Formation mechanism of asymmetric breather and rogue waves in pair-transition-coupled nonlinear Schr?dinger equations

Based on the developed Darboux transformation, we investigate the exact asymmetric solutions of breather and rogue waves in pair-transition-coupled nonlinear Schr?dinger equations. As an example, some types of exact breather solutions are given analytically by adjusting the parameters. Moreover, the interesting fundamental problem is to clarify the formation mechanism of asymmetry breather solutions and how the particle number and energy exchange between the background and soliton ultimately form the breather solutions. Our results also show that the formation mechanism from breather to rogue wave arises from the transformation from the periodic total exchange into the temporal local property.

240 nm AlGaN-based deep ultraviolet micro-LEDs:size effect versus edge effect

240 nm AlGaN-based micro-LEDs with different sizes are designed and fabricated.Then,the external quantum efficiency(EQE)and light extraction efficiency(LEE)are systematically investigated by comparing size and edge effects.Here,it is revealed that the peak optical output power increases by 81.83%with the size shrinking from 50.0 to 25.0μm.Thereinto,the LEE increases by 26.21%and the LEE enhancement mainly comes from the sidewall light extraction.Most notably,transversemagnetic(TM)mode light intensifies faster as the size shrinks due to the tilted mesa side-wall and Al reflector design.However,when it turns to 12.5μm sized micro-LEDs,the output power is lower than 25.0μm sized ones.The underlying mechanism is that even though protected by SiO2 passivation,the edge effect which leads to current leakage and Shockley-Read-Hall(SRH)recombination deteriorates rapidly with the size further shrinking.Moreover,the ratio of the p-contact area to mesa area is much lower,which deteriorates the p-type current spreading at the mesa edge.These findings show a role of thumb for the design of high efficiency micro-LEDs with wavelength below 250 nm,which will pave the way for wide applications of deep ultraviolet(DUV)micro-LEDs.

Symmetry and asymmetry rogue waves in two-component coupled nonlinear Schrdinger equations

By means of the modified Darboux transformation we obtain some types of rogue waves in two-coupled nonlinear Schrodinger equations.Our results show that the two components admits the symmetry and asymmetry rogue wave solutions,which arises from the joint action of self-phase,cross-phase modulation,and coherent coupling term.We also obtain the analytical transformation from the initial seed solution to unique rogue waves with the bountiful pair structure.In a special case,the asymmetry rogue wave can own the spatial and temporal symmetry gradually,which is controlled by one parameter.It is worth pointing out that the rogue wave of two components can share the temporal inversion symmetry.

A polarization mismatched p-GaN/p-Al_(0.25)Ga_(0.75)N/p-GaN structure to improve the hole injection for GaN based micro-LED with secondary etched mesa

A low hole injection efficiency for InGaN/GaN micro-light-emitting diodes(μLEDs) has become one of the main bottlenecks affecting the improvement of the external quantum efficiency(EQE) and the optical power. In this work, we propose and fabricate a polarization mismatched p-GaN/p-Al_(0.25)Ga_(0.75)N/p-GaN structure for 445 nm GaN-based μLEDs with the size of 40 × 40 μm^(2), which serves as the hole injection layer. The polarization-induced electric field in the p-GaN/p-Al_(0.25)Ga_(0.75)N/p-GaN structure provides holes with more energy and can facilitate the non-equilibrium holes to transport into the active region for radiative recombination. Meanwhile, a secondary etched mesa for μLEDs is also designed, which can effectively keep the holes apart from the defected region of the mesa sidewalls, and the surface nonradiative recombination can be suppressed. Therefore, the proposed μLED with the secondary etched mesa and the p-GaN/p-Al_(0.25)Ga_(0.75)N/p-GaN structure has the enhanced EQE and the improved optical power density when compared with the μLED without such designs.

Increasing the hole energy by grading the alloy composition of the p-type electron blocking layer for very high-performance deep ultraviolet light-emitting diodes

It is well known that the p-type AlGaN electron blocking layer(p-EBL) can block hole injection for deep ultraviolet light-emitting diodes(DUV LEDs). The polarization induced electric field in the p-EBL for [0001] oriented DUV LEDs makes the holes less mobile and thus further decreases the hole injection capability. Fortunately,enhanced hole injection is doable by making holes lose less energy, and this is enabled by a specifically designed p-EBL structure that has a graded AlN composition. The proposed p-EBL can screen the polarization induced electric field in the p-EBL. As a result, holes will lose less energy after going through the proposed p-EBL, which correspondingly leads to the enhanced hole injection. Thus, an external quantum efficiency of 7.6% for the 275 nm DUV LED structure is obtained.

Domain wall dynamics in magnetic nanotubes driven by an external magnetic field

We use the Landau-Lifshitz-Gilbert equation to investigate field-driven domain wall propagation in magnetic nan- otubes. We find that the distortion is maximum as the time becomes infinite and the exact rigid-body solutions are obtained analytically. We also find that the velocity increases with increasing the ratio of inner radius and outer radius. That is to say, we can accelerate domain wall motion not only by increasing the magnetic field, but also by reducing the thickness of the nanotubes.

A method for nonlinearity compensation of OFDR based on polynomial regression algorithm

A method based on polynomial regression algorithm(PRA) is proposed in this paper to compensate the nonlinear phase noise in optical frequency domain reflection(OFDR) systems. In this method, the nonlinear phase of OFDR systems is represented by the polynomial phase function, and then the coefficients of the polynomial phase function are estimated by PRA. Finally, the nonlinearity is compensated by match Fourier transform(MFT). Simulation results demonstrate that the proposed algorithm has good performance in compensating both weak and strong nonlinear phase noises of OFDR systems.

Impact of p-AlGaN/GaN hole injection layer on GaN-based vertical cavity surface emitting laser diodes[Invited]

The hole injection capability is essentially important for GaN-based vertical cavity surface emitting lasers[VCSELs]to enhance the laser power.In this work,we propose GaN-based VCSELs with the p-AlGaN/p-GaN structure as the p-type hole supplier to facilitate the hole injection.The p-AlGaN/p-GaN heterojunction is able to store the electric field and thus can moderately adjust the drift velocity and the kinetic energy for holes,which can improve the thermionic emission proc-ess for holes to travel across the p-type electron blocking layer[p-EBL].Besides,the valence band barrier height in the p-EBL can be reduced as a result of usage of the p-AlGaN layer.Therefore,the better stimulated radiative recombination rate and the increased laser power are obtained,thus enhancing the 3 dB frequency bandwidth.Moreover,we also inves-tigate the impact of the p-AlGaN/p-GaN structure with various AIN compositions in the p-AlGaN layer on the hole injection capabilit,the laser power,and the了dB frequency bandwidth.

Moving bright solitons in a pseudo-spin polarization Bose-Einstein condensate

We study the moving bright solitons in the weak attractive Bose-Einstein condensate with a spin-orbit interaction. By solving the coupled nonlinear Scbrodinger equation with the variational method and the imaginary time evolution method, two kinds of solitons （plane wave soliton and stripe solitons） are found in different parameter regions. It is shown that the soliton speed dominates its structure. The detuning between the Raman beam and energy states of the atoms decides the spin polarization strength of the system. The soliton dynamics is also studied for various moving speed and we find that the shape of individual components can be kept when the speed of soliton is low.

Versatile nanosphere lithography technique combining multiple-exposure nanosphere lens lithography and nanosphere template lithography

A versatile nanosphere composite lithography（NSCL） combining both the advantages of multiple-exposure nanosphere lens lithography（MENSLL） and nanosphere template lithography（NSTL） is demonstrated. By well controlling the development, washing and the drying processes, the nanosphere monolayer can be well retained on the substrate after developing and washing. Thus the NSTL can be performed based on MENSLL to fabricate nanoring, nanocrescent and hierarchical multiple structures. The pattern size and the shape can be systemically tuned by shrinking nanospheres by using dry etching and adjusting the tilted angle. It is a natural nanopattern alignment process and possesses a great potential in the scope of nano-science due to its low cost,simplicity, and versatility for variuos nano-fabrications.

Polarization-enhanced AlGaN solar-blind ultraviolet detectors

AlGaN solar-blind ultraviolet detectors have great potential in many fields,although their performance has not fully meet the requirements until now.Here,we proposed an approach to utilize the inherent polarization effect of AlGaN to improve the detector performance.AlGaN heterostructures were designed to enhance the polarization field in the absorption layer,and a high built-in field and a high electron mobility conduction channel were formed.As a result,a high-performance solar-blind ultraviolet detector with a peak responsivity of 1.42 A/W at 10 V was achieved,being 50 times higher than that of the nonpolarization-enhanced one.Moreover,an electron reservoir structure was proposed to further improve the performance.A higher peak responsivity of 3.1 A/W at30 V was achieved because the electron reservoir structure could modulate the electron concentration in the conduction channel.The investigation presented here provided feasible approaches to improve the performance of the AlGaN detector by taking advantage of its inherent property.

Polarization assisted self-powered GaN-based UV photodetector with high responsivity

In this work, a self-powered GaN-based metal-semiconductor-metal photodetector(MSM PD) with high responsivity has been proposed and fabricated. The proposed MSM PD forms an asymmetric feature by using the polarization effect under one electrode, such that we adopt an AlGaN/GaN heterojunction to produce the electric field,and by doing so, an asymmetric energy band between the two electrodes can be obtained even when the device is unbiased. The asymmetric feature is proven by generating the asymmetric current-voltage characteristics both in the dark and the illumination conditions. Our results show that the asymmetric energy band enables the selfpowered PD, and the peak responsivity wavelength is 240 nm with the responsivity of 0.005 A/W. Moreover, a high responsivity of 13.56 A/W at the applied bias of 3 V is also achieved. Thanks to the very strong electric field in the charge transport region, when compared to the symmetric MSM PD, the proposed MSM PD can reach an increased photocurrent of 100 times larger than that for the conventional PD, even if the illumination intensity for the light source becomes increased.

Multiple-quantum-well-induced unipolar carrier transport multiplication in AlGaN solar-blind ultraviolet photodiode

AlGaN solar-blind ultraviolet(SBUV)detectors have potential application in fire monitoring,corona discharge monitoring,or biological imaging.With the promotion of application requirements,there is an urgent demand for developing a high-performance vertical detector that can work at low bias or even zero bias.In this work,we have introduced a photoconductive gain mechanism into a vertical AlGaN SBUV detector and successfully realized it in a p-i-n photodiode via inserting a multiple-quantum-well(MQW)into the depletion region.The MQW plays the role of trapping holes and increasing carrier lifetime due to its strong hole confinement effect and quantum confinement Stark effect.Hence,the electrons can go through the detector multiple times,inducing unipolar carrier transport multiplication.Experimentally,an AlGaN SBUV detector with a zero-bias peak responsivity of about 0.425 A/W at 233 nm is achieved,corresponding to an external quantum efficiency of 226%,indicating the existence of internal current gain.When compared with the device without MQW structure,the gain is estimated to be about 103 in magnitude.The investigation provides an alternative and effective approach to obtain high current gain in vertical AlGaN SBUV detectors at zero bias.

On the polarization self-screening effect in multiple quantum wells for nitride-based near ultraviolet light-emitting diodes

The tilted energy band in the multiple quantum wells（MQWs） arising from the polarization effect causes the quantum confined Stark effect（QCSE） for [0001] oriented III-nitride-based near ultraviolet light-emitting diodes（NUV LEDs）. Here, we prove that the polarization effect in the MQWs for NUV LEDs can be self-screened once the polarization-induced bulk charges are employed by using the alloy-gradient InxGa1-xN quantum barriers. The numerical calculations demonstrate that the electric field in the quantum wells becomes weak and thereby flattens the energy band in the quantum wells, which accordingly increases the spatial overlap for the electron-hole wave functions. The polarization self-screening effect is further proven by observing the blueshift for the peak emission wavelength in the calculated and the measured emission spectra. Our results also indicate that for NUV LEDs with a small conduction band offset between the quantum well and the quantum barrier,the electron injection efficiency for the proposed structure becomes low. Therefore, we suggest doping the proposed quantum barrier structures with Mg dopants.