self-deflection cannot occur for photovoltaic bright solitons if the strength of the photovoltaic field and the intensity of the input beam are appropriately selected. We provide some relevant examples to confirm our prediction.