Momentum Conservation Conditions Research Articles

A comparative study of impact ionization and avalanche multiplication in InAs, HgCdTe, and InAlAs/InAsSb superlattice

We use an ensemble Monte Carlo transport approach to calculate and compare the impact ionization and avalanche photodiode excess noise characteristics in three materials—a band-engineered InAlAs/InAsSb type-II superlattice, bulk InAs, and HgCdTe—all with an identical bandgap of 370 meV at 250 K. The electronic band structures and energy–momentum conservation conditions are used to calculate the impact ionization rates, carrier histories, multiplication gains, and excess noise characteristics. The calculated impact ionization coefficients and excess noise factors indicate a single carrier species multiplication in all three materials under low applied electric fields. We find the ratio of impact ionization coefficients to be k=7×10−4 for InAs and 3×10−4 for HgCdTe under an applied field of 50 kV/cm, and the superlattice to be k<10−6 at fields up to 400 kV/cm. The bulk materials experience avalanche breakdown as the applied field increases, transitioning to Geiger mode behavior at gains above 103 for InAs and 104 for HgCdTe. However, this breakdown is absent from the superlattice at the highest fields considered in this study due to hole confinement, indicating superior performance compared to the bulk materials. Our results demonstrate the role of superlattice band engineering in designing quality avalanche photodiode materials.

Terahertz wave excitation by nonlinear coupling of intense laser field with magnetized plasma

In this article, the plasma density, the laser intensity and the external magnetic field are playing vast roles firstly to generate a coherent THz wave then to enhance the stability of generated THz. Due to the relativistic increase of electron mass, the relativistic self-focusing of a right circular polarized (RCP) laser beam inside magnetized plasma will occur which leads to raising the laser power to enough limits for exciting the terahertz wave. By fulfilling the energy–momentum conservation conditions, a terahertz wave frequency at the difference between the laser pump wave frequency and plasma wave frequency is obtained. More stabilization and higher power (reaching to tens of gigawatts) of terahertz field amplitude have been observed whenever the plasma density, the laser intensity, and the external magnetic field are increased. Better results are recorded at high THz frequency (5 THz) compared with low THz frequency (1 THz).

Interface-Induced Conservation of Momentum Leads to Chiral-Induced Spin Selectivity.

We study the nonequilibrium dynamics of electron transmission from a straight waveguide to a helix with spin-orbit coupling. Transmission is found to be spin-selective and can lead to large spin polarizations of the itinerant electrons. The degree of spin selectivity depends on the width of the interface region, and no polarization is found for single-point couplings. We show that this is due to momentum conservation conditions arising from extended interfaces. We therefore identify interface structure and conservation of momentum as crucial ingredients for chiral-induced spin selectivity, and we confirm that this mechanism is robust against static disorder.

Adaptive particle refinement strategies in smoothed particle hydrodynamics

Adaptive particle refinement (APR) can increase the efficiency of mesh-free particle methods by splitting mother particles into smaller daughter particles. However, most APR algorithms are based on solving a minimization problem in which a single mother particle is split into several daughter particles. This may cause insufficient accuracy because APR is typically adopted in problems where many mother particles are involved in splitting, and a superposition effect may occur in the refinement error field between neighboring mother particles. Hence, we develop a novel APR strategy for the smoothed particle hydrodynamics (SPH) method based on direct splitting multiple mother particles to obtain more accurate optimal APR parameters. In addition, angular momentum conservation conditions are derived and a new algorithm is proposed to implement periodic boundary conditions for open-channel flows. Furthermore, for the first time, we conduct a rigorous parametric study to investigate factors affecting the optimal APR parameters, including the split patterns, kernel types, the density calculation algorithms as well as refinement error definitions. This APR strategy is validated through five benchmark test cases. Test results show that the proposed algorithm can both improve the simulation accuracy, reduce the computational cost and show good flexibility for implementation.

Crystal symmetry based selection rules for anharmonic phonon-phonon scattering from a group theory formalism

Anharmonic phonon-phonon scattering serves a critical role in heat conduction in solids. Previous studies have identified many selection rules for possible phonon-phonon scattering channels imposed by phonon energy and momentum conservation conditions and crystal symmetry. However, the crystal-symmetry-based selection rules have mostly been \textit{ad hoc} so far in selected materials, and a general formalism that can summarize known selection rules and lead to new ones in any given crystal is still lacking. In this work, we apply a general formalism for symmetry-based scattering selection rules based on the group theory to anharmonic phonon-phonon scatterings, which can reproduce known selection rules and guide the discovery of new selection rules between phonon branches imposed by the crystal symmetry. We apply this formalism to analyze the phonon-phonon scattering selection rules imposed by the in-plane symmetry of graphene, and demonstrate the significant impact of symmetry-breaking strain on the lattice thermal conductivity. Our work quantifies the critical influence of the crystal symmetry on the lattice thermal conductivity in solids and suggests routes to engineer heat conduction by tuning the crystal symmetry.

Collision-geometry-based 3D initial condition for relativistic heavy-ion collisions

We present a simple way to construct 3D initial conditions for relativistic heavy-ion collisions based on the Glauber collision geometry. Local energy and momentum conservation conditions are imposed to set non-trivial constraints on our parameterizations of longitudinal profiles for the system's initial energy density and flow velocity. After calibrating parameters with charged hadron rapidity distributions in central Au+Au collisions, we test model predictions for particle rapidity distributions in d+Au and peripheral Au+Au collisions in the Beam Energy Scan (BES) program at Relativistic Heavy-Ion Collider (RHIC). Simulations and comparisons with measurements are also made for Pb+Pb collisions at Super Proton Synchrotron (SPS) energies. We demonstrate that elliptic flow measurements in heavy-ion collisions at $\sqrt{s} \sim 10$ GeV can set strong constraints on the dependence of Quark-Gluon Plasma shear viscosity on temperature and net baryon chemical potential.

Properties of the surface layer after high-energy treatment by powder particles

Experiments were conducted on high-energy surface treatment of a structural steel substrate with a flow of tungsten, nickel, and titanium nitride powder particles. The impact pressure of the steel target and particles accelerated by explosion energy was estimated using the momentum conservation equation and the linear equation of the particle material shock adiabat. It was found that the impact pressure of the target and particles is 62 GPa for a tungsten particle, 48 GPa for a nickel particle, and 41 GPa for a titanium nitride particle. The heating temperature of particles during their collision with the steel target surface was calculated taking into account the conditions of mass and momentum conservation at the shock wave front. The maximum heating temperature of particles at the point of their collision with the substrate surface (at a particle velocity of 2000 m/s) is 1103 K for tungsten particles, 755 K for nickel particles, and 589 K for titanium nitride particles. It was shown that the steel target strength increases when it is subjected to high-energy treatment with a flow of particles. The maximum hardening of the steel target surface layer increases by 32–55 % compared to initial microhardness and is observed at a depth of 2–4 mm from the treatment surface. Then it decreases to the value of starting material microhardness (170 HV) at a distance of 15–20 mm from the treated surface.

Direct Generation of Narrow-band Hyperentangled Photons.

In quantum communication and photonic quantum information processing, the requirement of quantum repeaters and quantum memory often imposes a strict bandwidth prerequisite for the entangled photons. At the same time, there is ever more increasing demand for entangling more degrees of freedom, i.e., hyperentanglement, for a photon pair. In this Letter, we report the direct generation of narrow-band orbital angular momentum (OAM) and polarization hyperentangled photons from cold atoms. The narrow-band photon pair is naturally entangled in polarization and OAM, in addition to time-frequency, degrees of freedom due to spin and orbital angular momentum conservation conditions in the spontaneous four-wave mixing process in a cold atom ensemble. The narrow-band hyperentangled photon pair source reported here is expected to play important roles in quantum memory-based long-distance quantum communication.

Observation of spontaneous decay of Alfvénic fluctuations into co- and counter-propagating magnetosonic waves in a laboratory plasma

We show experimentally that Alfvénic fluctuations can spontaneously decay into a fast and a slow magnetosonic wave in an inhomogeneous plasma. The fast wave of higher frequency propagates in the same direction, while the slow wave of lower frequency propagates in the opposite direction with the pump wave. Both of the daughter waves are characterized by strong parallel but modest perpendicular fluctuations. The measured frequencies and wavenumbers are found to satisfy the energy and momentum conservation conditions for a nonlinear three wave interaction. The evidence of energy flowing from pump fluctuations into daughter waves is also presented in this paper. The results may shed light on the origin of inward Alfvén waves observed in the solar corona and chromosphere and how shear Alfvén waves deposit its energy by driving compressional perturbations.

Transversely Divergent Second Harmonic Generation by Surface Plasmon Polaritons on Single Metallic Nanowires.

Coherently adding up signal wave from different locations are a prerequisite for realizing efficient nonlinear optical processes in traditional optical configurations. While nonlinear optical processes in plasmonic waveguides with subwavelength light confinement are in principle desirable for enhancing nonlinear effects, so far it has been difficult to improve the efficiency due to the large momentum mismatch. Here we demonstrate, using remotely excited surface plasmon polaritons (SPPs), axial collimated but transversely divergent second harmonic (SH) generation in a single silver nanowire-monolayer molybdenum disulfide hybrid system. Fourier imaging of the generated SH signal confirms the momentum conservation conditions between the incident and reflected SPPs and reveals distinct features inherent to the 1D plasmonic waveguides: (i) the SH photons are collimated perpendicular to the nanowire axis but are divergent within the perpendicular plane; (ii) the collimation (divergence) is inversely proportional to the length of the active region (lateral confinement of the SPPs); and (iii) the SH emission pattern resembles that of an aligned dipole chain on top of the substrate with an emission peak at the critical angle. Our results pave the way to generate and manipulate SH emission around subwavelength waveguides and open up new possibilities for realizing high efficiency on-chip nonlinear optics.

Experimental investigation of ejecta on melted Sn sample under shock loading

Ejecta production from the metal surface under shock-loading is currently a focused issue both at home and abroad. However, the traditional experimental techniques, such as piezoelectric pin, only diagnose the ejected data for low-density ejecta but not for high-density ones, giving a poor understanding of this process. Particularly, when ejecta production increases significantly as the loaded metal melts on release or shock, the measurement carried out by the traditional piezoelectric pin becomes worse, and brings further missing knowledge in the ejecta evolution.In this paper, an Asay-F window designed earlier by the authors based on the traditional Asay-window, is employed to investigate the formation process of the ejecta from the melted Sn metal. As indicated by previous experimental findings on shocked Pb sample, the Asay-F window is a reliable and effective tool for measuring the high-density ejecta by comparing the result with those of the piezoelectric pin. The interface velocity within the Asay-F window measured by Doppler pin system, is obtained. On the basis of momentum conservation condition, the physical quantities of ejecta, such as accumulative areal mass, volume density and velocity, are derived from the interface velocity. By analyzing the experimental data diagnosed by the Asay-F window, which is placed at different offsets from the free surface of Sn sample, the expansion evolution of the ejecta is obtained. Through transforming the dynamic volume density to the static one, the picture of the ejecta density distribution changes with the spatial distance at a specific moment, which is explicitly displayed. It is found that the ejecta density distributions gained from the different offsets at the uniform moment are consistent. As a consequence, the self-similar expansion evolution of the ejecta is experimentally confirmed, which successfully avoids the unclear understanding of this process if only examined by the piezoelectric pin. This experiment may lay the foundation of the formation of the ejecta production for the metal sample subjected to high pressure loading.

Continuous-wave 1.57 m/3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN

Continuous-wave (CW) coherent sources emitting two wavelengths of 1.57 m and 3.84 m have aroused much interest of scientists due to their many applications such as military multiband composite guidance, remote monitoring of the special environment, etc. Quasi-phase matching (QPM) optical parametric oscillator (OPO) device with periodically inverted structure of nonlinear coefficient can implement an efficient and wavelength conversion at arbitrary wavelength in the transparent range of the QPM material. Nowadays, using MgO:PPLN for QPM, various MgO:PPLN-OPOs pumped by conventional 1.06 m laser source can produce 1.57 m and 3.84 m laser and also achieve good results. But as a result of the limitation of momentum conservation condition and periodically poled structure, 1.57 m and 3.84 m laser can only meet a single band. To obtain the two-wavelength laser output at the same time, the MgO:PPLN-OPO could not be applied. In this paper, a CW 1.57 m and 3.84 m intra-cavity multiple optical parametric oscillator based on MgO:APLN is reported. The cross period parameter light is obtained by using a folded type doubly cavity which consists of 1064 nm resonator and multiple optical parametric oscillator. Considering both its thermal stability under high power pump and the light spot mode matching of multiple optical parametric oscillation process, through numerical simulation and theoretical analysis of two sub cavities, the optimum parameters of the cavity structure are determined. On this basis, the influences of output coupler transmittance on oscillation threshold, the down-conversion efficiency, output power stability are investigated in experiment. With T=10% at 1.47 m and 3.3 m output coupler used, the maximum output powers of 3.13 W at 1.57 m and 0.85 W at 3.84 m are obtained, corresponding to slope efficiencies of 6.8% and 1.9%, respectively. The power stabilities are better than 1.8% and 3% at the maximum output power in half an hour. The experimental results show that the intra-cavity multiple optical parametric oscillator based on a single poled crystal MgO:APLN is an effective method of obtaining a 1.57 m and 3.84 m CW laser.

Four-wave mixing with Bose-Einstein condensates in nonlinear lattices

The theory of the four-wave mixing in nonlinear lattices is presented. Nonlinear lattice modifies the momentum conservation condition and leaves energy conservation unaffected, contrary to the more conventional linear modulation. We study the dynamics of the moving wavepacket in the presence of a nonlinear lattice and observe the formation of two new waves which is not possible in the conventional four-wave mixing process. We design an analytical plane-wave model which agrees well with numerical simulations.

Method of extreme surfaces for optimizing geometry of acousto-optic interactions in crystalline materials: Example of LiNbO3 crystals

We suggest a method for optimizing geometry of acousto-optic (AO) interactions in anisotropic crystalline materials. Within the framework of this method, one gets global maximums of AO figure of merit M2 and their spatial orientations, proceeding from so-called “extreme” indicative surfaces, which are obtained after finding such an acoustic wave propagation direction that maximizes the M2 parameter for each propagation direction of the incident electromagnetic wave. The method improves earlier indicative surface-based techniques in several aspects, particularly in properly accounting for the momentum conservation condition for the AO diffraction, and yields a higher accuracy in assessing spatial anisotropy of the AO effect. We have constructed the extreme surfaces of LiNbO3 crystals for all possible cases, including those of isotropic/anisotropic AO diffractions and longitudinal/transverse acoustic waves. The anisotropy of the AO figure of merit for LiNbO3 is analyzed for the acoustic frequencies 0.01–2.0 GHz and the light wavelengths 405–1444 nm. The absolute M2 maximums refer to ‘indirect crystal cuts' and are equal to 26.3 × 10−15 s3/kg at 2 GHz and 405 nm, and 15.4 × 10−15 s3/kg at 0.4 GHz and 1444 nm.

The Phonon Thermal Conductivity of Single-Layer Graphene From Complete Phonon Dispersion Relations

In this paper, the phonon scattering mechanisms of single-layer graphene are investigated based on the complete phonon dispersion relations. According to the selection rules that a phonon scattering process should obey the energy and momentum conservation conditions, the relaxation rates of combining and splitting umklapp processes can be calculated by integrating the intersection lines between different phonon mode surfaces in the phonon dispersion relation space. The dependence of the relaxation rates on the wave vector directions is presented with a three-dimensional surface over the first Brillouin zone. It is found that the reason for the optical phonons contributing little to heat transfer is attributed to the strong umklapp processes but not to their low phonon group velocities. The combining umklapp scattering processes involving the optical phonons mainly decrease the acoustic phonon thermal conductivity, while the splitting umklapp scattering processes of the optical phonons mainly restrict heat conduction by the optical phonons themselves. Neglecting the splitting processes, the optical phonons can contribute more energy than that carried by the acoustic phonons. Based on the calculated phonon relaxation time, the thermal conductivities contributed from different mode phonons can be evaluated. At low temperatures, both longitudinal and in-plane transverse acoustic phonon thermal conductivities have T2 temperature dependence, and the out-of-plane transverse acoustic phonon thermal conductivity is proportion to T3/2. The calculated thermal conductivity is on the order of a few thousands W/(m K) at room temperature, depending on the sample size and the edge roughness, and is in agreement well with the recently measured data in the temperature range from about 350 K to 500 K.

Self-established noncollinear oscillation and angular tuning in a quasi-phase-matched mirrorless optical parametric oscillator

We experimentally demonstrate self-established noncollinear interactions in a quasi-phase-matched mirrorless optical parametric oscillator using a periodically-poled KTiOPO4 crystal with submicrometer periodicity. It is shown that the oscillation is established in a very specific geometric configuration where the two possible solutions of the energy and momentum conservation conditions become spatially degenerate and contribute coherently to the gain. In this configuration, the signal and the idler remain mutually counter-propagating. The tuning capabilities of a noncollinear mirrorless optical parametric oscillator are investigated.

Three-dimensional short-range interactions of strongly nonlocal optical spatial solitons

Basing on the accurate Gaussian analytical solution of a pair of (1+2)D optical spatial solitons with symmetrical oblique incidence,we investigated the short-range interactions in the strongly nonlocal nonlinear media.Two solitons in close proximity can be intertraped via the strong nonlocality ,and propagate together in a stable spiral and the trajectory of mass center of solitons will deflect because of the phase difference and the ratio of the power of two solitons under the condition of momentum conservation. It is possible to find its application in planar all-optical interconnection in bulk media.We analyszed the evolution of the Poynting vector as they propagate through the strongly nonlocal media.

Power controlled short-range interactions between strongly nonlocal spatial soliton

We investigate the power controlled short-range interactions between spacial optical solitons in the strongly nonlocal nonlinear media. For the short-range interactions, the trajectory of mass center of solitons will deflect because of the phase difference under the condition of momentum conservation. We verify by experiment that the degree of deflection depends on the ratio of the power of two solitons, and the case of equal power corresponds to the maximum deflection. Using the characteristics of power control on the interaction between solitons, we measure the photic-induced response time of nematic liquid crystals, and find that it is much shorter than the bias-voltage-induced response time.

A lattice Boltzmann model for the Korteweg–de Vries equation with two conservation laws

A lattice Boltzmann model for the Korteweg–de Vries (KdV) equation is presented by using the higher-order moment method. In contrast to the previous lattice Boltzmann model to the KdV equation, our method has higher-order accuracy. Two key steps in the development of this model are the addition of a momentum conservation condition, and the construction of a correlation between the first conservation law and the second conservation law. The numerical example shows the higher-order moment method can be used to raise the truncation error of the lattice Boltzmann scheme.

Orbital elements of the mild and strong barium stars formed through a wind accretion scenario

Taking account of the metallicity dependence of the s-process nucleosynthesis in the AGB stars, we adopted the wind accretion model with the condition of total angular momentum conservation and used the Monte-Carlo method to study the variations and the distributions of the orbital elements of the mild and strong Ba stars. The calculated results show that the level of heavy-element overabundance in a Ba star depends on the orbital period. Since there is a strong dependence of s-process yields on the initial stellar metallicity of the AGB star and a strong increase of the s-process yields in AGB stars with decreasing metallicity, the calculated results strongly suggest that the initial metallicity of the Ba star systems is another important parameter for the level of heavy-element overabundance in a Ba star. The strong Ba stars generally have lower metallicities than mild Ba stars. The masses of AGB progenitor and Ba star are other two parameters which also have some impact on the heavy-element overabundance in the Ba star.