Free-space Behavior Research Articles

Influence of thermal blooming on the beam quality of an array of Hermite-Gaussian beams propagating in the atmosphere.

The influence of thermal blooming on the quality of an array of Hermite-Gaussian (H-G) beams propagating in the atmosphere is studied, where the incoherent combination is considered. An analytical expression of the equivalent distortion parameter of such an array is derived and validated. As the mode order or the inverse radial fill factor of an array of H-G beams increases, the thermal blooming effect weakens, requiring more time to reach steady-state thermal blooming. The focal shift of an array of H-G beams in the atmosphere is also investigated. Owing to the thermal blooming effect in the atmosphere, the actual focus moves away from the geometric focus as the mode order decreases, which is different from the behavior in free space. Additionally, for an array of multimode beams, the actual focus moves away from the target as the weighting factor of TEM00 increases.

Fast atom effect on helium gas/graphite interfacial energy transfer

Energy transfer between gases and solid surfaces plays a vital role in many technologies. Thermal accommodation coefficient (TAC), which specifies the energy transferred between a surface composed of thermally-vibrating atoms and a colliding gas molecule, can be calculated by sampling the incident velocities of gas molecules by the Maxwell-Boltzmann (MB) distribution. However, the MB distribution describes gas behavior in free space rather than near a solid surface. In this work, we calculated the TAC between helium and graphite by a modified Maxwell-Boltzmann distribution accounting for the fast atom effect where gases of higher out-of-plane velocity have a larger chance of reaching the solid surface at the same period. A significant reduction in TAC is found by correcting the MB distribution with the modified-MB distribution, 32.7% for helium at 500 K and 10.74% at 300 K. Further analysis shows that the out-of-plane velocity component dominates the scattering process and energy transfer. This result underlines the importance and necessity to correct the MB distribution to incorporate the fast atom effect. Moreover, the lower and upper boundaries of the scattering events were depicted. These findings can provide a fundamental understanding of gas-surface energy transfer and guidelines for molecular dynamics simulations for TAC.

Quantum control of quasi-collision states: A protocol for hybrid fusion

When confined to small regions quantum systems exhibit electronic and structural properties different from their free space behavior. These properties are of interest, for example, for molecular insertion, hydrogen storage and the exploration of new pathways for chemical and nuclear reactions. Here, a confined three-body problem is studied, with emphasis on the study of the “quantum scars” associated to dynamical collisions. For the particular case of nuclear reactions, it is proposed that a molecular cage might simply be used as a confining device with the collision states accessed by quantum control techniques.

Dynamical tuning of energy transfer efficiency on a graphene monolayer

In this contribution we present a theoretical investigation of the energy transfer efficiency between quantum systems placed in proximity of a monolayer of conducting graphene. We calculate the spontaneous emission rate of a quantum system and the energy transfer rate between a donor-acceptor pair, and thus the energy transfer efficiency, using a Green's tensor formalism. The direct interaction between the donor and acceptor dominates when they are close to each other, but is modified from its free-space behavior due to the presence of the graphene monolayer and its interaction with the donor and acceptor. We report on a very large influence of the graphene monolayer on the energy transfer efficiency due to both the F\"orster mechanism and the propagating graphene plasmon mode. In particular, the F\"orster radius ${R}_{0}$ is modified from its free-space value of 20 nm and can reach values of 120 nm when close to a graphene monolayer. As the donor-acceptor separation is increased, their direct interaction is overshadowed by the interaction via the surface plasmon mode. Due to the large propagation length of the surface plasmon mode on graphene, an energy transfer efficiency as high as 50% can still be achieved for distances as large as 300 nm. The interaction via the surface plasmon mode is tunable via the doping of the graphene monolayer and the surface plasmon channel can also be switched off this way.

Influence of polychroism and decentration on spreading of laser beams propagating in non-Kolmogorov turbulence

Taking the mean-squared beam width, the angular spread, the Rayleigh range and the turbulence distance as characteristic parameters, the spreading of partially coherent polychromatic decentered laser beams propagating through non-Kolmogorov turbulence is studied. The influence of the bandwidth Ω of spectrum, the beam decentered parameter β, and the fractal constant α of the atmospheric power spectrum on the beam spreading is examined in detail. It is found that polychromatic laser beams are less sensitive to turbulence than monochromatic ones, and decentered laser beams are less sensitive to turbulence than centered ones. The larger the Ω and β are, the larger the beam spreading is in free space, but the less the beam spreading is affected by turbulence. In turbulence laser beams with different values of β will have the same directionality if the angular spread is chosen as the characteristic parameter of beam directionality, which is quite different from the behavior in free space. The beam spreading is largest and the influence of turbulence on the beam spreading is also largest when α is in a region about 3.1. In addition, the dependence of the turbulence distance on Ω and β is also investigated, and some interesting results are obtained.

Effects of atmospheric turbulence on the polarization of apertured electromagnetic Gaussian Schell-model beams

The effects of atmospheric turbulence on the polarization of apertured electromagnetic Gaussian Schell-model (EGSM) beams are studied. The closed-form expression for the degree of polarization of apertured EGSM beams propagating through atmospheric turbulence is derived. It is shown that on propagation over sufficient long distances the degree of polarization of apertured EGSM beams with different values of beam truncation parameter and beam coherence parameter tends to the same value at the source plane and becomes uniform due to turbulence, which is quite different from the behavior in free space. The smaller the beam truncation parameter is, the less the degree of polarization is affected by turbulence. In particular, the degree of polarization of apertured EGSM beams is less sensitive to the effects of turbulence than that of unapertured EGSM beams.

Diffraction properties of optical localized structures

The propagation properties of transverse localized structures are studied experimentally and numerically, showing that their free-space behavior is determined by the features acquired during the nonlinear light-matter interaction at their origin. For a Kerr-like nonlinearity in the optical loop, localized structures show a focalization followed by the formation of a local minimum of the intensity, these features reappearing outside the loop in the near-field region of their free-space propagation.

Dyson's equations for nucleon-pion-delta interactions: Toward a self-consistent solution in nuclear matter

A formalism is developed for the self-consistent solution of Dyson's equations for the pion and the delta in cold nuclear matter. The nucleon-delta and nucleon-pion “loop integrals” are evaluated and are inserted into the pion and delta-particle propagators, respectively. The resulting pion propagator displays two distinct families of “spikes” which are dramatically different from the free-space behavior. In particular, there are significant high-momentum components of the pion which give large contributions to the nucleon-pion loop integral. Such contributions lead to severe spikes in the iterated delta propagator. Studies are made too of the sensitivity of the results of the cutoff range of the “form factor” used to evaluate the loop integrals, to the starting solution, and to the density. Also, for the densities studies here, the pion propagator exhibits pure delta-function singularities, which occur over a small range of energies just above the mass of the pion. However, it is shown that such delta functions have a negligible effect on the main calculation. Finally, this paper gives a comprehensice treatment of the main methods used to solve the nuclear matter Dyson's equations.

The dielectric-filled tubular monopole

The integral equation for the current in a dielectric-filled tubular monopole when driven by a coaxial line with an assumed pure TEM mode is formulated and solved by a numerical method. It is found that when the dielectric-filled tube can support a TM <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">01</inf> circular waveguide mode, the characteristics of the antenna are drastically different. In this case a very sharp resonance behavior is observed. The resonant length depends primarily on the properties of the internal waveguide mode rather than on the free-space behavior of the antenna.