Additional Angular Momentum Research Articles

Pulse Slippage of Resonant Third Harmonic Generation in Electron-Hole Plasma**Supported by a financial grant from CSIR, New Delhi, India, under Project No. 03(1438)/18/EMR-II

Effect of pulse slippage on resonant third harmonic generation of a short pulse laser in electron-hole plasma in the presence of wiggler magnetic field has been investigated. The group velocity mismatch of the third harmonic pulse and the fundamental pulse is significant in electron hole plasma. As the third harmonic pulse has higher group velocity than that of fundamental pulse, therefore, it moves faster than the fundamental pulse. It gets slipped out of the domain of fundamental pulse and its amplitude saturates. Phase matching condition is satisfied by applying wiggler magnetic field, which provides additional angular momentum to the third harmonic photon to make the process resonant. Enhancement in the efficiency of third harmonic generation of an intense short pulse laser in electron-hole plasma embedded with a magnetic wiggler is seen.

Exponential density transition-based enhanced second harmonic generation in plasma

AbstractEnhanced harmonic generation of second order based on exponential density ramp in plasma has been noticed. It is well known that self-focusing of pump laser induces a transverse intensity gradient, which gives rise to the plasma wave at the pump frequency. Highly intense Gaussian laser with exponential density ramp profile generates a second harmonic pulse in plasma. It is noticed that the amplitude of second-harmonic pulse varies periodically with the distance and reaches its peak value in the focal region. Further, it is found that exponential plasma density ramp is useful for stronger self-focusing, which lead to rise second harmonic generation (SHG). Wiggler magnetic field provides the additional angular momentum required for second harmonic photon and aids in growing of resonant process. One may clearly see that exponential plasma density ramp imparts a major role in enhancing the SHG in plasma. Further, the combined effect of exponential density ramp and wiggler magnetic field boosts the SHG in plasma.

Radiative capture of cold neutrons by protons and deuteron photodisintegration with twisted beams

We consider two basic nuclear reactions: capture of neutrons by protons, n + p → γ + d, and its time-reversed counterpart, photodisintegration of the deuteron, γ + d → n + p. In both of these cases we assume that the incoming beam of neutrons or photons is ‘twisted’ by having an azimuthal phase dependence, i.e., it carries an additional angular momentum along its direction of propagation. Taking a low-energy limit of these reactions, we derive relations between corresponding transition amplitudes and cross sections with plane-wave beams and twisted beams. Implications for experiments with twisted cold neutrons and twisted photon beams are discussed.

Can plume-induced internal gravity waves regulate the core rotation of subgiant stars?

Context.The seismic data provided by the space-borne missions CoRoT andKeplerenabled us to probe the internal rotation of thousands of evolved low-mass stars. Subsequently, several studies showed that current stellar evolution codes are unable to reproduce the low core rotation rates observed in these stars. These results indicate that an additional angular momentum transport process is necessary to counteract the spin up due to the core contraction during the post-main sequence evolution. For several candidates, the transport induced by internal gravity waves (IGW) could play a non-negligible role.Aims.We aim to investigate the effect of IGW generated by penetrative convection on the internal rotation of low-mass stars from the subgiant branch to the beginning of the red giant branch.Methods.A semi-analytical excitation model was used to estimate the angular momentum wave flux. The characteristic timescale associated with the angular momentum transport by IGW was computed and compared to the contraction timescale throughout the radiative region of stellar models at different evolutionary stages.Results.We show that IGW can efficiently counteract the contraction-driven spin up of the core of subgiant stars if the amplitude of the radial-differential rotation (between the center of the star and the top of the radiative zone) is higher than a threshold value. This threshold depends on the evolutionary stage and is comparable to the differential rotation rates inferred for a sample of subgiant stars observed by the satelliteKepler. Such an agreement can therefore be interpreted as the consequence of a regulation mechanism driven by IGW. This result is obtained under the assumption of a smooth rotation profile in the radiative region and holds true even if a wide range of values is considered for the parameters of the generation model. In contrast, on the red giant branch, we find that IGW remain insufficient, on their own, to explain the observations because of an excessive radiative damping.Conclusions.IGW generated by penetrative convection are able to efficiently extract angular momentum from the core of stars on the subgiant branch and accordingly have to be taken into account. Moreover, agreements with the observations reinforce the idea that their effect is essential to regulate the amplitude of the radial-differential rotation in subgiant stars. On the red giant branch, another transport mechanism must likely be invoked.

Optimization of wiggler wave number for density transition based second harmonic generation in laser plasma interaction

We present the numerical analysis of optimization of wiggler wave number for density transition based second harmonic generation during laser plasma interaction. Wiggler wave number required for phase matching increases with plasma density and the intensity of pump wave. It is due to the fact that wiggler magnetic field offers the additional angular momentum which is necessary to make the second harmonic generation process resonant. A considerable enhancement in the resonant second harmonic generation of an ultra-short pulse laser in plasma with density transition surrounded by a magnetic wiggler is observed. Density transition in plasma plays a key role in self-focusing which further enhances the harmonic generation of second order in plasma. Also we have examined the effect of magnetic wiggler on the harmonic generation of second order in plasma.

Insights from Asteroseismology of Massive Stars: The Need for Additional Angular Momentum Transport Mechanisms

In massive stars, rotation and oscillatory waves can have a tight interplay. In order to assess the importance of additional angular momentum transport mechanisms other than rotation, we compare the asteroseismic properties of a uniformly rotating model and a differentially rotating one. Accordingly, we employ the observed period spacing of 36 dipole g-modes in the Kepler $\sim3.2$ M$_\odot$ target KIC 7760680 to discriminate between these two models. We favor the uniformly rotating model, which fully satisfies all observational constraints. Therefore, efficient angular momentum transport by additional mechanisms such as internal gravity waves, heat-driven modes and magnetic field is needed during early main sequence evolution of massive stars.

On the fragmentation boundary in magnetized self-gravitating discs

We investigate the role of magnetic fields in the fragmentation of self-gravitating discs using 3D global ideal magnetohydrodynamic simulations performed with the "phantom" smoothed particle hydrodynamics code. For initially toroidal fields, we find two regimes. In the first, where the cooling time is greater than five times the dynamical time, magnetic fields reduce spiral density wave amplitudes, which in turn suppresses fragmentation. This is the case even if the magnetic pressure is only a tenth of the thermal pressure. The second regime occurs when the cooling time is sufficiently short that magnetic fields cannot halt fragmentation. We find that magnetised discs produce more massive fragments, due to both the additional pressure exerted by the magnetic field, and the additional angular momentum transport induced by Maxwell stresses. The fragments are confined to a narrower range of initial semimajor axes than those in unmagnetised discs. The orbital eccentricity and inclination distributions of unmagnetised and magnetised disc fragments are similar. Our results suggest the fragmentation boundary could be at cooling times a factor of two lower than predicted by purely hydrodynamical models.

Localized high-Q modes in conical microcavities

We carry out three-dimensional (3D) numerical simulation on a conical microcavity with a half-angle of 11 degree, where there exists high-Q modes by introducing a thin high refractive index film on a conical surface. Our study reveals that, rather than the surface profile of the microcavity, the effective radius plays crucial role in whether the cavity may support localized modes. Specifically, the change of high refractive index film thickness creates an additional angular momentum barrier, so that the conical microcavity may sustain localized high-Q modes. Our study offers a new degree of freedom to control the properties of 3D microcavities, which is useful for microlaser or sensor applications.

Generation of Plasmonic Vortex with Linearly Polarized Light

A nanoslit-arrayed plasmonic lens that can generate plasmonic vortex with linearly polarized light is theoretically and numerically studied. The topological charge (TC) of generated plasmonic vortex is the sum of the geometrical charge of the proposed plasmonic lens and additional angular momentum of ±1. The plasmonic vortex is determined by the pitch of the plasmonic lens, the nanoslit rotation directions, as well as the polarization direction of the incident light. Combination of the geometry parameters of the plasmonic lens and the polarization direction of linearly polarized incident light, plasmonic vortexes with arbitrary TC are flexibly generated. Moreover, by cutting the continuous nanoslit arrays into segments, highly ordered plasmonic vortexes are realized with small plasmonic lens size. Comparisons with two slit plasmonic lenses are also presented to demonstrate the unique properties of the proposed plasmonic lens in plasmonic vortex generation.

Optical absorption by Dirac excitons in single-layer transition-metal dichalcogenides

We develop an analytically solvable model able to qualitatively explain nonhydrogenic exciton spectra observed recently in two-dimensional (2d) semiconducting transition metal dichalcogenides. Our exciton Hamiltonian explicitly includes additional angular momentum associated with the pseudospin degree of freedom unavoidable in 2d semiconducting materials with honeycomb structure. We claim that this is the key ingredient for understanding the nonhydrogenic exciton spectra that was missing so far.

Effect of pulse slippage on density transition-based resonant third-harmonic generation of short-pulse laser in plasma

The resonant third-harmonic generation of a self-focusing laser in plasma with a density transition was investigated. Because of self-focusing of the fundamental laser pulse, a transverse intensity gradient was created, which generated a plasma wave at the fundamental wave frequency. Phase matching was satisfied by using a Wiggler magnetic field, which provided additional angular momentum to the third-harmonic photon to make the process resonant. An enhancement was observed in the resonant third-harmonic generation of an intense short-pulse laser in plasma embedded with a magnetic Wiggler with a density transition. A plasma density ramp played an important role in the self-focusing, enhancing the third-harmonic generation in plasma. We also examined the effect of the Wiggler magnetic field on the pulse slippage of the third-harmonic pulse in plasma. The pulse slippage was due to the group-velocity mismatch between the fundamental and third-harmonic pulses.

Spin-Direction Control of High-Order Plasmonic Vortex With Double-Ring Distributed Nanoslits

A plasmonic vortex lens (PVL) consisting of circularly distributed nanoslits is proposed. It is shown that the tilted angle variance of nanoslits along the azimuthal direction gives an opportunity to add an additional orbital angular momentum (OAM) number to the plasmonic vortex. Using the double-ring nanoslit distribution, homogeneous amplitude of excited surface plasmon along the azimuthal angle is achieved. The proposed structure has advantages compared with conventional PVLs since the spin-assisted OAM number can be an arbitrary integer value, which can enable the spin-direction control of high-order plasmonic vortex.

CLOSE STELLAR BINARY SYSTEMS BY GRAZING ENVELOPE EVOLUTION

I suggest a spiral-in process in which a stellar companion grazes the envelope of a giant star while both the orbital separation and the giant radius shrink simultaneously, forming a close binary system. The binary system might be viewed as evolving in a constant state of "just entering a common envelope (CE) phase." In cases where this process takes place, it can be an alternative to CE evolution where the secondary star is immersed in the giant's envelope. Grazing envelope evolution (GEE) is made possible only if the companion manages to accrete mass at a high rate and launches jets that remove the outskirts of the giant envelope, hence preventing the formation of a CE. The high accretion rate is made possible by the accretion disk launching jets which efficiently carry the excess angular momentum and energy from the accreted mass. The orbital decay itself is caused by the gravitational interaction of the secondary star with the envelope inward of its orbit, i.e., dynamical friction (gravitational tide). Mass loss through the second Lagrangian point can carry additional angular momentum and envelope mass. The GEE lasts for tens to hundreds of years. The high accretion rate, with peaks lasting from months to years, might lead to a bright object referred to as the intermediate luminosity optical transient (Red Novae; Red Transients). A bipolar nebula and/or equatorial ring are formed around the binary remnant.

Long gamma ray bursts from binary black holes

We consider a scenario for the longest duration gamma ray bursts, resulting from the collapse of a massive rotating star in a close binary system with a companion black hole. The primary black hole born during the core collapse is first being spun up and increases its mass during the fallback of the stellar envelope just after its birth. As the companion black hole enters the outer envelope, it provides an additional angular momentum to the gas. After the infall and spiral-in towards the primary, the two black holes merge inside the circumbinary disk. The second episode of mass accretion and high final spin of the post-merger black hole prolongs the gamma ray burst central engine activity. The observed events should have two distinct peaks in the electromagnetic signal, separated by the gravitational wave emission. The gravitational recoil of the burst engine is also possible.

Additional Invariants and Statistical Equilibria for the 2D Euler Equations on a Spherical Domain

The role of the domain geometry for the statistical mechanics of 2D Euler flows is investigated. It is shown that for a spherical domain, there exists invariant subspaces in phase space which yield additional angular momentum, energy and enstrophy invariants. The microcanonical measure taking into account these invariants is built and a mean-field, Robert–Sommeria–Miller theory is developed in the simple case of the energy-enstrophy measure. The variational problem is solved analytically and a partial energy condensation is obtained. The thermodynamic properties of the system are also discussed.

Black hole from merging binary neutron stars: How fast can it spin?

The merger of two neutron stars will in general lead to the formation of a torus surrounding a black hole whose rotational energy can be tapped to potentially power a short gamma-ray burst. We have studied the merger of equal-mass binaries with spins aligned with the orbital angular momentum to determine the maximum spin the black hole can reach. Our initial data consists of irrotational binaries to which we add various amounts of rotation to increase the total angular momentum. Although the initial data violates the constraint equations, the use of the constraint-damping conformal and covariant Z4 formulation yields evolutions with violations smaller than those with irrotational initial data and standard formulations. Interestingly, we find that a limit of $J/{M}^{2}\ensuremath{\simeq}0.89$ exists for the dimensionless spin and that any additional angular momentum given to the binary ends up in the torus rather than in the black hole, thus providing another nontrivial example supporting the cosmic censorship hypothesis.

Long-term luminosity behavior of 14 ultracompact X-ray binaries

Context. X-ray binaries are usually divided in persistent and transient sources. For ultracompact X-ray binaries (UCXBs), the mass transfer rate is expected to be a strong function of orbital period, predicting persistent sources at short periods and transients at long periods. Aims. For 14 UCXBs including two candidates, we investigate the long-term variability and average luminosity with the purpose of learning how often a source can be expected to be visible above a given luminosity, and we compare the derived luminosities with the theoretical predictions. Methods. We use data from the RXTE All-Sky Monitor because of its long-term, unbiased observations. Many UCXBs are faint, i.e., they have a count rate at the noise level for most of the time. Still, information can be extracted from the data, either by using only reliable data points or by combining the bright-end variability behavior with the time-averaged luminosity. Results. Luminosity probability distributions show the fraction of time that a source emits above a given luminosity. All UCXBs show significant variability and relatively similar behavior, though the time-averaged luminosity implies higher variability in systems with a period longer than 40 min. Conclusions. There is no large difference in the statistical luminosity behavior of what we usually call persistent and transient sources. UCXBs with a period below ~30 min have a time-averaged bolometric luminosity that is in reasonable agreement with estimates based on the theoretical mass transfer rate. Around 40 min the lower bound on the time-averaged luminosity is similar to the luminosity based on the theoretical mass transfer rate, suggesting these sources are indeed faint when not detected. Above 50 min some systems are much brighter than the theoretical mass transfer rate predicts, unless these systems have helium burning donors or lose additional angular momentum.

Final stages of accretion onto non-Kerr compact objects

The $5--20{M}_{\ensuremath{\bigodot}}$ dark objects in x-ray binary systems and the ${10}^{5}--{10}^{9}{M}_{\ensuremath{\bigodot}}$ dark objects in galactic nuclei are currently thought to be the Kerr black holes predicted by general relativity. However, direct observational evidence for this identification is still elusive, and the only viable approach to confirm the Kerr black hole hypothesis is to explore and rule out any other possibility. Here we investigate the final stages of the accretion process onto generic compact objects. While for Kerr black holes and for more oblate bodies the accreting gas reaches the innermost stable circular orbit (ISCO) and plunges into the compact object, we find that for more prolate bodies several scenarios are possible, depending on the spacetime geometry. In particular, we find examples in which the gas reaches the ISCO but then gets trapped between the ISCO and the compact object. In this situation, accretion onto the compact object is possible only if the gas loses additional angular momentum, forming toruslike structures inside the ISCO.

On the Cauchy problem for nonlinear Schrödinger equations with rotation

We consider the Cauchy problem for (energy-subcritical) nonlinear Schrödinger equations with sub-quadratic external potentials and an additional angular momentum rotation term. This equation is a well-known model forsuperfluid quantum gases in rotating traps. We prove global existence (in the energy space) for defocusing nonlinearities withoutany restriction on the rotation frequency, generalizing earlier results given in [11, 12]. Moreover, we find that the rotation term has a considerable influence in proving finite time blow-up in the focusing case.

Rotating properties of Bragg reflections and spatial lattice solitons in rotating photonic lattices

We demonstrate the rotating properties of Bragg reflections and spatial lattice solitons in rotating photonic lattices by analyzing the linear and nonlinear propagations of light. It reveals that the Bragg reflection of the light waves rotates synchronously with the lattices, leading to the rotation of the Bloch waves during propagations. In the presence of nonlinearity, rotating lattice solitons from different transmission bands can propagate in a relatively stable manner. However, reduced-symmetry solitons at point X2 cannot easily rotate synchronously with the lattice, owing to Coriolis forces. Moreover, additional angular momenta are added to the off-axis propagating solitons.