Domain Wall Solitons Research Articles

Generation of spatiotemporal dark pulse or polarization domain wall solitons in a partial multimode mode-locked fiber laser

In this paper, we have experimentally investigated different operational states of a partial multimode Er-doped spatiotemporal mode-locked (STML) fiber laser. The STML is enabled by nonlinear multimode interference (NL-MMI) effect from the partial multimode cavity structure. It is found that by adjusting the multimode polarization controller (PC) and the pump power, the STML dark pulses, bright pulses, as well as the polarization domain wall solitons can all be obtained, alternatively. The fused-tapered multimode coupler is responsible for the dark pulses generation via the enhanced modal cross-phase coupling. It is believed that the present findings can offer an important insight into the spatiotemporal dynamics of the STML solitons.

Giant Atomic Swirl in Graphene Bilayers with Biaxial Heterostrain.

The study of moiré engineering started with the advent of van der Waals heterostructures in which stacking two-dimensional layers with different lattice constants leads to a moiré pattern controlling their electronic properties. The field entered a new era when it wasfound that adjusting the twist between two graphene layers led to strongly-correlated-electron physics and topological effects associated with atomic relaxation. Twist is now used routinely to adjust the properties of two-dimensional materials. Here, weinvestigate a new type of moiré superlattice in bilayer graphene when one layer is biaxially strained with respect to the other - so-called biaxial heterostrain. Scanning tunneling microscopy measurements uncover spiraling electronic states associated with a novel symmetry-breaking atomic reconstruction at small biaxial heterostrain. Atomistic calculations using experimental parameters as inputs reveal that a giant atomic swirl forms around regions of aligned stacking to reduce the mechanical energy of the bilayer. Tight-binding calculations performed on the relaxed structure show that the observed electronic states decorate spiraling domain wall solitons as required by topology. This study establishes biaxial heterostrain as an important parameter to be harnessed for the next step of moiré engineering in van der Waalsmultilayers. This article is protected by copyright. All rights reserved.

Temporal manipulation of period-2 polarization domain wall solitons in a nonlinear fiber Kerr resonator

We report on the experimental demonstration of temporal manipulations of dissipative polarization domain wall solitons in a Kerr nonlinear resonator operated in a period-doubled configuration. Our experiments are performed in a coherently driven normally dispersive fiber ring cavity, in which polarization domains emerge in virtue of a spontaneous symmetry breaking phenomenon. The addition of a lumped birefringent defect into the cavity forces their polarization components to swap on a two-round-trip cycle, thus protecting them from the deleterious impact of the system’s asymmetries. We exploit these symbiotic dynamics in proof-of-concept buffering experiments, in which these dissipative structures are addressed individually and temporally manipulated through phase-encoded perturbations imprinted onto the driving beam. Our results provide new insights into the practical exploitation of spontaneous symmetry breaking mechanisms and the use of two-component dissipative structures for all-optical data storage applications.

Fractional topological solitons in nonlinear viscoelastic ribbons with tunable speed

A unique type of domain-wall solitons, fractional topological solitons, have been recently theoretically shown to arise in minimal surface ribbons and demonstrate topological robustness. Here we study these fractional topological solitons experimentally using 3D printed helicoid ribbons and computationally using finite-element analysis (FEA). We show that these fractional solitons propagate at constant speed despite the strong dissipation in the viscoelastic material the ribbon consists of, and this speed can be sensitively controlled by an axial load. The nonlinear viscoelastic FEA quantitatively captures fine features of this fractional soliton. The results verify the predicted fractional solitons, and more importantly, open a new pathway of realizing and controlling topological fractional excitations in systems with complex material features including nonlinearity and viscoelasticity.

The dynamics of domain wall strings

We study the dynamics of domain wall solitons in (2+1)d field theories. These objects are extended along one of the spatial directions, so they also behave as strings; hence the name of domain wall strings. We show analytically and numerically that the amount of radiation from the propagation of wiggles on these objects is negligible except for regions of high curvature. Therefore, at low curvatures, the domain wall strings behave exactly as the Nambu-Goto action predicts. We show this explicitly with the use of several different numerical experiments of the evolution of these objects in a lattice. We then explore their dynamics in the presence of internal mode excitations. We do this again by performing field theory simulations and identify an effective action that captures the relevant interactions between the different degrees of freedom living on the string. We uncover a new parametric resonance instability that transfers energy from the internal mode to the position of the domain wall. We show that this instability accelerates the radiation of the internal mode energy. We also explore the possibility of exciting the internal mode of the soliton with the collision of wiggles on the domain wall. Our numerical experiments indicate that this does not happen unless the wiggles have already a wavelength of the order of the string thickness. Finally, we comment on the possible relevance of our findings to cosmological networks of defects. We argue that our results cast some doubts on the significance of the internal modes in cosmological applications beyond a brief transient period right after their formation. This, however, should be further investigated using cosmological simulations of our model.

Tantalum disulfide nanosheets for the generation of polarization domain wall solitons and polarization locked vector solitons

Tantalum disulfide (TaS2) has been employed for catalysis and optical switch due to its tunable band gap, strong photoluminescence, and controllable size. However, its nonlinear optical response and applications in ultrafast optics have not been exploited. In this work, TaS2 based saturable absorber (TaS2-SA) is prepared, which exhibits strong saturable absorption characteristics with modulation depth of 8.5%. Based on the TaS2-SA, polarization domain wall solitons and polarization locked vector solitons are successfully realized. To the best of our knowledge, this is the first study of the polarization characteristics of TaS2 based fiber lasers. Our findings suggest that TaS2 is a promising nonlinear optical material and provide a valuable method for the development of TaS2 based ultrafast photonic devices.

Generation of domain-wall solitons in an anomalous dispersion fiber ring laser* *Project supported by the National Natural Science Foundation of China (Grant Nos. 11874040, 11904327, and 61905222), Foundation for Leading Talents of Minhang, Shanghai, and the Specialized Research Fund for the Doctoral Program of Zhengzhou University of Light Industry (Grant No. 0170-13501050036).

We report experimental observations performed using a net anomalous dispersion Er-doped fiber ring laser without polarization-selective elements, highlighting the domain-wall solitary pulses generated under the incoherent polarization coupling. By adjusting the pump power and the polarization state appropriately, bright and dark solitons can stably co-exist in the cavity, both centered at 1562.16 nm with a 3-dB spectral width of ∼ 0.15 nm and a repetition rate of 3.83 MHz. Moreover, the 0.8 mm long thulium-doped fiber (TDF) facilitated the mode-locking and self-starting of the laser. This is the first demonstration of a laser being used to generate bright and dark solitons synchronously while using TDF as the saturable absorber (SA). Except possessing the all-fiber structure, the laser exhibits good stability, which may have a significant influence on improvement of the pulse-laser design, and may broaden practical applications in optical sensing, optical communication, and soliton multiplexed systems.

Collision between soliton and polarization domain walls in fiber lasers.

We report on the experimental observation of coexistence of solitons and polarization domains, as well as features of soliton collision with polarization domain walls (or kink solitons) in a single mode fiber laser. Depending on the sign of cavity dispersion, either bright or dark solitons have been formed in our fiber laser. Under suitable laser operation conditions, they could even coexist with polarization domains and collide with the domain walls. We show experimental evidence of both the elastic and inelastic soliton collisions with the domain wall solitons. Some interesting features of soliton interaction with polarization domains in a fiber laser are also firstly revealed.

Harmonic Mode-Locking in Bidirectional Domain-Wall Soliton Fiber Lasers

We report on the first experimental observation of passively harmonic mode-locking (HML) in a bidirectional domain-wall dark-soliton fiber laser. The domain-wall solitons (DWSs) are characterized, in which, the strong coupling between the two directions takes great responsibility for the generation. By appropriately altering the polarization state in the cavity, the stable fundamental domain-wall solitons and HML states could be obtained and switched with increasing the pump power. In our experiment, the repetition rate can be tunable from the fundamental mode locking of 2.96 MHz up to eighth-order HML of 23.68 MHz when the pump power is increased from 120 to 542.5 mW. Importantly, this HML operation of dark-pulse DWSs confirms that the DWSs are the intrinsic feature of the mode- locked lasers, as well as the conventional bright solitons. This work could provide a new perspective to the understanding of the formation and transformation of dark solitons, which may also find great potential applications in fiber sensing and future optical communication systems.

Bidirectional dark-soliton fiber lasers for high-sensitivity gyroscopic application.

Bidirectional mode-locked lasers are very useful in laser sensing and optical communications. Here we report a bidirectional domain wall soliton (DWS) fiber laser with an anomalous dispersion cavity. Two mode-locked dark pulse trains propagating in the opposite directions have been generated. Moreover, the specific application as a gyroscopic effect has been demonstrated by mounting this DWS laser on a rotating platform. The beat frequency of the two dark pulse DWS beams is measured as a linear function of the rotation velocity. The rotation sensitivity reaches 3.31kHz/(deg/s), which are comparable with the one in a bright pulse laser gyroscope. Without the limit of using tunable delay lines, the DWS laser gyroscope has the advantages of simple structure, high sensitivity, and low cost, while possessing the entire superiority of a mode-locked laser gyroscope. It is more promising for the applications in modern inertia navigation systems.

Soliton Fractional Charges in Graphene Nanoribbon and Polyacetylene: Similarities and Differences.

An introductory overview of current research developments regarding solitons and fractional boundary charges in graphene nanoribbons is presented. Graphene nanoribbons and polyacetylene have chiral symmetry and share numerous similar properties, e.g., the bulk-edge correspondence between the Zak phase and the existence of edge states, along with the presence of chiral boundary states, which are important for charge fractionalization. In polyacetylene, a fermion mass potential in the Dirac equation produces an excitation gap, and a twist in this scalar potential produces a zero-energy chiral soliton. Similarly, in a gapful armchair graphene nanoribbon, a distortion in the chiral gauge field can produce soliton states. In polyacetylene, a soliton is bound to a domain wall connecting two different dimerized phases. In graphene nanoribbons, a domain-wall soliton connects two topological zigzag edges with different chiralities. However, such a soliton does not display spin-charge separation. The existence of a soliton in finite-length polyacetylene can induce formation of fractional charges on the opposite ends. In contrast, for gapful graphene nanoribbons, the antiferromagnetic coupling between the opposite zigzag edges induces integer boundary charges. The presence of disorder in graphene nanoribbons partly mitigates antiferromagnetic coupling effect. Hence, the average edge charge of gap states with energies within a small interval is , with significant charge fluctuations. However, midgap states exhibit a well-defined charge fractionalization between the opposite zigzag edges in the weak-disorder regime. Numerous occupied soliton states in a disorder-free and doped zigzag graphene nanoribbon form a solitonic phase.

Generation of Bright/Dark Pulses in an Erbium-Doped Fiber Laser Mode-Locked With Glycerin

A passively mode-locked Erbium-doped fiber laser using glycerin as a saturable absorber (SA) has been proposed and experimentally demonstrated. The glycerin-SA is made of two fiber ferrules matched into a cannula, and the gap between them is filled with glycerin. Such an SA incorporated into the cavity leads to stable mode-locking operation. The generation of both bright and dark pulses is observed. The bright pulse is centered at 1568.92 nm, with a repetition rate of 23.08 MHz, the maximum output power of 33.5 mW, duration of 3.57 ps, and a signal-to-noise ratio of 55.6 dB, which exhibits good performance on self-starting and stability. Moreover, the dark pulses with the same repetition rate are achieved by adjusting the polarization state properly. The dark pulses are contributed to generating the domain-wall solitons induced by the orthogonal polarization coupling. The effectiveness of glycerin-SA as a mode-locker is validated. Considering its superiority in terms of low cost, easy fabrication, and non-volatile feature, we believe that glycerin-SA can be found many practical applications in ultrafast photonics.

Domain Wall Solitons Arising in Classical Gauge Field Theories

Domain wall solitons are basic constructs realizing phase transitions in various field-theoretical models and are solutions to some nonlinear ordinary differential equations descending from the corresponding full sets of governing equations in higher dimensions. In this paper, we present a series of domain wall solitons arising in several classical gauge field theory models. In the context of the Abelian gauge field theory, we unveil the surprising result that the solutions may explicitly be constructed, which enriches our knowledge on integrability of the planar Liouville type equations in their one-dimensional limits. In the context of the non-Abelian gauge field theory, we obtain some existence theorems for domain wall solutions arising in the electroweak type theories by developing some methods of calculus of variations formulated as direct and constrained minimization problems over a weighted Sobolev space.

Recent progress of study on optical solitons in fiber lasers

Solitons are stable localized wave packets that can propagate long distance in dispersive media without changing their shapes. As particle-like nonlinear localized waves, solitons have been investigated in different physical systems. Owing to potential applications in optical communication and optical signal processing systems, optical solitons have attracted intense interest in the past three decades. To experimentally study the formation and dynamics of temporal optical solitons, fiber lasers are considered as a wonderful nonlinear system. During the last decade, several kinds of theoretically predicted solitons were observed experimentally in fiber lasers. In this review, we present a detailed overview of the experimentally verified optical solitons in fiber lasers, including bright solitons, dark solitons, vector solitons, dissipative solitons, dispersion-managed solitons, polarization domain wall solitons, and so on. An outlook for the development on the solitons in fiber lasers is also provided and discussed.

Manipulation of domain-wall solitons in bi- and trilayer graphene.

Topological dislocations and stacking faults greatly affect the performance of functional crystalline materials1-3. Layer-stacking domain walls (DWs) in graphene alter its electronic properties and give rise to fascinating new physics such as quantum valley Hall edge states4-10. Extensive efforts have been dedicated to the engineering of dislocations to obtain materials with advanced properties. However, the manipulation of individual dislocations to precisely control the local structure and local properties of bulk material remains an outstanding challenge. Here we report the manipulation of individual layer-stacking DWs in bi- and trilayer graphene by means of a local mechanical force exerted by an atomic force microscope tip. We demonstrate experimentally the capability to move, erase and split individual DWs as well as annihilate or create closed-loop DWs. We further show that the DW motion is highly anisotropic, offering a simple approach to create solitons with designed atomic structures. Most artificially created DW structures are found to be stable at room temperature.

Pulse dynamics of dual-wavelength dissipative soliton resonances and domain wall solitons in a Tm fiber laser with fiber-based Lyot filter.

We report on the first demonstration of dual-wavelength square-wave pulses in a thulium-doped fiber laser. Under appropriate cavity parameters, dual-wavelength dissipative soliton resonances (DSRs) and domain wall solitons (DWSs) are successively obtained. Meanwhile, dark pulses generation is achieved at the dual-wavelength DWSs region due to the overlap of the two domain wall pulses. The fiber-based Lyot filter, conducted by inserting PMF between an in-line PBS and a PD-ISO, facilitates the generation of dual-wavelength operation. The polarization-resolved investigation suggests that the cross coupling between two orthogonal polarization components in the high nonlinear fiber plays an important role in the square-wave pulses formation. The investigation may be helpful for further understanding the square-wave pulse formation and has potential in application filed of multi-wavelength pulsed fiber lasers.

Observation of chirality transition of quasiparticles at stacking solitons in trilayer graphene

Trilayer graphene (TLG) exhibits rich novel electronic properties and extraordinary quantum Hall phenomena owning to enhanced electronic interactions and tunable chirality of its quasiparticles. Here, we report direct observation of chirality transition of quasiparticles at stacking solitons of TLG via spatial-resolved Landau level spectroscopy. The one-dimensional stacking solitons with width of the order of 10 nm separate adjacent Bernal-stacked TLG and rhombohedral-stacked TLG. By using high field tunneling spectra of scanning tunneling microscopy, we measured Landau quantization in both the Bernal-stacked TLG and the rhombohedral-stacked TLG and, importantly, we observed evolution of quasiparticles between the chiral degree l = 1&2 and l = 3 across the stacking domain wall solitons. Our experiment indicates that such a chirality transition occurs smoothly, accompanying the transition of the stacking orders of TLG, around the domain wall solitons. This result demonstrates the important and hitherto neglected relationship between the crystallographic stacking order and the chirality of quasiparticles in graphene systems.

Treatment of Oil Wastewater and Electricity Generation by Integrating Constructed Wetland with Microbial Fuel Cell.

Conventional oil sewage treatment methods can achieve satisfactory removal efficiency, but energy consumption problems during the process of oil sewage treatment are worth attention. The integration of a constructed wetland reactor and a microbial fuel cell reactor (CW-MFC) to treat oil-contaminated wastewater, compared with a microbial fuel cell reactor (MFC) alone and a constructed wetland reactor (CW) alone, was explored in this research. Performances of the three reactors including chemical oxygen demand (COD), oil removal, and output voltage generation were continuously monitored. The COD removals of three reactors were between 73% and 75%, and oil removals were over 95.7%. Compared with MFC, the CW-MFC with a MnO2 modified cathode produced higher power density and output voltage. Maximum power densities of CW-MFC and MFC were 3868 mW/m3 (102 mW/m2) and 3044 mW/m3 (80 mW/m2), respectively. The plants in CW-MFC play a positive role for reactor cathode potential. Both plants and cathode modification can improve reactor performance of electricity generation.

Soliton-dependent plasmon reflection at bilayer graphene domainwalls.

Layer-stacking domain walls in bilayer graphene are emerging as a fascinating one-dimensional system that features stacking solitons structurally and quantum valley Hall boundary states electronically. The interactions between electrons in the 2D graphene domains and the one-dimensional domain-wall solitons can lead to further new quantum phenomena. Domain-wall solitons of varied local structures exist along different crystallographic orientations, which can exhibit distinct electrical, mechanical and optical properties. Here we report soliton-dependent 2D graphene plasmon reflection at different 1D domain-wall solitons in bilayer graphene using near-field infrared nanoscopy. We observe various domain-wall structures in mechanically exfoliated graphene bilayers, including network-forming triangular lattices, individual straight or bent lines, and even closed circles. The near-field infrared contrast of domain-wall solitons arises from plasmon reflection at domain walls, and exhibits markedly different behaviours at the tensile- and shear-type domain-wall solitons. In addition, the plasmon reflection at domain walls exhibits a peculiar dependence on electrostatic gating. Our study demonstrates the unusual and tunable coupling between 2D graphene plasmons and domain-wallsolitons.

Direct observations of the photoinduced change in dimerization in K-TCNQ

The photoinduced dynamics of a potassium-tetracyanoquinodimethane (K-TCNQ) single crystal in the generalized Peierls phase are evaluated via time-resolved vibrational spectroscopy. The transient reflectivity spectrum of the photoinduced state in the mid-IR range shows a decrease in the height and width of the reflectivity band because of the electron-molecular-vibration-coupled CN stretching mode at approximately 2180 ${\mathrm{cm}}^{\ensuremath{-}1}$. This spectral change suggests that the photoexcitation of the charge transfer in TCNQ molecules induces melting of the dimerization of the molecules. From detailed analysis of the spectral evolution, the relaxation time constant from the photoinduced state to the dimerized state is estimated to be approximately 0.6 ps. Even after the recovery of the dimerization, a fluctuation is still observed, probably because of a domain-wall soliton. The fluctuation gradually dissipates with a time constant of approximately 2.3 ps. Direct observation of the dimerization process reveals the true dynamics of the photoinduced cooperative phenomenon within this system.