Soliton Diode Research Articles

Dark soliton scattering in symmetric and asymmetric double potential barriers

Motivated by the recent theoretical study of (bright) soliton diode effects in systems with multiple scatterers, as well as by experimental investigations of soliton-impurity interactions, we consider some prototypical case examples of interactions of dark solitons with a pair of scatterers. In a way fundamentally opposite to the case of bright solitons (but consonant to their “anti-particle character”), we find that dark solitons accelerate as they pass the first barrier and hence cannot be trapped by a second equal-height barrier. A pair of unequal barriers may lead to reflection from the second one, however trapping in the inter-barrier region cannot occur. We also give some examples of dynamical adjusting of the barriers to trap the dark soliton in the inter-barrier region, yet we show that this can only occur over finite time horizons, with the dark soliton always escaping eventually, contrary again to what is potentially the case with bright solitons.

Discrete soliton diode induced by synthetic gauge phase

We demonstrate the nonreciprocal dynamics of discrete solitons (DSs) induced by a uniform synthetic gauge phase (SGP) in one-dimensional waveguide array. The SGP is embedded into the coupling constant between the waveguides, which provides a directional momentum to the propagating fields and creates a nonreciprocal for the system. When a kicked Gaussian wave packet propagates through the waveguide arrays, diode effects are formed. Dependence between the diode effect and the SGP is studied. We found that the threshold of the reverse breakdown of this optical diode exhibits a perfect linear dependence against the SGP, and an extremely high diode quality can be achieved when the initial phase tilt exactly compensates the initial momentum provided by the SGP. Moreover, the direction of the diode can be switched by the direction of the SGP, which brings additional freedom to control the one-way propagation of the DSs. Our finding may have potential application to realizing new devices in high-speed all-optical communications.

Femtosecond soliton diode on heterojunction Bragg-grating structure

We numerically propose a scheme for realizing an all-optical femtosecond soliton diode based on a tailored heterojunction Bragg grating, which is designed by two spatially asymmetric chirped cholesteric liquid crystals. Our simulations demonstrate that with the consideration of optical nonlinearity, not only the femtosecond diode effect with nonreciprocal transmission ratio up to 120 can be achieved but also the optical pulse evolving into soliton which maintains its shape during propagation through the sample is observed. Further, the influence of pulse width and the carrier wavelength to the femtosecond diode effect is also discussed in detail. Our demonstrations might suggest a direction for experimentally realizing the femtosecond soliton diode based on the cholesteric liquid crystals.

All-optical switches, unidirectional flow, and logic gates with discrete solitons in waveguide arrays.

We propose a mechanism by which a number of useful all-optical operations, such as switches, diodes, and logic gates, can be performed with a single device. An effective potential well is obtained by modulating the coupling between the waveguides through their separations. Depending on the power of a control soliton injected through the potential well, an incoming soliton will either completely transmit or reflect forming a controllable switch. We show that two such switches can work as AND, OR, NAND, and NOR logic gates. Furthermore, the same device may also function as a perfect soliton diode with adjustable polarity. We discuss the feasibility of realising such devices with current experimental setups.

Slow-light all-optical soliton diode based on tailored Bragg-grating structure.

All-optical soliton diode (AOSD) is proposed based on a sandwich nonlinear Bragg-grating structure: a linearly chirped Bragg-grating linked to a uniform Bragg-grating and again to a chirped Bragg-grating. The nonreciprocity is achieved by introducing two spatially asymmetric chirped Bragg-gratings with optical nonlinearity. High transmission ratio up to 150 is obtained when launching a picosecond Gaussian pulse into the setting. We find that such pulses in a form of solitons propagate at a rather small velocity (<0.03c) when pulse wavelength is selected in the vicinity of photonic bandgap.

A comparator based on soliton antisoliton pair generation in soliton diode

Modified sine-Gordon equation is solved for a soliton diode, which is a long Josephson junction in which the magnetic field of the control current reverses its direction at the center. Applying a properly directed bias current can start the spontaneous generation of soliton and antisolitons at the transition region of the diode and leads to a flux-flow voltage. The threshold bias current at which such a voltage generation starts is a function of the control current and the size of the transition region. Such phenomena can find applications in high-speed decision making electronic comparators.

Numerical investigation of soliton dynamics, injection into the transition region of a soliton diode

We investigate soliton–antisoliton interactions by numerical simulation of a nonlinear transmission line. Long Josephson junction acts as a nonlinear transmission line in which soliton solutions are obtained form Sine–Gordon equation. By proper initial and boundary conditions solitons and antisolitons are obtained inside the same junction, simulating the so called soliton diode. The role of loss in the annihilation of solitons and antisolitons injected to the transition region of the soliton diode is investigated by numerical simulation of their dynamics and phase revolution along the junction. It is shown that such loss can greatly reduce the speed of solitons and antisolitons, lowering the frequency response of the device. A so called “locally-lossy” soliton diode is proposed and it is shown that this configuration maintains annihilation process without much degradation of the device speed.

Directional flow of solitons with asymmetric potential wells: Soliton diode

We study the flow of bright solitons through two asymmetric potential wells. The scattering of a soliton by a certain type of single potential wells, e.g., Gaussian or Rosen-Morse, is distinguished by a critical velocity above which solitons can transmit almost completely and below which solitons can reflect nearly perfectly. For two such wells in series with certain parameter combinations, we find that there is an appreciable velocity range for which solitons can propagate in one direction only. Our study shows that this directional propagation or diode behavior is due to a combined effect of the sharp transition in the transport coefficients at the critical velocity and a slight reduction in the center-of-mass speed of the soliton while it travels across a potential well.